ORIGINAL PAPER

Developments in the Pd Catalyzed Hydrogenation of OxygenatedOrganic Compounds

Charles Sumner Jr. • William Burchett

Published online: 21 June 2012

� Springer Science+Business Media, LLC 2012

Abstract This manuscript briefly reviews some key

developments in hydrogenation catalysis and their appli-

cation from an industrial perspective. The interaction of Pd

(and Pt) with the support material has been previously

reported as a factor that determines the activity and

selectivity of Pd hydrogenation catalysts. Temperature

programmed reduction experiments carried out on

Pd/Al2O3 catalysts in our laboratory showed a correlation

between hydrogen consumption and selectivity for the

hydrogenation of acetophenone.

Keywords TPR � Pd/Al2O3 � Hydrogenation �Selectivity � Acetophenone

1 Introduction

The economic value and impact of hydrogenation catalysts

is tremendous. Each day we consume products derived

from catalyzed hydrogenations ranging from the drugs we

need, the clothes we wear, to the containers that store our

food and beverages. The purpose of this paper is to give a

brief overview of some key developments in hydrogenation

catalysis and how they relate to the industrial manufacture

of chemical products. We will also describe results from

our laboratory pertaining to the interaction of palladium

with an alumina support and the selectivity of the catalyst.

Over the last 30 years, surface chemistry has evolved

into a molecular-level science. The recognition that

chemisorption (and reaction) of a substrate can lead to

reconstruction of a single crystal surface has resulted in the

development of many in situ techniques of spectroscopic

analysis [1, 2]. The characterization of the catalyst surface

during the course of the catalytic reaction (referred to as

Operando techniques) has the obvious advantage of pro-

viding a look at the catalyst while it is in its working form.

Operando measurements and methods are constantly

improving, and many research groups specialize in this

field. Using these techniques, one can potentially identify

reaction intermediate species adsorbed on the surface and

determine other parameters important to the reaction

mechanism. Having a true picture of the reaction mecha-

nism and the structure of the catalyst surface would ulti-

mately lead to the determination of structure–reactivity

relationships. Such insights will greatly aid in the design of

better catalysts and in the improvement of existing com-

mercial catalysts where often a very small increase in

selectivity or rate results in very large economic returns.

2 Catalyst Modeling

Recent advances in the use of density functional theory for

modeling surface reactions have led to the discovery of

several new hydrogenation catalysts. Work published by

Norskov and collaborators described a systematic, com-

putational method to screen potential catalysts for a desired

application [3]. Descriptors (‘‘the most important micro-

scopic materials properties of the catalyst which determine

the macroscopic catalytic performance’’; for example,

reactant heat of adsorption) [4] were identified for the

desired reaction. The descriptors were able to predict trends

in the activity, selectivity, and cost of the catalyst. The

descriptor values were calculated using DFT simulations

and pre-existing computational databases. The materials

that were identified with descriptor values closest to the

C. Sumner Jr. (&) � W. Burchett

Eastman Chemical Company, Kingsport, TN 37662, USA

e-mail: cesumner@eastman.com

123

Top Catal (2012) 55:480–485

DOI 10.1007/s11244-012-9820-4

optimal values were evaluated experimentally [4]. Several

new catalysts were identified using this approach and

include a Ni–Fe methanation catalyst [5] that is cheaper

and more active than Ni only; a Ni–Zn catalyst [6] for the

selective hydrogenation of ethylene that has selectivity

comparable to the commercial Pd–Ag catalyst but is

cheaper, and a Pt–Bi electrocatalyst [7] for the hydrogen

evolution reaction that is more active than pure Pt. The

advantage of the computational approach to screening

catalysts is that it could save money and time by elimi-

nating most of the experimental screening reactions. A

disadvantage is that it requires access to high performance

computing systems. The approach does not work well with

processes that have several competing reactions, or with

metal oxide surfaces [3]. Improvements to the method

would include the formation of larger databases and

development of better computer code. Computational

methods used to model catalyst surfaces continue to

improve and can be a powerful tool when combined with

experimental work. So, will the day come when experi-

ments are no longer needed? Probably not, since no matter

how good computation becomes, someone will still have to

figure out how to make and then test and scale up the

structures predicted by a computer.

3 Chiral Catalysis

Hydrogenation of prochiral substrates to produce chiral

products is commonly accomplished using homogeneous

catalysts comprised of precious metals complexed with

chiral phosphine ligands. While very good enantiomeric

excess can be achieved with the homogeneous catalysts,

there are many obvious advantages for the use of hetero-

geneous catalysts such as ease of separation, catalyst

recovery and recycle, and handling. These can be particu-

larly important for precious metal catalysts. Advances in

the development and understanding of heterogeneous cat-

alysts for asymmetric hydrogenations have been made

through the combination of surface science techniques and

practical applications [8, 9].

Enantioselective heterogeneous catalysis is generally

thought to involve the formation of a chiral environment at

the catalyst surface [10]. One way chirality can be induced

is by the modification of the surface with adsorbed chiral

molecules to give an ordered array of chiral molecules that

influences the adsorption of a prochiral reactant in a

geometry that favors one of the enantiomeric products. For

example, heterogeneous Pt catalysts modified with chiral

cinchona alkaloids have been found to be effective cata-

lysts for the enantioselective hydrogenation of activated

ketones and a,b-unsaturated carboxylic acids. R,R-tartaric

acid was used as a chiral modifier in the Ni-catalyzed

hydrogenation of b-ketoesters [11, 12].For the case of

acidic modifiers, it is possible that chiral etching of the

metal surface can take place under certain conditions.

Scanning tunneling microscopy (STM) studies have shown

that the adsorption of chiral amino acids onto an achiral

Cu(001) surface can induce restructuring of the surface to

develop intrinsically chiral facets [13]. A similar phe-

nomenon was observed with tartaric acid [14].

The effectiveness of the chiral modifier was reported to

be influenced by the ionicity of the catalyst support mate-

rial for the cinchonidine-modified Pt/Al2O3 system [15].

The enantiomeric excess in the hydrogenation of methyl

benzoyl formate increased with decreasing basicity. The

reason for the effect was thought to be due to the electronic

state of Pt that was dependent on the acidity of the support.

In related work, the diastereoselectivity of cinchonidine

hydrogenation was controlled by tuning the acid–base

properties of the alumina support [16].

4 Metal Support Interaction

Subtle changes in the composition of the catalyst support

material can have a dramatic effect on the performance of

the catalyst. Vannice [17, 18] reported that the reduction

temperature of Pt/TiO2 influenced the selectivity and

activity for the hydrogenation of carbonyl groups to alco-

hols. The catalysts that were reduced at 500 �C were more

selective than catalysts reduced at 200 �C. The origin of

the enhanced selectivity was thought to be the formation of

a Ti3? cation or an oxygen vacancy adjacent to a Pt atom.

The carbonyl oxygen atom was thought to be activated at

the metal-support interfacial region. The Bronsted acidity

or basicity (ionicity) of the support material can have a

great influence on the performance of the catalyst. For

example, Pd supported on high purity silica support was

reported to catalyze the hydrogenation of carbon monoxide

to mainly methane; however, if the silica support contained

as little as 600 ppm of Ca the major product was methanol

[19]. The more acidic silica favored the production of

methane while the more basic silica favored the formation

of methanol.

Mojet and Koningsberger [20] applied a combination of

computational, surface and sorptive characterizations, and

reactivity studies to formulate a model to describe the

metal support interaction for Pt and Pd. In this model, the

ionicity of the support (high ionicity is equivalent to low

acidity) can affect the strength of the metal–oxygen bond

and thus the morphology of the metal particle. A stronger

bond with the support as a result of low ionicity was cal-

culated to cause the metal particles to be flat and raft like,

while a weaker metal oxygen bond caused by high ionicity

caused the particles to be rounder and hemisphere shaped

Top Catal (2012) 55:480–485 481

123

[21]. The ionicity can also affect the bonding modes of

hydrogen and carbon monoxide with the metal particle. For

example, FTIR showed significant l-CO with Pt supported

on basic alumina but not with acidic alumina. The Pt–H

bond strength is higher for ionic supports and the Pt–O

bond strength is higher for acidic supports [22]. The dif-

ference in electron density in the supports affects the

sp/(sp ? d) Pt orbital hybridization and causes the Pt 5d

states to shift to lower binding energy with increased ion-

icity [23]. The hydrogen adsorption energy increases for Pt

on supports with higher ionicity, and thus the ionicity of

the support influences the hydrogen coverage and type of

empty site available for adsorption of reactants. Experi-

mentally, this results in greater activity for hydrogenolysis

reactions on acidic supports [24–26].

5 Experimental Section

5.1 Preparation of Catalysts

The alumina used in this work was 1/16th inch pellets and

had a surface area of 40 m2/g. Solutions (1.0 wt% Pd) of

Pd(NH3)4(OH)2 were prepared by ion exchange of

Pd(NH3)4Cl2 over Amberlyst� A-26(OH) ion exchange

resin. The solutions were analyzed by ICP for Pd and Cl

and stored under an argon atmosphere prior to use. The

concentration of the Pd solution was adjusted by dilution

with deionized water and the resulting solution was added

to the alumina (via incipient wetness technique). The

resulting alumina was vacuum dried first on a rotovap

followed by 12 h at 115 �C in an oven. The dried material

was calcined at 300 �C for 4 h in air, cooled under nitro-

gen, and activated by heating at a rate of 1�/min to 200 �C

under a flow of H2/N2 (1:1). The Pd dispersion as measured

by CO chemisorption (stoichiometry factor of one) ranged

from 33 to 40 %. The average particle size measured by

TEM ranged from 2.3 to 2.5 nm.

5.2 Hydrogenation of Acetophenone

The hydrogenations were carried out in cyclohexane

solution in a 300 mL stainless steel autoclave equipped

with a catalyst basket and a dip tube that enabled sampling

during the reaction. The reaction temperature was 170 �C

and the hydrogen pressure was 1,000 psig. The reactants

were added in one portion to the autoclave at reaction

conditions. The mixture was sampled at time intervals and

analyzed by gas chromatography for 1-phenylethanol,

acetophenone, cyclohexylmethyl ketone (CMK), 1-cyclo-

hexylethanol, ethylbenzene, ethylcyclohexane, and styrene

(no styrene was detected). The initial rate for the formation

of CMK and 1-cyclohexylethanol (the products resulting

from the hydrogenation of the benzene ring) were deter-

mined by taking the slope of the sum of the products

plotted against time at the zero order portion of the curve.

The initial rate for the formation of products resulting from

hydrogenolysis (ethyl benzene and ethyl cyclohexane) was

determined similarly.

6 Results and Discussion

Alumina supported Pd catalysts are used commercially in

continuous processes for the hydrogenation of carbon–

carbon double bonds and for the hydrogenation of aromatic

rings to cyclohexane derivatives. A common problem

encountered during the hydrogenation of compounds that

possess an oxygen-containing functional group is the

hydrogenolysis of the carbon–oxygen bond to give unwanted

Fig. 1 Overlay of H2TPR plots illustrating the variation in H2

consumption for 1 % Pd/Al2O3 catalysts

O OH

O OH

+

+

acetophenone phenylethanol

ethylbenzene

ethylcyclohexane

CMK cyclohexylethanol

ring hydrogenation

hydrogenolysis

Fig. 2 Products from the hydrogenation of acetophenone over 1 %

Pd/Al2O3 at 170 �C and 1,000 psig of H2 in cyclohexane solution

482 Top Catal (2012) 55:480–485

123

byproducts. Research in our laboratory has been focused on

improving the selectivity of the catalyst while at the same

time increasing the activity. The work presented here

involved the characterization and evaluation of catalysts

comprised of alumina supports with differing acidities that

had been decorated with (1 %) Pd.

Temperature programed reduction (TPR) of oxidized Pd

supported on gamma-alumina was reported by Lieske and

Volter [27]to reflect an interaction of Pd with the alumina

support, presumably as the form of a two dimensional

surface species of palladium aluminate. The reduction

temperature of the palladium aluminate was reported to be

around 300 �C, and its formation was thought to be

involved in the redispersion of Pd on alumina by high

temperature calcination. XPS data reported by Otto et al.

[28] were consistent with the presence of two forms of

oxidized Pd. In our labs, we noticed a similar high tem-

perature reduction (at around 300 �C) for alumina sup-

ported Pd hydrogenation catalysts prepared by incipient

wetness using an aqueous solution of Pd(NH3)4(OH)2. The

amount of hydrogen that was consumed at around 300 �C

(250–310 �C) in the TPR experiment varied depending

upon the amount of alkali metal present on the alumina

(that is, the support ionicity). For example, alumina that

had been treated with sodium carbonate (or nitrate) fol-

lowed by calcination at 900 �C prior to impregnation with

Pd gave catalysts that consumed greater amounts of

hydrogen at 300 �C than catalysts prepared from the alu-

mina that had no additive. It was assumed that the supports

with higher ionicity would also have lower acidity. Fig-

ure 1 shows an overlay of TPR plots for five catalysts

comprised of 1 % Pd/alumina that had different hydrogen

consumption amounts. While the hydrogen consumptions

are in the temperature range given by Lieske and Volter for

palladium aluminate, we cannot say with certainty what is

responsible for it. Whether the higher temperature hydro-

gen consumption was due to the presence of palladium

aluminate or the reduction of a surface species in contact

with the metal, the amount of hydrogen that was consumed

at around 300 �C was an indication of the selectivity of the

catalyst when it was applied to the hydrogenation of an

oxygenated organic compound.

The selectivity of the catalysts was evaluated using the

hydrogenation of acetophenone as a model reaction. This

reaction was chosen because of its ability to determine

differences in selectivity that arise as a result of ring

hydrogenation and hydrogenolysis of the carbon–oxygen

bond (Fig. 2). The hydrogenations were carried out in

cyclohexane solution at 170 �C and 1,000 psig H2 in a

stirred autoclave. Saturated products that retained the car-

bon–oxygen bond, cyclohexyl methyl ketone (CMK) and

1-cyclohexylethanol, were grouped as products of ring

hydrogenation, and products that did not contain oxygen,

ethyl benzene and ethyl cyclohexane, were grouped as

Table 1 Hydrogen

consumption and initial rate for

the hydrogenation of

acetophenone for different

1 %Pd/Al2O3 catalysts

Catalyst H2

consumption @

300� (lmol/g)

Initial rate of ring

hydrogenation

(mmol/g/min/gcat)

Initial rate of

hydrogenolysis

(mmol/g/min/gcat)

Total rate

(mmol/g/

min/gcat)

Selectivity

to ring (%)

A 0 0.0013 0.0130 0.0143 9

B 6 0.0027 0.0055 0.0082 33

C 10 0.0017 0.0057 0.0074 23

D 22 0.0023 0.0041 0.0065 36

E 51 0.0016 0.0025 0.0041 39

Fig. 3 Selectivity to ring hydrogenation as a function of hydrogen

consumption Fig. 4 Total initial rate as a function of hydrogen consumption

Top Catal (2012) 55:480–485 483

123

hydrogenolysis products. Under these reaction conditions,

the initial product appeared to be 1-phenylethanol which

was converted to the mixture of final products (similar

kinds of product distributions were obtained when starting

with 1-phenylethanol).

The data for the acetophenone hydrogenations are listed

in the Table 1. The selectivity for ring hydrogenation

appeared to increase with increasing hydrogen consump-

tion measured in the TPR (Fig. 3), while the total rate of

reaction decreased with increasing TPR hydrogen con-

sumption (Fig. 4) at around 300 �C. The selectivity for ring

hydrogenation increased, but the overall activity of the

catalyst with respect to the conversion of acetophenone

decreased. As shown in Fig. 5, the initial rate of hydrog-

enolysis decreased with hydrogen consumption. A plot of

the initial rate of ring hydrogenation against hydrogen

consumption does not appear to show any correlation

(Fig. 6). The hydrogenolysis reaction and ring hydroge-

nation are presumably parallel reactions. The hydrogenol-

ysis reaction appears to be slower on catalysts that have

higher TPR hydrogen consumption. While hydrogenolysis

is slowed, the rate of the ring hydrogenation (in these

examples) does not appear to be affected as much by the

properties of the support, thus the selectivity for ring

hydrogenation increases and the activity as measured by

conversion of acetophenone decreases.

7 Conclusion

Catalysis of industrial hydrogenation processes almost

always involves a balance between activity and selectivity.

Recent progress in the field of surface science has shed

light on some of the properties that are responsible for

both. The interaction of the catalyst metal with the support

material has been recognized as a phenomenon that can

have a great effect on catalyst selectivity, and new models

have been developed to explain how the interaction

operates on a molecular level. In processes where parallel

reactions take place, it is possible that the nature of the

support can favor or inhibit one pathway over the other and

thereby influence the selectivity (and activity) of the cat-

alyst. In the work described here, the observation of a

reduction at 300 �C in the TPR experiment might reflect an

interaction of Pd with the support or be the result of other

factors1. The reduction appears to correlate with the

selectivity of the catalyst for ring hydrogenation, and the

absence of the reduction correlates with a tendency toward

hydrogenolysis.

Acknowledgments We thank Eastman Chemical Company for

permission to publish this work.

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