Chemoselective hydrogenation of carbonyl compounds over heterogeneous catalysts

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Applied Catalysis A: General 292 (2005) 1–49

Review

Chemoselective hydrogenation of carbonyl compounds over

heterogeneous catalysts

P. Maki-Arvela, J. Hajek, T. Salmi, D.Yu. Murzin *

Laboratory of Industrial Chemistry, Abo Akademi, Abo/Turku, Finland

Received 29 March 2005; received in revised form 13 May 2005; accepted 20 May 2005

Available online 1 August 2005

Abstract

Chemoselective hydrogenation of unsaturated aldehydes and ketones over heterogeneous catalysts is a demanding task. The achieved

selectivity levels depend both on the electronic and geometric structures of reactants and metal surfaces. Recent development in catalyst

preparation, selection of new solvents, catalyst and reactor structures have been reviewed in this work. Additionally, catalyst characterization

technique, especially alloy formation and in situ characterizations have been discussed in detail. The deep understanding of the catalyst

structure is necessary for development of tailor-made catalysts for chemoselective hydrogenations. The state of art in both kinetic and

molecular modelling used in chemoselective hydrogenations was presented. Some special features, like conversion–selectivity relationship

and the performance in gas and liquid phase hydrogenations were discussed.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Chemoselective hydrogenation; Unsaturated aldehyde; Heterogeneous catalyst

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Reactant structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Choice of catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1. Metal selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1. Monometallic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.2. Bi- and multimetallic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2. Metal particle size and shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3. Metal precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.4. Support selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4.1. Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4.2. Zeolites and mesoporous materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.4.3. Clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4.4. Porous metal catalysts of Raney type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4.5. Carbons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4.6. Colloidal metal catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.4.7. Structured supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.5. Catalyst preparation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.6. Catalyst pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6.1. Catalyst reduction temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6.2. Catalyst calcination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.6.3. Catalyst aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

* Corresponding author.

E-mail address: [email protected] (D.Yu. Murzin).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.05.045

P. Maki-Arvela et al. / Applied Catalysis A: General 292 (2005) 1–492

3.7. Catalyst modifiers and promoters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.7.1. Adsorbed modifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4. Hydrogenation kinetics and catalyst deactivation in gas and liquid phase hydrogenations . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.1. Effect of reaction parameters on the catalytic activity and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1.1. Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1.2. Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1.3. Initial substrate concentration and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.2. The choice of solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3. Side reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.4. Deactivation and catalyst recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.5. Comparison of gas and liquid phase hydrogenations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.6. Metal leaching and the role of homogeneous catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5. Experimental and theoretical adsorption studies as a tool for proposing the surface reaction mechanisms. . . . . . . . . . . . . . . 39

5.1. Adsorption studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.1.1. Experimental adsorption studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.1.2. Theoretical calculations in adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.1.3. Bond order conservation Morse potential model in chemoselective hydrogenations . . . . . . . . . . . . . . . . . . . 41

6. Kinetic modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7. Reactor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

8. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

1. Introduction

The chemoselective hydrogenation of a carbonyl bond in

multi-unsaturated aldehydes and ketones is a difficult task [1–

4], since thermodynamics favors C C hydrogenation over

C O by ca. 35 kJ/mol [5]. These unsaturated alcohols are

used as fragrances and drugs and thus this field has an

industrial interest. In principle these compounds can be

selectively hydrogenated by using homogeneous catalysts,

but the heterogeneous catalysts are environmentally more

friendly and easier to separate and re-use than their

homogeneous counterparts. The rational design of active

and selective heterogeneous metal supported catalyst is not

however very easy task. There are several factors, which can

affect the activity and selectivity of a catalyst. These are metal

and support selection, metal precursor, catalyst preparation

and activation methods, selection of reaction conditions and

operation mode (e.g. gas or liquid phase system).

It is thus important to understand the catalyst structure

and relate performance of the catalyst (e.g. activity and

selectivity) to its structure. This task is quite demanding,

because hydrogenation over heterogeneous catalysts pro-

ceeds via several surface reaction steps, like adsorption,

reaction and desorption. Additionally, the reaction mechan-

ism is rather complicated including competitive/non-

competitive and dissociative/non-dissociative adsorption

as well as adsorption of solvents, formation of coke etc.

In order to rationalize the catalyst preparation and to be able

to achieve higher production capacities in intermediate

products with high selectivities, it is important to get a whole

picture of all these phenomena on the catalyst surface.

The selectivity to an intermediate product is defined as

a ratio between the yield of the desired product and the

conversion. High selectivity to an intermediate product (in

this case an unsaturated alcohol) at high conversion level

is from synthetic point of view a very important topic. It

should, however be pointed out that the selectivity can be

strongly dependent on the conversion level. This issue is

summarized in more detail in Section 4.5. Regarding

the effect of other parameters, like substrate structure,

catalyst type and structure as well as solvent etc., on

product selectivity, these parameters are interdependent.

These effects will be treated separately in the review with

the aim to show the influence of each parameter on

selectivity.

Kinetic modelling is important for optimization of the

reactor performances. Such models are usually based

primarily on kinetic data, but recently physical methods of

in situ catalyst characterization have given additional

information for proposing, for example reaction inter-

mediates, which can be used in kinetic models. Moreover,

kinetic data can be interpreted by means of molecular

modelling, which gives the most stable adsorption modes.

These results can be combined with spectroscopic adsorp-

tion measurements. The adsorption mode of the reactant

can, for example vary with increasing initial concentration

of the reactant.

There exist several reviews from chemoselective hydro-

genations of carbonyl compounds. One very recent review

on kinetics of liquid phase hydrogenation reactions is

published by Singh and Vannice [1]. Other reviews found in

the literature are from Gallezot [2], where the following

items, like metals, metal particle sizes, supports, catalyst

modifiers, graphite, solvents and side reactions were

presented. Additionally, Ponec [3] considered effect of

promoters, alloys, poisons, structure sensitivity, zeolite,

P. Maki-Arvela et al. / Applied Catalysis A: General 292 (2005) 1–49 3

alkali, TiO2 and Kluson and Cerveny [4] discussed

ruthenium catalysts in selective hydrogenation, addressing

the following topics: bimetallic catalysts, influence of

promoters effect, particle size, effect of titania, different

support, impact of the active metal and chlorides.

Furthermore, Mohr and Claus [5] have published a review

regarding hydrogenation properties of gold particles. The

aim of the present paper is to review the trends in

chemoselective hydrogenations of carbonyl compounds

appeared in the literature mainly after 1997 covering a

very broad range of topics, e.g. catalyst properties,

optimization of reaction conditions, kinetics and modeling.

Physico-chemical methods of catalyst characterization and

the adsorption phenomena will be described in the present

work as well. Finally, the reactor selection will be presented

in Section 7.

New support materials used in catalytic hydrogenations,

like silica fibers, polymer and carbon nanofibers, monoliths,

clays, supported Pd-complex, as well as impregnated

catalyst modifiers, heteropolyacids will be also considered

as well as application of non-conventional metals, like silver

and gold in chemoselective hydrogenations. The structure

sensitivity, i.e. the effect of metal particle size on the activity

and selectivity will be discussed in Section 3.2. Bimetallic

catalysts and reducible supports have been used quite for

long time to boost chemoselectivity. Recent advancements

in their applications along with a possibility to combine

these two properties will be covered.

Kinetics analysis, i.e. the analysis of conversion–

selectivity relationship, which gives information about the

surface reactions on the catalyst, will be also discussed. The

adsorption strength and concentrations of different com-

pounds on the surface as well as the solvent selection in

liquid phase hydrogenations can change both the activity

and selectivity.

Gas and liquid phase hydrogenations are compared in this

work, revealing the differences in gas and liquid phase

reactions, for instance in the changes of product selectivities

as a function of reactant conversion. The possible reasons for

these differences are discussed. Additionally, catalyst

deactivation and new reactor concepts in fine chemical

applications, like recirculating reactor, trickle bed and

monolithic reactors used the hydrogenations will be

discussed.

2. Reactant structure

Reactant structure can affect the maximum selectivities

for desired products obtained over heterogeneous catalysts.

The branching in the vicinity of ethylenic double bond

favors high selectivities for unsaturated alcohols, which

means that the selectivities for unsaturated alcohols decrease

in the following reactants: cinnamaldehyde >3-methylcro-

tonaldehyde > crotonaldehyde > acrolein [6]. There are

several reasons, which might affect the product selectivity

with different substrates, like electronic and inductive

effects in the reactant, adsorption mode of the reactant and

the geometrical restrictions of the metal surface. Adsorption

mode of the reactant can change with changing the initial

concentration of the reactant [7]. Moreover the metal surface

has geometrical limitations for reactant adsorption, i.e.

metal particle size and shape, as well as amount of steps,

edges and kinks is essential for high selectivity. The metal

surface properties are discussed in Section 3.2.

The selectivities to unsaturated alcohols starting from

different substrates over conventional monometallic cata-

lysts are reported in Table 1. Besides unsaturated aldehydes

also data on aromatic aldehydes are presented. Additionally,

the maximum selectivities to the desired products over

heterogeneous catalysts are given in Table 2. Citral (1)

hydrogenation has been investigated in the liquid phase over

different catalysts. In citral hydrogenation the desired

products can be unsaturated alcohols, nerol (2) and geraniol

(3) or citronellol (4) as well as unsaturated aldehyde

citronellal (6) (Fig. 1) [8]. Over Ru/C the selectivities to

nerol and geraniol are quite low (Table 1) [14], but with Pt/

SiO2 the selectivity is enhanced to 76% [10]. Recently very

high selectivities to nerol and geraniol have been reported by

Milone et al. [11], Reyes et al. [12] and Malathi et al. [13]

over Au/Fe2O3 [11], Ce/AC [14], Pt-Sn/MgO [15], Ir/TiO2

[12], Pt/TiO2 [13,16]. In the two latter catalysts, the role of

reducible support is important (see Section 3.4.1.2). The

bimetallic catalysts, like Ir-Ge/TiO2–HTR, were both active

and selective [12]. The higher activity of the bimetallic

catalyst was attributed to the presence of ionic species of Ge

(see Section 3.1.2.6). Very high selectivity to nerol and

geraniol, 95.2% at 95.6% conversion was obtained over Ru-

Fe/C catalyst in citral hydrogenation in methanol under

40 bar hydrogen [17,18]. The reaction mixture contained,

however, a basic additive, triethyl amine, which can increase

the chemoselectivity (see Section 3.7.1). Similarly bime-

tallic Pt-Sn/MgO catalyst was active and selective for

producing nerol and geraniol in a continuous trickle bed

reactor [15] (Section 7). One drawback with this catalyst was

the long induction period of 20 h, during which the

conversion and selectivity increased. The third desired

product from citral hydrogenation, an unsaturated alcohol

citronellol can be achieved over Ni/Al2O3 catalyst with the

selectivity of 95% in ethanol at 70 8C [8]. The catalyst

pretreatment is an important factor for achieving high

selectivities to citronellol over this catalyst in alcohol

solvents (see Section 4.3). Citronellal, a partially unsatu-

rated aldehyde, which is used as an intermediate for

production of fragrances, can be synthesized over supported

Pd catalysts, like Pd supported on polyethylene fibers

containing grafted 4-vinylpyridine groups [19], and Pd/

SiO2/AlPO4 [20]. In general, Pd as the main metal is very

selective for hydrogenating aliphatic unsaturated aldehydes,

like citral to citronellal (5) with selectivity of 86% and minor

formation of alcohols [19]. Pd/C catalyst was used in citral

hydrogenation to citronellal in methanol and in the presence


Table 1

The selectivities to the desired product at one particular time corresponding to the given conversion in the chemoselective hydrogenation of aldehydes and

ketones over conventional monometallic catalysts

Reactant Product Catalyst Selectivity (%) Conversion (%) Ref.

Ru/C 39 70 [14]

Pt/SiO2 76 30 [10]

Os/SiO2 100 5 [21]

Ir/SiO2 60 8 [21]

Ni/Al2O3 94 100 [8]

Pd/polymer fiber 85 40 [19]

Pd/SiO2 69 5 [21]

Rh/SiO2 74 5 [21]

Cu/SiO2 83.5 68.2 [22]

Ni/SiO2 90 25 [23]

Ni/Y 74 70 [24]

Ag/SiO2 94.8 n.m. [25]

Ni/Y 90 10 [24]

Ni/Y 90 8 [24]

Ru/Al2O3 30 40 [27]

Pt/Y 92 100 [158]

Pt/MCM-48 96.6 100 [142]

Pt/Al2O3 8 50 [30]

Pt 98 98 [33]

Cu/MgO


Table 1 (Continued )


Pt/Al2O3 0 <5 [34]

Rh/SiO2 2 10 [35]

Rh/SiO2 15 30 [58]

Rh/SiO2 0 100 [40]

Pt/Al2O3 92 85 [41]

Pt/C 3 100 [43]

Pd/SiO2 64 100 [81]

of triethylamine as additive the selectivity to citronellal was

94% at 99.5% conversion [209].

Molecules containing besides a carbonyl group also a

phenyl ring can be quite selectively hydrogenated to

corresponding aromatic due to the fact that aromatic ring

is less prone to hydrogenation. Data on benzaldehyde (6)

[22,23], 2-methylbenzaldehyde (7) [23] and acetophenone

(10) [24,25] are available in the literature. Benzaldehyde

hydrogenation over Pd/polymer [26] and Cu/SiO2 catalysts

was very selective to benzyl alcohol being 95 and 83%,

respectively [22]. Not only liquid phase but gas phase

hydrogenation of benzaldehyde and 2-methylbenzaldehyde

over Ni/SiO2 resulted in over 90% selectivities to

corresponding unsaturated alcohols at 120 8C, whereas

application of higher temperatures decreased the selectivity

[23]. Hydrogenation of 2-methylbenzaldehyde was slower

than the hydrogenation of benzaldehyde. This observation

was interpreted as an increased stability of 2-methylben-

zaldehyde exhibiting an ortho-methyl substituent compared

to benzaldehyde. The decreased rate for hydrogenation of 2-

methylbenzalaldehyde is due to electronic effect [23]. In

benzaldehyde the carbonyl group is electron deficient

because of the mobile B cloud form benzyl ring and thus

it is easier to reduce the polarized carbonyl group. Methyl

group in O-position donates electron density to the ring

retarding hydrogenation. Additionally, methyl group intro-

duces a new adsorption centre in the molecule. Benzalde-

hyde hydrogenation was also investigated over a bimetallic

Pt-Sn/SiO2 catalyst [29] and in acetophenone hydrogenation

in the liquid phase over Ni/Y [24] (Fig. 2) and in the gas

phase over Ag/SiO2 leading to very high selectivities to

phenyl ethanol, i.e. 100% [24] and 94.8% [25], respectively.

Chemoselective hydrogenation of p-hydroxyl- (12) and p-

(2-methylpropyl)-derivatives (14) of acetophenone has also

been quite selective to corresponding unsaturated alcohols

(13) and (15), respectively over Ni/Y catalyst, but the

achieved conversions were low [24]. Cinnamaldehyde (16)

hydrogenation yielded, however, quite low selectivities of


Table 2

The maximum selectivities to the desired product at one particular time corresponding to the given conversion in the chemoselective hydrogenation of aldehydes

and ketones


Ir/TiO2 100 (geraniol) 37 [12]

Pt/TiO2 100 (geraniol) 30 [13]

Os/SiO2 100 5 [21]

Ru/C, MgO 100 98 [14]

CeO2 98 [14]

100 50 [100]

Ce/AC 70 [14]

97.7

Pt-Sn/MgO 100 [15]

96.98

Au/Fe2O3 90 [11]

Ni/Al2O3 94 100 [8]

Pd/polymer fiber 87.5 95 [19]

Pd/SiO2 69 5 [21]

Cu/SiO2 83 68 [22]

Ni/SiO2 92 5 [23]

Ni/Y 74 70 [24]

Ag/SiO2 94.8 n.m. [25]

Ni/Y 90 10 [24]

Ni/Y 90 8 [24]

Ra-Co 83 30 [28]

Mo-Co-B Close to 100 100 [32]

CuPMo on Ra-Ni 98.5 98.1 [31]

Cu/MgO 98 98 [33]


Table 2 (Continued )


Au/ZrO2 37 5 [95]

Pt/ZnO 80 <20 [36]

90 7.3 [67]

Rh/SiO2 0 100 [40]

Pt/Al2O3 92 85 [41]

Pt/Sn/C 100 Very low [43]

Pt/C 96 100 [43]

cinnamyl alcohol (17) (30%) over Ru/Al2O3 catalyst [27]

(Fig. 3). Over Raney cobalt catalyst the selectivity to

cinnamyl alcohol (12) was 83% (Table 2) [28]. In

cinnamaldehyde the phenyl group, which is conjugated to

the ethylenic double bond, creates electronic effect inside

the molecule. This electronic effect is larger than in

crotonaldehyde (22) [29] (Fig. 4).

The selectivity of 85% in cinnamylalcohol was

achieved over bimetallic 5%Ru-5%Sn/SiO2 catalyst in

2-propanol [122]. The high yield was attributed to the

promotional effect of tin as well as to decreased reaction

temperature. Higher selectivity of 92% to the unsaturated

alcohol was recently obtained in hydrogenation of

cinnamaldehyde over ion-exchanged 14% Pt/NaY [158].

In this case, the selectivity is most assumably improved by

the solvent (2-propanol/H2O/sodium acetate) and by the

ion-exchange technique used during the preparation of the

catalyst. The microporous, montmorillonite based 5% Pt/

K-10 gave cinnamylalcohol in very high selectivities.

Typically the selectivity was over 90%, the highest one,

99% was obtained at 25 8C under the pressure of 4 bar

[146]. Promissing results were obtained in supercritical

CO2. The references indicate selectivity 98% [195].

Application of bimetallic Ru-Pt catalyst supported on

MCM-48 is reported to give only cinnamylalcohol [196].

The high selectivity is explained by the interactions

between the carbonyl group and the CO2 molecules and by

the improved mass transfer.

The hydrogenation rate of benzaldehyde was higher than

that for butyraldehyde or butanone over Pt/SiO2 catalyst

[29]. This behavior was interpreted as an inductive effect of

the phenyl group lowering the bonding strength of carbonyl

on the metal surface. In fact, bonding strength of the

carbonyl was characterized by FTIR showing the change in

the CO stretching. Opposite to phenyl containing molecules

furfural (18) is not easy to hydrogenate selectively to

furfuryl alcohol (19) over Pt/Al2O3 [30] (Table 1), but high

selectivities have been obtained over CuPMo-RaNi [31],

over Mo-CoB catalyst [32] and over Cu/MgO catalyst [33]

(Table 2) (Fig. 5).


Fig. 1. Reaction scheme for citral hydrogenation [8].

One of the most challenging tasks is to hydrogenate

selectively acrolein (20), in which there are no large steric

hindrances to hydrogenate ethylenic double bond. Absence

of selectivity in carbonyl group hydrogenation was obtained

in case of acrolein over Pt/Al2O3 catalyst [34], whereas very

low chemoselectivities have been reported for crotylalcohol

(23) over Rh/SiO2 catalyst [35]. In crotonaldehyde hydro-

genation in the gas phase the highest selectivities have been

80% over Pt/ZnO [36] and over Pt supported mesoporous

silica containing CeO2 [37], whereas only 34% selectivity to

crotyl alcohol was obtained over Ir/TiO2 catalyst reduced at

Fig. 2. Reaction scheme for hydrogenation of acetophenone [56].

high temperature [38], i.e. when SMSI state was achieved

(see Section 3.4.1.2). Over a monometallic Co/SiO2 catalyst

the selectivity to crotyl alcohol was maximally 20% [39]. Co

exists in this catalyst in four different surface structures

according to DRIFTS. The DFT (density functional theory)

calculations have been used as a tool to predict the

adsorption geometries of acrolein, prenal and crotonalde-

hyde on metal surfaces [135] and to interpret the obtained

product selectivities (see Section 5.1.2).

Selective hydrogenation of a,b-unsaturated ketones to

a,b-unsaturated alcohols was proven also to be a diffucult

task [6]. One example can be seen in the hydrogenation of 2-

cyclohexenone (24), which yields over Rh/SiO2 catalyst

only cyclohexanone and cyclohexanol as products, but not

an unsaturated alcohol, 2-cyclohexenol (25) [40]. Unsatu-

rated alcohols were, however, produced in the chemoselec-

tive hydrogenation of 2,6,6-trimethyl-2-cyclohexen-1,4-

dione (26) [41], where 92% selectivities to an allylic

alcohol (27) were obtained over Pd/alumina catalyst.

Monometallic Pd/Y [42] and Pt/C [43] catalyst resulted

only in carvoneacetone (30) in carvone (28) hydrogenation,

but over a bimetallic Pt/Sn catalyst very high initial

selectivities to carveol (29) were obtained. The selectivity

dropped at higher conversions due to further hydrogenation

to carvotanalcohol and carvomenthol [43].

3. Choice of catalyst

Heterogeneous catalysts used in hydrogenation reactions

contain usually a metal supported on a carrier. The metal is


Fig. 3. Reaction scheme for cinnamaldehyde hydrogenation [52].

able to adsorb hydrogen thus making hydrogenation reaction

possible. The carrier is able to disperse the metal to smaller

particles as compared to bulk metal enhancing the specific

metal surface area. Additionally, smaller metal particles

partially behave as non-metals, e.g. have higher electron

densities, leading to higher hydrogenation rates compared to

larger particles. In addition to electronic effects, which are

different for different metals, geometric properties of the

metal particles can affect the hydrogenation rate and

selectivity if the size of reacting molecule is close to the

size of metal crystallites. The electronic properties of the

active metal can be modified by a promoter. In this paper the

promoter is defined very broadly, i.e. a promoter can be a

non-inert support, second metal, oxide or other additive,

which can change the geometric and the electronic structure

of the metal surface. The metal phase composition and the

amount of edges and corners vary with different metal

particle sizes. Activity and selectivity depend not only the

metal type and dispersion but also on the support. The

Fig. 4. Reaction scheme for crotonaldehyde hydrogenation [35].

catalyst carriers should be able to disperse and stabilize the

active metal. Support materials can exhibit inert, basic or

acidic properties. Furthermore, non-inert carriers can form

an alloy with an active metal. In this section, metal selection

and the recent catalytic results by using monometallic and

bimetallic catalysts from different groups are presented. A

separate section is devoted to catalysts containing both one

and several metals on reducible supports. Furthermore,

different methods for characterization of bimetallic catalysts

are shortly discussed in Section 3.1.2.1. The most important

catalyst properties, like metal particle size and shape, metal

precursor, support selection, catalyst pretreatment and the

effect of modifiers and promoters and their influence on the

chemoselective hydrogenation of carbonyl compounds are

reviewed below.

3.1. Metal selection

Different geometric and electronic properties of metals

can affect the hydrogenation activities and selectivities by

influencing not only surface reactions, but also adsorption,

the latter being a prerequisite of any heterogeneous catalytic

Fig. 5. Reaction scheme for furfural hydrogenation [31].


reactions. The adsorption phenomenon itself is limited by

many factors, e.g. by the mode of reactant adsorption on a

specific metal surface. Different crystal phases of metals

have different geometrical constraints, which explain the

need to study performance of distinct crystallographic faces.

In face-centered cubic (FCC) materials the most studied

metal faces are (1 1 1), (1 0 0) and (1 1 0) faces. The amount

of steps is lower in (1 1 1) face than in (1 0 0) face [44].

Pt(1 1 1) surface is a flat, dense surface, while number of

edges and kinks is increasing with decreasing metal particle

size. Transition metals, which are used in catalysts, have

partially filled or filled d-orbital. When the d-orbital is filled,

the metal is in relative scale catalytically inactive, like in

case of silver, gold and copper. The interstitial electron

density in metals having filled d-orbital is low and they are

not eager to form bonds. However, the electronic structure of

the metal can be changed by adding a second metal, leading

for instance to alloy formation, by changing the metal

particle size or by enhancing interactions with the support.

In this section the importance of metal selection in

chemoselective hydrogenation of unsaturated compounds

will be presented by taking into account the structure of the

metal, i.e. in which group in the periodic table it is located.

Additionally, the bimetallic catalysts, the metal dispersion,

the catalyst pretreatment as well as the effect of support and

additives will be considered with the main aim to correlate

the catalyst structure with activity and selectivity. As it will

be seen a rational catalyst design for chemoselective

hydrogenation yielding predefined selectivities and activ-

ities is a very demanding task.

3.1.1. Monometallic catalysts

In the conventional monometallic catalysts the metals are

selected from the group 10, e.g. Ni, Pd and Pt. In addition,

Rh from group 9 and Ru from group 8 have been used in

catalysts. Very high selectivities to unsaturated alcohols

(83–100%) at high conversions have also been reported over

a pyridine poisoned Re catalyst [197], which belongs to

group 7. The supports are usually alumina, silica and carbon.

The catalytic activity of different metal supported catalysts

in hydrogenation is determined by ability to activate C C

and C O bonds as well as the activity of hydrogen to react

on the metal surface. Hydrogen activity on different metals

is relatively well understood on the basis of adsorption

studies and fundamental theories [44]. According to Masel

[44], hydrogen is reactive in the surfaces of Co, Ni, Ru, Rh,

Pd, Os, Ir, Pt as well as on Sc, Ti, V, Y, Zr, Nb, Mo, La, Hf,

Ta, W, Cr, Mn, Fe, Tc and Re. A slower uptake of hydrogen

was observed with Cu, whereas Ag and Au were inert [44].

Interestingly to note that ability of hydrogen to react and to

dissociate on the surfaces of Au and Ag depends on the size

of metal nanoparticles. Exceptional catalytic activities of

gold nanoparticles in hydrogenation and oxidation reactions

were recently reported [46,47]. Moreover the filling of

d-orbital affects very much the adsorption mode. The two

latter metals, Au and Ag have completely filled d orbitals,

5d10 and 4d10, and thus they exhibit very low catalytic

activities. No adsorption can occur on the metal having a low

interstitial electron density [44]. Generally there has been a

correlation between the width of the d bands of metal

catalysts and selectivities in the hydrogenation of a,b-

unsaturated aldehydes [45]. The d bandwidth increases in

the following order Pd < Pt < Ir ffi Os. The repulsive

interaction of the metal with the C C bonds is stronger

with the metals exhibiting larger d bandwidth. Thus, the

chemisorption of C C bond becomes less probable. The

hydrogenation activities and selectivities are presented for

different groups in the periodic table by starting from group

8. Note that the selectivities to the desired products over

metal supported catalysts depend not only on the metal, but

also on the reactant structure [29]. In some hydrogenation

reactions, like in the hydrogenation of citral (8), the

monometallic supported catalysts are very selective,

whereas selective hydrogenation of acrolein [34] and

crotonaldehyde [35] cannot be achieved over conventional

supported monometallic catalysts. Additionally, it could be

pointed out that large selectivity changes have been

observed in, e.g. citral hydrogenation over a monometallic

Pt/SiO2 catalyst just by changing the reaction temperature

[10] (see Section 4.1.1).

3.1.1.1. Chemoselective hydrogenation over group 8 me-

tals, Ru, Os. The group 8 metals are electropositive,

because they have unfilled d orbitals. Supported ruthenium

catalysts have been recently used in chemoselective

hydrogenations [8,14,27,45,48–53], like in hydrogenation

of cinnamaldehyde [27,51,52], crotonaldehyde [14] and

citral [8,14,49,50].

Os has been used in very few papers [21,45,183] as a

catalytic metal supported on SiO2 [21,45] and on TiO2 [183].

In the hydrogenation of oxopromegestone Os was the most

selective among tested metals, but the catalytic activity was

low. Catalytic activity of Os in hydrogenation reactions is,

however, well documented for a long period of time. For

instance, F. Haber applied Os catalysts, in ammonia

synthesis already a century ago. In citral hydrogenation at

27 8C under 1 bar, hydrogen in hexane the selectivity to

nerol and geraniol was close to 100% at 5% conversion [21].

The initial TOF-values were, however, very low over Os/

SiO2 catalyst [21].

The performance of Ru/C and Ru/Al2O3 catalysts was

compared in citral hydrogenation [14]. They exhibited the

same TOF in gas phase hydrogenation of crotonaldehyde,

but alumina supported Ru catalyst was more selective for

crotyl alcohol than Ru/C. The selectivities were however

relatively low, i.e. 17 and 3%, respectively. At the same time

these two catalysts afforded in citral hydrogenation at the

conversion level of 70% selectivities to nerol and geraniol 54

and 38%, respectively [14]. The selectivities in citral

hydrogenation at 60 8C are additionally lowered by

simultaneous side reactions, like cyclisation and acetaliza-

tion of citronellal, when the solvent was 2-propanol. The Ru/


C catalyst exhibited three times higher activity than Ru/Al2O3

calculated per gram of Ru [14]. Ru/C was tested also in citral

hydrogenation in the work of Maki-Arvela et al. [8]. This

catalyst gave about 22% selectivity to nerol and geraniol in

citral hydrogenation in 2-pentanol at 70 8C at 75% conver-

sion. The lower selectivity in [8] compared to [14] might be

explained by the origin of the metal precursor, in the former

work the precursor was chloride whereas in the latter one it

was nitrate. In particular, chloride precursor is known to favor

acetalization [51], which in principle could be avoided in

2-pentanol. However, in hydrophobic 2-pentanol quite

large amounts of isopulegol (12%) are formed [8]. High

selectivities for nerol and geraniol over Ru/Al2O3 and Ru/

TiO2, 47.6 and 73.4% at 100% conversion, respectively, were

obtained in citral hydrogenation in n-heptane at 126 8C and

50 bar [49]. The selectivity to nerol and geraniol was 73.4%

over Ru/TiO2 catalyst reduced at high temperature, when the

hydrogenation was carried out at 126 8C and at 50 bar in n-

heptane as a solvent [49]. In [21] the selectivity to nerol and

geraniol was 58% at 5% conversion of citral in citral

hydrogenation at 27 8C under 1 bar hydrogen in hexane. The

initial hydrogenation rate, however, over this catalyst was

quite low. Relatively low selectivities were obtained over Ru

supported catalysts [27,48]. Only 13% selectivity of cinnamyl

alcohol at 20% conversion was obtained over Ru/SiO2

catalyst in 2-propanol at 60 8C and 10 bar [27], whereas

Ru/Al2O3 catalyst gave higher selectivity (about 40%) to

cinnamyl alcohol. Cinnamyl alcohol selectivity over Ru/CNF

catalyst (CNF stands for carbon nanofiber) was 48% at 60%

conversion of cinnamaldehyde at 110 8C and 45 bar in

2-propanol [48].

3.1.1.2. Chemoselective hydrogenation over group 9 me-

tals, Co, Rh, Ir. Monometallic catalysts containing a metal

from group 9 have been used in several chemoselective

hydrogenations. Applications of Rh [8,40,54–58], Co

[21,39,59] and Ir [12,21,38,60] were reported. Monome-

tallic catalysts including a metal from group 9 can act quite

selectively in some cases, like in citral, acetophenone and

cinnamaldehyde hydrogenations. The achieved selectivities

depend, however, very much on the reactant structure. Ir

catalysts have been very selective to unsaturated alcohols,

but rather inactive.

Monometallic Rh catalysts have been relatively selec-

tive in acetophenone [56] and in cinnamaldehyde [61]

hydrogenations, but low selectivities were obtained in

crotonaldehyde, citral and cyclohexenone hydrogenations.

Acetophenone hydrogenation over Rh/C catalyst resulted in

about 70% selectivity to unsaturated alcohol at 25 bar and

80 8C in cyclohexane [56]. Similarly, 70% selectivity to

cinnamyl alcohol was achieved over Rh/SiO2 catalyst at 50 8Cand 1 bar in ethanol [61]. In case of crotonaldehyde the

selectivities to crotyl alcohol were low over Rh/SiO2 [57] and

over Co/SiO2 [39], i.e. Rh/SiO2 catalyst yielded relatively low

selectivities to crotyl alcohol (18% over Rh/SiO2 catalyst

prepared by sol–gel method from Rh(acac)) [57] at 100 8C,

and 2% over 0.5 wt.% Rh/SiO2 at 35 8C (Rh from RhCl3prepared by impregnation) [35]. The initial selectivity to

crotyl alcohol was, however, close to 100% at the conversion

of 0.2% in crotonaldehyde hydrogenation at 40 8C over a

calcined Rh/Al2O3 catalyst indicating that catalyst pretreat-

ment is important [62] (see Section 3.6.2). The selectivity to

crotyl alcohol decreased linearly with increasing time-on-

stream. In citral hydrogenation over Rh/Al2O3 the main

product was citronellol (60% selectivity) in 2-pentanol at

70 8C and 1 bar and below 1% nerol and geraniol were

obtained [8]. Polymer supported Rh catalyst gave as a major

product citronellal [55]. No unsaturated alcohols were also

formed over Rh/SiO2 catalyst in the hydrogenation of 2-

cyclohexenone at 25 8C and at 6.8 bar in 2-propanol [40]. The

reason for this is the presence of sterically hindered carbonyl

bond. In [21] over Rh/SiO2 citronellal was the main product in

citral hydrogenation at 27 8C under atmospheric pressure in

hexane. Polymer supported Rh catalyst gave as a major

product citronellal [55].

As the d bandwidth of Ir is larger than for Pd or Pt, it

could be expected that the hydrogenation of ethylenic bond

decrease. In fact Ir has been very selective in citral

hydrogenation to nerol and geraniol, but the drawback has

been in the low reaction rate [12]. Ir/SiO2 was very selective

in the hydrogenation of citral at 70 8C and 4.1 bar in ethanol

affording 100% selectivity of nerol and geraniol. On the

other hand only 7% selectivity to nerol and geraniol was

observed in citral hydrogenation at 27 8C and 1 bar

hydrogen in hexane at 23% conversion [21]. It might be

that the solvent also has an effect on selectivity (see Section

4.2). On Ir/TiO2 selectivity to nerol was also 100% [12], but

over both catalysts the total conversion was very low.

Ir/graphite catalyst was very selective to cinnamyl alcohol in

the hydrogenation of cinnamaldehyde at higher tempera-

tures, i.e. 110 8C and 130 8C in the pressure range of 10–

30 bar in 2-propanol [60]. Additionally, the selectivity to

cinnamyl alcohol increased with increasing conversion.

Co/SiO2 resulted in maximally 20% selectivity of crotyl

alcohol at 120 8C and 1 bar in gas phase [39]. In general,

however, it can be stated that Co being not a noble metal, is

more selective to unsaturated alcohols than Rh, Ru or Pt. The

selectivity to unsaturated alcohols can be tuned by changing

the surface structure of Co, which exists in four different

forms [39]. Relatively high selectivities to nerol and geraniol

(55%) were obtained at 5% conversion over Co/SiO2 in

citral hydrogenation under 1 bar hydrogen at 27 8C in

hexane [21].

3.1.1.3. Chemoselective hydrogenation over group 10

metals, Ni, Pd, Pt. Platinum has been the most intensively

used as the active metal in chemoselective hydrogenations

[1,10,13,15,16,21,28,30,34,36,43,48,64–80,112,113],

whereas Ni [8,21,23,24,72] and Pd supported catalysts

[19,20,21,26,27,74,81–84] are less applied. Some recent

results in chemoselective hydrogenations over Pd, Ni and Pt

supported catalysts are reviewed below.


Pt is not intrinsically selective to produce unsaturated

alcohols, although high selectivities to unsaturated alcohols

could be still achieved depending very much on the sterical

structure of the reactant. No allyl alcohol was formed in the

gas phase hydrogenation of acrolein over Pt/black at 50 8Cand the main product was propanal [34]. Relatively high

selectivities have, however, been achieved in cinnamalde-

hyde, citral and furfural hydrogenations. In cinnamaldehyde

hydrogenation over Pt on graphite and Pt/SiO2 catalysts the

selectivities to cinnamyl alcohol were 81.5% [85] in toluene/

water mixture at 25 8C and 80% in ethanol. Similarly high

selectivity (92%) was obtained over Pt supported on NaY

zeolite; the catalyst was prepared by ion-exchange [158].

Even higher selectivity, about 99% towards cinnamylalcohol

has been reported over montmorillonite supported Pt [146].

High selectivities to nerol and geraniol can be achieved in

citral hydrogenation over Pt/TiO2 at 45 8C and 1 bar in

ethanol and Pt/SiO2 catalysts at 150 8C and 20 bar in hexane,

i.e. 100% (selectivity to geraniol) and 76%, respectively

[13,77]. Furfural can be selectively hydrogenated to furfuryl

alcohol (96% at 58% conversion) in 2-propanol–water

solvent at 150 8C and 20.6 bar hydrogen over a Pt/C catalyst

[79].

Pd differs from the other metals in group 10 from the

selectivity point of view, as it is very selective to hydrogenate

ethylenic bond [19]. It is believed in [27] that the reason for

this is the small d bandwidth of Pd, which affects the

adsorption mode of the reactant. Pd favors tetrahepto di-p

geometry through both double bonds. Citronellal was the

major product in citral hydrogenation over Pd/polymer

catalyst (see Section 3.4.7.1) at 70 8C and 4.5 bar in 2-

propanol [19] as well as over Pd/SiO2 catalyst at 27 8C under

1 bar hydrogen in hexane [21]. One exception is hydrogena-

tion of a carbonyl group in aromatic compounds [26,27],

where selective hydrogenation of carbonyl group can be

achieved. For instance 95% selectivity to benzyl alcohol was

reported by [26] in benzaldehyde hydrogenation over a cross-

linked Pd polymer catalyst at 25 8C and 10 bar hydrogen

pressure in methanol. In the liquid phase hydrogenation of

cinnamaldehyde over a Pd/SiO2 catalyst, however the

maximum selectivity to cinnamyl alcohol was around 32%

at 90% conversion at 60 8C and 10 bar in 2-propanol [27].

Ni supported catalysts are very selective in some

chemoselective hydrogenations. Ni favors formation of

citronellol in citral hydrogenation at 70 8C and 1 bar in

ethanol [8]. Citronellal was, however, the main product in

citral hydrogenation over Ni/SiO2 in hexane at 27 8C under

1 bar hydrogen [21]. The selectivity to citronellal was 77%

at 48% conversion [21]. Acetophenone [24] and benzalde-

hyde hydrogenation [23] was very selective to unsaturated

alcohols over Ni/H-Y [24] and Ni/SiO2 [23] catalysts,

respectively. In acetophenone hydrogenation at 100 8Cand 30 bar the highest selectivity to 1-phenylethanol was

100% in n-hexane, but the conversion remained rather low

(10%) [24]. In alcohol solvents the highest selectivity to

1-phenylethanol was 74% in methanol under the same

reaction conditions. It should, however, be pointed out that

Ni/Y catalyst was not stable after catalyst recycling, whereas

the bimetallic Ni-Pt/Y catalyst exhibited constant activity

after recycling. Selectivities close to 100% in benzyl alcohol

were achieved in gas phase hydrogenation of benzaldehyde

over Ni/SiO2 catalyst at 120 8C [23]. Chemoselective

crotonaldehyde hydrogenation is more difficult than pre-

vious cases over monometallic supported Ni catalysts.

Maximally 59% selectivity to crotyl alcohol was obtained in

crotonaldehyde hydrogenation at 60 8C and 1 bar over Ni/

TiO2 reduced at 500 8C [72]. The relatively high selectivity

over Ni/TiO2 was achived due to the existence of interfacial

Ni-TiOx sites exhibiting strong metal support interactions

(see Section 3.4.1.2). The promoted Ni-P/SiO2 catalyst was

very active in benzaldehyde hydrogenation at 100 8C and

10 bar in ethanol, but the only reaction product was toluene

[86]. The reason for this could be the low pH of the reaction

mixture (pH 4).

3.1.1.4. Chemoselective hydrogenation over group 11

metals, Cu, Ag, Au. Catalysts having an active metal from

group 11 are usually less active than those having a metal

from the other groups. This originates from the electronic

structure of the d band, which is filled. Recent catalytic

results from chemoselective hydrogenations using copper

[22,33,57,87,88]; silver [25,89–92]; and gold [5,11,94–

96,98,99] are briefly overviewed here.

Supported monometallic copper catalysts have been used

in chemoselective hydrogenations [22,33,57,87,88]. Cu/

SiO2 was very selective towards benzyl alcohol (83%) in

benzaldehyde hydrogenation in gas phase at 100 8C at

conversion of 68% [22]. No benzyl alcohol was formed over

Cu catalysts supported on TiO2, ZrO2, g-Al2O3 and traces of

benzyl alcohol were formed on Cu/CeO2 [22]. The highest

selectivity to cinnamyl alcohol over 13.7 wt.% Cu/SiO2 was

48% at 60% conversion level of cinnamaldehyde in decalin

at 140 8C [88]. The highest selectivity to cinnamyl alcohol

was obtained over a Cu/SiO2 prepared by impregnation. The

catalyst exhibited large copper crystallites and small

amounts of Cu2O at the metal support interface. On the

other hand high selectivity to cinnamyl alcohol (78%), but

very low activity in cinnamaldehyde hydrogenation was

obtained over Cu/ZrO2 catalyst at 140 8C and under

atmospheric pressure in propylene carbonate [87]. Very

high conversion of furfural and selectivity to furfuryl alcohol

(98%) was obtained over Cu/MgO catalyst in the gas phase

hydrogenation at 180 8C [33]. The reason for high activity

and selectivity was stated to be the presence of defective

sites at Cu and MgO in interfacial region.

Silver supported catalysts have been used in the

hydrogenation of acetophenone [25], acrolein [212],

crotonaldehyde [89–92] and furfural [25]. Unusually high

selectivities to unsaturated alcohols have been reported over

nanosized silver catalysts. According to Claus [25], silver is

able to adsorb conjugated double bond via bifunctional

bonding to both C C and C O bond resulting into


formation of 2-propenyloxo intermediate. Possibility of

hydrogen adsorption with dissociation was recently con-

firmed by isotope jumping technique [93]. Selectivity of

94.8% to 1-phenylethanol was reported in acetophenone

hydrogenation over Ag/SiO2 catalyst at 140 8C and 2 bar

[25], whereas in crotonaldehyde hydrogenation over Ag/

SiO2 catalyst at 20 bar and at 140 8C the selectivity to crotyl

alcohol was 62.8% at 16% conversion [92]. Ag/SiO2 catalyst

was quite selective in the hydrogenation of furfuryl alcohol.

The selectivity to furfuryl alcohol was 79% in the gas phase

hydrogenation of furfural at 1.9 bar and at 200 8C over

12.1% Ag/SiO2 catalyst [25].

Gold has been used in the chemoselective hydrogenation

of crotonaldehyde [5,94,99], trans-3-buten-4-phenyl-2-one

(benzalaldehyde) [98], acrolein [95–97] and citral [11]. Since

gold in powder form has been catalytically active in gas phase

hydrogenation of acrolein [95], thus its catalytic activity is not

exclusively originated from quantum size effect or from

specific metal-support interactions. Quantum size effect alters

the electronic properties of a metal particle [5,97]. Moreover

the binding energy shifts of core levels have been observed in

supported gold nanoparticles [213]. Gold supported on Fe2O3

yielded to very high selectivities of nerol and geraniol in citral

hydrogenation at 60 8C in and 1 bar ethanol (95% at 90%

conversion of citral) and the selectivity was independent on

the metal loading [11]. In the gas phase hydrogenation of

acrolein at 240 8C and 20 bar the maximum selectivity to allyl

alcohol was 37% over Au/ZrO2 catalyst, but the conversion of

acrolein was only 5% [95]. The hydrogenation rate was

decreasing, while the selectivity to unsaturated alcohol

increased with increasing gold particle sizes. The observed

structure sensitivity is discussed in detail in Section 3.2.

3.1.2. Bi- and multimetallic catalysts

Bimetallic catalysts have been very selective in several

chemoselective hydrogenations, like Rh-Sn/SiO2 for nerol

and geraniol (98%) [100], Pt/ZnO for crotyl alcohol (90%)

[67], Rh-Cu/SiO2 for crotyl alcohol (62%) [57]. The origin for

the high selectivities to unsaturated alcohols has been

associated with electron transfer from the less noble metal

to more noble metal as well as alloy formation [68] and /or an

intimate contact between two metals. This concept was,

however, challenged by [207,208]. The second metal can exist

as an adatom [29], in alloy [58,68], or in ionic state [29,101] as

well as in partially oxidized form [58,61]. The difference in

the electronegativity between two metals can enable the

polarization of the carbonyl bond [35]. Additionally,

geometrical effects, like a change in metal dispersion

[15,57], decoration of the main metal by the second metal

via surface enrichment [58,61,102] and organic fragments in

the vicinity of an active metal [29,58], have been observed in

bimetallic catalysts. It is important to note that these effects

are often coexistent in bimetallic catalysts and thus one effect,

like for instance alloy formation in a model catalyst Sn/

Pt(1 1 1), is not alone responsible for improved selectivity

[107], because nearly the same selectivities to crotyl alcohol

were obtained over non-supported Pt and Pt-Sn model

catalysts. Increased selectivities to crotyl alcohol were,

however, obtained over supported Pt-Sn catalyst [107].

There exists controversial information of the role of alloys

in chemoselective hydrogenation and even bimetallic

catalysts non-selective for formation of unsaturated alcohols

have been reported [54,100]. Alloys are defined as

compounds formed by two or more metals; they can also

be formed between a metal and another metal originating

from a non-inert support [36]. The alloy formation is strongly

influenced by the selection of metal precursor and support,

catalyst pretreatment as well as the preparation method.

In order to uncover both the selective hydrogenation site

and the reaction mechanism it is important to know the

chemical nature of the active site. Several physico-chemical

methods have been used as tools for characterization of the

active sites. The benefits and drawbacks of different

characterization methods are discussed in the subsequent

section. The following bi- or multimetallic catalysts have

been recently investigated in chemoselective hydrogena-

tions from group 8: Ru-Sn [49], Ru-Fe/C [14] from group

9 Rh-Sn/SiO2 [35,54,58,109,110], Rh-Cu/SiO2 [57], Rh-

Mo/SiO2, Ir-Ge/SiO2, Ir-Fe/SiO2 [12], Mo-Co-B [32], Sn-

Co-B [108], Ra-Co [28], from group 10 Pt-Sn/Al2O3

[43,64], Pt-Sn/C, Pt-Ge/Al2O3 [43], Pt/ZnO [36], Pt-Sn/

MgO [15], a model catalyst Sn-Pt(1 1 1) without support

[107], Pt-Co/graphite [85], Ni-P-B [111], Ni-Cu/SiO2 [102],

from group 11 Ag-In/SiO2 [211], Au/ZnO [96], Au-In/ZnO

[96], Cu-Zn-Al, Cu-Ni(Co)-Zn-Al [101]. Additionally, there

exist catalysts exhibiting a combined effect of both two

metals and reducible oxides, like Pt-Sn/CeO2-SiO2

[112,113] and Ru-Sn/TiO2 [49]. The examples are presented

below on a group-by-group basis and the combined effect of

bimetallic catalysts on reducible supports is discussed

separately in Section 3.1.2.4.

3.1.2.1. Methodology to characterize bimetallic cata-

lysts. Catalytic properties are related to the structural

properties of bimetallic catalysts, which means that several

factors should be investigated in these catalysts including

surface and bulk composition, different oxidation states of

both metals, as well as existence of alloy phases [54]. There

are many physico-chemical methods, which have been

applied for characterization of bimetallic catalysts related to

chemoselective hydrogenations, like XRD [24,36,61,65,

68,112], ED [35,58], HRTEM [15,35,66,96], HRTEM/EDX

[96], TPR [9,14,24,35,36,49,57,61,64,85,100,101,112],

Moessbauer spectroscopy [214], EXAFS [54,115–117],

XPS [24,29,35,36,57,58,76,96,102,107,109,112,117,136],

calorimetric measurements [14] and FTIR [81] for CO

adsorption. Some examples of the use of these methods are

described below.

X-ray diffraction method is especially useful in

confirmation of the presence of alloys. Different metal

phases give corresponding reflections of an alloy phase, like

(1 1 1), (2 0 0) and (0 0 2) phases of CePt5 can be seen in


Fig. 6. XRD patterns for 5 wt.% (A) Pt/ZnO prepared from

Pt(NH3)4(NO3)2 and (B) H2PtCl6 [36].

Fig. 7. ED pattern of Rh4Sn/SiO2 catalyst [35].

Fig. 8. TPR profiles of Pt/Al2O3, Pt-Sn/Al2O3 and Sn/Al2O3 [64].

XRD pattern [65] or formation of Pt-Zn alloy with

increasing reduction temperature of the catalyst was

reported (Fig. 6) [36]. Electron diffraction has also been

used as a tool to confirm different alloy phases [58]), like

RhSn2 phase (Fig. 7). The change in chemical shift in XPS

measurements is an indicative sign from either alloy

formation and/or an intimate contact between two metals

[57]. Additionally, XPS can reveal the surface composition

of different metals. The second metal is often enriched on

the surface of the main metal, which can be seen in Pt-Sn

[43], Ni-Cu [102] catalysts. TPR can be also used as a tool to

reveal the change in the reduction temperature of a metal

[64] (Fig. 8), although this method is only indicative.

Changes in the position of the temperature maximum could

be indicative for formation of alloys with different

stoichiometry, as discussed in [100] for Rh-Sn systems.

HRTEM image analysis can reveal the metal particle size

distribution and the metal particle shapes (Fig. 9). The metal

phases can be identified by measuring the lattice spacing

perpendicular to the surface with HRTEM image analysis

[96]. Additionally, together with HRTEM images different

types of particle shape models have been used for taking into

account the particle shape effects in catalysis (Fig. 10). It

turned out, however, from the work of Mohr et al. [95] that

cubohedral single crystal model is not generally applicable

for gold particles.

EXAFS can give structural information of a bimetallic

catalyst in a short-range ordering scale [54,115,116]. The

interaction between Pt-W and the support was observed with

EXAFS in the catalysts prepared from a bimetallic

precursor, like {Pt[W(CO)3(C2H5)]2(PhCN)2}, whereas no

interactions were observed when the catalyst was prepared

from two monometallic precursors [116]. These catalysts


Fig. 9. HRTEM image of Au/ZrO2 exhibiting a rounded gold particle (A) and a particle with facets (B). Right: separate size distribution of single crystalline

(SC) giold particles, single twinned particles (ST), multiple twinned particles (MTP) together with overall size distribution. Lower row: left: HRTEM images of

multiple twinned particles of Au/ZrO2, left: gold particle size distribution [96] (reprinted with permission, copyright (2003) American Chemical Society).

were not, however, selective in crotonaldehyde hydrogena-

tion (see Section 3.1.2.3). Alloy formation between Pt and

Sn could be confirmed by EXAFS [117]. In a bimetallic Rh-

Sn/SiO2 catalyst the EXAFS data revealed that the tin ions

present in the catalyst were acting as a growth limiting factor

for Rh [54] (see Section 3.1.2.3).

The position of the second metal on the main metal surface

was analyzed via CO adsorption and FTIR [109]. FTIR

DRIFT measurements of CO adsorption give information

about the electronic state of metals in bimetallic catalysts [59].

Additionally, adsorption of different aldehydes was investi-

gated by FTIR [109]. One of the reported examples was

adsorption of propionaldehyde on Rh-Sn/SiO2 catalyst, where

it was shown that in selective hydrogenation carbonyl group is

adsorbed on Sn and hydrogenation occurred by hydrogen

atoms spilt over to the support from Rh sites. Dependence

between the catalyst reduction temperature and the optimal

catalyst properties can be shown by FTIR studies [109].

DRIFTS technique and adsorption of CO were used for

Fig. 10. Ball models of truncated cubotahedron (A), decahedron (B), and

icosahedron (C) [95].

measuring the electronic state of Co particles and the

geometry in Co-Zn/SiO2 catalysts [59].

3.1.2.2. Bimetallic catalysts from group 8 metals, Ru and

Os. Several bimetallic Ru-catalysts, like Ru-Cr/SiO2 [118],

Ru-Sn/TiO2 [49], Ru-Sn/Al2O3 [119], Ru-Sn/SiO2 [53,120–

122], Ru-Fe/C [9,17] and Ru-Ce/Al2O3, Ru-Ce/C [14] have

been recently used in chemoselective hydrogenations. The

catalyst activity decreased by addition of Cr and Fe to the

monometallic Ru catalysts [9,49], whereas it passed through a

maximum with increasing Sn-content in Ru-Sn/TiO2 catalyst

[49] in citral hydrogenation. An additional effect of reducible

TiO2 will be discussed in Section 3.1.2.2. Another bimetallic

Ru-Sn/SiO2 catalyst, which was used in cinnamaldehyde

hydrogenation was prepared by the sol–gel method [53,120–

122]. The best selectivity to cinnamyl alcohol (85% at 50%

conversion) in cinnamaldehyde hydrogenation at 160 8C and

70 bar in cyclohexane was achieved over a Ru-Sn/SiO2

reduced chemically with NaBH4. Additionally, this catalyst

exhibited a high hydrogenation rate. The decreased catalytic

activity in case of Ru-Cr/SiO2 and Ru-Fe/C compared to

corresponding monometallic catalysts can be explained by the

decrease in hydrogen adsorption capacity of the catalysts,

when a second metal is added. The selectivity towards

unsaturated alcohols was, however, enhanced in both cases

indicating an inverse activity to selectivity correlation. The

Ru-Cr/B/SiO2 catalyst gave enhanced selectivity to formation

of 1-phenylethanol in acetophenone hydrogenation at 60 8Cand 9 bar hydrogen in hexane, but the hydrogenation rates


decreased due to the presence of Ru3+ ions [118]. Bimetallic

Ru-Fe catalysts supported on inert activated carbon and on an

electron donor support, graphite (see Section 3.4.5) were

studied in citral hydrogenation in 2-propanol at 60 8C and

1 bar hydrogen [9]. In this work [9] the highest selectivity

towards nerol and geraniol was maximally 73%. An inverse

correlation between the initial hydrogenation rate and the

selectivity to nerol and geraniol was noticeable. Over the Ru-

Fe/C the alloyed phase was important for achieving

unsaturated alcohols, since iron in the alloy acts as a Lewis

acid site for activating the carbonyl group. Although the alloy

could not be detected by XRD, the TPR profiles showed

simultaneous reduction of both metals [9]. Additionally, the

calorimetric study of the CO adsorption supported the alloy

formation. Iron was enriched on the Ru surface, which was

due to the lower surface energy of iron compared to Ru [9]. In

the recent patent literature very high selectivities to nerol and

geraniol in citral hydrogenation over a Ru-Fe/C catalyst at

100 8C and 50 bar in methanol were reported [18]. However

in this case a basic promoter, triethylamine was used as an

additive (see Section 3.6.3).

3.1.2.3. Bimetallic catalysts from group 9 metals, Co, Rh

and Ir. The following bimetallic Rh catalysts have been

investigated in chemoselective hydrogenation: Rh-Sn/SiO2

[25,35,54,58,100,103,109,110], Rh-Ge/Al2O3 [124], Rh-

Mo/SiO2 [61], Rh-Cu/SiO2 [57].

Bimetallic Rh-Sn/SiO2 catalysts were more selective than

monometallic catalysts in the hydrogenation of acetophe-

none [103], crotonaldehyde [57,58], whereas also non-

selective Rh-Sn catalysts have been reported [54,100]. Not

surprisingly in bimetallic Rh-Sn catalysts the hydrogen

adsorption capacity is usually lower than in monometallic

Rh-catalyst, because tin is not adsorbing hydrogen and thus

it is diluting the hydrogen adsorption capacity of Rh [35,58].

A lowered hydrogen adsorption capacity lead to reduced

catalytic activity, but a higher selectivity to unsaturated

alcohols can be obtained compared to the monometallic

catalysts [54]. However, there can also exist an optimum tin-

rhodium ratio in bimetallic catalysts giving the highest

activity and selectivity [35,54]. For catalysts with tin content

above 60% the selectivity of crotyl alcohol dropped close to

zero [35]. In some Rh-Sn catalysts the existence of several

Rh-Sn alloys (RhSn2, Rh3Sn2) as well as metallic Rh0, Sn0

and oxidized species, like SnOx has been confirmed by

electron diffraction [58]. A partial electron transfer from tin

to Rh was suggested based on the slight shift in BE of Rh

3d5/2. Due to the presence of SnOx species confirmed by XPS

the carbonyl group was polarized and a selectivity

enhancement towards carbonyl hydrogenation was obtained.

Some alloys exhibiting a different stoichiometry between

Rh-Sn might be unselective in chemoselective hydrogena-

tion [100]. TPR showed different maximum temperature for

hydrogen consumption as reported for Rh-Sn alloy and thus

it was suggested that a different bimetallic site was formed.

Selectivity to unsaturated alcohols, nerol and geraniol

increased with increasing conversion over some Rh-Sn

catalysts [54]. This induction period can be due to tin

oxidation in the presence of silica surface silanols or due to

reaction between aldehyde CO and zero valent Rh-Sn. In the

work of Nishiyama et al. [110] crotonaldehyde and

cinnamaldehyde were hydrogenated at 130 8C in 2-butanol

with Sn/SiO2 catalyst and the selectivities to corresponding

unsaturated alcohols were 95 and 100%, respectively at the

conversion levels of 23 and 42%, clearly showing intrinsic

selectivity of Sn in hydrogenation of C O group. At the

same time, the hydrogenation activities were lower than for

Rh-Sn/SiO2 [110].

Presence of second metal has several functions in the

catalysts. Tin ions act as growth limiting agents for Rh

ensembles lowering simultaneously hydrogen adsorption

capacity of the catalyst. In spite of the latter fact only minor

increase in metal particle size was observed in Rh-Sn/SiO2

catalyst prepared from Rh(Acac)3 and silica via sol–gel

process and followed by the reaction of tetra-n-butyl-tin and

the reduced Rh/SiO2 [58]. In the work of Sordelli et al. [54],

the Rh-Sn/SiO2 catalyst was prepared from corresponding

metal chlorides. The presence of ionic tin was confirmed by

XPS [58]. The larger metal particles in Rh-Sn/SiO2 catalyst

exhibited higher hydrogen adsorption capacity than the

smaller ones and thus the decrease in hydrogen adsorption

capacity was not correlating with the metal particle size

effect (see below).

Organometallic complexes used in catalyst synthesis

create beneficial steric effects in bimetallic catalysts [58].

Such steric effects originated from the butyl groups were

beneficial for increasing the selectivity to nerol and geraniol

over a bimetallic Rh-Sn/SiO2 catalyst [100] as well as

phenyl ethanol in acetophenone hydrogenation [103] and in

crotonaldehyde hydrogenation [58]. Over Rh-Sn/SiO2

catalyst prepared from tetra-n-butyl tin [58] the achieved

TOF-values were higher than with a monometallic Rh/SiO2

catalyst exhibiting nearly four times higher hydrogen

adsorption capacity. It was suggested that new sites were

developed in the bimetallic catalysts being responsible for

enhanced activity and selectivity [58]. Both electronic and

steric effects were important in this catalyst. The former one

is caused by the difference in the electronegativity between

Rh and Sn, while the latter one originates from the alkyl

fragments on tin. The phenyl ring hydrogenation was

inhibited with a bimetallic catalyst having tin adatoms as

well as alkyl tin fragments [103].

Metal particle sizes are important in chemoselective

hydrogenations over bimetallic catalysts. Although larger

metal particles have been beneficial in chemoselective

hydrogenations [58], the smaller bimetallic Rh-Sn and Rh-

Cu particles have been selective in liquid phase hydrogena-

tion of citral [54] and crotonaldehyde [58], respectively.

However, the metal particle size is not the only factor

determining the product selectivity. The pretreatment of Rh-

Sn catalysts affected the activity and the selectivity in citral

hydrogenation [54]. A comparative study demonstrated that


when a Rh-Sn/SiO2 catalyst was either calcined and reduced

or only reduced the latter catalyst was non-selective towards

unsaturated alcohols in citral hydrogenation, whereas the

former catalyst gave selectivities to nerol and geraniol of

about 70% [54]. This catalyst contained smaller metal

particles than the catalyst being only reduced. It was

additionally observed that the suppression of hydrogen

chemisorption capacity was not related to particle size

effect: the former catalyst exhibited smaller hydrogen

adsorption capacity than the latter one. EXAFS character-

ization of both the catalysts revealed that the former catalyst

had some patches of tin decorating Rh surface, whereas in

the latter catalyst tin existed on more external surfaces of Rh

[54]. Additionally, the calcined and reduced Rh-Sn/SiO2

catalyst had each Rh atom surrounded by other Rh atoms,

whereas the catalyst being only reduced exhibited an alloy

type phase and the tin-enriched phase located at the external

surface of Rh. It was stated that the alloy type phase hindered

the formation of SnOx. In bimetallic Rh-Sn catalyst both

larger [57] and smaller [54] bimetallic particles have been

reported depending on the preparation method. Cu as a

second metal for Rh has also promoted the Rh dispersion and

higher conversion levels of crotonaldehyde were obtained

than over monometallic Rh/SiO2 catalyst [57]. Rh-Cu/SiO2

catalysts exhibited higher selectivities to crotyl alcohol than

obtained over a monometallic Rh/SiO2 catalyst in gas phase

hydrogenation of crotonaldehyde at 100 8C [57]. Addition-

ally, the most selective bimetallic Rh-Cu/SiO2 catalyst

exhibited the highest TOF-value, which was 1.5 times higher

than that for a monometallic catalyst [57]. This catalyst

possessed higher hydrogen adsorption capacity and smaller

metal particles than a monometallic one. The reason for

higher selectivities over bimetallic catalysts is the change of

the electronic state of metal clusters in bimetallic catalysts

[57], since an electron transfer from Cu to Rh was suggested

based on XPS measurements.

There exist some analogies between Rh-Sn and Rh-Mo

bimetallic catalysts in their catalytic activity and selectivity.

In Rh-MoSiO2 catalyst addition of Mo decreased the

hydrogenation rate and simultaneously increased the desired

selectivity to unsaturated alcohols [61] compared to

monometallic Rh/SiO2 catalyst. The second metal Mo

was mainly in non-zero valent state, i.e. in Mo4+ and Mo6+,

Rh was positively charged and the carbonyl hydrogenation

was enhanced [61]. Relatively high selectivities to nerol and

geraniol (75%) in citral hydrogenation in 2-propanol at

70 8C and 70 bar of hydrogen presence were obtained also

over bimetallic Rh-Ge/Al2O3 catalyst, whereas a mono-

metallic Rh/Al2O3 catalyst exhibited very low selectivity to

unsaturated alcohols [124].

Other bimetallic catalysts reported recently have been

Co-Mo and Co-Ni catalysts [32,125]. These bimetallic,

amorphous Co-Mo and Co-Ni catalysts containing B were

very selective in the hydrogenation of furfural to furfuryl

alcohol [32,125]. The former catalyst contained Mo in the

form of MoO3, since B was electron deficient; Co nearby B

was exhibiting electron rich. The highest selectivity to

furfuryl alcohol was close to 100% in furfural hydrogenation

at 100 8C and 10 bar of hydrogen in ethanol [32]. When the

amorphous catalyst was treated above 300 8C, it crystallized

with simultaneous decrease in catalytic activity. In

amorphous Ni-Co-B catalyst, which was used in furfural

hydrogenation with selectivity of 99% to furfuryl alcohol,

both Co and Ni were present according to XPS measure-

ments metallic form, B was positively charged being able to

donate electrons to Ni and Co [125]. Over a bimetallic

amorphous CoSn(II)B catalyst maximally about 22% crotyl

alcohol at 80% conversion was obtained in the hydrogena-

tion of crotonaldehyde at 100 8C and under 10 bar hydrogen

in ethanol [108]. Over the monometallic amorphous Co-B

catalyst the maximum selectivity to crotyl alcohol was

45.6% at 79.9% conversion, when SnCl4 salt was added into

the reactor. The higher selectivity of the latter catalyst was

explained by the presence of adsorbed tin ions, which

suppress hydrogenation of C C and accelerate hydrogena-

tion of C O, whereas in the former case the presence of

metallic tin the hydrogenation rate of C O was decreased

more than that of C C [108]. Bimetallic Co-Zn/SiO2

catalysts where Zn was mainly in ZnO form were used in

crotonaldehyde hydrogenation in gas phase [59]. The

highest selectivity to crotyl alcohol (67%) in this reaction

at 120 8C and 1 bar was obtained over 25% Co/10% Zn/SiO2

catalyst, but the catalyst activity was diminished compared

to monometallic catalyst.

3.1.2.4. Bimetallic catalysts with group 10 metals, Ni, Pd,

Pt. Several bimetallic catalysts having metal from group 10

have been investigated in chemoselective hydrogenations,

like Pd-Cu/SiO2 [81,104], Pd-Sn/SiO2 [105,106], Ni-Cu/

SiO2 [102], Ni-Co-B [125], Ni-Pt/Y [24], Pt-Sn/Al2O3

[43,64,127], Pt-Sn/SiO2 [29], Pt-W/Al2O3 [116], Pt-Ge/

Al2O3 [43,127], Pt-Sn/C [43,126], Pt-Co/SiO2 [128], Pt/

ZnO [36,67], Pt-Sn-B [108], Pt/SnO2 [68,69], Pt-Sn/SiO2

[29,129–132], Pt-Sn/MgO [15], Sn/Pt(1 1 1) model catalyst

[107] and Pd-Au/C [133].

Bimetallic nickel catalysts have been tested in acet-

ophenone [24] and in citral hydrogenation [102]. A

bimetallic Ni-Pt/Y catalyst exhibited stable activity in the

hydrogenation of acetophenone compared to its mono-

metallic counterpart, which was not active after catalyst

recycling. The selectivity to unsaturated alcohol was,

however higher over Ni/Y than over Ni-Pt/Y [24]. The

highest selectivity (70%) to 1-phenylethanol at high

conversion (81%) was obtained in methanol over 10 wt.%

Ni-0.5 wt.% Pt/Y at 100 8C and 30 bar [24]. The Ni was

better dispersed in the bimetallic catalyst and according to

XPS it was completely reduced. Some side reactions, like

ether formation took, however, place on the Ni supported

zeolite catalyst (see Section 4.3).

The performance of bimetallic Ni-Cu/SiO2 catalyst was

compared with Ni/SiO2 in citral hydrogenation in ethanol at

70 8C and at 2.3 bar [102]. The former catalyst gave slightly


higher TOF-values and selectivities to citronellol than the

latter one. The higher activity in bimetallic Ni-Cu/SiO2

catalyst correlated with the lower temperature for maximum

amount of hydrogen desorbed from the catalyst compared to

the monometallic catalyst, i.e. 170 and 225 8C, respectively.

XPS measurements revealed that copper was mainly in

metallic form (90%), whereas only 11.5% of nickel was in

metallic form although the catalyst was reduced at 400 8C.

Additionally, surface enrichment of copper was observed.

The better performance of the bimetallic Ni-Cu/SiO2

catalyst might originate from the electronic effect of Cu.

Several Pt catalysts modified with tin [15,29,43,

64,76,107,126,127,129–132], Pt-Co [128] and Pt-Zn cata-

lysts [36,67,114] have been recently tested. The high

chemoselectivities in carbonyl bond hydrogenation over Pt-

Sn can originate from the alloy formation, presence of ionic

tin [29,130], surface enrichment of tin on Pt, changing the

geometry of Pt particles [43] as well as from the lowered

hydrogen adsorption capacity caused by tin addition [64].

Very often the addition of tin reduces the initial

hydrogenation rate [64], which is due to the lowered

hydrogen adsorption capacity of the catalyst. However, also

higher rates have been reported over bimetallic Pt-Sn

catalysts than observed over monometallic Pt catalysts

[29,117,131]. Some recent catalytic data from the use of Pt-

Sn catalysts in citral [15], carvone [43], cinnamaldehyde

[29], benzaldehyde [29] and crotonaldehyde hydrogenations

[29,130–132] are presented below. Additionally, Pt/ZnO

catalyst was used in crotonaldehyde hydrogenation [36,67].

Pt-Sn/MgO catalyst was investigated in citral hydrogena-

tion [15]. The selectivities to nerol and geraniol were about

97.5% over this catalyst at 100 8C and 20 bar when citral was

hydrogenated without a solvent in a trickle bed reactor [15].

The high selectivity towards nerol and geraniol was achieved

over Pt particles with size of ca. 1 nm having a tin decorated

surface. It was suggested that tin prevents sintering of Pt

particles compared to the monometallic Pt/MgO catalyst

having larger Pt particles. Influence of metal loading was

investigated in Pt/ZnO catalysts in the hydrogenation of

crotonaldehyde at 80 8C and 1 bar in the gas phase [67]. Over

5 wt.% Pt/ZnO catalyst the catalytic activity decreased by

reducing the catalyst at high temperature and simultaneously

the selectivity to crotyl alcohol increased [36]. The formation

of Pt-Zn alloy was confirmed by XRD measurements. The

lower loaded (1 wt.%) Pt/ZnO catalyst exhibited higher

activity, selectivity and stability than 5 wt.% Pt/ZnO catalyst

[67]. One reason for this is the lower chlorine content in the

former catalyst. Additionally, crotyl chloride formation was

observed with 5 wt.% Pt/ZnO catalyst [36].

Ionic tin species in Pt-Sn catalysts were reported to be

responsible for selectivity enhancement in chemoselective

hydrogenations [130]. The selectivities to cinnamyl alcohol

and crotyl alcohol were 60 and 56%, respectively at 80%

conversion in 2-propanol at 40 8C and 10 bar 60 and 56%,

respectively [29]. Snd+ species act as Lewis acid sites

promoting the attack of hydrogen to C O group leading to

higher formation rate and selectivity to the unsaturated

alcohol [29]. Electron transfer from tin to platinum was

observed by XPS. Additionally, existence of ionic tin has

been confirmed by Mossbauer spectroscopy [130]. Both

ionic tin and zero valent tin can increase the electron density

of platinum [29]. At the same time the hydrogenation of

C C was retarded and the chemisorption capacity of the

bimetallic catalyst was lower than that for the monometallic

catalyst. An additional promoting effect in this catalyst was

the tin catalyst precursor, namely SnBu4. The catalyst

preparation technique changed the location of tin as well as

the presence of alkyl fragments, for instance at higher

preparation temperature the alkyl fragments disappeared

[29]. In the work of Margitfalvi et al. [131] the performance

of a bimetallic Pt-Sn/SiO2 catalyst prepared from tin

tetraethyl precursor was investigated in gas phase hydro-

genation of crotonaldehyde at 80 8C and 1 bar. The

selectivity to crotyl alcohol was 78% at 5% conversion

level and decreased with increasing conversion, while

selectivity at the same conversion level was zero over a

monometallic Pt/SiO2 catalyst. Interestingly higher initial

rates were obtained over a bimetallic catalyst than over Pt/

SiO2 catalyst. These results were interpreted as formation of

new type of active sites. The importance of ionic tin was

additionally confirmed, when low selectivity to crotyl

alcohol was obtained over Pd-Sn/SiO2 exhibiting no ionic

Sn [106]. This catalyst was prepared by solvated metal atom

dispersion technique and no Snn+ species was observed.

Different supports (Al2O3, C) have been investigated in

Pt-Sn catalysts [43,126,127]. The surface enrichment of tin

was different in Pt-Sn/C and Pt-Sn/Al2O3 catalyst. In the

former catalyst ten-fold larger amount of tin measured by

XPS was observed on the metal surface than in the latter one.

Due to the larger surface enrichment in the former catalyst

the catalytic activity over this catalyst in the hydrogenation

of carvone was very low, but the initial selectivity to carveol

was close to 100%.

Combination of Pt and Co has enhanced the selectivity to

crotyl alcohol over Pt-Co/SiO2 catalyst in gas phase

hydrogenation of crotonaldehyde at 100 8C, when its

performance was compared to the monometallic Pt/SiO2

catalyst [128]. The reported selectivities over the former and

the latter catalysts were 17% at 8% conversion and

decreasing with conversion and 3% being nearly constant

with increasing conversion, respectively [128].

Bimetallic Pd-Sn/SiO2 catalyst was used in cinnamalde-

hyde hydrogenation [105]. The catalytic activity decreased

with increasing Sn content, whereas more cinnamaldehyde

was produced over a bimetallic catalyst compared to

monometallic Pd/SiO2 catalyst. The selectivity was, how-

ever, quite low because cinnamyl alcohol reacted further.

Additionally, benzaldehyde was hydrogenated over Pd-Au/

C catalyst [133].

3.1.2.5. Bimetallic catalysts with group 11 metals, Cu, Ag,

Au. Cu-Ni-Zn-Al, Cu-Co-Al [101] and Cu-Pd/V2O5/SiO2


catalysts have been used in cinnamaldehyde hydrogenation

[88], while copper chromite catalyst was applied in furfural

hydrogenation [134]. Additionally, bimetallic Cu-Cd/SiO2

catalyst was investigated in crotonaldehyde hydrogenation

in gas phase [91]. The Cu-Ni-Zn-Al and Cu-Co-Al catalysts

were both more active and selective in cinnamaldehyde

hydrogenation than a monometallic Cu/SiO2 catalyst. The

selectivity enhancement to unsaturated alcohol was

explained by the presence of Cu0-M2+ sites. M2+ sites

adsorb cinnamaldehyde via h1 on-top adsorption and a close

contact to copper is needed, because copper is able to

dissociate hydrogen. Copper chromite catalyst gave selec-

tivity to furfuryl alcohol about 80% at 140 8C and about 3%

selectivity to cinnamyl alcohol in cinnamaldehyde hydro-

genation at 60 8C [134]. During gas phase hydrogenation of

furfural the catalyst deactivation was very severe. The other

products formed were 2-methylfuran, furan, tetrahydro-

furan, tetrahydrofurfuryl alcohol as well as ring decom-

position products, like pentanols and pentanediols [134].

Low selectivity to crotyl alcohol obtained over the copper

chromite catalyst in gas phase hydrogenation of furfural was

explained by fast isomerization of crotyl alcohol to

butyraldehyde [134]. Ca 40% selectivity to cinnamyl

alcohol was recorded over a Pd/Cu/SiO2 catalyst at 90%

conversion in decalin as a solvent at 140 8C and at 1 bar

hydrogen [88]. However, the Pd addition had a negative

effect on the selecivity. In gas phase hydrogenation of

crotonaldehyde over Cu-Cd/SiO2 at 240 8C and 20 bar

Fig. 11. HRTEM image of Au-In/ZnO [96] (reprinted with per

hydrogen 72.2% selectivity to crotyl alcohol was obtained at

94.6% conversion [91].

Bimetallic gold catalyst, Au-In/ZnO, has been used in

acrolein hydrogenation yielding to very high selectivities of

allyl alcohol (63.3%) [96]. However, the catalytic activity

was decreased compared with a monometallic Au/ZnO

catalyst. The catalyst was characterized by EDX revealing a

homogeneous distribution of In on the Au particles.

According to HRTEM image analysis the formation of a

separate indium phase was observed. Indium decorates the

outer faces of gold leaving the edges uncovered (Fig. 11).

Relatively high selectivities to crotyl alcohol, 68.5% at

86.9% conversion, were obtained over Ag-Cd/SiO2 catalyst

in gas phase hydrogenation of crotonaldehyde at 260 8C and

20 bar hydrogen [91].

3.1.2.6. Combined effect of bimetallic catalysts on reduci-

ble oxide. The catalysts exhibiting both two active metals

and a reducible support have been tested in the hydrogena-

tion of crotonaldehyde over Pt-Zn/CeO2-SiO2 [112,113], Pt-

CeO2 embedded on silica [37], Pt-Sn/TiO2 [76], Pt-Zn/TiO2

[114] and citral over Ir-Ge/TiO2 [12], Ru-Sn/TiO2 [49].

Some additional benefits were obtained over such bimetallic

catalysts supported on reducible oxides compared to the

monometallic catalysts supported on reducible oxides.

Pt-Zn system supported on CeO2-SiO2 exhibited higher

catalytic activity in crotonaldehyde hydrogenation than a Pt/

CeO2-SiO2 catalyst. The catalytic activity of this bimetallic

mission, copyright (2003) American Chemical Society).


Fig. 12. TOF in acrolein hydrogenation as a function of mean gold particle

size on different Au/ZrO2 catalysts [95].

catalyst increased by increasing the reduction temperature,

which means that the partially reduced CeO2 had a positive

effect. However, in Pt-Zn/CeO2-SiO2 catalyst Pt is affected

not only by CeO2, but also by Zn, as Zn can be in oxidized or

in metallic state, moreover Pt-Zn alloy is formed at higher

reduction temperatures. The higher maximum selectivities

to crotyl alcohol were achieved over the bimetallic system

compared to the monometallic Pt/CeO2-SiO2 catalysts

[112]. Bimetallic Pt-Zn-TiO2 catalyst reduced at 500 8C[114] exhibited similarly to Pt-Zn/CeO2-SiO2 catalyst [112]

higher activity than Pt-Zn/TiO2 catalyst reduced at low

temperature [114]. The highest selectivity to crotyl alcohol

(75%) in crotonaldehyde hydrogenation at 60 8C and 1 bar

hydrogen, was however obtained over a Pt-Zn/TiO2 catalyst

reduced at low temperature, since this catalyst contained

more ionic Zn compared to the Pt-Zn/TiO2 catalyst reduced

at high temperature (500 8C). Over Pt-Sn/TiO2 catalyst high

catalytic activities and stable selectivities to crotyl alcohol

were observed, when the catalyst was reduced at high

temperature [76] most probably due to the surface

enrichment of tin as observed by XPS measurements. As

a comparison a monometallic Pt/TiO2 catalyst gave lower

conversion levels and lower selectivities to crotyl alcohol.

A bimetallic Ru-Sn/TiO2 catalyst was very selective

(87.8%) for producing nerol and geraniol in citral hydro-

genation in n-heptane at 126 8C and 50 bar [49]. The

hydrogenation activity was, however, more than five times

lower over a monometallic Ru/Al2O3 catalyst. The catalytic

activity of the bimetallic Ru-Sn catalysts reduced at lower

temperatures decreased with increasing tin amount. When the

bimetallic catalyst was reduced at higher temperature, the

initial citral hydrogenation rate increased with an increase of

the Sn/Ru ratio level 0.2, after which it started to decrease

[49]. The beneficial effect of interfacial Ru-TiOx created at

high reduction temperatures was achieved with the catalysts

exhibiting low amounts of tin. Analogously higher catalytic

activity was obtained in citral hydrogenation at 1 bar

hydrogen and 100 8C in ethanol over a bimetallic Ir-Ge/

TiO2 system reduced at high temperature than over a Ir/TiO2

catalyst [12]. The high temperature reduction of Ir-Ge/TiO2

formed interfacial Ir-TiOx sites, which were not developed in

Ir-Ge/TiO2 after low temperature reduction (LTR) and in Ir/

SiO2 catalysts. The high selectivities over Ir-Ge/TiO2 after

high temperature reduction (HTR) to nerol and geraniol

(100%) were originated from the presence of ionic Ge4+

species, which were able to polarize C O bond.

In general, it can be concluded that the bimetallic catalysts

supported on reducible oxides can be more active and selective

in chemoselective hydrogenations than the corresponding

monometallic catalysts. The reason for this positive behavior

is the formation of interfacial Me-support sites and/or alloys.

3.2. Metal particle size and shape

The concept of structure-sensitivity, which applies that the

hydrogenation activity and selectivity are dependent on the

metal particle size or metal dispersion, has been intensively

debated under recent years [1]. The caution was given

regarding the effect of particle size on the reaction kinetics,

because several side reactions can take place inhibiting the

hydrogenation and thus the initial rates without catalyst

deactivation could not be obtained [1]. Moreover the

selectivity can vary as a function of TOF. In order to exclude

the possible change of selectivity with changing TOF the tests

with varying the reciprocal space velocity and with different

metal particle sizes were used as a tool [96]. Additionally, the

change in metal particle size can affect the electronic and

geometrical properties of metal particles. The smaller metal

particles are known to be more electron deficient than larger

ones. Furthermore, the number of edges and corners, which

could have different activities and selectivities is increased in

the smaller particles. The change in metal particles can

simultaneously change other properties of the metal.

There are several reports on chemoselective hydrogena-

tions, which have been considered as structure-sensitive, like

citral [77] and acrolein [123], However, some hydrogenations

have also been reported to be structure insensitive, like

hydrogenation of 2-cyclohexenone [40], as TOF did not

change with varying metal dispersion. Citral hydrogenation

was considered as a secondary structure-insensitive reaction,

where TOF increased with increasing Pt particle sizes varying

between 1 and 5 nm, whereas only minor effect was observed

between Pt particle sizes between 5 and 30 nm [1].

The antiphatetic structure sensitivity, which means that

TOF increases with increasing metal particle size [123] was

observed in the hydrogenation of acrolein over Au/TiO2, Au/

ZrO2 [97] (Fig. 12) and over Ag/TiO2 catalysts [123]. It was

concluded in their work that the origin for antiphatetic

structure sensitivity was quantum size effects, which altered

the electronic properties of gold when the metal particle size

was below 2 nm. When the valence band vanishes the drop

in repulsion is obtained as a result leading to lowered

selectivities to C O hydrogenation [5]. These particles

exhibited electron donating character according to EPR

measurement. Contradictory data from the effect of particle

size on TOF were, however, presented by Zanella et al. [94].

In their work, an optimum Au particle size was obtained in


crotonaldehyde hydrogenation providing the maximum in

TOF with increasing Au particle size over Au/TiO2 catalyst

[94]. This maximum was explained by the participation of

edges and corners in the hydrogenation processes [94]. The

results regarding the effect of gold particle size on TOF and

selectivity to unsaturated alcohols [94] were thus not

consistent with each other probably due to different reaction

conditions and catalyst supports.

Selectivity to unsaturated alcohols has increased with

increasing metal particle size in the hydrogenation of

crotonaldehyde [70,89] and cinnamaldehyde [88]. It was

observed for liquid phase crotonaldehyde hydrogenation that

larger Pt particles were more active and they deactivate slower

than the smaller Pt particles [70]. Additionally, smaller Pt

particles favored side reactions (see Section 4.5) [70]. Smaller

Pt particles can be more selective for formation of

butyraldehyde due to electronic and steric effects. The

electronic effect in small Pt particles, i.e. a decrease in

electron density of the d-orbitals favors the interactions of

both the double bonds in unsaturated aldehyde. Smaller metal

particles, exhibiting low coordination, have more edges and

corners compared to the larger Pt particles. These sites allow

unconstrained adsorption of both double bonds in crotonal-

dehyde [70]. Overall it was concluded [70] that selectivity to

unsaturated alcohols is increased with increasing metal

particle size. Small Pt particles (1.1 nm) in a colloidal catalyst

gave low selectivity to cinnamyl alcohol at 60 8C and 40 bar in

ethanol [154]. Additionally, Pt particles below 2 nm in

bimetallic Pt-W/Al2O3 catalysts were non-selective in gas

phase hydrogenation of crotonaldehyde at 60 8C and 1 bar

hydrogen (selectivity below 10%) [116]. According to

theoretical calculations [135], the steric constraints on

Pt(1 1 1) induce the preferential adsorption of C O on metal

surface [70]. In the work of Englisch et al. [70] the fraction of

Pt(1 1 1) surfaces in Pt particles was estimated by a

cubooctahedral shape model. A correlation between the

abundance of Pt(1 1 1), which increased by increasing Pt

particle size [70], and the selectivity for formation of crotyl

alcohol was presented. Large Pt(1 1 1) particles have also

relatively small number of corners and edges. Over small Pt

particles the rate of hydrogenation of olefinic bonds was five

times faster than hydrogenation rate of carbonyl bond.

However, XANES measurements indicated that the strength

of interaction between carbonyl-Pt and ethylenic C C and Pt

was about the same and the differences in the hydrogenation

rates could not be originated from the differences in

adsorption constants. Analogously high index planes with

low-coordinated atoms in smaller sized silver particles were

not selective in crotonaldehyde hydrogenation to crotyl

alcohol [89], while dense Ag[111] were selective. Large Cu

particles in Cu/SiO2 catalyst (about 11–31.5 nm) and

presence of Cu2O could lead to selective hydrogenation of

cinnamaldehyde to cinnamyl alcohol in decalin at 140 8C and

1 bar hydrogen [88]. The reactant structure can, however,

affect the achieved selectivity, because very high selectivities

to benzyl alcohol were obtained in gas phase hydrogenation of

benzaldehyde over Ni/SiO2 catalyst exhibiting relatively

small (1.4 nm), mean Ni particles [23].

3.3. Metal precursor

The most common metal precursors are chlorides,

because they are relatively inexpensive. Other precursors

frequently used are, e.g. ammonium nitrates. Systematic

comparisons between chloride and non-chloride containing

Pt catalysts, where the effect of catalyst reduction

temperature and the nature of metals precursor affected

the catalytic performance, have been investigated [36,68].

Residual chloride, which remains in the catalyst after

pretreatment, can have several effects on catalyst properties

and performance, for instance diminishing the metal

dispersion [36]. An alloy formation in case of reducible

oxides is also affected by the presence of chloride [36,67].

Additionally, product distribution can change depending on

the amount of residual chloride, even leading to chlorine

containing products [36]. It thus could be anticipated that the

effect of chloride in chemoselective hydrogenations is

complex. Enhancement of the selectivity to unsaturated

alcohols was stated in the hydrogenation of crotonaldehyde

[36,136], whereas negative effect of chlorine was observed

in the hydrogenation of the same molecule over Pt/CeO2

[65] and over Pt/SnO2 [68], as well as in cinnamaldehyde

hydrogenation over Ru/Y and Ru/MCM-41 catalysts.

Alteration of selectivity in case of reducible oxides can

be rationalized, since chloride can control the migration of

support onto the metal and/or formation of alloy [68].

Metal precursor can have opposite types of effects in

catalysts having different reducible oxides as supports

[36,67]. The performance of Pt/ZnO catalysts prepared

either from chloride or from nitrate precursors was

compared in crotonaldehyde hydrogenation [67]. The

calcined (at 400 8C) and reduced (at 400 8C) Pt/ZnO

catalyst prepared via impregnation from H2PtCl6 exhibited

88% selectivity to crotyl alcohol in gas phase hydrogenation

at 40 8C [67]. With a higher metal loading the Pt dispersion

was lowered due to the larger amount of residual chloride,

leading to reduced hydrogenation rates and formation of

crotylchloride [36]. The reason for higher crotyl alcohol

selectivity over chloride containing catalysts compared to

the catalysts prepared from nitrate was the Pt-Zn alloy

formation in the former catalyst (Fig. 6). An opposite type of

chloride effect was observed in Pt/CeO2 in crotonaldehyde

hydrogenation. The catalyst prepared from nitrate precursor

exhibited higher selectivity to crotyl alcohol than the

chloride containing catalyst. Over the latter catalyst the

maximum selectivity to crotyl alcohol was 30%, whereas

over Pt/CeO2 catalyst made from nitrate 80% selectivities to

crotyl alcohol were obtained. The reason for the high

selectivity to crotyl alcohol was the presence of CePt5,

which was inhibited in the presence of chloride [65].

Residual chloride can have both positive and negative

effects on chemoselectivity. The positive effect was observed


with chloride containing Pt/graphite catalysts, which were

treated at different temperatures. The selectivity over this

catalyst to crotyl alcohol was higher than over Pt/graphite

catalyst prepared from nitrate precursor [136], when both the

catalysts exhibited the same Pt particle sizes. It should,

however, be stated that the catalyst pretreatment temperature

in case of graphite changes the amount of oxidic surface

groups. The negative chloride effect could be seen in

cinnamaldehyde hydrogenation over Ru/Y and Ru/MCM-

41. The calcined Ru/Y and Ru/MCM-41 catalysts exhibited

lower amount of chloride and higher selectivity to cinnamyl

alcohol [52].

Additional complication in rationalizing the effect of

chloride is that the residual chloride can catalyze homo-

geneous side reactions. The catalysts made from chloride

containing precursors favored side reactions, like acetaliza-

tion [8,19]. It was shown by Maki-Arvela et al. [8] that large

amounts of citronellal acetals formed in citral hydrogenation

over the Ru/C catalyst prepared from chloride precursor

most probably originated from residual chloride. No acetals

were, however, formed when the Ru/Al2O3 catalyst was

prepared from Ru(acac) precursor [51].

3.4. Support selection

The main task for the support is to disperse the metal,

because usually highly dispersed small metal particles are

more active in activating organic molecules. The most

conventional supports are acidic or basic oxides and different

types of carbons. In terms of their reactivity supports can be

either inert or non-inert. The latter group can form with the

metal an alloy or segregate on the metal surface via forming

partially reduced groups, like TiOx. Moreover, supports can

have very different properties, like specific surface areas

ranging from very low 10 to ca. 1200 m2/gcat or even higher,

pore volumes, acidities, electronic and geometrical proper-

ties. Additionally, the shape of catalyst particles can vary from

powders and pastes to pellets, fibers and monoliths. The

disadvantage with pellets is the diffusional limitations leading

to lower reaction rates, at the same time they are used in

industrial scale because of lower pressure drops in continuous

reactor operations. Recently new support structures, like

several types of fibers [19,48,137,138] and monoliths

[139,140] have been applied in liquid phase hydrogenations

of a,b-unsaturated aldehydes. Additionally, bifunctional

catalysts, exhibiting both acidic sites and an active metal,

like zeolites [24,50,52,141], mesoporous materials [52,142–

144] and clays [7,146] have been used in chemoselective

liquid phase hydrogenations. In this section recent catalytic

results by using different support materials are summarized. A

separate section is devoted to catalysts exhibiting both two

active metals and a reducible oxide as a support.

3.4.1. Oxides

3.4.1.1. Conventional oxide supports. The most conven-

tional supports are acidic Al2O3 [8,14,22,27,30,41,43,63,

116,147–149], SiO2 [10,12,22,23,27,29,30,39,40,45,54,59,

70,71,74,88,89,99,100,102–105,109,110,118,130–132,147,

150,151] as well as basic MgO [15]. Functionalized

supports, like fullerene grafted silica were also used in

cinnamaldehyde hydrogenation [152]. Other oxides used in

chemoselective hydrogenations are Fe2O3 [11] and Cr2O3

[134]. Mixed oxides have been applied as supports, like

SiO2/AlPO4 [20] in citral hydrogenation and zinc-aluminate

spinel, i.e. Cu-Zn-Al or Cu-Ni(Co)Zn-Al [101] prepared by

coprecipitation in cinnamaldehyde hydrogenation. A variety

of reducible oxides, like TiO2 [12,13,16,22,49,70,72,76,77,

89,99,153,155], CeO2 [13,22,65,66,112], MoO3 [34], WO3

[34], ZrO2 [5,22,95,97] and alloy forming oxides, i.e. ZnO

[36,67,96], SnO2 [68,69] have been studied in chemose-

lective hydrogenations.

A comparison of different oxides in chemoselective

hydrogenations was performed with Cu [22], Pt [30] and Pd

as active metals [147]. In general it can be stated that oxide

supports can provide stronger interactions with the main

metal than carbon (see Section 3.4.5) [14,43]. Additionally,

the full reduction of the metal might be more difficult on an

oxide than on carbon [14]. Monometallic catalysts supported

on Al2O3 and SiO2 have produced very often saturated

aldehydes as the main products in chemoselective hydro-

genation of unsaturated aldehydes [11,36,99]. In acrolein

hydrogenation at 50 8C and 1 bar pressure 100 and 99%

propanal at 100% conversion were obtained over Pt/Al2O3

and Pt/SiO2 catalyst, respectively [34].

Furthermore, acidic oxides can promote side reactions, for

instance alumina favors cyclisation of citronellal in citral

hydrogenation [49] and formation of condensation products

from furfural [30]. Support effects have been investigated in

the hydrogenation of benzaldehyde [147] and in the

hydrogenation of crotonaldehyde [14]. Oxidic supports, like

SiO2 and Al2O3 were efficient for belzaldehyde hydrogena-

tion, whereas low selectivity was obtained over Pd/C [147].

Alumina and silica provided higher metal dispersion than Pd/

C, the reason for this being the stronger interaction between

Pd and alumina compared to carbon. The highest selectivity to

benzyl alcohol (100%) was achieved over a Pd/Al2O3 catalyst

calcined at 500 8C exhibiting the lowest rate. When the higher

rate was obtained, i.e. for Pd/C and Pd/Al2O3 calcined at

800 8C, the selectivity to benzyl alcohol decreased [147]. In

gas phase crotonaldehyde hydrogenation over Ru/C and Ru/

Al2O3 the latter catalyst exhibited higher selectivity to crotyl

alcohol than the former one [14]. One possible explanation to

this was the presence of partially oxidized Rud+ in Ru/Al2O3,

whereas in Ru/C ruthenium was completely reduced.

Additionally, the interaction between Ru and alumina was

stronger than between Ru/C confirmed by the lower reduction

temperature of Ru in Ru/C. It should be noted that carbon

support per se was not inert in citral hydrogenation, which was

elucidated by testing only support in the hydrogenation,

resulting in formation of acetals [14].

The nature of the support affects the mobility of different

metals during catalyst pretreatment. The surface enrichment


of a second metal in bimetallic catalyst, like Sn in Pt-Sn was

larger in carbon supported catalyst than in alumina-

supported catalyst [43].

Iron oxide has been applied as a support in chemose-

lective hydrogenations [11,98]. This idea originated from

the fact that iron has been used as an additive for enhancing

chemoselective hydrogenation of carbonyl bonds. When

Fe2O3 was used as a support for Au, very high selectivities

were obtained over Au/Fe2O3 catalysts prepared by

coprecipitation in benzalacetone and citral hydrogenation

to unsaturated alcohols [11,98]. The role of iron sites

modifying the electronic properties of gold was claimed to

be important.

Basic supports can exhibit electron rich properties

towards the active metal [15]. One of such supports is

MgO, which is a non-reducible alkaline earth oxide [54]

enhancing selectivity to unsaturated alcohols in the

hydrogenation of citral over Pt-Sn/MgO catalyst [15]. In

fact MgO, even in the absence of metal, was able to convert

benzaldehyde to benzyl alcohol at 300 8C in the presence of

hydrogen [156]. Very high activity in furfural hydrogenation

and selectivity to furfuryl alcohol was obtained over Cu/

MgO catalyst [33], where the presence of defect sites or

oxygen vacancies at Cu-MgO interface was stated to be

responsible for selective hydrogenation. However, in the

hydrogenation of citral over Rh/MgO and Rh/SiO2 catalysts

the same selectivities to nerol and geraniol were obtained

[54], speaking against the possible electron enrichment

effect. The Rh/MgO catalyst exhibited, however, a reduced

hydrogen chemisorption capacity, which was attributed to

the Rh-migration onto the support after calcinations at

500 8C [54]. Most of residual chloride located according to

EXAFS measurements on the Mg surface, thus not affecting

the hydrogen chemisorption capacity.

The textural properties of the support can have an effect

on chemoselectivity. This was the case in cinnamaldehyde

hydrogenation, where different selectivities were achieved

with silicas having different pore structures [88]. The most

selective silica in Cu/SiO2 exhibited bimodal pore structure

with an average pore size of 20 nm and a lower specific

surface area [88], whereas lower selectivity was obtained

over Cu/SiO2, with 10 nm pores. [60] fullerene grafted on

silica Pt catalyst has been used in cinnamaldehyde

hydrogenation [152] and 89% selectivity to cinnamyl

alcohol was obtained at 80% conversion.

3.4.1.2. Alloy forming and reducible oxidic sup-

ports. There are several alloy forming supports, which

have recently been reported in the literature, like SnO2

[68,69] and ZnO [36,67], CeO2 [37,65,66]. The effect of

metal precursor (see Section 3.3) and the effect of catalyst

reduction temperature (see Section 3.6.1) can be different

for catalysts having different supports. In this section the

catalytic results obtained in chemoselective hydrogenations

over metal supported catalysts on reducible oxides are

related to the catalyst properties.

The alloy formation and the state of the active metal [36]

is very much dependent on the metal precursor and the type

of support. In case of SnO2 the Pt-Sn alloy was formed in Pt/

SnO2 catalyst prepared form chloride precursor [67],

whereas chloride inhibited the alloy formation between Pt

and Ce in Pt/CeO2 catalyst [65]. Very selective Pt/CeO2

catalyst prepared from tetraammine platinum(II) nitrate

included CePt5 alloyed epitaxial particles on the support

[65]. These particles were able to catalyze crotyl alcohols

formation (consant 80% selectivity at between 4 and 45%

conversion as a function of time-on-stream at 180 8C). In the

work of Concepcion et al. [37] high activities and

selectivities to crotyl alcohol were obtained in the gas

phase hydrogenation of crotonaldehyde over Pt/CeO2-SiO2

at 30 8C and under 1 bar hydrogen. The high selectivity was

achieved due to the presence of metal support interactions

and small Pt particles. In this catalyst the mesoporous silica

contained 8 nm pores. In case of Pt/ZnO the better

performance was obtained over a catalyst prepared form a

chloride precursor than from a nitrate one. XPS revealed the

presence of chlorinated species on the former catalyst,

whereas Pt was in both oxidized and metallic form in the

latter catalyst [36]. Additionally, the increase in catalyst

reduction temperature can change the type of alloy formed

[68]. Pt/SnO2 exhibited higher selectivity than was obtained

over Pt/Al2O3 or Pt/SiO2 [68]. The selectivity to crotyl

alcohol underwent trough a maximum with increasing

reduction temperature of the catalyst. It was confirmed by

XRD that an alloy started to form at 170 8C between Pt and

Sn. Above the reduction temperature 250 8C another alloyed

phase, PtSn2 was formed, which lowered both the catalytic

activity and selectivity to crotyl alcohol. SnO2 itself is inert

in crotonaldehyde hydrogenation at 80 8C [68].

Several metals supported on reducible oxides have been

applied in chemoselective hydrogenations, i.e. TiO2

[12,13,16,22,30,37,38,49,70,72,77,80,89,91,99,114,123,15-

3,155], CeO2 [13,37,65,66,112,113], WO3, MoO3 [34] and

ZrO2 [5,97] have been used in chemoselective hydrogena-

tions. The use of reducible supports, like TiO2 or Nb2O5, has

increased the selectivity towards unsaturated alcohols [157].

The effect achieved by reducing a metal supported on

reducible oxide at high temperature is called strong support

metal interaction (SMSI). The origin for SMSI effect has

been proposed as follows: (a) a suppression of chemisorp-

tion ability and modification of catalytic activity, (b)

formation of an alloy as well as decoration of metal

particles with partially reduced support. The strong metal

support interactions (SMSI) are affecting both the electronic

structure and the metal particle sizes [123]. Gold [97], silver

[89,123], platinum [70,153] and Ir [12] have been used as

active metals with TiO2 support. The presence of partially

reduced support at the metal interface region results in

polarization of C O bond and the selectivity to activate

C O hydrogenation increases.

The performance of gold supported on TiO2 and ZrO2

catalyst was compared in acrolein hydrogenation [95,97].


The presence of Zr4+ and Ti4+ in Au/ZrO2 or Au/TiO2

catalyst favored formation of allyl alcohol. The selectivities

to allyl alcohol were at 240 8C and 20 bar and 100%

conversion 43 and 26%, respectively [97]. Additionally, gold

was negatively charged [97]. ZrO2 support exhibits less

strong interaction with Au than TiO2 [95] and thus Au

particles are most probably in (1 1 1) surfaces in Au/ZrO2

catalyst. It was observed that Au/TiO2 (prepared by sol–gel

method) and Au/ZrO2 (prepared by coprecipitation) catalyst

exhibited no catalytic activity in acrolein hydrogenation, if

they were treated above 180 8C [97]. Additionally, ZrO2 as

such was not inert in acrolein hydrogenation at 320 8C [95].

Another support used with gold was ZnO, which was

selected, since it favors mainly the formation of single

crystalline particles [96]. In general particle morphology can

affect the chemoselectivity [95]. In particular very high

selectivity to allyl alcohol was obtained over Au-In/ZnO

[96] (see Section 3.1.2.4). Over this catalyst the selectivity to

allyl alcohol was stated to be enhanced due to decoration of

gold faces with indium, whereas the edges of cubooctahedral

gold remain uncovered [96].

Ti suboxide formation was observed in Ag/TiO2 catalysts

after high temperature reduction, inhibiting the growth of Ag

particles compared to the catalyst reduced at lower

temperatures [123]. The former catalyst exhibited Ag

particles of the size 1.5 nm, whereas the catalyst reduced at

200 8C had about 3 nm Ag particles. The higher TOF-values

and higher selectivities to allyl alcohol (41.8%) were obtained

over the larger Ag particles reduced at low temperature [123].

From the mechanistic point of view it was stated in the work of

Claus et al. [89] that the active species for carbonyl

hydrogenation in Ag/TiO2 catalyst are not TiOx/Ti3+ species

present in the metal-support interface, which is the case in Pt/

TiO2 catalyst, because the Ag/TiO2 (LTR) catalysts exhibited

higher activity than the corresponding HTR catalysts.

Furthermore, the catalytic activity and selectivity to crotyl

alcohol increased with increasing metal particle size showing

that crotonaldehyde hydrogenation is structure-sensitive [89].

It was concluded that the active sites in Ag/TiO2 catalysts can

be formed via structural changes, like faceting, multiple

twinning or selective coverage by TiOx.

Platinum catalysts supported on several reducible

supports have been applied in chemoselective hydrogena-

tions [34,70,153]. In the hydrogenation of crotonaldehyde

over a Pt/TiO2 catalyst, the higher selectivity to crotyl

alcohol was obtained due to the interaction between a

carbonyl group and the Lewis acid site created from TiOx

[70]. Opposite to Ag/TiO2 [89] larger Pt particles supported

in TiO2 were formed after reduction at 500 8C (HTR

conditions) than after reduction at 200 8C (LTR) [70].

Crotonaldehyde hydrogenation was investigated at 80 8Cover Pt/TiO2 (HTR) and Pt/TiO2 (LTR) catalysts. The

former catalyst exhibited both higher TOF-values and higher

selectivities to crotyl alcohol (43%) than the latter catalyst

with selectivity about 41%. The larger Pt particle size

together with the interfacial sites of TiOx is beneficial for

promoting selective hydrogenation of crotonaldehyde [70].

Analogous results were obtained in the hydrogenation of

phenylacetaldehyde over Pt/TiO2 (HTR) catalyst, where

both activity and selectivity to 2-phenylethanol were

enhanced [153]. It should be pointed out that according

to [70] SMSI state is, however, not stable in liquid phase

hydrogenation due to the presence of moisture. Another

beneficial effect of Pt/TiO2 (HTR) catalyst in the hydro-

genation of phenylacetaldehyde was the lowered hydro-

genolysis activity due to the lack of large Pt ensembles

[153]. In crotonaldehyde hydrogenation over Pt/TiO2

reduced at 500 8C the maximal selectivity to crotyl alcohol

was about 70% [72]. The presence of coordinatively

unsaturated Ti cations enhanced the adsorption strength

of C O bond and the carbonyl group interacted with the

Lewis acid sites yielding higher selectivities to crotyl

alcohol [70]. The Pt/TiO2 (HTR) catalyst exhibited as

expected lowered hydrogen adsorption capacity and lowered

catalytic activity due to the migration of TiOx species on the

metal surface [72]. The promotion effect of TiOx was,

however, dependent on Pt particle sizes [70]. When larger Pt

particles were promoted with TiOx both higher TOF and

higher selectivities to crotyl alcohol were obtained, while the

effect of TiOx was minor with smaller Pt particles [70].

According to EXAFS study the larger Pt particles exhibited

a disordered overlayer of TiOx on Pt surface, but neither

regular Pt-O no Pt-Ti were formed [70]. Additionally, these

large Pt-particles had Pt(1 1 1) surfaces.

In the work of Englisch et al. [70], the performance of Pt/

TiO2 catalyst was compared in the gas and in the liquid

phase hydrogenation of crotonaldehyde. The selectivity

behavior was analogous, but the enhanced (SMSI) activity

over Pt/TiO2 catalyst in liquid phase hydrogenation of

crotonaldehyde was not observed, which was explained by

the presence of moisture in the liquid phase. In a case of

bimetallic catalyst, like Pt-Sn/TiOx (HTR) catalyst, which

was more selective in crotyl alcohol than a monometallic

catalyst [76], tentative explanations of the catalytic

behavior, were both the presence of Sn partially in oxidized

state and the decoration of Pt surface with TiOx.

Ir/TiO2 reduced at high temperature exhibited higher

activity and selectivity to crotyl alcohol in the gas phase

hydrogenation of crotonaldehyde than one reduced at a

lower temperature [38,202]. The better performance of the

formed catalyst was explained by the formation of

interfacial Ir-TiOx species, which could stabilize the

formation of s-bonded C O complex [38]. The existence

of this complex was confirmed by DRIFTS measurements.

3.4.2. Zeolites and mesoporous materials

Zeolites and mesoporous materials, which exhibit struc-

tured ordering at the nanometer scale, have been used for a

long time in petrochemical applications. Due to their channel

system and shape selective properties these materials have a

potential for synthesis of fine chemicals. Zeolites can,

however, suffer from pore diffusional limitations. Mesopor-


ous materials have tunable pore sizes between 2 and 10 nm

[194], therefore they can be more suitable for hydrogenation

of large organic molecules than zeolite pores.

Zeolites and mesoporous molecular sieves have been

used as supports in the hydrogenation of acetophenone [24],

citral [102,145] and cinnamaldehyde [50,52,142,143]. Citral

was hydrogenated over Ni/Y catalyst, but the hydrogenation

resulted mainly in undesired by-products, whereas less side

reactions occurred in the hydrogenation of cinnamaldehyde

[52]. The reason for lower amount of side products formed in

cinnamaldehyde hydrogenation is its stabilization due to the

electronic effect of phenyl ring [29]. The highest selectivity

to cinnamyl alcohol in cinnamaldehyde hydrogenation in

cyclohexane at 100 8C and 50 bar hydrogen was obtained

over Ru/Y, whereas Ru/MCM-41 resulting in about seven

times low selectivities [52]. The possible reason for higher

selectivity to cinnamyl alcohol over Ru/Y might be the

channel system of zeolite compared to the mesoporous

MCM-41. A systematic study on the dependence of the

support acidity and catalytic performance in cinnamalde-

hyde hydrogenation was carried out by Lashdaf et al. [50]. It

was shown that the Ru particle size decreased with

increasing total acidity of the support. These Ru/b zeolite

catalysts were, however, not selective at all to cinnamyl

alcohol. The only products were hydrocinnamaldehyde and

acetals [50].

Pt/Y zeolite prepared by ion-exchange was very selective

in cinnamaldehyde hydrogenation [158]. The selectivity to

cinnamyl alcohol increased with increasing conversion and

was maximally 92% (Fig. 13). One additional reason for

high cinnamyl alcohol selectivities was the presence of

sodium acetate, since the solvent was 2-propanol/H2O/

sodium acetate mixture (see Section 3.7.1). Alkali modified

(Rb, Sr) L zeollite gave very high selectivity to cinnamyl

alcohol (over 90%) in cinnamaldehyde hydrogenation [198].

Fig. 13. Selectivity to cinnamyl alcohol as a function of conversion in the

hydrogenation of cinnamaldehyde at 60 8C and 40 bar over ion-exchanged

14% Pt/NaY catalyst [158].

A Pt modified mesoporous molecular sieve, Pt-MCM-48,

which has cubic array structure, has been used in the

hydrogenation of cinnamaldehyde in supercritical carbon

dioxide [142,143,145]. Very high selectivity to cinnamyl

alcohol (96.6%) was obtained in cinnamaldehyde hydro-

genation over 1 wt.% Pt-MCM-48 at 50 8C and 40 bar

hydrogen in the presence of CO2. The conversion level

remained, however, low, at 30.8% [142]. Furthermore,

cinnamaldehyde hydrogenation was investigated over

copper-containing crystalline silicate mesoporous material

at 50 8C in supercritical CO2 in a pressure range of 70–

120 bar [143]. High selectivity to cinnamyl alcohol 90.5%

was obtained at 30% conversion. The benefits of using

supercritical carbon dioxide as a solvent are described later

on in Section 4.2.

3.4.3. Clays

Metals supported on clays have been used as catalysts in

cinnamaldehyde [146] and in crotonaldehyde hydrogena-

tions [73]. Such bifunctional catalysts can be not only very

active but can also afford high selectivity. Very high

selectivities to cinnamyl alcohol (>95%) at high conver-

sions (>95%) were obtained in 2-propanol at 25 8C and

15 bar H2 over 5 wt.% Pt/K-10, which is an acidic material,

exhibiting both Brønsted and Lewis acidity [73]. The high

selectivity was achieved due to Lewis acid sites located in

the vicinity of metal. Carbonyl oxygen is able to adsorb on

the Lewis acid sites and the simultaneous adsorption of C C

and C O does not occur. It was however stated by the

authors [146] that in cinnamaldehyde hydrogenation the

high selectivity to cinnamyl alcohol was obtained over Pt/K-

10 clay catalyst in spite of the fact that Pt particles in K-10

were not immobilized in the interlamellar space of the clay.

In case of crotonaldehyde maximally about 40–45%

selectivity to crotyl alcohol was obtained over Pt/H-Ben

(bentonite) and Pt/K-10 [73], i.e. the reactant structure is

crucial for achieving high selectivities to unsaturated

alcohols.

3.4.4. Porous metal catalysts of Raney type

Raney cobalt [28] and Raney nickel catalysts [31] have

been used in the hydrogenation of cinnamaldehyde and

furfural, respectively. The selectivities to the unsaturated

alcohols have been relatively low over these catalysts, 23%

to cinnamyl alcohol [28] and 75% to furfuryl alcohol [31].

Interestingly when these catalysts were modified with

heteropolyacid salts the selectivities to intermediate

unsaturated alcohols were increased (see Section 3.7.1).

3.4.5. Carbons

Several types of carbons have been applied in chemo-

selective hydrogenations. The most common is activated

carbon [9,14,43,56,126,147,155,159], although graphite

[9,85] and diamond have been reported as well [159].

Application of carbon nanotubes is described in more detail

in Section 3.4.7.1. Additionally, carbon-coated wafers


containing Ru were tested in gas phase hydrogenation of

acrolein [160]. As mentioned already in Section 3.4.1, the

interactions between carbon and metals are not so strong as

between oxides and metals. Activated carbon surfaces

posess rather complex structures, containing oxygen surface

groups [161]. The metal dispersion was found to be lower on

the supports exhibiting high amounts of oxygenated acidic

surface groups. Such Pt/C catalysts were catalytically active

in gas phase hydrogenation of crotonaldehyde and they

exhibited at the same time higher selectivities to crotyl

alcohol than catalysts containing less surface acidic groups

[161]. Analogous results were obtained in crotonaldehyde

hydrogenation, where the highest TOF was obtained over the

Cu catalyst on an activated carbon treated with nitric acid

[159]. The higher TOF over this catalyst was interpreted by

creation of additional adsorption sites and possible

formation of spill-over hydrogen [159]. Over this catalyst

the selectivity to crotyl alcohol was very low, about 3% after

30 min reaction time at 150 8C [159]. Five times higher

selectivity to crotyl alcohol after 30 min was obtained over

ion-exchanged Cu graphitic carbon fibers, but the TOF of

crotonaldehyde over this catalyst was very low. The low

TOF could be due to very small Cu particles stabilized along

the edges of the graphitic basal planes. Selectivity to crotyl

alcohol was enhanced over more oxidized catalysts [161].

However, an opposite result was obtained in the same

reaction over supported graphites, i.e. the presence of

oxygen containing surface groups did not have any effect on

the selectivity towards unsaturated alcohols. On the other

hand the surface oxygen groups were acting as anchoring

sites for residual chloride ions, which in turn enhanced the

selectivity to unsaturated alcohols [136].

3.4.6. Colloidal metal catalysts

Polymer stabilized metal catalysts with a narrow metal

particle size distribution have been used as catalysts in

chemoselective hydrogenations [154]. One example is a Pt

catalyst stabilized with poly-(N-vinyl-2-pyrrolidone). The

mean Pt particle size was 1.1 nm and very high activities and

selectivities (99.8%) in cinnamaldehyde hydrogenation at

60 8C and 40 bar in ethanol have been reported over this

catalyst [154]. The high selectivities to cinnamyl alcohol

originate, however, partially also from the presence of metal

salts used as promoters (Section 3.7). Another example

reported in the literature was metallic platinum in polymer

resins [162]. Citral hydrogenation was investigated over

several Pt catalysts supported on polymer resins at 60 8C and

under 1 bar hydrogen in ethanol. High selectivities (80–

90%) to nerol and geraniol were obtained over Co(II)

promoted Pt-resin catalysts at conversion levels of 80–90%.

The drawback of the added ionic metal promoters was,

however, the catalyst deactivation. Unpromoted Pt-resin

catalysts exhibited relatively high selecivities to nerol and

geraniol (45–55%) at low citral conversions. The reason for

this could be the interaction of Pt with the basic cyano and

pyridyl goups in the support [162].

3.4.7. Structured supports

Structured supports can exhibit either microscale or

nanoscale pores/cavities. Fibers, monoliths, microstructured

reactors [160] and catalytic membranes belong to the former

group, whereas zeolites and mesoporous materials exhibit

porosity in nanoscale. In this section application of the

microstructured materials as catalyst supports is described.

In general it can be noted that structured catalysts have some

benefits compared to conventional catalysts structures. For

instance they can facilitate lower pressure drop, catalyst

separation is not needed and the geometrical surface area is

large [140]. The major drawback is the lack of the practical

experience in using these types of structures in liquid phase

hydrogenations.

3.4.7.1. Fibrous catalysts. Several types of fibers have been

recently applied in chemoselective hydrogenations of

carbonyl compounds [19,48,137,138,203]. The fiber mate-

rial can be polymer [19], woven glass fiber [137] activated

carbon fibers [203,204] even silica [138]. The major benefit

of fibrous catalyst is the short diffusion length of the

reactants to the active site, since the fiber diameters can vary

from 25 nm [48] to 6 mm for silica fibers [138] and 30 mm

for polymer fibers [19]. Thermal stability of the fibers is also

an important factor, especially in gas phase applications at

high temperature. For instance silica fibers exhibit much

higher thermal stability than polymer fibers, the advantage in

polymer fibers is the surface modification via grafting

different surface groups, whereas carbon nanotubes are

relatively inert. Therefore, in order to attach a metal to

carbon nanofibers their surfaces have to be oxidized with

mineral acids [48].

Activated Kynol carbon fibers are attractive support

materials due to their large specific surface area (1500 m2/

gcat) and small diameter (9 mm) [203]. Synthetic insoluble

novoloid fibers are cured phenol-aldehyde fibers prepared by

acid-catalyzed cross-linking of amorphous novolac resin.

The pores in activated carbon fibers are straight and uniform

[203]. Activated carbon fibers impregnated with either Ni or

Pt as active metals were tested as catalysts in citral

hydrogenation in a pressurized reactor in hexane in a

temperature and pressure range of 80–100 8C and 6 to

51 bars, respectively [204]. The main products over 20 wt.%

Ni/activated carbon fibers were citronellal, menthol and

isopulegol, whereas over 5.9 or 10.9 wt.% Pt/activated

carbon fibers geraniol and nerol were obtained. The main

disadvantage in metal modified activated carbon fibers was

the catalyst deactivation [204].

Woven glass fibers have been used as catalyst supports in

chemoselective hydrogenation of benzaldehyde. The woven

glass fibers allow low-pressure drop and high flow rates,

because of the macro structure of the catalyst. The specific

surface area can vary from 15 to 75 m2/g. The threads of the

fabrics consist of 50–100 filaments with a diameter of 3–

15 mm twisted to bundles exhibiting a diameter of several

hundreds of microns. Pt supported on woven glass fibers


(0.2 wt.% Pt) was used in the liquid phase hydrogenation of

benzaldehyde at 50 8C under 1 bar hydrogen in 2-propanol

[137]. Total conversion to benzyl alcohol was obtained, but

unfortunately only a comparison of the initial mass-based

rates in the hydrogenation of benzaldehyde over Pt/fiber and

over 5 wt.% Pt/C were given. The corresponding initial rates

were 55.8 mmol/min/gPt and 7.5 mmol/min/gPt, respec-

tively.

Carbon nanofibers (CNF) are chemically inert and

mechanically strong. They have the external surface area

of 100–200 m2/g, since the fiber diameters are 10 to 50 nm.

Carbon nanofibers have no pores, but they have, however,

larger mesoporous skeins, because they are interwoven

during the growth [48]. As carbon nanofibers are chemically

inert, it is difficult to impregnate metal particle on those

fibers. One of the utilized procedures to overcome this

property is the treatment with nitric acid creating surface

oxygen groups on the carbon nanofiber surface, which is

then wettable for metal precursors to be attached. The

ruthenium loaded carbon nanofibers have been prepared by

using homogeneous deposition precipitation method starting

from RuNO(NO3)3�nH2O. This catalyst exhibiting a narrow

ruthenium particle size distribution of< 3 nm, has been used

in the hydrogenation of cinnamaldehyde [48]. The best

selectivities to cinnamyl alcohol have been about 48% at

60% conversion in 2-propanol and at 110 8C and 45 bar. The

selectivity to cinnamyl alcohol dropped with increasing

treatment temperature of Ru/CNF. Simultaneously the total

hydrogenation activity increased and the amount of surface

Fig. 14. (A) Recirculating reactor used for citral hydrogenation over (B) kn

oxygen-containing acidic groups decreased [48]. Close to

60% selectivity to cinnamyl alcohol was observed in

cinnamaldehyde hydrogenation over Pt/CNF catalyst at 70%

conversion at 110 8C and under 30 bar total pressure [189].

The carbon nanofiber surface changed from polar to non-

polar when the catalyt treatment temperature increased,

which in turn changes the adsorption mode of cinnamalde-

hyde and alters the selectivity. Another possible explanation

for the enhanced selectivity towards cinnamyl alcohol could

be the change of the electronic state of Ru. Further studies

are needed in order to fully explain the obtained selectivities

[48].

Knitted silica fibers were used as a support in citral

hydrogenation [138] (Fig. 14). Silica fibers were prepared

from a hybrid organic–inorganic fiber material containing

67% cellulose and 30% polysilicic acid and 3% sodium

aluminate. The preparation method of the fibers is reported

in [163]. A knitted silica fiber catalyst exhibited the diameter

of 6 mm and the tunable specific surface area in a range of

56–177 m2/gcat. The support was impregnated with

Ni(NO3)2 salt and used as a catalyst support in citral

hydrogenation in a recirculating tube reactor (see Section 7).

The main benefit of using this support is the short diffusional

distance and the low-pressure drop in a catalyst bed.

Catalytic performance of fibrous Ni/SiO2 was compared

with batch reactor results over a Ni/Al2O3 powder catalyst.

The comparative performance of these two catalysts was

observed in terms of activities [138]. The initial citral

hydrogenations rates in ethanol at 70 8C and 1 bar hydrogen

itted silica fibers impregnated with Ni, (C) a single silica fiber [138].


were 22 mmol/min/gNi and 12 mmol/min/gNi over 5 wt.% Ni/

SiO2 fiber and over 16.7 wt.% Ni/Al2O3. The maximum

selectivity to citronellol in citral hydrogenation over the

former and the latter catalysts were 92 and 70%, respectively.

Polymer fibers with Pd as an active metal [19] have been

used in citral hydrogenation. Polymer fibers were prepared

by electron-beam-induced pre-irradiation grafting of 4-

vinylpyridine onto polyethylene fibers. Metal was intro-

duced via ion-exchanging the fibers using a metal salt, like

Pd(NO3)2 as a precursor. The fiber diameter was 30 mm.

High selectivities to citronellal (maximally close to 90%)

were obtained in citral hydrogenation at 5 bar hydrogen and

70 8C 2-propanol over a 12 wt.% Pd/ pyridine modified

polyethylene fiber catalyst prepared from a PdCl2 precursor

[19]. As a comparison the selectivity to citronellal over Pd/C

catalyst was only 67.5% [19].

3.4.7.2. Monoliths. Monolith catalysts are attractive alter-

natives to slurry and trickle bed reactors. The main

advantages with monoliths are the absence of catalyst

separation from the products, lowered catalyst attrition

compared to slurry reactors, low-pressure drop, easy scale-

up, cost effectiveness due to absence of intensive stirring in

large scale operation and safe operation [205]. The main

disadvantage is the lack of practical experience in large-

scale production of chemicals.

Monolith catalysts have been investigated in the hydro-

genation of benzaldehyde [139,140] and citral [19]. In the

former case monolith was prepared from cordierite with a

surface area of 0.7 m2/g. Because of the low surface area of

cordierite it was washcoated with g-alumina. Further

pretreatment of the Ni/monolith catalyst involves deposition

precipitation techniques. A pilot-scale study using a mono-

lithic reactor in the hydrogenation of benzaldehyde showed

97% selectivity to benzyl alcohol at 50% conversion, while

hydrogenation in a trickle bed reactor gave only 75%

selectivity at the same conversion [140]. The higher selectivity

was attributed to the sharper residence time distribution

compared to the trickle bed reactor similar to fiber catalysts.

Additionally, 600 cpsi monoliths exhibited higher selectivities

to benzylalcohol than those with the cell density 400 cpsi,

which was due to the shorter diffusion path in 600 cpsi

monoliths. In citral hydrogenation over 4.5 wt.% Ni on

alumina washcoated monolith nickel was equally distributed

throughout the monolith having the thickness of 106 nm [164].

3.4.7.3. Membrane reactors. Membrane reactors have been

applied in liquid phase hydrogenations of cinnamaldehyde

[74,165]. Analogously to monolithic systems, when the

catalyst is impregnated onto the membrane wall the catalyst

separation can be avoided [165]. The technical features of

membrane reactors will be discussed in Section 7. Catalytic

tubular membrane made from a-Al2O3 was coated with Co-

Pt/g-Al2O3 layer and used for cinnamaldehyde hydrogena-

tion [165]. The highest selectivity to cinnamyl alcohol was

75% at conversion of 43.7% at 50 8C in ethanol.

Hydrogen permeating palladium membrane was used as a

catalyst in furan hydrogenation at 150 8C [166]. The

conversion of 16% was achieved with tetrahydrofuran as

a main product.

3.5. Catalyst preparation methods

There are several techniques applied in laboratory and

industrial practice for catalyst preparation, like gas and

liquid phase deposition, ion exchange, incipient wetness,

deposition precipitation method, sol–gel method, metal

introduction into mesoporous materials via in situ synthesis

etc. [167,171]. Catalyst preparation method affects very

much on the metal dispersion, which could be crucial for

achieving high activity and selectivity. A systematic

comparative study of preparing catalysts via gas phase

deposition and via wet impregnation impregnation and

testing in cinnamaldehyde hydrogenation was performed by

Lashdaf et al. [27]. Pd and Ru catalysts supported on silica

and alumina were prepared with Pd and Ru b-diketonates as

metal precursors. In gas phase preparation (atomic layer

epitaxy method) small Pd metal crystallites were formed

even with high metal loadings, whereas larger Pd particles

were achieved via impregnation. Additionally, ALE-

prepared Pd catalysts were more selective to cinnamyl

alcohol formation than the impregnated catalysts exhibiting

larger metal particles. The Ru catalysts prepared by ALE

technique resulted in higher selectivities than corresponding

impregnated catalysts. The highest selectivity to cinnamyl

alcohol, 42% at 20% conversion, was obtained with a Ru/

Al2O3 catalyst exhibiting 2 nm Ru particles.

Ni/Al2O3 and Ni/SiO2 catalysts prepared by ALE were

tested in citral hydrogenation [102]. The results showed that

by this method a more even metal distribution can be achieved

(Fig. 15). A high maximum selectivity to citronellol (76%)

was obtained over 17.6 wt.% Ni/SiO2 (ALE) catalyst at 70 8Cand 5.3 bar in ethanol [102], whereas over a conventional

10 wt.% Ni/SiO2 catalyst the maximum selectivity to

citronellol was nearly the same, i.e. 74%.

A sol–gel technique is suitable for preparing active and

selective catalysts [53,90,92,97,120–122,171]. Over 60%

selectivities were reported at high cinnamaldehyde con-

version (90%) over a bimetallic 5%Ru-10%Sn/SiO2

catalyst at 160 8C and 70 bar [168]. Another system, where

sol–gel technique was used, is Au/TiO2 catalyst. This

catalyst was tested in acrolein hydrogenation at 240 8Cunder 20 bar. The selectivity to allyl alcohol was 19% at

100% conversion [97]. Additionally, Ag/SiO2 catalyst

prepared by sol–gel method and exhibiting the Ag particle

size of 4.5 nm used in crotonaldehyde hydrogenationat

140 8C and 20 bar resulting in more than 60% selectivity to

crotyl alcohol [89].

In situ metal introduction into mesoporous MCM-41

material was compared to an ion exchange and an

impregnation method. Additionally, these catalysts were

tested in cinnamaldehyde hydrogenation [169]. The largest


Fig. 15. (A) SEM image and (B) elemental analysis profile from a Ni/Al2O3

catalyst particle prepared by atomic layer epitaxy [102].

Ru particle size was observed in situ synthesized Ru/MCM-

41, which also provided the highest selectivity to cinnamyl

alcohol. The highest activity was obtained over the ion

exchange Ru/MCM-41 catalyst, but no cinnamyl alcohol

was produced over this catalyst. An inverse relationship was

observed between the final cinnamaldehyde conversion and

the selectivity to cinnamyl alcohol.

There is a growing interest in the application of ionic

liquids in various field of catalyst preparation [170]. For

instance, a colloidal Pd catalyst was prepared by stabilizing

it with an ionic liquid, 1-butyl-3-methyl imidazolium

bistrifluorosulphonylimide, after which it was treated with

(EtO)4Si and formic acid in order to form monolithic

structure. Cinnamaldehyde was hydrogenated over this Pd

catalyst affording 100% selectivity to hydrocinnamaldehyde

at 100% conversion.

3.6. Catalyst pretreatment

Catalyst pretreatment is often necessary, because the solid

materials containing metal compounds in non-metallic state

can exhibit only low catalytic activity or be catalytically

inactive [171]. Metal precursors can react at the interface of

the support and metal, like in reduction of NiO to Ni [171].

Catalyst pretreatment affects both catalytic activity and

selectivity [36,52,62,94,124], since it can change metal

particle size, morphology [94], amount of residual chloride

[36,52], influence alloy formation [36,65–67], lead to

reduction of reducible oxides, which decorate the metal

surface [72] as well as in case of carbon can alter the amount

of oxygen containing surface groups [48,136]. One crucial

parameter in catalyst preparation is the origin of the metal. If

the metal originates from chloride precursor the amount of

residual chloride can be decreased with calcination [52].

Additionally, metal dispersion can be changed with catalyst

pretreatment [136]. The selective catalyst can also be

obtained by catalyst aging [132].

3.6.1. Catalyst reduction temperature

Catalyst reduction temperature has been very intensively

investigated in the case of reducible supports exhibiting

SMSI effect (see Section 3.4.1.2). However, even over

conventional supports, e.g. alumina and silica the catalyst

reduction temperature can have dramatic effects on the

catalytic performance. These effects are connected to the

degree of metal reduction and the type of metal precursor.

Especially, chloride precursors are more difficult to reduce

than metals originated from nitrates and other sources. Alloy

formation in bimetallic catalysts, which was shown to be

beneficial for high chemoselectivity is strongly dependent

on the metal ratio as well as on the catalyst reduction

temperature. Additionally, both reduction time and tem-

perature influence the formation of spill-over hydrogen

[210], which can be of crucial importance in inhibiting side

reactions on acidic support surfaces [8].

On conventional oxide supports, the metal morphology

and degree of reduction can be varied by varying the

reduction temperature. Higher selectivities to cinnamyl

alcohol were obtained in liquid phase hydrogenation in

ethanol at 35 8C and 1 bar hydrogen over an impregnated Pt/

Al2O3 catalyst reduced at lower temperatures [63], whereas

more by-products were obtained over Pt/Al2O3 catalyst

reduced at high temperatures. The extent of Pt reduction,

measured by XPS and TPR, was higher over Pt catalyst

reduced at lower temperatures than at higher temperature.

One possible explanation for these results could be the

presence of Na and B in the former catalyst, which could

change the size and structure of Pt ensembles [63]. The Pt

particles reduced at lower temperatures are most probably in

high Miller index planes, which favor hydrogenation of

C O group [63]. Additionally, the adsorption of phenyl ring

is suppressed via steric hindrance decreasing the probability

for hydrogenation of C C bond.

Alloy formation in bimetallic catalysts can be tuned by

changing the reduction temperature. The correlation was

found between the catalyst reduction temperature, catalytic

activity and selectivity in the gas phase hydrogenation of

crotonaldehyde over Pt/ZnO catalyst prepared from

Pt(NH3)4(NO3)2 [36], i.e. with increasing reduction tem-

perature of the catalyst the catalytic activity decreased and

simultaneously the selectivity to crotyl alcohol increased.

These results were correlated with the alloy formation and

the migration of reducible oxide onto the metal surface.

When comparing these results with the performance of the


chloride-containing catalysts, it was concluded that Cl� ions

were present at each reduction temperature and the Pt-Zn

alloy was formed already at a lower reduction temperature

200 8C. The selectivity to crotyl alcohol was thus remark-

able higher over the latter catalyst than over the former

catalyst.

Reducibility of oxides in metal supported catalysts can be

affected via changing the catalyst reduction temperature.

The catalytic activity and selectivity to crotyl alcohol in

crotonaldehyde hydrogenation over Au/TiO2 catalyst

exhibited a maximum with increasing catalyst reduction

temperature [94] (see Section 3.4.1.2). This was explained

by the change in Au metal size as well as changes in Au

particle morphology [94]. The well-developed facets were

not formed after reduction at 200 8C, whereas reduction at

500 8C resulted in formation of these facetted index planes.

When the Au/TiO2 catalyst was reduced at 500 8C, both the

catalytic activity and selectivity to crotyl alcohol decreased,

because of the smoothing of the outer gold surfaces.

3.6.2. Catalyst calcination

Catalyst calcination affects the amount of residual

chloride as well as the hydrogen adsorption capacity of

the catalyst [52]. In case of chemoselective hydrogenations

the effect of calcination was investigated for cinnamalde-

hyde [52]. Two methods were used: the catalyst was either

calcined in air at a higher temperature thereafter and reduced

or directly reduced without calcinations. The catalyst

calcination suppressed the hydrogen adsorption in Ru/Y

[52], but the final conversion in cinnamaldehyde hydro-

genation was higher over the calcined Ru/Y catalyst than

over a non-calcined one. The amount of residual chloride

decreased with calcinations and the higher selectivity

obtained over a calcined Ru/Y catalyst correlated well with

the decreased catalyst acidity [52]. Analogously selectivity

to cinnamyl alcohol was higher over calcined Ru/MCM-41

than over non-calcined Ru/MCM-41 [52].

3.6.3. Catalyst aging

In gas phase hydrogenation of unsaturated aldehydes the

catalyst deactivation is very fast, since as already discussed

previously higher selectivity can be obtained over less active

catalysts, it is not surprosing that the selectivity to

unsaturated alcohols increases with increasing time-on-

stream, e.g. with a decrease in conversion. In order to

decrease the transient period the catalyst can be aged. The

effect of catalyst aging has been investigated in detail by

Margitfalvi et al. [132]. The catalyst was purged periodically

with pulses of a mixture of substance and hydrogen and

hydrogen alone. A partial restoration of activity could be

obtained with pure hydrogen pulse. After 15–20 pulses the

constant activity of the catalyst together with enhanced

steady state selectivity was achieved in crotonaldehyde

hydrogenation at 80 8C and 1 bar over Pt-Sn/SiO2 catalyst.

Catalyst aging was tested also in the liquid phase

hydrogenation of cinnamaldehyde [172]. The catalyst (Ru/

Y) was aged before the liquid phase hydrogenation at 100 8Cand 50 bar in cyclohexane with 1-butene in the gas phase.

The conversion levels remained lower (about 25%)

compared to the fresh catalyst (resulting in the conversion

level of 85%). The selectivity to cinnamyl alcohol was the

highest (18%) over the catalyst aged for 10 min with 1-

butene at 10% conversion followed by the catalyst aged for

2 h (selectivity about 9%) and the lowest selectivity (about

3%) was achieved with the fresh catalyst.

3.7. Catalyst modifiers and promoters

The catalytic activity and selectivity can be affected by

introducing modifiers onto the catalyst or into the reaction

mixture. In the former case modifiers are adsorbed, whereas

in the latter one they can be dissolved in the liquid phase.

The adsorbed modifiers can be solid acids, sulphur

compounds or alkali metal and transition metal oxides.

The dissolved promoters are inorganic bases and salts. Some

recent examples from these modifiers and promoters are

given below.

3.7.1. Adsorbed modifiers

Adsorbed heteropolyacids have been used as catalyst

modifiers in chemoselective hydrogenations. Raney nickel

[31] and Raney cobalt [28] catalysts were modified with

heteropolyacids exhibiting Keggin type structure. The

highest selectivity for furfuryl alcohol at the conversion

of 98.1% was obtained in furfuraldehyde hydrogenation in

ethanol at 20 bar over a modified Raney nickel catalyst. Both

activity and selectivity were enhanced by addition of the

modifier. The amount of adsorbed modifier was 7.1 wt.%

Cu1.5PMo12. At the moment the mechanism for activity and

selectivity enhancement is not clear. A modified Raney

cobalt catalyst modified with 2.8 wt.% Cu1.5PMo12O40 used

in cinnamaldehyde hydrogenation at 10 bar hydrogen and

80 8C in ethanol gave higher selectivity than a non-modified

Raney cobalt catalyst, the selectivities were 83 and 70%,

respectively. The conversion of cinnamaldehyde was,

however, decreased very much within the same reaction

time compared to a non-modified Raney cobalt catalyst [28].

According to XPS measurements the modifier was reduced

during adsorption on Raney cobalt, i.e. copper and

molybdenium introduced initially as Cu2+ and Mo6+ were

present respectively as Cu0 and Mo5+ and Mo4+.

Thiophene has been used as a promoter in crotonaldehyde

hydrogenation over Au/ZnO and Au/ZrO2 catalysts [173].

The formation rate of crotyl alcohol was enhanced with

thiophene promotion, whereas in the case of Au/SiO2

decreased hydrogenation rates were observed. It was

concluded that sulphur promotion depends very much on

the type of the support. Additionally, the activation of

carbonyl bond over Au/ZnO and Au/ZrO2 may be due to

formation of interfacial sites, which thiophene can modify

[173]. The correct catalyst preparation method was very

important in formation of an active catalyst, since the Au/


Fig. 17. Adsorption of cinnamaldehyde via donation of a lone pair of

electrons from the oxygen atom (M+ = Li+, Na+, K+) [85].

ZnO catalyst prepared by impregnation was inactive in

crotonaldehyde hydrogenation.

Alkali metal oxides or transition metal oxides (MgO and

CeO2) have been used as promoters in gas phase

hydrogenation of crotonaldehyde and in liquid phase

hydrogenation of citral [14]. The Ru-catalysts have been

prepared via consecutive impregnation of the active metal

and the promoter. The following catalysts were investigated:

Ru-Mg or Ru-Ce supported on either Al2O3 or C. The

promoter oxides were well dispersed on the supports, as no

oxide peaks were visible in XRD-patterns. When adding

oxide the catalytic activities decreased with the simulta-

neous increase in the selectivity to crotyl alcohol in the gas

phase and corresponding increase in nerol and geraniol

selectivity in citral hydrogenation in 2-propanol over Ru-Ce/

Al2O3. Interestingly the activity of Ru-Ce/C catalyst was,

however, increased with the simultaneous increase in the

selectivity towards unsaturated alcohols: nerol and geraniol.

The different behavior in Ru-Ce/Al2O3 and Ru-Ce/C, i.e. the

latter catalyst exhibited both higher activity and selectivity

to unsaturated alcohols than the former catalyst, was

explained by the fact that the interactions between Ru and

alumina are stronger than in Ru/C, confirmed by TPR. This

facilitates the more effective Ce-promoter effect in Ru/C.

The possible promoting mechanisms could be an increased

electron density of the metal, and presence of Lewis acid

sites near the metal particle. Additionally, it should be noted

that CeO2 and Ce/C exhibited some catalytic activity and

very high selectivity to crotyl alcohol [14].

Inorganic bases, e.g. NaOH [24], KOH [85] and salts,

such as KCl, CH3COOK [71,85], CH3COONa [158], FeCl3[154], KNO3, NH4OH [85] have been used as promoters in

liquid phase chemoselective hydrogenations. Some organic

amines, like piperidine and pyridine inhibited the hydro-

genation [24], whereas other organic amines and phosphines

had beneficial effects [17,71].

NaOH increased the selectivity to 1-phenylethanol in the

hydrogenation of acetophenone, whereas activity was

simultaneously decreased [24]. It was stated that in the

presence of NaOH an enolate ion was formed. A hydride ion

transfer to the adsorbed species followed by protonation

from the solution gives an unsaturated alcohol (Fig. 16).

KOH showed a first-order dependence on the initial

hydrogenation rate of cinnamaldehyde over Pt/G catalyst

in toluene/water solvent. The nature of metal cation was

important. The activation of C O was enhanced by using an

electropositive metal with respect to Pt and the most

effective was Li+ followed by Na+ and K+ [85].

Cinnamaldehyde adsorbs on the modifier surface via

Fig. 16. Mechanism for formation of an unsaturated alcohol in the presence

of NaOH [24].

donation of a lone pair of electrons from the oxygen atom

(Fig. 17) [85].

Organic bases, which have an amine structure, could have

either positive or negative effect on the catalytic perfor-

mance. Piperidine and pyridine inhibited hydrogenation of

acetophenones [24], whereas triethylamine used as a

modifier in citral hydrogenation resulted in very high

conversions of citral (95%) and selectivities to nerol and

geraniol (>95%) over Ru-Fe/C catalyst at 100 8C and 50 bar

of hydrogen pressure in methanol [17]. Triphenylphosphine

as a modifier decreased both hydrogenation of C C and

C O bond, but the selectivity to crotyl alcohol was,

however, enhanced [71].

Metal salts were used as promoters in cinnamaldehyde

hydrogenation over colloidal Pt catalyst [154]. In the

absence of metal salt this catalyst gave 12% selectivity to

cinnamyl alcohol at 37.5% conversion in ethanol at 40 bar of

hydrogen and 60 8C. When Zn2+ cations were added into the

reaction mixture with the same catalyst and reaction

conditions, the selectivity to cinnamyl alcohol was

99.8%. The final conversion was, however, only 13%

[154] indicating that Zn2+ acts as a catalyst poison.

Replacement of Zn2+ by Fe3+ resulted in higher conversions

of cinnamaldehyde and selectivities to cinnamyl alcohol

(98.5%).

4. Hydrogenation kinetics and catalyst deactivation

in gas and liquid phase hydrogenations

The hydrogenation kinetics is briefly reviewed in this

section with the emphasis on the influence of the main

parameters, e.g. temperature, pressure, solvent for liquid

phase reactions and the initial reactant concentration. From

industrial point of view the deep understanding of catalyst

deactivation is important. Therefore, special attention is given

to the dependence on catalytic activity and the product

selectivity on reaction time or time-on-stream. Additionally, a

separate section is devoted to comparison of gas phase and

liquid phase especially the activity–selectivity conversion

relationship. This comparison is rather complicated, since

usually the catalyst deactivation is directly visible in gas phase

kinetic data, whereas in liquid phase batch reactions the

kinetics and the deactivation are lumped together. Moreover,

different concentrations, temperatures and pressures are used

in liquid- and in gas phase hydrogenations.


4.1. Effect of reaction parameters on the catalytic

activity and selectivity

The kinetic behavior during hydrogenation and the trends

in activity and selectivity with varying the reaction

parameters, temperature, pressure, solvent as well as catalyst

can reveal the relative adsorption strength between reactant

and different products. Hydrogen order in chemoselective

hydrogenations is usually one indicating the weak adsorp-

tion of hydrogen, whereas the reaction order with respect to

organic reactant is close to zero [10,20,40,56,74,134]. It was

also shown that the reactant structure [134] and the catalyst

support can affect the reaction order with respect to reactant.

In particular, the SMSI effect (see Section 3.4.1.2) has a

large impact on the reaction kinetics as shown in citral [1]

and crotonaldehyde [89] hydrogenation. In some cases the

order with respect to reactant can vary with changing initial

reactant concentration. Not only activity, but also selectivity

depends on the reaction parameters, for instance the product

selectivity can vary with different initial reactant concentra-

tions, the reason for this being the concentration dependent

adsorption mode of the reactant.

4.1.1. Temperature

The hydrogenation rates usually increase with increasing

temperatures, as reported for the hydrogenation of furfural

[31], cyclohexenone [40], crotonaldehyde [159], citral [10]

hydrogenation. There was, however, an activity minimum at

100 8C in citral hydrogenation over Pt/TiO2-LTR and over

Pt/SiO2 catalysts, whereas Pt/TiO2-HTR catalyst gave an

Arrhenius type temperature dependence [1]. The reason for

this unconventional behavior was the observed catalyst

deactivation caused by unsaturated alcohols since with

temperature increase more CO is formed and remained on

the surface due to faster decoarbonylation. When tempera-

ture is further elevated faster CO desorption results in higher

hydrogenation rate at 150 8C than at 100 8C. In liquid phase

hydrogenation of crotonaldehyde over Pt/SiO2 the hydro-

genation activity decreased with increasing temperature

[71]. The reason for this phenomenon was once again

deactivation, namely the irreversible adsorption of CO

during decarbonylation of crotonaldehyde.

Table 3

Reaction order with respect to the reactant and hydrogen over different catalysts

Reactant Catalyst Solvent Temperature

range (8C)

Citral 3 wt.% Pd /80%:20%

SiO2/AlPO4

Tetrahydrofuran 10–50

Citral Pt/SiO2 Hexane 25–150

Citral Pt/TiO2 (HTR) Hexane 100

Acetophenone Rh/C Cyclohexane 60–100

Cyclohexenone Rh/SiO2 2-propanol 25

Cinnam-aldehyde Pt/SiO2 Ethanol 50

Furfural Pt/C 2-propanol, water 130–175

The selectivity to an intermediate product, unsaturated

alcohol can be independent on temperature or alternatively

increase or decrease. In fact, temperature is one of the

means to change the selectivity. This was visible in citral

hydrogenation over monometallic Pt/SiO2 catalyst at

different temperatures, where large selectivity differences

were observed [10]. The selectivity of cyclohexanone in

cyclohexenone hydrogenation was independent on tem-

perature [40], whereas selectivity to crotyl alcohol

increased slightly with increasing temperature [71]. An

opposite trend was observed in gas phase hydrogenation of

crotonaldehyde over Rh-Cu/SiO2 catalyst [57]. Further-

more, the selectivity towards unsaturated alcohol decreased

only slightly with increasing temperature in furfural

hydrogenation in ethanol under 20 bar of hydrogen pressure

over a Raney nickel catalyst modified with heteropoly acids

[31], which was explained by the carbon-carbon double

bonds stabilization due to conjugating with the ring. In gas

phase hydrogenation of benzaldehyde over Ni/SiO2 catalyst

the maximum TOF for formation of benzyl alcohol was

achieved around 170–190 8C [23]. At higher temperatures

the hydrogenolysis reactions took place. Due to the higher

stability of 2-methylbenzaldehyde compared to benzalde-

hyde the maximum in TOF-values for formation of

unsaturated alcohols was shifted to higher temperatures

(ca. 187–197 8C).

4.1.2. Pressure

The experimentally obtained reaction order with respect

to hydrogen pressure gives information for proposing a

kinetic model. The reaction order with respect to hydrogen is

often close to one for citral [10], cinnamaldehyde [60,148],

2-cyclohexenone [40] and for acetophenone [56] (Table 3).

This indicates weakly bonded hydrogen on the metal surface

[40]. The order for hydrogen decreases, however, with

decreasing temperature in crotonaldehyde hydrogenation

[159] (Fig. 18). It should be pointed out here that if the

catalyst deactivation is very extensive, the reaction orders

with respect to hydrogen and to reactant should be corrected

by taking into account the catalyst deactivation. This type of

treatment has been applied to gas phase hydrogenation of

furfural over copper chromite [134]. The obtained activity

Pressure

range (bar)

Order with respect

to reactant

Order with respect

to hydrogen

Ref.

0.68–1.37 0.36 1.25 [20]

7–41 0 1 [10]

7–41 �0.86 Near zero [1]

5–40 0 1 ! 0 [56]

6.9–20.7 0 1 [40]

10–120 0 1 [74]

10–20 0, when c0 > 0.13

kmol/m3 slighly <1,

when c0 <0.13 kmol/m3

1.1 at 130 8C; 1.16

at 175 8C[79]


Fig. 18. Activity as a function of H2 partial pressure at different tempera-

tures during crotonaldehyde hydrogenation over Cu/AC catalyst [159].

Fig. 19. Initial hydrogenation rates for acetophenone as a function of the

initial acetophenone concentration at 60 8C (&); at 80 8C (^); and at

100 8C (~) and 25 bar hydrogen [56].

was corrected by normalizing it with the activity obtained at

standard conditions.

The selectivity to unsaturated alcohols in chemoselective

hydrogenations can either be constant [40,85], decrease

[71,74], or pass through a maximum [73] with increasing

hydrogen pressure. The constant selectivity to nerol and

geraniol was observed in citral hydrogenation to nerol and

geraniol over Pt/SiO2 catalyst [78] indicating the same

reaction orders in parallel routes for formation of citronellal

and unsaturated alcohols, nerol and geraniol. On the other

hand in liquid phase hydrogenation of crotonaldehyde the

selectivity to crotyl alcohol decreased with increasing

pressure [71] and at the same time more butyraldehyde was

formed over Pt/SiO2 catalyst.

Interestingly hydrogen pressure dependence of selectiv-

ity depends also on support selection, leading to different

product distribution. This indicates that the relative

adsorption and desorption strengths of reactants and

products can vary depending on the type of the catalyst

support [73], as observed in crotonaldehyde hydrogenation.

The selectivity to crotyl alcohol went through a maximum

and decreased with increasing hydrogen pressure in the

hydrogenation of crotonaldehyde over Pt-clays [73]. The

reason for the selectivity decrease at high hydrogen

pressures was, however, different over Pt/SiO2 catalyst

than over Pt/K-10 and Pt/Bentonite, since over the latter

catalysts the major product at high hydrogen pressures was

1-butanol, whereas over Pt/SiO2 butyraldehyde was mainly

produced [71]. Analogously to crotyl alcohol the selectivity

to cinnamyl alcohol decreased with increasing hydrogen

pressure in cinnamaldehyde hydrogenation over Pt/SiO2

catalyst in ethanol at 50 8C and 29 bar H2 [74], but over Pt/

graphite catalyst no significant selectivity change was

observed with increasing hydrogen pressure [85]. Addi-

tionally, the pressure change can affect the adsorption mode

of the reactant (crotonaldehyde, cinnamaldehyde).

Gas phase hydrogenations have often been carried out

mainly under atmospheric pressure [72]. This information is

not, however, adequate, if the pressure change alters the

adsorption mode of the reactant on the metal surface [25].

The pressure effect was investigated in the hydrogenation of

acrolein at two different pressure levels over Rh-Sn/SiO2

catalyst [25]. The selectivity enhancement to allyl alcohol

was observed when increasing the total pressure from 1 to

20 bar. It was concluded [25] that the intramolecular

selectivity is not only controlled by the properties of active

sites, but also by the pressure conditions. Additionally,

acrolein was hydrogenated over Ag/SiO2 catalyst in pressure

range of 50 mbar to 20 bar [174]. Higher selectivities to allyl

alcohol were obtained with increasing acrolein partial

pressure and hydrogen pressure, which was rationalized by

assuming that both hydrogen adsorption and surface

coverage are pressure dependent.

4.1.3. Initial substrate concentration and selectivity

The reaction order with respect to the initial reactant

concentration is very important not only for understanding

the reaction mechanism, but also for the process design.

Zero order dependence on the reactant concentration has

been observed for ketones [40] (Fig. 19) and aldehydes

[10,60,74,79,85,147,175], indicating strong adsorption of

organic molecule on the metal surface. The reactant

structure [134] and the nature of support [1] as well as

the initial reactant concentration can, however, change the

observed reaction order.

Zero order dependence on the initial reactant concentra-

tion was observed for cinnamaldehyde hydrogenation over

Pt/SiO2 catalyst in ethanol at 50 8C and 29 bar hydrogen

[74], over Pt/G catalyst in toluene/water mixture at 25 8Cand 11 bar [85] as well as over Pd/C at 50 8C and 1 bar

[175], for 2-cyclohexanone in 2-propanol at 25 8C and

6.8 bar [40], for belzaldehyde over Pd/C, Pd/Al2O3 and over

Pd/SiO2 in ethanol at 20 8C and 1 bar [147], for furfural

hydrogenation over Pt/C in 2-propanol/water solvent within

temperature and pressure range of 130–175 8C and 10–

20.6 bar [79] above the initial concentration of 0.13 kmol/

m3. The increase in reactant order from zero to 0.86 was


observed, when the initial furfural concentration was below

this level.

Both reactant structure and catalyst support can affect the

observable reaction order with respect to reactant. Furfural

hydrogenation kinetics was near one order over a copper

chromite catalyst, whereas for crotonaldehyde it was near

zero-order over the same catalyst [134]. Slightly negative

order close to zero was observed in citral hydrogenation in

hexane over several monometallic Pt catalysts supported on

SiO2 or TiO2 [1], whereas over Pt/TiO2 catalyst the reaction

order with respect to citral was �0.9 consistent with a

kinetic equation including the CO surface coverage [1]. CO

originates from the decarbonylation reaction, which inhibits

the hydrogenation.

The selectivity to unsaturated alcohols can change with

varying the initial concentration of the reactant. This was the

case in cinnamaldehyde hydrogenation over Pt/SiO2 in

ethanol at 50 8C and 29 bar hydrogen [74] and over Pt/C/

monolith in toluene at 30 8C and 50 bar, where selectivity to

cinnamyl alcohol increased with increasing initial cinna-

maldehyde concentration. This effect might be explained by

different adsorption mode of cinnamaldehyde at higher

initial reactant concentrations [7,74] namely the self-

assembling of the aromatic ring preferring the end-on

adsorption.

4.2. The choice of solvent

Solvent effects in heterogeneously catalyzed reactions

have been reviewed by Singh et al. [1]. The most important

solvent effects in the hydrogenation of a,b-unsaturated

aldehydes are solvent polarity, hydrogen solubility, inter-

actions between the catalyst and the solvent [41] as well as

solvation of reactants in the bulk liquid phase. During recent

years solvent effects have been investigated in the

hydrogenation of cinnamaldehyde [52,74,176], crotonalde-

hyde [71,73], citral [8,63] a,b-unsaturated ketones [41] and

acetophenones [24]. Additionally, hydrogenations have been

carried out in the absence of solvent [15], in the presence of

super critical medium [142,143,145] as well as using ionic

liquids [177,178]. At the moment it is difficult to draw a

complete picture of different types of phenomena occurring

during the catalytic hydrogenation in the presence of

solvents. The rationalization of solvent effects is very

difficult due to the lack of systematic experimental data

including different solvents, reactants, reactions conditions,

catalysts, adsorption modes of reactants, side reactions etc.

However, the recent trends found in literature regarding

solvent effects are presented below.

Controversial results were reported in literature regarding

the effect of solvent polarity on the hydrogenation rates.

Solvent polarity has increased the hydrogenation rates

[41,52,74], whereas high citral hydrogenation rates were

obtained with solvents, exhibiting low values for dielectric

constants over Pd/SiO2/AlPO4 catalyst [20]. Polar solvents

activated the hydrogenation of C O in cinnamaldehyde at

50 8C and 1 bar, whereas non-polar solvents favored

hydrogenation of C C bond over Pd/C, Pt/C and Co/

Al2O3 [176]. Analogously polar solvents increased the

selectivity to crotyl alcohol formation in crotonaldehyde

hydrogenation at 25 8C and 20 bar, whereas in ethers more

butyraldehyde was formed over Pt/clay catalysts [73].

Additionally solvents might promote transfer hydrogena-

tion. This was investigated in crotonaldehyde hydrogenation

by using He instead of hydrogen, but the extent of transfer

hydrogenation was very minor [73].

Support selection can affect the selectivity to inter-

mediate products in different solvents. This was observed in

the work of Hajek et al. [52] in cinnamaldehyde hydro-

genation at 100 8C and at 50 bar in three different solvents,

cyclohexane, hexane and 2-propanol. Over Ru/C the lowest

selectivity to cinnamyl alcohol was obtained in non-polar

solvents (about 10%), whereas in 2-propanol the selectivity

was 15%. Over Ru/Y the selectivities nearly 30% were

achieved in non-polar solvents, whereas acetals were formed

in 2-propanol over Ru/Y. Acetalization can be related to the

acidity of Ru/Y. Over Ru/H-MCM-41 only selectivities

below 15% were achieved. This might originate from the

mild acidity of the mesoporous material. Additionally, it has

been stated that acids enhance the hydrogenation of carbonyl

bonds due to partial protonation of carbonyl oxygen [41].

Citral hydrogenation was investigated at 70 8C over Ni/

Al2O3 catalyst in ethanol, 2-propanol, 2-methyl-2-propanol

and 2-pentanol. The hydrogen solubilities were determined

with a gas chromatograph equipped with a TC-detector. The

highest hydrogenation rate was obtained over in 2-propanol,

whereas the highest hydrogen solubility was measured in 2-

pentanol [8] showing that hydrogenation rates could not be

correlated with hydrogen solubility. Similar observations

were done in case of cinnamaldehyde [52]. There are other

effects, like adsorption of solvent and discussed above

nature of support, which might affect the initial hydrogena-

tion rates.

The initial concentration of the reactant and the selection

of solvent have large effect on the product selectivity in

chemoselective hydrogenations. Catalyst deactivation and

formation of by-products are more pronounced in lower

chain alcohols methanol, ethanol and 2-propanol than in 2-

methyl-2-propanol. With high initial concentrations of

crotonaldehyde large amounts of dehydration products

and acetals were formed [73]. When using bulky alcohols,

i.e. 2-methyl-2-propanol, less poisoning of the selective

metal support interfacial surface sites was occurring than

with, e.g. 2-propanol.

Not only conventional solvents, but also supercritical

carbon dioxide has been used in the hydrogenation of

cinnamaldehyde [143]. Hydrogenation of organic com-

pounds in super critical CO2 proceeds only in one phase and

the solubility of hydrogen is much higher in scCO2 than in

conventional solvents. The benefits in replacing organic

solvents by supercritical carbon dioxide are the easy

separation of solvent, higher selectivity to cinnamyl alcohol


Fig. 20. In situ DRIFTS spectra during crotonaldehyde hydrogenation over

Pt/TiO2 as a function of time-on-stream. The peak at 1660 cm�1 decreases

with increasing time-on-stream [72].

than in the absence of scCO2. The concentration of hydrogen

on the catalyst surface is increased in the presence of scCO2

and thus the hydrogen pressure is an important factor. There

was, however, an optimum hydrogen pressure, which gave

the highest selectivity to cinnamyl alcohol.

Ionic liquids, which along with scCO2 are considered as

green solvents, have been investigated in cinnamaldehyde

hydrogenation. The specific advantage of ionic liquids is

associated with their lower vapor pressure. The selectivity to

cinnamyl alcohol was 83% with conversion of 72% over Pt/

graphite at 60 8C and 40 bar hydrogen in 1-hexyl-3-

methylimidazolium bis(trifluoromethyl)amide [177].

Besides two and three phase systems, also four-phase

systems, comprising gas, organic and aqueous liquid phases

and solid catalyst have been applied in the hydrogenation of

cinnamaldehyde. Additionally, alkaline earth salts were used

as additives [175]. When cinnamaldehyde was hydrogenated

over Pt/C catalyst in the presence of water and KOH in

organic phase the selectivity to cinnamyl alcohol was

maximally close to 80%. When the hydrogenation was

carried out in the absence of water and KOH was adsorbed

on the Pt/C surface, no cinnamyl alcohol was observed.

These two experiments showed that the presence of water

was essential for formation of unsaturated alcohol [175].

4.3. Side reactions

Typical side reactions in the liquid phase hydrogenations

are acetalization in alcohol solvents [9,14,29,35,52,63,73],

dimerisation [97], hydrogenolysis [23,30], formation of

esters and ethers [24] as well as formation of CO and C3

hydrocarbons especially from crotonaldehyde [70,76].

Additionally, cyclisation of citronellal to isopulegol has

been observed in citral hydrogenation [8,14], in particular

over Ru/C.

Hydrogenolysis is very prominent in the hydrogenation

of aromatic carbonyl compounds [23,153], e.g. in benzal-

dehyde hydrogenation over Ni/SiO2 catalyst [23] as well as

in phenylacetaldehyde hydrogenation over Pt/SiO2 and Pt/

Al2O3 [153], especially at higher temperatures [23].

Hydrogenolysis of phenylacetaldehyde was dramatically

suppressed over Pt/TiO2 (HTR) catalyst [153], explained by

suppression of large Pt ensembles formation. Furthermore,

hydrogenolysis products propane, propene as well as butane

and butenes were formed in liquid phase hydrogenation of

crotonaldehyde over Pt/SiO2 catalyst at 25 8C and 25 bar

hydrogen in ethanol/water mixture [71].

Decarbonylation is observed in gas phase hydrogenation

of crotonaldehyde [70,132]. Interestingly a bimetallic Pt-Sn/

SiO2 catalyst is more stable in the hydrogenation of

crotonaldehyde than the corresponding monometallic Pt/

SiO2 catalysts. The decarbonylation reaction was investi-

gated with FTIR via adsorption of crotonaldehyde on the

catalyst surface. The peaks at 2047 cm�1 and 1838 cm�1

were attributed to the adsorbed CO originated from

decarbonylation [132]. The intensity of these peaks

decreased dramatically when changing the catalyst from

Pt/SiO2 to Pt-Sn/SiO2. Decarbonylation was also observed

in the TPD of acrolein over Pt(1 1 1) surface [179,180] and a

decarbonylation mechanism was proposed besides TPD also

by RAIRS [179]. In Fig. 20 the peak at 1681 cm�1

corresponds to the n(C O) stretching vibration. After

annealing a series of surface species, like CO at 7 8C and

ethylidyne at 67 8C were formed. Analogously Hirschl et al.

[181] observed CO and ethylene formed from acrolein.

Decarbonylation of furfural was observed over Pt supported

catalysts [30].

Homogeneous side reactions occur only in the liquid

phase. Side reactions involving solvents can proceed on

acidic catalysts [8,24], leading for instance to acetals

[8,52,71,74]. Additionally, the residual chloride enhanced

acetalization [8,19] and hydrogenolysis reactions [148]. No

acetals were formed in citral hydrogenation over Ru/Al2O3

prepared from Ru-acetylacetonate precursor [51]. Acetali-

zation could by suppressed by reducing the catalyst on an

oxidic support containing hydroxyl groups, like Ni/Al2O3, at

a high enough temperature for a long time (2 h) in order to

form spill-over hydrogen (see Section 3.6.1). Other ways to

avoid acetalization are to use hydrophobic solvents or

secondary longer chain (C5) alcohols [8]. At higher reaction

temperatures (77 8C) in citral hydrogenation in ethanol

under atmospheric pressure acetalization over Ni/SiO2

catalyst was more pronounced compared to lower reaction

temperatures (70 8C) [102] corresponding to the citronellal

acetal yields of 70 and 6%, respectively. The presence of

SnO2 in a bimetallic Rh-Sn/SiO2 catalyst promoted the

formation of acetals and esters in the hydrogenation of

crotonaldehyde [35], which was due to the formation of

Brønsted acid sites induced by the presence of SnO2. The

following mechanism for formation of acetals was proposed:

protonation of alcohol molecule by acid sites and loss of

proton from a surface intermediate [35].


Cyclisation of citronellal to isopulegol is an acid

catalyzed reaction, which can occur during citral hydro-

genation. Over Ru/C catalyst isopulegol formation was

related to the presence of acidic surface oxygen groups,

mainly carboxyl groups confirmed by TPD experiments [9].

Cyclisation was favored in more hydrophobic solvents, i.e.

2-pentanol [8].

Other side reactions can be cyclisation, cracking and

dehydrogenation [102] especially over acidic zeolite sup-

ports. For instance, over an acidic Ni/Y catalyst oxygen was

cleaved away by dehydration [24]. In cinnamaldehyde

hydrogenation over Pt/Al2O3 in ethanol at 35 8C and 1 bar

hydrogen a cyclic trimer was formed as a by-product [63].

Additionally, in crotonaldehyde hydrogenation over Pt/Clay

supported catalysts an aldol was formed, which was converted

further via dehydration and hydrogenation. Furthermore,

acrylic acid was formed in acrolein hydrogenation over Au/

TiO2 catalyst by oxygen transfer from the support [97]. A

choice of a proper solvent can diminish formation of

unwanted products, e.g. dioxane was used instead of alcohols

in cinnamaldehyde hydrogenation over Pd/CNF in order to

avoid formation of heavier by-products [83].

4.4. Deactivation and catalyst recycling

Catalyst deactivation is a very important issue due to

often required catalyst durability, meaning that both activity

and selectivity should remain at high levels with time on-

stream or in successive batches. There are several

phenomena causing catalyst deactivation [181,182]. These

are changes in catalyst structure, i.e. sintering, leaching off

the active metal, coking or fouling as well as poisoning. The

metal particles can sinter due to thermal effects, however

sintering is seldom observed, since temperatures applied for

the liquid and even gas phase hydrogenations are not

sufficiently high. Coking is caused by formation and

cracking of hydrocarbons on metal surfaces. Coke can be

formed on the external surface of the catalyst and inside the

pores, dramatically decreasing the specific surface area of

the used catalysts due to total blockage of the micropores.

Poisoning is known to be caused by strong chemisorption of

compounds on the metal surface. In this section catalyst

deactivation in both gas and liquid phase hydrogenations is

considered presenting the trends in catalyst activity and

selectivity due to catalyst deactivation. Furthermore, in situ

catalyst characterization can bring new information about

the deactivation mechanisms. The re-use and regeneration of

catalysts, which is industrially an important issue, will be

also addressed.

Catalyst deactivation has been investigated both in the

liquid [10,29,51,184,193] and in gas phase hydrogenations

[36,38,70,134,159]. Due to catalyst deactivation both

activity and selectivity of the catalyst can change and it is

important to understand the kinetics and mechanism for

catalyst deactivation. The in situ catalyst characterizations

during gas phase hydrogenations by DRIFT [38,134] and in

situ IR [70] as well as in liquid phase by ATR-IR [184] have

given new knowledge about the chemical structure of the

compounds adsorbed on the catalyst surface, which cause

deactivation [38,70]. Some recent catalytic data together

with catalyst characterization are presented below for both

gas and liquid phase reactions.

Catalyst deactivation studies during gas phase hydro-

genations were performed in the following systems:

crotonaldehyde over Ir/TiO2 [38], copper chromite [134],

and Pt/ZnO [36], Pt/SiO2 [70], Pt/TiO2 [72], Ni/TiO2 [72]

and Rh-Cu/SiO2 [57]; furfural over copper chromite [134].

Deactivation was attributed to the coke formation on the

active metal sites [187]. By in situ technique, it was observed

that in crotonaldehyde hydrogenation over Ir/TiO2 catalyst

both asymmetric carboxylate and heavy products were

formed, progressively blocking the active sites on the

catalyst surface [38]. Crotonaldehyde was hydrogenated

over Pt/TiO2 catalyst and the rate for formation of crotyl

alcohol could be correlated with the concentration of

adsorbed crotonaldehyde species at 1660 cm�1 in the di-sCO

configuration on the interfacial Pt-TiOx and Ni-TiOx sites,

which imply a strong interaction between C O and the

surface. This carbonyl group stabilizes the Pt-TiOx inter-

action. With increasing time on-stream the surface coverage

of this species at 1660 cm�1 decreased in parallel to the drop

in crotyl alcohol selectivity (Fig. 20) [72]. The deactivation

during crotonaldehyde hydrogenation could be suppressed

over Pt/TiO2 catalyst reduced at high temperature [72]. The

reason for slower catalyst deactivation might be the

ensemble effect, when TiOx sites decorate the Pt sites

active for aldehyde decarbonylation.

Platinum catalysts deactivated fast during gas phase

hydrogenations. Over Pt/SiO2 catalyst the crotonaldehyde

conversion decreased with increasing time-on-stream and

the conversion was correlated with the increased amount of

the adsorbed CO measured by in situ IR [70]. At the same

time the selectivity to crotyl alcohol increased with

increasing time-on-stream. The catalytic activity of the

Pt/SiO2 catalyst could be restored, if the catalyst was

reduced or evacuated at 400 8C to remove the adsorbed CO.

The IR characterization of the Pt/SiO2 catalyst during

crotonaldehyde hydrogenation revealed that several irrever-

sible adsorbed organic molecules are present on the metal

surface. These molecules originated from aldol condensa-

tion, dimerisation and condensation reactions. The selectiv-

ity enhancement to crotyl alcohol during catalyst

deactivation might be due to the change in the adsorption

mode of crotonaldehyde.

Copper supported catalysts retained their catalytic

activity quite well during the gas phase hydrogenation of

crotonaldehyde. Only slight catalyst deactivation was

observed in crotonaldehyde hydrogenation over Rh-Cu/

SiO2 catalyst prepared by so–gel method [57] and over

copper chromite [134]. In the characterization of the former

catalyst Cu(I) species were observed indicating the presence

of metal-support interactions. Reactant structure affected


also catalyst deactivation. Thus, over copper chromite

catalyst it was noticed that this catalyst deactivated less in

crotonaldehyde than in furfural hydrogenation [134]. In the

latter case the possible reasons for catalyst deactivation are

coke formation and/or catalyst poisoning. Additionally, the

oxidation state of copper can change during hydrogenation.

In liquid phase hydrogenation catalyst deactivation has

been investigated in the cases of citral [51,138,184],

cinnamaldehyde [172], acetophenone [24], crotonaldehyde

[29,70], and benzaldehyde [29]. Deactivation was found to

originate in crotonaldehyde hydrogenation from the

reversible poisoning via formation of aldolic condensation

products [29] or via irreversible poisoning and formation of

CO. In the latter case the catalyst washing can recover the

original catalytic activity [29]. Irreversible catalyst poison-

ing was observed in citral hydrogenation over Pd/Al2O3

catalyst [184]. Citral decarbonylated over 5 wt.% Pd/Al2O3

catalyst under 1 bar hydrogen in hexane at 40 8C [184]. The

reoxidation of the catalyst could inhibit the further catalyst

deactivation, but the original activity could not be obtained

indicating that the reason for catalyst deactivation in liquid

phase citral hydrogenation is the formation of oligomeric

surface products. Several factors, like nature of the metal

[51], metal particle size [70], and reactant structure affect the

extent of catalyst deactivation.

Additionally, the electronic properties of the metal have

an influence on deactivation. This was observed in citral

hydrogenation over different metal supported catalysts. The

spent catalysts Ru/C, Rh/Al2O3 and Ni/Al2O3 after citral

hydrogenation were characterized by TPR and the formed

methane was quantified. The amount of methane decreased

in the following order: Ru/C > Rh/Al2O3 > Ni/Al2O3,

which also correlated to the final conversion levels [8].

As already indicated in the previous section hydro-

genolysis of unsaturated alcohols occurs in the liquid phase

leading to deactivation. Hydrogenolysis of crotonaldehyde

was for instance confirmed in liquid phase hydrogenation,

by analyzing the gas phase composition over Pt/SiO2

catalyst [70], showing the presence of propane and propene

formed by decarbonylation of crotonaldehyde. It was

noticed that deactivation was metal size dependent, in

particular smaller Pt particles deactivated faster than larger

ones [70].

Catalyst reusability is a very important issue for the

industrial applications. Catalyst recycling in hydrogenation

of carbonyl compounds has been recently investigated by

Hajek et al. [172], Malyala et al. [24] and Vaidya et al. [79].

The catalytic activity in cinnamaldehyde hydrogenation was

dependent on the type of support. Ru/Y exhibited a constant

catalytic activity in cinnamaldehyde hydrogenation at

100 8C and 50 bar, whereas loss of activity was obtained

in Ru/H-MCM-41, as well as in Pt/Y in the three consecutive

experiments with the same catalyst [172]. The selectivity to

cinnamyl alcohol increased in consecutive experiments. The

reason for this selectivity enhancement to cinnamyl alcohol

might be the adsorbed coke, which can in turn cause the self-

assembling effect of adsorbed molecules via changing the

adsorption mode of cinnamaldehyde [172]. Pt/C catalyst

showed the same activity in consecutive hydrogenations of

furfural in 2-propanol at 130 8C and 10 bar [79]. The

bimetallic Ni-Pt/Y catalyst exhibited constant activity in

catalyst recycling tests in the liquid phase hydrogenation of

acetophenone in alcohols at 100 8C and 30 bar [24], whereas

a monometallic Ni/Y catalyst lost its activity in recycling.

When the Pt loading remained quite low, high dispersion

was observed, but with increasing Pt content the indepen-

dent Pt particles were formed [24].

Both irreversible [51] and reversible catalyst deactivation

[29,138] has been recorded. The former type of deactivation

was observed in successive citral hydrogenations over Ru/

Al2O3 catalyst. Even when the catalyst was reduced with

hydrogen at 350 8C between the hydrogenations, both the

catalytic activity and selectivity to citronellol decreased and

thus the catalyst reduction was not sufficient to remove

impurities from the used catalyst [51]. Reversible catalyst

deactivation was on the other hand observed in citral

hydrogenation over Ni/Al2O3 and in crotonaldehyde

hydrogenation over Pt/SiO2 [29]. When the Ni/Al2O3

catalyst was used twice in citral hydrogenation nearly the

same catalytic performances were obtained in two con-

secutive experiments [138], when the catalyst was reduced

between the two hydrogenations at 400 8C. The pore size

distribution of Ni/Al2O3 catalyst indicated that the mean

pore size had increased and the smaller pores were blocked

[138]. Analogously in crotonaldehyde hydrogenation over

Pt/SiO2 catalyst, the activity of catalyst could be restored by

washing it between the experiments. The reversible catalyst

poisoning caused by oligomers could be removed via

washing the catalyst at room temperature [29].

4.5. Comparison of gas and liquid phase

hydrogenations

The comparison of chemoselectivity and activity in the

liquid and gas phase hydrogenations is not very straightfor-

ward, because the reaction conditions and the concentrations

are different, a further complication results from the

presence of a solvent in liquid phase reactions. In this

section, the conversion–selectivity relationships in gas and

in liquid phase hydrogenations are compared keeping in

mind the differences of the two systems.

The direct comparison of the catalytic activities in

chemoselective gas and liquid phase hydrogenations of

unsaturated aldehydes is not available over the same

catalysts and under similar reaction conditions. Activities

were compared in gas and liquid phase hydrogenations of

crotonaldehyde over different Pt/SiO2 catalysts [70]. The

reaction conditions were not, however the same. In this

work, the following TOF-values were given over the same

Pt/SiO2 catalyst for liquid phase hydrogenation at 27 8C and

25 bar hydrogen and for gas phase hydrogenation at 80 8Cand 1 bar, i.e. 0.016 and 0.013 s�1 [70], respectively.


Fig. 21. Selectivity to cinnamyl alcohol in cinnamaldehyde hydrogenation

at 100 8C and 50 bar in cyclohexane [52].

In spite of the fact that catalyst deactivation during liquid

and gas phase hydrogenations of unsaturated aldehydes has

been investigated, there exists no direct comparison of

catalyst deactivation phenomenon in these phases under the

same reaction conditions. In general gas phase hydrogena-

tions are carried out at higher temperatures in the absence of

solvent compared to liquid phase hydrogenations. Catalyst

deactivation can be retarded in the presence of solvent by

retarding the coke formation because solvent can remove

adsorbed hydrocarbons on the catalyst surface. Higher

temperatures favor side reactions, like hydrogenolysis,

decarbonylation and cracking. The catalyst deactivation is

very severe during several gas phase hydrogenations of

unsaturated aldehydes [36,70], whereas in liquid phase

hydrogenations the catalyst deactivation depends on the

support and metal [52] and similar catalytic activities were

obtained in repeated cinnamaldehyde hydrogenation experi-

ments over Ru/Y catalyst at 100 8C and 50 bar cyclohexane,

whereas over Ru/MCM-41 catalyst the catalyst deactivation

was extensive [52]. Catalyst deactivation was compared in gas

and in liquid phase hydrogenation of crotonaldehyde over the

same Pt/SiO2 catalysts under different reaction conditions

[70]. The results indicated that the catalyst deactivation was

more prominent in liquid phase after the same number of

turnovers [70]. The reasons for this are the lower hydrogen

concentration as well as higher reactant concentration in the

liquid phase compared to the gas phase [70].

Similarly to catalyst activity and deactivation there is no

direct comparison of the selectivities to unsaturated alcohols

in gas and in liquid phase hydrogenations. In the work of

Englisch et al. [70] comparison of selectivities to

unsaturated alcohols in gas phase and in liquid phase

hydrogenations of crotonaldehyde was performed over the

same catalysts [70]. In gas phase hydrogenation of

crotonaldehyde at 80 8C over two different Pt/SiO2 catalysts

the selectivities to crotyl alcohol were 34 and 43%, whereas

the comparative selectivities to crotyl alcohol in liquid phase

hydrogenation of crotonaldehyde at 27 8C and under 25 bar

hydrogen were 35 and 41% at 50% conversion, respectively.

This result indicated that there are no large differences in the

selectivity values in crotonaldehyde hydrogenation over two

different Pt/SiO2 catalysts in gas and in liquid phase.

Selectivity to an intermediate product depends usually on

the conversion level. Typically in the hydrogenation of a,b-

unsaturated alcohols the selectivity to the intermediate

desired product can be dependent on the conversion level

and in order to be able to increase the selectivity to the desired

product it is important to understand the reasons for different

relationships between conversion and selectivity. When

comparing gas and liquid phase hydrogenations, it can be

noted that high conversion levels are obtained in the former

case in the beginning of the reaction at low time-on-stream

levels decreasing thereafter due to deactivation, whereas in

liquid phase hydrogenation the conversion increases with

increasing reaction time. Very often, however, the selectivity

to unsaturated alcohol increases due to catalyst deactivation,

when the most active metal sites are poisoned [132].

Simultaneously new active sites are formed in situ during

the hydrogenation [69].

Selectivity to an intermediate product, unsaturated

alcohol in liquid phase hydrogenation of unsaturated

aldehydes typically is constant [14] or sometimes goes

trough a maximum [10] with increasing conversion. The

former case was observed in the hydrogenation of citral over

several Ru catalysts supported on alumina or active carbon

[14], although at very high conversion levels the selectivity

drops fast. In the work of Singh et al. [10] the selectivity to

nerol and geraniol in citral hydrogenation over Pt/SiO2 went

through a maximum with increasing citral conversion.

Nearly constant or slightly decreasing selectivities were

observed in crotonaldehyde hydrogenation over Pt/SiO2 and

Pt-Sn/SiO2 catalysts [29]. In cinnamaldehyde hydrogenation

over Ru/C and Ru/H-MCM-41, which did not exhibit strong

acidity, the selectivity to cinnamyl alcohol was nearly

independent on the conversion level. An exceptional

relationship, increasing selectivity towards unsaturated

alcohol with increasing conversion was, however observed

over a bifunctional Ru/Y catalyst, e.g. the selectivity to

cinnamyl alcohol increased with increasing conversion [52]

(Fig. 21). This result was interpreted by the formation of new

active sites selective for C O hydrogenation.

In gas phase hydrogenations the selectivity to inter-

mediate unsaturated alcohols very often decreased with

increasing conversion, like in crotonaldehyde hydrogenation

over Pt/SnO2 [69] (Fig. 22) and over Ti4Co12 [188]. The

support selection can, however, change the selectivity–

conversion relationship also in gas phase hydrogenation

[157], indicating that different supports exhibit different

adsorption strengths towards different compounds. This was

the case in crotonaldehyde hydrogenation over Pt/rutile and

Pt/anatase in crotonaldehyde hydrogenation, where Pt/rutile

favored decreasing selectivity to crotyl alcohol with

increasing conversion, whereas constant selectivity or


Fig. 22. Crotylalcohol selectivity as a function of conversion in consecutive

crotonaldehyde hydrogenation over Pt/SnO2 catalyst at 170 8C [69].

decreasing selectivity were observed over Pt/anatase and

over a commercial Pt/TiO2 catalysts. The increasing

selectivity to crotyl alcohol with increasing conversion of

crotonaldehyde over Pt/TiO2 was interpreted by competitive

adsorption, whereas the stable selectivity as a function of

crotonaldehyde conversion was obtained over Pt/anatase. In

this case butyraldehyde was the most dominant product

[157]. It was proposed that butyraldehyde is formed on

metallic Pt sites, whereas crotyl alcohol is formed on special

sites exhibiting SMSI effect. Additionally, a maximum was

observed in selectivity to crotyl alcohol with increasing

conversion in gas phase hydrogenation of crotonaldehyde

over Pt-Zn/CeO2-SiO2 [112].

Superficially selectivity enhancement was observed both

in gas and in liquid phase hydrogenations with either

increasing time-on-stream in gas phase or increasing

reaction time in liquid phase, however selectivity–conver-

sion dependence is different, since in gas phase conversion

decreases due to deactivation. Some examples can be seen in

literature from this phenomenon in gas phase hydrogenation

of crotonaldehyde [69,112] over Pt/CeO2-SiO2 and Pt-Zn/

CeO2-SiO2 reduced at lower temperature, and over Pt/SnO2

catalyst [68,69] and in liquid phase hydrogenation of

cinnamaldehyde over Ru/H-Y [52]. This effect could be

either due to the poisoning the most active sites responsible

for C C hydrogenation and/or formation of the new active

sites selective for C O hydrogenation.

4.6. Metal leaching and the role of homogeneous

catalysis

Catalysis leaching is extremely important particularly for

liquid phase reactions. Often leached metals are responsible

for high catalytic activity in some reactions, which are

considered to be heterogeneous catalytic, but are in fact

homogeneous (for instance occurring with so-called

heterogenized homogeneous catalysts). Leaching is parti-

cularly a problem in oxidation catalysis, however, can occur

during liquid phase hydrogenation [120].

It was demonstrated that the extent of metal leaching is

dependent on the catalyst reduction method. In cinnamal-

dehyde hydrogenation under 70 bar hydrogen at 160 8C in

2-propanol, a lower activity was observed over Ru-Sn/SiO2

reduced with KBH4 than with NaBH4 [120]. The reason for

this was the lower content of Ru remaining in the catalyst

after reduction with KBH4. On the other hand neither

leaching of Ru nor Sn was observed, when the catalyst was

reduced with NaBH4. When Ru-Sn/SiO2 catalyst was in

the non reduced state, about 50 wt.% of Ru was leached

away, while presence of Sn in the fluid phase was not

detected.

Note that it cannot be claimed that there is no leaching,

even if a catalyst can be recycled several times in batch

experiments without loss of activity, as even small leaching

could be responsible for catalyst activity.

5. Experimental and theoretical adsorption studies

as a tool for proposing the surface reaction

mechanisms

The ultimate goal in developing new catalysts and

investigating hydrogenation kinetics is to rationalize the

catalyst design and develop active and selective processes

for preparation of fine chemicals. On the way to reach these

goals one method is to investigate adsorption of reactants

and products on the metal surfaces via theoretical modeling

and experimental adsorption studies. This information

combined with the real kinetic data facilitates the proposal

of possible reaction pathways for heterogeneously cata-

lyzed hydrogenations. The different reaction mechanisms

can be validated by kinetic modeling, which is reviewed in

Section 6.

5.1. Adsorption studies

Adsorption studies of unsaturated aldehydes can give

information about the different adsorption modes of

aldehydes on the metal surfaces. Challenge is to link this

information with the catalytic activities and selectivities

during hydrogenation and, finally develop active and

selective catalysts. Adsorption of unsaturated aldehydes

has been investigated by TPD [179,180], RAIRS [179,180],

IR [150] and DRIFTS techniques [72]. One of the recently

used tools is quantum chemical calculation, which gives the

most stable adsorption structures [135] and reveal the

activation barriers for the reaction [190].

5.1.1. Experimental adsorption studies

Adsorption studies have been performed both under ultra-

high vacuum conditions as well as under real hydrogenation

conditions. The surfaces have been single crystals or

polycrystalline surfaces as well as supported catalysts. The

effect of substrate structure, metal surface structure, as well

as concentration of the substrate and temperature was


Fig. 23. RAIRS data for acrolein on Pt(1 1 1) surface at �183 8C as a

function of initial dose [179].

Fig. 24. Different adsorption modes of unsaturated aldehydes: (A) disCC,

(B) disCO, (C) h4-trans, (D) top, (E) h3-cis and (F) dis-14 [181].

investigated. These investigations can give information from

the adsorption mode and interactions with the metal surface.

Adsorption of acrolein and crotonaldehyde has been

investigated by RAIRS and TPD [179,180] as well as with

infrared reflection absorption [192]. Acrolein was adsorbed

in a flat geometry on Pt(1 1 1) according to RAIRS

measurement at low coverage (Fig. 23), whereas with

higher initial doses the t(CH2) twisting motion at 990 cm�1

becomes visible. Crotonaldehyde had both flat and via C C

double bond adsorbed geometries. Additionally, TPD

measurements revealed that both of these aldehydes showed

thermal decomposition to CO, starting for acrolein and

crotonaldehyde, respectively at 47 and at 87 8C [179].

Interestingly no hydrogenation was observed on Pt(1 1 1)

surface, which had preadsorbed hydrogen [179]. Low

coordination metal sites favor production of saturated

aldehydes while (1 1 1) flat terraces lead to formation of

unsaturated alcohols [179]. Acrolein adsorption was also

investigated on Au(1 1 1), Ag(1 1 1) and on polycrystalline

gold surfaces at 80 K with infrared reflection absorption

method [192]. Analogously to Jesus et al. [179] the parallel

adsorption of acrolein was observed on Au(1 1 1) surfaces

[192], whereas on polycrystalline gold surfaces y(C O) and

v(CH2) bands were observed. Additionally, acrolein was

adsorbed in less ordered state on Ag(1 1 1) than on

Au(1 1 1). This could be due to a stronger chemical

potential on Ag(1 1 1) compered to Au(1 1 1) reducing the

mobility of the adsorbate on Ag(1 1 1). It was observed that

some adsorption modes are metastable with increasing

temperature.

In situ catalyst characterization has been applied as a tool

to identify the adsorbed species during the hydrogenations.

In situ DRIFT spectra have given evidences to the existence

of monohydrogenated crotonaldehyde species on Cu/C

surface [159] and on copper chromite surface. Finally,

reaction mechanism was proposed based on this information

(Section 6) [134]. The results revealed that both C O and

C C were adsorbed simultaneously on the metal surface.

Because butanol was not among the products, the existence

of butyraldehyde should originate from the fast isomeriza-

tion of crotyl alcohol to butyraldehyde.

5.1.2. Theoretical calculations in adsorption

Quantum chemical calculations give information on the

most stable adsorption structures on metal surfaces, which

can be used in the interpretation of the catalytic data. Based

on the relative stabilities of adsorbed species on metal

surfaces the possible reaction mechanisms can be proposed.

It should be, however, pointed out that the theoretical

adsorption calculations by DFT-method have several

limitations, like difficulties to consider competitive reaction

pathways, coadsorption of hydrogen, as well as taking in

account in a realistic way the effect of metal surface.

Theoretical calculations for adsorption of unsaturated

aldehydes were carried out by using DFT method [135].

Adsorption energies and intermolecular and metal-adsorbate

bond lengths for different adsorption modes were calculated.

Additionally, calculations were carried out with different

surface coverage. These calculations are not made for the

real hydrogenation conditions. The following different

adsorption modes for acrolein on metal surface can be

obtained: di-sCO, h4-trans, top, h3-cis and di-s14 (Fig. 24)

[181]. Di-sCO adsorption means that both C and O atom are


interacting with different Pt atoms, whereas in h4-trans both

C O and C C bonds have interactions with the surface. In

top adsorption the molecule adsorbs vertically via lone pair

oxygen atoms and in h3-cis the adsorption takes place both

via C C and via oxygen atom. Finally, in di-s14 adsorption

occurs via C1 and oxygen atom. The adsorption modes of

different molecules have been investigated on Pt(1 1 1)

surface [135]. As a result it was shown that the molecular

structure affects the adsorption mode. According to these

calculations acrolein exhibited the main interaction via its

C C bond to Pt(1 1 1) surface and only weak interaction

with oxygen atom was observed. The substituents in

crotonaldehyde and prenal affected the adsorption geometry

by preferring both the adsorption of C C and C O. Higher

selectivities obtained in crotonaldehyde and prenal hydro-

genation compared to acrolein hydrogenation can partially

be explained by the change in the adsorption mode [135].

Additionally, the surface coverage can change the adsorp-

tion mode [190], as has been observed for acrolein [135,190]

and also for prenal and for crotonaldehyde on Pt(1 1 1)

surfaces [135]. The mode of adsorption changes in case of

prenal in such a way that under low coverage the most stable

adsorption mode is di-sCC, but with increasing coverage

there in not enough space on the metal surface for the bulky

prenal to be adsorbed via C C and thus the mode of

adsorption changes to a vertical atop form. For acrolein the

most stable adsorption mode is di-sCC [135]. Additionally,

vibrational spectroscopic measurements, which are used for

calculation of vibrational frequencies can reveal the

adsorption modes of molecules close to real conditions

[135].

Theoretical adsorption studies have been performed on

alloyed phases. In the work of Hirschl et al. [181] both

Pt(1 1 1) and Pt80Fe20 surfaces were used. In the latter case

adsorption geometries of acrolein were calculated by using

spin-polarized density-functional theory for Pt-Fe. For

bimetallic catalysts DFT modeling has suggested a charge

transfer from an electropositive iron to the adsorbate’s

oxygen atom thus activating the hydrogenation of a carbonyl

group [181]. The interaction between Fe and oxygen from

a,b-unsaturated aldehyde weakens C O bond, but this bond

weakening can only be calculated for a metal surface where

Fe is present. The higher selectivity to unsaturated alcohols

in case of prenal compared to acrolein was interpreted by

reduced interaction of the C C bond with the surface in

prenal [181]. Additionally, it was claimed that the lower

adsorption energy for disCC on the bimetallic Pt-Fe surface

facilitates the easier desorption of unsaturated alcohol from

a bimetallic surface than from a Pt(1 1 1) surface. The

consecutive hydrogenation will be inhibited on the

bimetallic surface.

5.1.3. Bond order conservation Morse potential model

in chemoselective hydrogenations

The energetics in chemoselective hydrogenation can be

calculated with an aid of bond-order conservation-Morse

potential model [190]. The estimates of different activation

barriers as well as their dependence on coverages and

strengths of adsorption are obtained via BOC-MP model.

This model was applied in acrolein hydrogenation over

Pt(1 1 1) and Ag(1 1 1) surfaces. The treatment was limited

to qualitative estimation of the energetics, since there is lack

of the knowledge from desorption of products. Additionally,

input data like heats of adsorption in intermediate states as

well as sticking coefficients were not available [190]. The

input data in this work was adsorption data of acrolein on

Pt(1 1 1) surface obtained by DFT calculations [135] and on

Ag(1 1 1) [191]. Only the formation of propanal and allyl

alcohol was treated in BOC-MP modeling [190]. Two

reaction steps were included both in the formation of allyl

alcohol and propanal:

CH2CHCHOs þ Hs$CH2CHCH2Os; h3

CH2CHCH2Os þ Hs$CH2CHCH2OHs; h3

CH2CHCHOs þ Hs$CH3CHCHOs; di � sCC

CH3CHCHOs þ Hs$CH3CH2CHO; di � sCC

The formation of propanol, as well as isomerization of

allyl alcohol to propanal was excluded from the model. The

results indicated that at low coverages a lower selectivity to

allyl alcohol was obtained than at higher coverages.

Furthermore, the selectivity to allyl alcohol was higher

over Ag(1 1 1) surface than over Pt(1 1 1) surface.

6. Kinetic modeling

A heterogeneously catalyzed hydrogenation over metal

supported surfaces exhibits several reactions steps, like

adsorption of reactants, surface reactions as well as

desorption of the products form the catalytic sites. Usually

a Langmuir–Hinshelwood mechanism is a good approxima-

tion for the kinetics [10,56,79,134,148,188]. In order to be

able to differentiate between different possible surface

reactions the complete kinetic curve should be taken as a

basis for kinetic analysis. The models can include either

competitive or non-competitive adsorption steps as well as

dissociative or non-dissociative adsorption of hydrogen [56].

Furthermore, the kinetic model can have either one or two

different types of adsorptions sites. Besides hydrogenation

also other reactions can take place. Additionally, catalyst

deactivation should be separated form the kinetics. In gas

phase hydrogenation the catalyst deactivation can be easily

modeled by semi-empirical equations [187], whereas in

liquid phase hydrogenation the separation of the kinetics and

catalyst deactivation is more difficult. Catalyst deactivation

in liquid phase batch reactors can be modeled by using the

data from consecutive hydrogenations [51].

A Langmuir–Hinshelwood type kinetic model was used

for the liquid phase hydrogenation of acetophenone over Rh/

C catalyst [56]. After writing the mass balances for


consumption and formation of each compound the rate

equations were presented by using Langmuir–Hinshelwood

formalism. The competitive adsorption of hydrogen and

organic species involves one type of catalytic sites, whereas

in non-competitive adsorption there are two types of sites

[56]. The following rate equations were applied:

r1 ¼ k1KACAC � X; r2 ¼ k2KACAC � X; r3

¼ k3KMCC � X; r4 ¼ k4KPEPE � X; r5 ¼ k5KPEPE � X

where the multiplicator X varied depending on the type of

model as follows: X = a, when the mechanism is non-

competitive and non-dissociative in hydrogen X = b, when

the mechanism is non-competitive and dissociative, X = g,

when the mechanism is competitive and non-dissociative

and X = d, when the mechanism is competitive and disso-

ciative. The following expressions were obtained:

a ¼ 1

ð1 þ KACAC þ KPEPE þ KMCCMCC þ KCECEÞ

� KH2PH2

ð1 þ KH2PH2

Þ (1)

b ¼ 1

ð1 þ KACAC þ KPEPE þ KMCCMCC þ KCECEÞ

�ffiffiffiffiffiffiffiffiffiffiffiffiffi

KHPH2

p

ð1 þ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

KH2PH2

p Þ (2)

g¼ KH2PH2

ð1þKACACþKPEPEþKMCCMCCþKCECEþKH2PH2

Þ2

(3)

d¼ffiffiffiffiffiffiffiffiffiffiffiffiffi

KHPH2

p

ð1þKACACþKPEPEþKMCCMCCþKCECEþ ffiffiffiffiffiffiffiffiffiffiffiffiffi

KHPH2

p Þ2

(4)

where AC = acetophenone, PE = phenyl-1-ethanol, MCC =

methylcyclohexylketone, CE = cyclohexyl-1-ethanol. The

best fit was obtained with a model based on non-dissociative

adsorption of hydrogen and non-competitive adsorption of

organic species [56]. A two-site model was used in liquid

Fig. 25. Reaction scheme in cinnamaldehyde hydrogenation [122] (reprin

phase hydrogenations of cinnamaldehyde [148] and furfural

[79]. In the former one competitive adsorption of substrate

and products was applied [148], whereas in the latter model

non-dissociative adsorption of hydrogen gave the best fit for

furfural hydrogenation [79]. However, Vergunst et al. [75]

have described cinnamaldehyde hydrogenation most satis-

factory with a single-site model for adsorption. Additionally,

the model included the competition between desorption and

the subsequent conversion of intermediates [75]. Experi-

mentally obtained negative reaction orders for citral in citral

hydrogenation over Pt/TiO2 catalyst were modeled by Singh

et al. [1] by applying a competitive adsorption model. The

following rate expression was used in their modeling:

r ¼ kKCitK1=2H CCitP

1=2H

ð1 þ KCitCCit þ K1=2H P

1=2H Þ2

ð1 �QCOÞ2(5)

which takes into account deactivation by including surface

coverage of CO [1].

Cinnamaldehyde is frequently used as a model substrate

for screening hydrogenation catalysts. However, the reaction

kinetics has been described to a limited extent. Below we

will overview kinetic modeling of cinnamaldehyde hydro-

genation reported recently [122]. A commonly accepted

reaction network with the three main reaction products is

following: A, B, C and D denote cinnamaldehyde, cinnamyl

alcohol, 3-phenylpropanal and 3-phenylpropanol. The

reaction scheme is in Fig. 25. Two models were discussed

in [122].

The preliminary modelling was based on Langmuir–

Hinshelwood reaction mechanism with an assumption of

individual sites for organic compounds and hydrogen

adsorption [122]. Since hydrogen molecules are much

smaller than the organic ones; particularly in the present

case, interstitial sites between adsorbed organic bulk

molecules remain accessible for hydrogen adsorption.

Therefore, a non-competitive adsorption model was

assumed. Adsorption steps of the organic compounds were

assumed to be rapid compared to hydrogenation steps

implying quasi-equilibrium for adsorption steps. Irreversible

hydrogenation steps were presumed to determine the rate of

ted with permission, copyright (2004) American Chemical Society).


product formation, which is consistent with the overall

reaction thermodynamics.

The following kinetic equations were obtained:

r1 ¼ k1KAKH2cA pH2

ð1 þ KAcA þ KBcB þ KCcC þ KDcDÞð1 þ KH2pH2

Þ(6)

r2 ¼ k2KAKH2cA pH2


Þ(7)

r3 ¼ k3KBKH2cB pH2


Þ(8)

r4 ¼ k4KCKH2cC pH2


Þ(9)

where k1 is the rate constant of reaction 1 (r1), KA is the

adsorption constant and cA concentration of component (A)

etc., pH2denotes the hydrogen partial pressure. Eqs. (6)–(9)

should be further combined with mass balances of the

components.

1

r

dcA

dt¼ �r1 � r2 (10)

1

r

dcB

dt¼ r1 � r3 (11)

1

r

dcC

dt¼ r2 � r4 (12)

1

r

dcD

dt¼ r3 þ r4 (13)

where r is the mass of catalyst-to-liquid volume ratio.

Preliminary parameter estimations indicated overpara-

metrisation, which was partially swept away by assigning

KA = KB = KC � KD ffi 0.

The parameter estimation results were in good agreement

with the experimentally obtained ones. The coefficient of

determination1 was 99.01%. The standard errors for the most

of estimated parameters were within reasonable limits.

Although the model predicted correctly the reaction

behavior, it did not fit properly the experimental conver-

sion–selectivity dependence for (C) at different hydrogen

pressures (Fig. 26).

The revised model in [122] was based on the reaction

mechanism advanced in recent years [200,201]. Kinetic

equations based on non-competitive hydrogen adsorption and

competitive adsorption of other compounds was proposed.

The adsorption and hydrogenation of C O and C C bonds on

different sites was suggested [53]. The mechanism is of first

order with respect to the hydrogen pressure, at low pressures.

1 The most common measure for the goodness of fit. The closer the value

is the number 100%, the more perfect is the fit.

At high hydrogen pressures, the reaction rate is independent of

the pressure and on the substrate concentration. All reaction

steps are considered to be irreversible:

r1 ¼ kIcA pH2

1 þ kVIcA pH2

(14)

r2 ¼ kIIcA pH2

1 þ kIIIcA pH2

(15)

r3 ¼ kIVcB pH2

1 þ kIIIcA pH2

(16)

r4 ¼ kVcB pH2

1 þ kIIIcA pH2

(17)

where k are lumped parameters [149,199–201]. Similarly to

the previous Model I, the parameter estimations demon-

strated a high degree of explanation (98.93%).

The selectivity analysis provided by Fig. 26 allows one to

discriminate between the two rival mechanisms, if statistical

data and graphical fitting give similar results. The selectivity–

conversion dependence is fitted better by Model II.

The intrinsic discrepancy between the estimated and

experimental selectivities for models becomes apparent

from the following analysis. The initial ratio of the

selectivity to components (B) and (C) at low conversions

is defined as,

Model I:rB

rC

¼ k1KAcA pH2

k2KAcA pH2

¼ k1

k2

(18)

Model II:rB

rC

¼ kI

1 þ kVIcA pH2

1 þ kIIIcA pH2

kII: (19)

It is clearly seen that the ratio in Eq. (18) is independent on

the conversion or on the hydrogen pressure. The ratio for

Model II depends on both, substrate conversion as well as on

the hydrogen pressure. Hence, it is not surprising that

regression analysis performed for a conventional Langmuir–

Hinshelwood Model I provided a reasonable description of

the concentration dependencies, but it failed to describe the

selectivity-conversion dependence. The more advanced

Model II based on the reaction mechanism, which takes into

account the adsorption and reaction of C O and C C on

different sites and the formation of adsorbed intermediates

between organic substances and hydrogen, accounts better for

the observed kinetics and explains all the essential features of

hydrogenation kinetics, including selectivity dependence.

In gas phase hydrogenation the kinetics and the catalyst

deactivation can be easily separated via modeling both the

transient and the steady state. Crotonaldehyde hydrogena-

tion was investigated over different Pt supported catalysts

and despite of the fact that different catalysts exhibited

different activities the selectivity–conversion dependency

showed the similar type of pattern [187]. The catalyst

deactivation could be therefore treated separately. In kinetic

modeling the formation rates for all the compounds,

crotonaldehyde, butyraldehyde, butanol including hydro-


Fig. 26. Selectivity to 3-phenylpropanal (C) as a function of cinnamaldehyde conversion: experimental vs. estimated. Conditions: solvent 2-propanol,

temperature 160 8C, pressure (D) 59 bar, (&) 37 bar and (^) 14 bar [122]. Reprinted with permission. Copyright (2004) American Chemical Society.

carbons were taken into account. Quasi-equilibrium

approximation was used in the description of adsorption/

desorption steps. The surface coverages were presented as a

function of partial pressure of crotonaldehyde, adsorption

equilibrium constant and the amount of vacant sites on the

catalyst surface. The differential equations for consumption

and formation rates for all the compounds were expressed

per catalyst weight and gas flow rate [187]. Parameter

estimation was carried out by using a numerical integration

programme. In the hydrogenation of 2-butenal the dis-

sociative adsorption of hydrogen was applied [188], which

corresponds to the hydrogen order 0.5. Additionally, the

surface reactions were assumed to be irreversible. A

confirmation of this assumption was made by testing the

possible dehydrogenation of 2-butenol, not leading to

formation of 2-butenal.

Based on the kinetic data from furfural hydrogenation in

the gas phase over copper chromite catalyst [134] two types

of kinetic models were proposed, the bimolecular surface

reaction being a rate determining step either over one type of

sites with competitive adsorption of furfural and hydrogen or

adsorption on two types of sites, one for each reactant. It

was, however, impossible to discriminate between different

models in this case, because a nearly first order dependence

was observed for both the reactants after correction for

deactivation.

A Langmuir–Hinshelwood type model containing for-

mation of an intermediate half-hydrogenated species of

crotonaldehyde as a rate-determining step was proposed by

Rao et al. [134]. The idea of an intermediate species in the

model was based on the in situ DRIFT measurements during

gas phase hydrogenation of crotonaldehyde [134] (see

Section 5.1.1). The following rate expression was obtained

by using the half-hydrogenated intermediate:

r¼ kK � KH2KcroaldPH2

Pcroald

ð1þK1=2H2

K1=2H2

þKcroaldPcroaldþK�K1=2H2

KcroaldK1=2H2

PcroaldÞ2

(20)

This equation was able to fit all the experimental results.

In order to account for the catalyst deactivation the

concept of deactivation and self-regeneration was applied

[51]. An exponential equation of the following type,

r ¼ r0QA ¼ a1 þ a2e�a3t (21)

where r0 and QA denote the hydrogenation rate under

deactivation free conditions and surface coverage of A,

respectively was used. The same equation was even applied

in the modeling of liquid phase catalyst deactivation in citral

hydrogenation [51].

In the case of three phase heterogeneous catalytic

reactions, the rate of the process and its selectivity can be

determined either by intrinsic reaction kinetics or by external

diffusion (on the gas–liquid and gas–solid interface) as well as

by internal diffusion through the catalyst pores. Hetero-

geneous catalytic hydrogenation reactions are a typical

example of such a process. Careful analysis of mass transfer is

important for the elucidation of intrinsic catalytic properties,

for the design of catalysts, and for the scale up of processes.

Unfortunately most of the papers on three-phase

hydrogenation of carbonyl compounds do not treat this

topic very deeply. It is well known that the presence or

absence of mass transfer limitations can be investigated both

experimentally and theoretically. An example of how gas/

liquid, liquid/solid mass transfer limitations as well as

internal diffusion can be verified was presented recently for

the case of cinnamaldehyde hydrogenation in propanol

[121]. It is worth to note that not only activity, but also

selectivity can depend on mass transfer, as experimentally

reported [121].

7. Reactor selection

Traditionally batch reactors have been applied in fine

chemicals production in the liquid phase [8,52], whereas gas

phase hydrogenation is usually carried out in fixed bed


Fig. 27. SISR reactor in citral hydrogenation [164].

reactors [36,68,69]. It could be, however, attractive from the

industrial point of view to produce fine chemicals in a

continuous mode. The benefits compared to the conventional

batch operation are the higher production capacity,

minimization of dead time during the start and end, as

well as more equal product quality. According to OECD

reference scenario the world chemical demand will increase

by 39% from the level in year 2000 to the level in year 2020

[206]. Additionally, from the scientific point of view the

continuous operation allows an easier way to record and

model catalyst stability and deactivation in case of liquid

phase reactions.

Liquid phase hydrogenations have been performed in

trickle bed [15,139,140], in monolithic [139,140,164] and in

bubble column [209] reactors as well as in a fixed bed reactor

with recirculating liquid phase [138] and in continuous flow

[184,185]. One decisive factor for achieving high selectivity

to intermediate products in consecutive hydrogenations is

the residence time of the reactant in the reactor. Trickle bed

reactors are widely used in chemical processes in hydro-

genations and oxidations. The drawback in using trickle bed

reactors is the poor wetting of the catalyst, which can lead to

ineffective use of the catalyst as well as to formation of hot

spots [186]. In trickle bed reactors the lower velocities are

applied in order to maintain the trickle-flow regime, whereas

in monolithic reactor higher velocities should be used in

order to avoid maldistribution at lower velocities [140].

Trickle bed reactors have been used in citral [15],

benzaldehyde [139,140] and in crotonaldehyde [186]

hydrogenations. Solvent free citral was hydrogenated in a

trickle bed reactor at 100 8C and 20 bar hydrogen [15].

Citral hydrogenation in this reactor exhibited 20 h induction

period during which the conversion level of citral increased.

After induction period a constant very high conversion

(97%) as well as very high selectivity to nerol and geraniol

(97%) over Pt-Sn/MgO catalyst was obtained at 100 8C and

20 bar [15]. Interestingly with a monometallic Pt/MgO

lower selectivity was achieved, but also no induction period

was observed. In order to avoid the poor catalyst wetting and

to enhance the productivity the trickle bed reactor with

periodic flow interruption has been applied [186]. As a result

an enhanced crotonaldehyde hydrogenation rate was

observed over Pd/g-Al2O3 at 25 8C and 11 bar in such

reactor compared to a steady state operation [186]. The

performances of a trickle bed and a monolithic reactor were

compared in benzaldehyde hydrogenation at 100 8C and

10 bar [140]. The former reactor contained Ni supported on

g-alumina extrudates as a catalyst, whereas a Ni-cordierite

catalyst was used in the monolithic reactor. Co-current gas

and liquid flows were applied. Monolithic reactor showed

higher productivity and selectivity than the trickle bed

reactor in the hydrogenation of benzaldehyde [140]. The

reason for this might be the sharper residence time

distribution in the monolithic reactor.

Monolithic reactors can operate either in single-pass

mode or with external liquid recycle [205]. The product

selectivity might decrease in a latter case with liquid back-

mixing and thus the single pass mode might be preferred.

Monolith catalysts were investigated in citral hydrogenation

in a screw-impeller tank reactor [164] and in benzaldehyde

hydrogenation over a monolith catalyst in a batch mode or in

continuous mode [139]. Ni-monolith was used as a catalyst

in citral hydrogenation in a screw-impeller stirred tank

reactor (SISR) (Fig. 27). Citral hydrogenation was

investigated in hexane at 5 and 40 bar hydrogen and at

temperatures 40, 80 and 100 8C. Catalyst deactivation was

observed after consecutive experiments. The selectivity to

citronellal was the highest at 40 8C and 5 bar hydrogen, i.e.

96% at 80% conversion. Higher hydrogen pressures favored

formation of 3,7-dimethyloctanal [164]. In benzaldehyde

hydrogenation over Ni-monolith catalysts the highest

selectivities to benzyl alcohol were obtained in a batch

operation without residence time distribution [139].

A continuous fixed bed reactor operation was demon-

strated in citral hydrogenation with a recirculating liquid

phase [138] and in a continuous flow [184,185]. In the former

case it was shown in citral hydrogenation at 70 8C in ethanol

over knitted Ni-silica fibers that the recirculating reactor

operation was equally good as a traditionally batch reactor. In

the latter case citral hydrogenation was carried out over

5 wt.% Pd/Al2O3 catalyst at 408 and 190 bar hexane,

supercritical CO2 or ethane [185]. Both catalyst deactivation

and changes in the product distribution were observed.

A packed bubble column reactor was used in hydro-

genation of citral to citronellal. The aim of using

recirculating product and circulating hydrogen was to

improve the space–time yield, because according to Brocker

et al. [209] the catalyst regeneration is not successful in citral


hydrogenation and the larger amounts of catalyst needed to

enhance the productivity would decrease the process

economy. In a packed bubble column reactor hydrogenation

of citral to citronellal over a powdered Pd/C catalyst in the

presence of triethyl amine in methanol at 70 8C and 8 bar

hydrogen gave 92.9% selectivity to citronellal at 99.7%

conversion.

Microstructured carbon-coated reactors have been

applied in gas phase hydrogenation of acrolein. The catalyst

contained microchannels of 700 mm depth and of 300 mm

width with length of 50 mm manufactured from copper wire.

The carbon-coating was obtained via carbonization the

polymer coating at 550 8C. Thereafter the support was

calcined at 350 8C and after fucntionalization of the carbon

surface with HNO3 Ru was deposited on the support via ion

exchange with [Ru(NH3)Cl]Cl2 [160]. Although the

comparative specific catalytic activities for microstructured

Ru-catalyst and for Ru/C were not measured, it was reported

that the selectivity to allyl alcohol was comparable for these

catalysts. The short overview of various reactor configura-

tions applied in the recent years for heterogeneous catalytic

hydrogenation of carbonyl compounds demonstrates the big

although largely unexplored potential of chemical reaction

engineering in the field of synthesis of fine chemicals.

8. Conclusions

Recent literature on the selective hydrogenation of

unsaturated aldehydes to unsaturated alcohols over hetero-

geneous catalysts is overviewed. Both the reactant structure

and the nature of metal surface affect the possibilities to

achieve high selectivities to unsaturated alcohols. The main

challenge in chemoselective hydrogenation is to develop a

stable catalyst, which is active and selective, since usually

there is an inverse relationship between activity and

selectivity.

For supported catalysts both metal and support influence

activity and selectivity of the final catalyst. Metals can have

different electronic properties for instance due to their

different origin, exhibiting various properties. Furthermore,

several metals or metal alloys can be used as active

components and electronic properties can be tuned by

adding promoters. The nature of support material, like

porosity, pore size distribution and reducibility is an

important parameter in catalyst preparation. Besides

conventional oxide and activated carbon supports more

advanced support materials have been investigated in the

synthesis of fine chemicals. Some examples of tailor-made

supports are bifunctional zeolites and mesoporous materials,

colloidal metals in polymers as well as reducible and alloy

forming supports. It should be pointed out that the final

catalysts properties are also a result of specific catalyst

pretreatment and a required catalyst could be obtained via

fine-tuning the catalyst pretreatment procedures. The

selectivity to unsaturated alcohols can in general be

increased via increasing the number of active sites activating

the carbonyl group.

Design of tailor-made catalysts with specific selectivity is

based on the knowledge in reaction mechanisms. Recent

studies on molecular modeling and adsorption along with

kinetic modeling are discussed. Besides being a way of

translating our understanding of the reaction mechanism,

kinetic modeling is efficiently used for process develop-

ment. Three-phase fine chemical syntheses are traditionally

carried out in batch reactors over conventional slurry

catalysts. There exist, as shown in this review, substantial

amount of kinetic data from chemoselective hydrogenations,

which can be used in process development. Additionally, the

knowledge from conversion–selectivity relationship and a

comparison of liquid and gas phase hydrogenations serve as

tools for understanding the catalytic phenomena on the

surface sites. Due to the increasing future demand of fine

chemicals there is, however, a need to develop continuous

production technologies for three-phase hydrogenations.

These technologies require structured catalysts, which have

low-pressure drop and avoiding at the same time maldis-

tribution, thus utilization of several structured supports in

chemoselective hydrogenation, like fibers prepared from

activated carbon, polymer, silica woven glass fibers has a

high potential. Recently, membrane reactors and monolithic

catalysts have been also applied in the field of fine

chemicals. Besides other advantages continuous technology

brings additionally a benefit of understanding the catalyst re-

use and deactivation in a well-defined, quantitative way.

Analysis of the state-of-the-art in heterogeneous catalytic

hydrogenation strongly emphasizes the necessity of a truly

multidisciplinary approach in catalyst development, when

the materials chemistry is tightly linked on one hand to

surface science and molecular modeling and on the other

hand to catalytic reaction engineering, including kinetic

modeling and reactor selection.

Acknowledgements

This work is part of the activities at the Abo Akademi

Process Chemistry Centre within the Finnish Centre of

Excellence Programme (2000–2005) by the Academy of

Finland. Figs. 1–8, 10–12, 14–24 are reproduced with

permission from Elsevier.

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Chemoselective hydrogenation of carbonyl compounds over heterogeneous catalysts

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