hydrogenation of a,b unsaturated aldehyde to allyl alcohol is very important industrial and academic topic to study.
47
This article was downloaded by: [Indian Institute of Technology Madras] On: 16 April 2012, At: 21:19 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Catalysis Reviews: Science and Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lctr20 Selective Hydrogenation of α,β- Unsaturated Aldehydes P. GALLEZOT a & D. RICHARD a a Institut de Recherches sur la Catalyse—CNRS, 2, Avenue Albert Einstein, 69626, Villeurbanne, Cedex, France Available online: 15 Aug 2006 To cite this article: P. GALLEZOT & D. RICHARD (1998): Selective Hydrogenation of α,β-Unsaturated Aldehydes, Catalysis Reviews: Science and Engineering, 40:1-2, 81-126 To link to this article: http://dx.doi.org/10.1080/01614949808007106 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Transcript
This article was downloaded by: [Indian Institute of Technology Madras]On: 16 April 2012, At: 21:19Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Catalysis Reviews: Science andEngineeringPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lctr20
Selective Hydrogenation of α,β-Unsaturated AldehydesP. GALLEZOT a & D. RICHARD aa Institut de Recherches sur la Catalyse—CNRS, 2, Avenue AlbertEinstein, 69626, Villeurbanne, Cedex, France
Available online: 15 Aug 2006
To cite this article: P. GALLEZOT & D. RICHARD (1998): Selective Hydrogenation of α,β-UnsaturatedAldehydes, Catalysis Reviews: Science and Engineering, 40:1-2, 81-126
To link to this article: http://dx.doi.org/10.1080/01614949808007106
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae, and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand, or costs or damages whatsoever or howsoever caused arising directly or indirectlyin connection with or arising out of the use of this material.
The synthesis of a large number of fine chemicals, particularly in the field of flavor and fragrance chemistry [1,2] and pharmaceuticals [ 3 ] , involves the selective hydrogenation of unsaturated carbonyl intermediates as a critical step. The hydrogenation of a$-unsaturated carbonyls into saturated carbonyls is comparatively easy to achieve because thermodynamics favor the hydro-
81
Copyright 0 1998 by Marcel Dekker, Inc.
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82 GALLEZOT AND RICHARD
genation of the C=C bonds; therefore, research efforts were more directed at improving the selectivity to unsaturated alcohols. When a substituent is present on the carbon atom of the carbonyl group (i.e. with ketones), there is no chance to hydrogenate selectively the C=O bond, and saturated ketones are obtained with a high yield. This review is thus mostly restricted to the hydrogenation of a$-unsaturated aldehydes into the corresponding unsatu- rated alcohols.
Selective reductions can be achieved using stoichiometric amounts of reducing agents such as metal hydrides [4]. Thus, cinnamaldehyde was re- duced into cinnamic alcohol with a 99% selectivity [5]. These methods are useful only for the small-scale production of highly priced products because they involve costly chemicals. Therefore, research efforts were mainly di- rected at developing hydrogenation processes based on heterogeneous or ho- mogeneous catalysis. The pioneering work of Adams and collaborators [6- 91 in the twenties paved the way for subsequent investigations on heteroge- neous catalysts. Early works in this field were mainly conducted in industrial laboratories. However, during the last decade, many academic investigations were carried out because hydrogenations of unsaturated aldehydes were cho- sen as model reactions to establish relations between selectivity and catalyst structure.
In spite of the large number of articles published in recent years, the subject has been hardly reviewed. The classic treatises of Rylander on cata- lytic hydrogenation [ 10,111 deal briefly with hydrogenation of unsaturated aldehydes and ketones. Stereochemical aspects of the hydrogenation of a,p- unsaturated ketones were discussed by Augustine with a special attention to cyclic ketones and steroids [12]. Cordier et al. [13] briefly reviewed some of the factors which may affect the selectivity to unsaturated alcohols in the hydrogenation of a,P-unsaturated aldehydes, such as the nature of the cata- lysts (metal, support, and preparation conditions), the solvent, and the addi- tion of promoters. Kluson and Cerveny [14] reviewed the use of ruthenium catalysts in the hydrogenation of the C=O group. Finally, Gallezot and Richard [ 151 presented a short review on the chemoselective hydrogenation of unsaturated aldehydes, pointing out the effect of the structure of hetero- geneous catalysts on the selectivity to unsaturated alcohols.
The aim of the present review is to extend this previous work and to present the state of the art in the selective reduction of a$-unsaturated al- dehydes on heterogeneous catalysts. Stress will be placed on the influence of all the factors playing a role on the final chemoselectivity or regioselectivity [i.e., influence of molecular structure (steric and/or electronic effects produced by the substituents) as well as of catalyst structure (nature of the metal atoms, geometry of their arrangement, local structure and texture of the support, electronic and geometric effects of a second metal or of surface ligands)]. Although this review deals mostly with heterogeneous catalysts, the most significant selectivity achievements obtained with homogeneous catalysts, or with transition metal complexes immobilized on supports, will be briefly re-
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SELECTIVE HYDROGENATION OF ~u,P-UNSATURATED ALDEHYDES 83
viewed. New ways of selectively reducing a$-unsaturated compounds using biotransformations caused by yeast or fungi, as recently reported [16,17], are beyond the scope of the present review.
11. HYDROGENATION ON METAL CATALYSTS
A. Generalities on Mechanisms
The hydrogenation of unsaturated a$-unsaturated aldehydes proceeds via different reaction pathways given schematically in Fig. 1. The 172-addition of hydrogen gives the unsaturated alcohol, the 3,4-addition gives the saturated aldehyde, and the 1,4-addition gives the enol, which isomerizes into a satu- rated aldehyde. Subsequent hydrogenation of the C=C or C-0 bonds leads to the saturated alcohol. These reaction pathways determine the chemoselec- tivity or regioselectivity of the transformation. Note that the term regioselec- tivity can be applied in the case of a,P-unsaturated carbonyl compounds because the selectivity depends on hydrogen addition at a given position of a conjugated system. Unsaturated alcohols can isomerize into saturated al- dehydes. Isomerization reactions were mainly reported in gas-phase hydro- genation. Thus, detailed kinetics studies of vapor-phase crotonaldehyde hy- drogenation conducted by Simon& and Berdnek [18], Vannice and Sen [19], and Noller and Lin [20] showed that significant amounts of crotyl alcohol isomerize into butyraldehyde. However, Campelo et al. [21] have shown that unsaturated alcohols may also isomerize into saturated aldehydes under liq- uid-phase hydrogenation conditions on Rh/AIPO, catalysts.
The reaction scheme given in Fig. 1 can be further complicated by side reactions occurring either on metals or on supports. There are few reports of reactions conducted in liquid phase mentioning the formation of hydrocar- bons by hydrogenolysis of the C-0 bonds. In contrast, hydrogenolysis re- actions were reported in the vapor-phase hydrogenation of acrolein even at low conversion [22,23]. Formation of up to 20% of hydrogenolysis product
"4. H2 ~ " 4 H
I I -% RIP H
R2 R2 I
'"t H 4 R2 R 1 d H 1.1 b>
R2
FIG. 1. urated aldehydes.
Scheme of the reaction pathways in the hydrogenation of a,p-unsat-
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84 GALLEZOT AND RICHARD
was observed in gas-phase reactions carried out on platinum single-crystal faces [24,25].
It is widely assumed that hydrogenation of a,P-unsaturated carbonyls on metal surfaces occurs via the Horiuti-Polyani mechanism involving either di-a,=, q , di-ac,c q , or di-T q2 (q4) adsorbed states as described in the reaction scheme given in Fig. 1. This mechanism was substantiated by studies of reduction reactions with deuterium [22,26-291. These studies [22,27] also accounted for the hydrogenolysis of C-0 bonds. There are few theoretical studies on the mechanism of hydrogenation of a,P-unsaturated carbonyl com- pounds although quantum-chemical analysis could provide valuable insight on the pathways of hydrogenation of unsaturated compounds and, thus, on the origin of regioselectivity. Chuvylkin et al. [30] used the INDO (intermediate neglect of differential overlap) method to investigate the influence of the elec- tronic structure of a,P-unsaturated aldehydes (citral and cinnamaldehyde) on their mechanism of hydrogenation. Various hypotheses were made for the ad- sorption of molecules on the metal surface, but the nature, electronic properties, and surface geometry of metals were not taken into account.
Delbecq and Sautet [31] have used semiempirical extended Huckel cal- culations to study the adsorption of various a,P-unsaturated aldehydes (acro- lein, crotonaldehyde, prenal, and cinnamaldehyde) on platinum and palladium crystal faces. The metal surface was modeled by finite clusters with a cor- rection for edges effects. It was found that the adsorption mode of the mol- ecules is strongly dependent on the nature of the metal and on the type of face exposed. Thus, a di-a form is preferred on Pt( l l l ) , a planar q4 one on Pd(ll1) and Pt(100), and a T,=, one on Pt(l l0) and on the steps of Pt(ll1). The selectivity to unsaturated alcohol was discussed in terms of the compet- itive adsorption of the C-C and C=O bonds on the metal surface. The selectivity can be improved by a decrease of the binding energy of the C=C bond as a result of an increase of the repulsive four-electron interac- tions with the metal; this may occur because of the presence of substituents on the C-C bond or with metals, such as osmium or iridium, presenting more extended &orbitals. A higher proportion of dense (1 11) faces, which occur on large faceted particles or by support epitaxy, can also decrease the C=C bond adsorption, thus increasing the selectivity. Alternatively, the se- lectivity can be improved by favoring the interaction of the C-0 T system with the surface; thus, the presence of Lewis-acid sites would activate the carbonyl group and lower the acceptor n,*,-orbital. Also, enrichment of the metal surface with electrons by interaction with a support or ligand will tend to favor the backbonding interaction with n& to a larger extent than with T&, thus favoring C-0 adsorption. On the other hand, a poor selectivity can be expected with the q4 adsorption mode because the hydrogenation of the C-C bond is then favored for kinetic reasons. This could happen when the four-electron repulsion between the surface and the molecule is small (e.g., with palladium or with the open faces or steps of platinum). In a sub- sequent article, the same authors studied the adsorption of unsaturated mol-
2 . 2
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SELECTIVE HYDROGENATION OF a,P-UNSATLJRATED ALDEHYDES 85
ecules on a model Pt,,Fe,,(lll) surface with the Fe atoms located in the second layer [32]. Calculations showed that the a,P-unsaturated aldehyde molecules are less strongly adsorbed and have a more pronounced tendency to adsorb via the C-0 bond on the alloy than on pure platinum.
The structure of adsorbed unsaturated carbonyls on single-crystal sur- faces were studied by HREELS (high-resolution electron energy loss spec- troscopy) and TPD (temperature programmed desorption) by Barteau and co- workers [33,34]. On the Rh(ll1) surface, acrolein was found to adsorb via the $(C,O) configuration at 91 K, the T~(C,C,C,O) configuration at 200 K, and decarbonylation occurred between 200 and 300 K [33]. On the Pd(ll1) face, acrolein was strongly rehybridized into q4(C,C,C,0)-bound species, in agreement with the theoretical prediction discussed above.
Because the selectivity to unsaturated alcohol can be improved either by inhibiting the hydrogenation of the C-C bond or by favoring the hydro- genation of the C-0 bond, the knowledge of the respective hydrogenation rates of C=C and C-0 bonds is helpful to interpret any selectivity change that may occur as a function of the different factors to be examined in Sec- tions 1I.B through 1I.H.
B. Metal Specificities
Early studies showed that unpromoted metals have specific selectivities to unsaturated alcohols: Iridium and osmium are rather selective; palladium, rhodium, and nickel are unselective or little selective; platinum, ruthenium, and cobalt are moderately selective [35]. These trends were confirmed by reaction data given by Sokol’skii [36] for crotonaldehyde hydrogenation (0s > Ir > Ru > Rh = Pt = Pd), and by Cordier for cinnamaldehyde hydroge- nation (0s > Ir > Pt > Ru > Rh > Pd) [13,37]. The selectivity to cinnamyl alcohol of well-dispersed Ir, Pt, Ru, Rh, and Pt catalysts were compared by Giroir-Fendler et al. [38]. The catalysts prepared by ion exchange of a high- surface-area graphite and of an active charcoal were obtained with similar particle size (1-2 nm). Table 1 gives the selectivity to unsaturated alcohol at 25% conversion and the turnover frequency (TOF) corresponding to the initial activity. The selectivity to unsaturated alcohol follows the series Ir > Pt > Ru > Rh > Pd on active carbon and graphite support. The large increase of selectivity for platinum and ruthenium on graphite was attributed to an elec- tronic effect, which will be discussed in Section 1I.E.
The factors governing intrinsic selectivities of metals, or metal specific- ities, have been recently highlighted by the theoretical calculations of Delbecq and Sautet [31], mentioned earlier, showing that metal selectivities can be rationalized in terms of the different radial expansion of their d bands; the larger the band, the stronger the four-electron repulsive interactions with the C=C bond and the lower the probability of its adsorption. Indeed, the d- band width increases in the series Pd c Pt < Ir, 0 s which accounts well for the experimental selectivities.
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TA
BL
E 1
Se
lect
ivity
at 0
% a
nd 2
5% C
onve
rsio
n an
d In
itial
Act
ivity
of
Plat
inum
-Gro
up M
etal
s in
Cin
nam
alde
hyde
Hyd
roge
natio
n on
Act
ive
Cha
rcoa
l (C
) an
d on
Gra
phite
(G
)
Con
vers
. 1"
2h
3'
TO
F 1"
2h
3'
TO
F C
atal
yst
(%)
(%)
(%)
(%)
(s-7
C
atal
yst
(%I
(%>
(%)
(s-9
2.
9% Ir
/C
0 60
36
4
0.04
4.
4% I
r/G
61
36
3
0.09
3.8%
Pt/C
0
0 95
5
0.02
3.
6% P
t/G
72
28
0 0.
02
4.6%
Ru/
C
0 0
95
5 0.
05
3.6%
Ru/
G
12
83
5 0.
09
25
81
13
6 72
25
3
25
33
60
7 78
20
2
25
5 88
7
39
57
4
25
0 97
3
7 90
3
25
0 55
45
0
83
17
2.7%
Rh/
C
0 0
95
5 0.
50
1.6%
Rh/
G
0 94
6
0.61
4.0%
Pd/
C
0 0
60
40
6.7
2.5%
Pd/
G
0 70
30
4.
4
r E
'Hyd
roci
nnam
yl a
lcoh
ol (
3-ph
enyl
pro
pano
l).
3 $
"Cin
nam
yl a
lcoh
ol (
3-ph
enyl
pro
peno
l).
"Hyd
roci
nnam
alde
hyde
(3-
phen
yl p
ropa
nal).
Rea
ctio
n co
nditi
ons:
T =
333
K, P
= 4
MPa
, 0.1
mol
of
cinn
amal
dehy
de in
37.
5 m
l is
opro
pano
l an
d 10
ml H
zO, 4
00 m
g of
cat
alys
t. So
urce
: Ref
. 38.
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SELECTIVE HYDROGENATION OF a,P-UNSATURATED ALDEHYDES 87
Table 2 shows that the selectivity to unsaturated alcohols of osmium catalysts are rather high and are in agreement with the theoretical provision given above. These data also clearly illustrate the importance of the effect of substituents on the C=C bond, which increase the selectivity to unsaturated alcohol in the series acrolein c crotonaldehyde c cinnamaldehyde.
At the other end of the series, palladium catalysts are very active for C-C bond hydrogenation and give almost a 100% yield in saturated alde- hyde [35]. This is well illustrated by the TOF and selectivity data of palladium catalysts given in Table 1. Even when there is a steric hindrance around the C=C bonds with very bulky R substituents as in unsaturated furan ketones, a 99% selectivity to saturated ketones was measured [41]. Selectivities of 100% in the reduction of C-C bonds in various unsaturated ketones were also obtained on Pd/C using hydrogen transfer reaction with limonene as the hydrogen donor [42]. The reason why palladium is so selective to saturated carbonyl can be understood from the theoretical work of Delbecq and Sautet [31] showing that the preferred adsorption mode for the conjugated system on palladium was q4, which leads to the saturated carbonyl. Rhodium cata- lysts are also mostly selective to saturated aldehydes even in the presence of a promoter. A noteworthy exception, rhodium modified by tetrabutyl tin, will be discussed in Section I1.D.
Unpromoted transition metals of the first row are also weakly selective to unsaturated alcohol. Thus, using a Ni/Al,O, catalyst to hydrogenate citral, Sokol’skii et al. [43] obtained citronellal, the corresponding saturated alde- hyde with a 99% selectivity. Hydrogenation of citral with Ni/Cr,O, gave primarily citronellol [44]. Copper catalysts are also more selective for the hydrogenation of the C=C bond than the C=O bond when used in the hydrogenation of a,P-unsaturated carbonyls. Thus, Ravasio et al. [45,46] selectively reduced the conjugated C-C bond of 1,4-androstadiene-3,17-di- one and other steroids with various supported copper catalysts. Coq et al. [47] found that in the gas-phase hydrogenation of acrolein, the selectivities
TABLE 2 Selectivity Data for Osmium Catalysts
T P Yield Selectivity Catalyst Substrate (K) (MPa) (%) Ref.
os/c o s / c os/c 0 s borided OS/AZO, Os/ZnO 0 s 0s + Fe,O,
Acrolein Crotonaldehy de Cinnamaldehy de Crotonaldehyde Crotonaldehy de Crotonaldehy de Crotonaldehy de Crotonaldehy de
of alumina-supported nickel and cobalt catalysts exhibited the highest selec- tivity to ally1 alcohol. However, secondary products, particularly acetone, were formed, indicating that the reaction scheme was complicated by side reactions which may modify the selectivity pattern.
The selectivities of platinum, ruthenium, and cobalt, which are inter- mediate between those of iridium and osmium on the one hand and palladium, rhodium, and nickel on the other hand, can be greatly modified by the various factors which will be examined in Sections 1I.C through 1I.G.
C. Steric Efsects on Metal Surfaces
This section deals with the effects associated with the mutual steric interactions between the reacting molecules and the structure and morphology of metal surfaces. They include the nature of the crystal planes exposed on the surface and morphological aspects such as shape and size of the metal particles.
Studies of gas-phase hydrogenation of a$-unsaturated aldehydes were undertaken on well-defined single-crystal surfaces in order to establish the specific modes of adsorption and to better understand the mechanism of se- lective hydrogenation. The importance of the crystal face and structure of the molecule on the selectivity were evidenced by Birchem et al. [24,25,48] and by Beccat et al. [49].
The hydrogenation of 3-methylcrotonaldehyde (3-methylbutenal or prenal) conducted on the Pt(ll1) face at 353 K led mainly to the unsaturated alcohol at low conversion (65% selectivity at 20% conversion) [24], whereas on Pt(llO), the main products were the saturated aldehyde and alcohol [25]. This structure sensitivity was accounted for by geometric effects. On the one hand, the close-packed structure of the (111) surface induces a steric hin- drance for the accommodation of the two methyl groups and, thus, for the adsorption of the C-C bond; the molecule is thus activated preferentially via the C=O group. On the other hand, the corrugated structure of the (110) surface removes this steric hindrance and enables the activation of the whole conjugated system of the molecule followed by a 1,4-addition of hydrogen that leads to the formation of the saturated aldehyde via the enol intermediate. Interestingly, as the pressure of prenal increased, both the selectivity to sat- urated and unsaturated alcohol increased; this indicates a progressive change of the chemisorption mode of the molecule. The hydrogenation of prenal conducted on the Pt(553) surface, which presents (111) terraces and steps, gave a lower yield of unsaturated alcohol than the Pt(l11) face (50% and 70%, respectively, at 10% conversion) and a higher yield of saturated alde- hyde (20% and 5%, respectively) [48]. This selectivity change is consistent with the interpretation given in terms of steric constraints, which are less stringent on the stepped surface.
Beccat et al. [49] compared the hydrogenation of crotonaldehyde and
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SELECTIVE HYDROGENATION OF CY,P-UNSATURATED ALDEHYDES 89
methyl crotonaldehyde on the Pt(ll1) single-crystal face; they found that, in the presence of the methyl group, the hydrogenation rate of the C=C bond decreased by an order of magnitude, whereas the hydrogenation of the C-0 bond was almost unchanged. The resulting increase of selectivity was also attributed to steric hindrance of the C=C bond rather than to the enhance- ment of C-0 bond hydrogenation.
Steric effects on flat metal surfaces are well documented in the case of cinnamaldehyde hydrogenation. The selectivity data on platinum and rhodium catalysts given in Table 3 indicates that the selectivity to cinnamyl alcohol improved as particle size increased. Thus, the initial selectivity of platinum on active charcoal increased from 0% to 32% as the particle size increased from 1.3 to 8 nm [50]. A 98% selectivity at SO% conversion was obtained on large, graphite-supported hexagonal Pt particles (3-6 nm) produced by sintering 1.3-nm Pt particles at 1173 K under vacuum [51,52]. The selectivity measured on unsupported Pt-Adams catalyst increased from 73% to 90% as the sponge metal was sintered at 100°C producing large faceted particles as evidenced by transmission electron microscopy (TEM) [S1,52]. This was at- tributed to a steric effect whereby the planar cinnamaldehyde molecule cannot adsorb parallel to a flat metal surface because of the steric repulsion of the aromatic ring. Indeed, theoretical calculations showed that aromatic rings that are chemisorbed on a metal surface must lie at a distance exceeding 0.3 nm because there is an energy barrier preventing a closer approach to the surface [S3]. Because of this energy barrier, the C-C bond cannot approach the surface as closely as the C-0 bond; the latter is then hydrogenated prefer- entially. This steric effect schematized in Fig. 2 does not operate on small particles where both the C=C and the C=O bonds can approach the surface.
TABLE 3 Selectivity in Cinnamyl Alcohol as a Function of Pt Particle Size
Preparation Particle size Catalyst of catalyst (nm) so (%) s25 (%) s50 (%)
Ion exchange Impregnation Ion exchange Sintering of Pt/G(l) Ion exchange Sintering of Rh/G(l) Reduction at 60°C Reduction at 100°C
1.3 8.0 1.3 5.0 2.5 7.0 2.0-6.0
20-200
0 33 32 45 72 78 91 96 12 14 32 36 73 74 90 90
55 60 83 98 18 42 81 93
Note: S o = Initial selectivity to cinnamyl alcohol (3-phenyl propenol); S25 and Ss" = selectivities at 25% and 50% conversion. C and G stand for active carbon and graphite, respectively. Reaction conditions: T = 333 K, P = 4 MPa, 0.1 mol of cinnamaldehyde in 37.5 ml isopropanol and 10 ml HZO, 400 mg of catalyst.
Source: Refs. 50 and 51.
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GALLEZOT AND F U O
h a - 1.5 nrn- / //////.A FIG. 2. Scheme of cinnamaldehyde adsorption on a small metal particle and
on a flat surface. (From Ref. 51.)
Accordingly, the selectivity improvement is due to the lower probability of activation of the C=C bond rather than to an increased activation of the C-0 bond because the rate of C=O bond hydrogenation did not change. Selectivity improvement due to steric effect on a flat surface was observed even with rhodium, even though this metal has a low intrinsic selectivity. Thus, the initial selectivities to cinnamyl alcohol rose from 12% to 32% (Table 3) as the mean particle size increased from 2.5 to 7 nm [51].
Similar selectivity improvements to unsaturated alcohols attributed to particle size or flat particle morphology were reported. Thus, in a study of crotonaldehyde hydrogenation on cobalt catalysts supported on silica, Nitta et al. [54] found a marked particle size effect: The selectivity to crotyl alcohol rose from 25% to 70% as the particle size increased from 2.5 to 18 nm. Particle size effects were also measured for cinnamaldehyde hydrogenation on chrysolite-supported cobalt catalysts [54].
Galvagno et al. [55,56] have prepared increasingly larger ruthenium par- ticles by impregnating an active carbon with increasing amounts of ruthenium chloride followed by hydrogen reduction at 673 K. Table 4 gives the selec- tivity to cinnamyl alcohol as a function of the particle sizes which were measured by three methods. The net effect of the particle size on the selec- tivity was interpreted as discussed previously [Sl]. On the other hand, for the hydrogenation of citral (E- and Z-isomers) into unsaturated alcohols, geraniol (E-isomer) and nerol (2-isomer), Galvagno et al. [56,57] found no effect of
TABLE 4 Initial Selectivity to Cinnamyl Alcohol as a Function of Ru Particle Size
Note: Reaction conditions: T = 333 K, P = 1 bar of H,. Source: Ref. 55.
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SELECTIVE HYDROGENATION OF a,P-UNSATURATED ALDEHYDES 91
particle size using the same series of catalysts. The different influence of particle size on the product selectivity in the hydrogenation of cinnamalde- hyde and citral was interpreted in terms of the difference of steric hindrance of the C=C bond (i.e., only the aromatic ring of cinnamaldehyde produces a significant steric effect).
Coq et al. [58] investigated the gas-phase hydrogenation of acrolein, 2- methylpropenal, 2-butenal, and 3-methylbutenal on alumina-supported ruthe- nium catalysts. They attributed the increase of selectivity to unsaturated al- cohols to the decreased adsorption of the C=C bond as it became more hindered by the larger substituents.
Arai et al. [59,60] found that small platinum particles have a higher selectivity to unsaturated alcohol in crotonaldehyde, methylcrotonaldehyde, and cinnamaldehyde hydrogenation than larger ones. This observation, in con- tradiction to those quoted above, could be due to the particular preparation mode of the small particles involving the reduction with sodium tetraborate of an alumina gel impregnated with Pt(NH,),Cl,. The higher selectivity may be explained by the presence of residual boron acting as Lewis sites to ac- tivate the hydrogenation of the C=O bond according to the mechanism dis- cussed in Section 1I.F.
Patil et al. [61] have carried out a very complete study of the gas-phase hydrogenation of 2-butenal (crotonaldehyde) on unsupported cobalt catalysts prepared by thermal decomposition of two different polynuclear organome- tallic cobalt complexes. The complex designated Co4Co18 consisted of a core of 4 Co atoms surrounded by a layer of 6 cobalt carbonyl cluster carboxylate ligands containing 18 Co atoms. The complex designated Co2Col, consisted of a core of 2 Co atoms surrounded by 4 cobalt carbonyl cluster carboxylate ligands containing 12 Co atoms. These clusters were thermally decomposed at 393 K (LT materials) or 493 K (HT materials). Table 5 gives the selectivity data for gas-phase hydrogenation of 2-butenal carried out at 393 and 423 K. The selectivities are very different particularly if one compares the selectivity
TABLE 5 Gas-Phase Hydrogenation of 2-Butenal on Cobalt Catalysts Issued from
Polynuclear Organometallic Clusters
Yield of Catalyst (K) (%) Butane Butanal 1-Butanol 2-Butenol 2-butenol
Note: Reaction conditions: 15 mg catalyst precursor, P2.bu,cnal = 9.3 Tom (1.2% in H2-
Source: Ref. 61. He flow).
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to the unsaturated alcohol on HT-Co,Co,, (27.9% yield at 82.2% conversion) and HT-Co,Co,, (0% yield). It was assumed that the different microstructures of the cobalt catalysts issued from different precursors account for the dif- ferent activities and selectivities because the adsorption mode of the reactant molecule depends on structural factors.
D. Steric Effects of Supports and Ligands
Mutual steric constraints between metal surfaces and molecules can modify the adsorption geometry and thus the selectivity to unsaturated alcohol (e.g., by hindering the adsorption of the C=C bond), as discussed in Section 1I.C. This section deals with the steric effects produced by the environment of metal particles, such as the spatial constraints in microporous materials or over metal surfaces covered by bulky ligands, which orientate the adsorption of the reactant molecules.
There are three well-documented examples of very selective catalytic systems involving metal clusters in the micropores of zeolite Y [62,63] and beta [64,65]. Gallezot et al. [62] prepared platinum and rhodium catalysts by ion exchange of a NaY zeolite with Pt(NH,)j' and Rh(NH,),C12', respec- tively. Catalysts Pt(l)/Y and Rh/Y were obtained by calcination under oxygen to decompose the amminocations followed by hydrogen reduction. In a sub- sequent study [63], ruthenium catalysts, prepared by ion exchange of NaY and KY zeolites with Ru(NH,);' cations, were treated in the same way. Under these conditions, Pt, Rh, and Ru clusters smaller than 1.2 nm were obtained in the zeolite cages. Catalyst Pt(2)/Y was obtained by reduction under H2, without 0, pretreatment, which resulted in large Pt particles occluded in the
TABLE 6 Selectivities to Cinnamyl Alcohol of Metal Clusters in
Zeolites and on Carbon
Metal Particle size Catalyst (wt%) (nm) sc<,125
PtIC Pt(1)N P t (2) N Rh/C R h N Ru/C RuNaN R u W
3.8 11.0 14.0 2.7 7.0
2.3 2.2
1.3 1.3 5 2.5 1 1 1 1
33 74 97 0
16 30 63 67
Note: Reaction conditions: T = 333 K, P = 4 MPa, 0.1 mol of cinnamaldehyde in 37.5 ml isopropanol and 10 ml H,O, 400 mg of catalyst.
Source: Refs. 62 and 63.
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SELECTIVE HYDROGENATION OF (Y,P-UNSATURATED ALDEHYDES 93
FIG. 3. Scheme showing the end-on approach of a cinnamaldehyde molecule toward a metal cluster encaged in Y zeolite. (From Ref. 62.)
zeolite bulk. The selectivities to cinnamyl alcohol of the zeolite catalysts were compared to those obtained, under the same conditions, on active-carbon- supported samples. Table 6 shows that the metal zeolite catalysts were always more selective. This was attributed to molecular constraints in the zeolite micropores which force the molecule to adsorb on the encaged aggregates via the C=O group, thus hampering the adsorption of the C=C bond as suggested in the scheme given in Fig. 3. The steric effect controlling this end-on adsorption depends on the relative sizes of the micropores and of the metal aggregates. Thus, the best selectivity can be obtained when there is no space left between the metal cluster and the cage walls, as in the catalyst Pt(2)/Y. Molecular constraints are also highly dependent on the relative sizes of pores and molecules; thus, the selectivity to unsaturated alcohol was much higher for cinnamaldehyde hydrogenation than for the less bulky 3-methyl- crotonaldehyde on encaged ruthenium particles in Y zeolite [63].
A similar study on cinnamaldehyde hydrogenation was conducted with a beta-zeolite containing two types of platinum clusters [64]. In the catalyst Pt/beta(l), obtained by direct reduction under H,, the platinum was in the form of 2-5-nm particles filling the cylindrical micropores. An 86% selec- tivity to cinnamyl alcohol was measured from 0% to 90% conversion because the molecule can only adsorb end-on (Fig. 4, left field). In the catalyst Pt/ beta(2), obtained by calcination under flowing 0, followed by H, reduction, platinum clusters were much smaller than pore diameter so they could interact both with the C-C and C-0 bonds (Fig. 4, right field); the C=C bond was then hydrogenated preferentially, thus resulting in a poor initial selectiv-
FIG. 4. Scheme showing a cinnamaldehyde molecule approaching metal clusters. Left field End-on approach of the molecule on platinum filling the pore [Ptl beta(1) catalyst]; right field: lateral interaction of the molecule with a Pt cluster smaller than the pore [Pt/beta(2) catalyst]. (From Ref. 64.)
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ity (5%). However, the selectivity improved steadily up to 55% as conversion proceeded, because the Pt clusters sintered into larger particles on which the molecules adsorbed end-on, thus favoring C=O hydrogenation. Accordingly, after recycling this catalyst in a second reaction run, the initial selectivity was 55%.
In a similar study, Tas et al. [65] studied geranial (3,7-dimethylocta-2,6- dien-1-al) hydrogenation on platinum particles that filled the pores of beta- zeolite. Geranial was hydrogenated with a high selectivity (97% at 22% conver- sion) to geraniol (3,7-dirnethylocta-2,6-dien-l-o1). These authors interpreted the data, as previously [62,64], in terms of end-on adsorption of the aldehyde molecule on the metal clusters.
In the three examples quoted above, a high selectivity was obtained because the size of the zeolite pores, of the metal particles, and of the mol- ecules were commensurate, thus forming a supramolecular catalytic system. Molecular constraints in zeolitic pores are helpful to enhance the selectivity, but they severely limit the diffusion of reactants and products, resulting in low activities. Thus, the reaction rates measured on zeolite catalysts were much smaller than on other supports even though faujasite and beta-zeolites have a three-dimensional wide-pore system, which is the most favorable to molecular circulation.
Supramolecular catalytic systems, where molecular constraints imposed by the metal environment would impair molecular diffusivity to a lesser de- gree, might consist of metal surfaces covered with bulky ligands, which im- pose steric constraints on the incoming reactant molecule. Silica-supported rhodium-tin catalysts prepared by surface organometallic reaction of Sn(n- C,H,), on rhodium particles followed by partial hydrogenolysis could be an example of such a catalyst [66-681. Table 7 gives the catalytic activities and selectivities in the hydrogenation of citral on RhSn(C,H,)JSiO, catalysts with
TABLE 7 Catalytic Activities and Selectivities of Rhodium-Organometallic Tin Catalysts
Note: Reaction conditions: 0.9 ml citral, 0.4 ml tetradecane, 10 ml heptane, 2.5 X lo-'
"Selectivities at 100% conversion. bCatalyst treated at 630 K under hydrogen. Source: Ref. 66.
mol Rh, 340 K, 7.6 MPa.
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SELECTIVE HYDROGENATION OF CN,P-UNSATURATED ALDEHYDES 95
different Sn/Rh ratios [66]. The selectivity to unsaturated alcohols (geraniol + nerol) increased up to a maximum of 96%, as more tetrabutyl tin reacted with the rhodium surface. The total removal of ligands obtained by treating under hydrogen at 630 K resulted in a dramatic decrease of the selectivity. The 96% selectivity measured for Sn/Rh = 0.92 is surprisingly high for rho- dium, which is known to be poorly selective to unsaturated alcohol (see Section 1I.B). In their first report [66], Didillon et al. interpreted the selectivity in terms of tin atoms on the rhodium surface acting as electrophile species activating the C=O bond, which is the usual interpretation given to account for the selectivity enhancement with bimetallic catalysts (see Section 1I.E). However, this would not explain why, after the thermal removal of organic ligands, the selectivity to unsaturated alcohol dropped to 6%. Indeed, in sub- sequent articles [67,68], where the surface modification was also achieved with tetrabutyl germanium or lead, the high selectivity to unsaturated alcohol was rather interpreted in terms of molecular constraints: the presence of Sn(C,H,),, or Ge(C4H& ligands on the rhodium surface favoring the ap- proach of the incoming molecule via its carbonyl group, whereas the adsorp- tion of the C=C bonds on the surface was hindered. The activity of the more selective catalyst was only reduced by a factor of 1.6 with respect to the ligand-free catalyst (Table 7), which indicates that activity loss experienced in the presence of bulky surface ligands is much less than that in zeolite micropores.
E. Electronic Effect of Supports and Ligands
The probability to hydrogenate either the C=C or the C=O bond of a$-unsaturated aldehydes, which governs the final selectivity to unsaturated alcohol, depends critically on the mode of adsorption of the molecule and, ultimately, on the electronic structure of the metal surface as shown in Sec- tions 1I.A and 1I.B. The local structure of metals can be modified by electron- donating species interacting with the surface atoms (electronic ligand effect). From the theoretical studies mentioned in Section 1I.A [31,32], it can be forecast that a higher charge density on metal surface atoms could, on the one hand, decrease the binding energy of the C-C bond via an increase of the repulsive four-electron interaction and, on the other hand, favor the back- bonding interaction with the n&,-orbital and the hydrogenation of the C-0 bond with respect to that of C-C. Thus, electron-donating species interacting with metal particles should improve the selectivity to unsaturated alcohol as shown in the following experimental investigations.
1. Effect of Basic Molecules
Cordier et al. [ 131 reported data on cinnamaldehyde hydrogenation [69] show- ing that the selectivity of Pt catalysts to unsaturated alcohol was improved in the presence of phosphines and arsines (Table 8). Although no interpretation
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TABLE 8 Effect of Bransted Bases on the Selectivity of Platinum Catalysts
Base added: None P(C,H,), OP(C,H,), P(OC,H,), P(OEt), As(C,H,),
Sunsaturated alcohol (%)I 50 94 52 96 93 88
Source: Ref. 69, as reported in Ref. 13.
was given, one may assume that these bases interact with the metal surface via their lone electron pair which may disfavor the hydrogenation of the C-C bond as discussed above. The beneficial role of amine addition [70] can be accounted for in the same way. However, if these surface ligands are strongly bonded to the metal surface, one cannot exclude a steric effect fa- voring the end-on adsorption of the molecule as discussed in Section 1I.D.
A beneficial effect of potassium hydroxide added to the reaction medium on the selectivity of platinum catalysts was reported in the patent literature [71,72]. Satagopan et al. [73] found that the addition of KOH to the reaction medium decreased the hydrogenation rate on Pd catalysts of the C-C bond of cinnamaldehyde, a-amyl cinnamaldehyde, and citral; the same treatment with Pt/C catalysts increased the selectivity to a-amyl cinnamyl alcohol from 0% to 92%. In a subsequent article [74], these authors found that the selec- tivity of Pt/C catalysts to cinnamyl alcohol was greatly improved by the addition of KOH or NaOH (Table 9). The improvement of selectivity to unsaturated alcohol is probably due to the electron-donor effect of the alkali hydroxides. However, the rate of reaction increased in the presence of alkali hydroxides, which means that they have a positive effect on the rate of C-0 bond hydrogenation. The enhanced activation of the C-0 bond could be interpreted by the polarization of the C=O bond resulting from the in-
TABLE 9 Cinnamaldehyde Hydrogenation on Pt/C Catalysts; Effect of Alkali Hydroxides
Alkali Conc. Time Conversion" S",," SC<,lh S",.,:. used (%I (min) (%I (%) (%) (%I
"Conversion. 'Initial selectivities to hydrocinnamylaldehyde, cinnamyl alcohol, and hydrocinnamyl
Source: Ref. 74. alcohol, respectively.
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SELECTIVE HYDROGENATION OF CX,P-UNSATURATED ALDEHYDES 97
teraction of the alkali cation acting as a Lewis site with the lone electron pair of the oxygen atom of the C-0 group. This mechanism of C=O bond activation accounts for the promoting effect of electropositive species and will be discussed in detail in Section 1I.G.
A positive effect of sodium hydroxide was also reported for the hydro- genation of cinnamaldehyde on polymer-protected Pt-Co colloids [75]. The selectivity increased from 87.6% at 69.2% conversion to 99.5% at 78.2% conversion as 1 mg of NaOH was added to 60 ml of the ethanol solution containing 15.2 mmol of cinnamaldehyde; retardation of the olefinic hydro- genation was invoked.
2. Effect of Electron-Donating Supports
Electronic ligand effects can be produced by supports acting as a macro- ligand interacting with the metal particles. Thus, one may expect that elec- tron-donating supports produce the same effect as basic molecules, provided metal particles are very small with a high proportion of atoms interacting with the support.
Indeed, Richard et al. [38,50] interpreted the higher selectivity to un- saturated alcohol of platinum-group metals supported on graphite compared to active carbon in terms of an electronic ligand effect. Platinum catalysts, Pt/G, were prepared by ion exchange with Pt(NH,);+ ions of a Lonza high- surface-area graphite previously treated with NaOCl solutions to create car- boxylic groups. After H2 reduction, Pt particles in the size range 1-1.5 nm were detected by TEM along the graphite steps. It was shown that their lattice was expanded as a result of an electron transfer from graphite to the anti- bonding orbitals of the metal clusters [50,76]. Also, by measuring the ratio KTIB of the adsorption coefficients of toluene and benzene using toluene- benzene competitive hydrogenation [77], it was shown [50] that the KT,B were 5.5 and 8 on Pt/G and Pt/C, respectively, which indicates that the electron density was higher on the graphite-supported catalyst. Although this electron transfer could be due partly to the equalization of the Fermi levels of graphite and platinum, the oxygenated functional groups at the extremities of graphite basal planes, where the Pt clusters are anchored, could also act as electron- donating ligands [78]. The comparison of the selectivities of Pt/G and Pt/C catalysts, prepared in the same way with the same particle sizes and with similar metal content, is given in Fig. 5 as a function of conversion and in Table 1 at 0% and 25% conversion. It clearly appears that the initial selec- tivity to cinnamyl alcohol (COL) is very weak on Pt/C catalyst, whereas it is as high as 72% on Pt/G. The higher selectivity of Pt/G catalyst was attrib- uted to the higher electron density on the metal particles, which decreases the probability of C-C bond hydrogenation. Similar enhancement of selec- tivity to unsaturated alcohol attributed to electronic effect of graphite support were observed for ruthenium and rhodium clusters grafted at the extremities of graphite basal planes (Table 1) [38]. The higher selectivity of the graphite-
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98 GALLEZOT AND RICHARD
60
40
20
(a) 0 20 40 CONV.(%) (b) 0 20 40 CONV.(%)
FIG. 5. Product distribution of cinnamaldehyde hydrogenation as a function of conversion on platinum catalyst: (a) graphite support; (b) active charcoal supports. (From Ref. 38.)
supported ruthenium catalysts, Ru/G, compared to a similar catalyst supported on active charcoal (39% and 5%, respectively at 25% conversion, Table 1) and even the significant increase from 0% on Rh/C to 7% on Rh/G at 25% conversion were also interpreted as resulting from the decrease of the C=C bond hydrogenation on the Ru or Rh clusters. This interpretation was also supported by the fact that the increase of selectivities measured on platinum, ruthenium, and rhodium on graphite support were not accompanied with an increase of catalysts activity (Table 1) such as those observed in bimetallic systems (see Section II.G), where the selectivity improvement is mainly due to a large increase of the rate of C=O hydrogenation.
In another investigation [79], a graphite-supported platinum catalyst, F'tl G,,,, was prepared by decomposition of the zero-valent platinum complex Pt(DBA), (DBA = dibenzylidene acetone) in CH,Cl, solution with the graph- ite powder in suspension. It was shown that the Pto atoms form colloidal particles which deposited under the form of flat, epitaxially oriented particles on the graphite basal planes [76]. Table 10 indicates that KTIB and selectivity of Pt/G,,,, are intermediate between those of Pt/C catalyst and Pt/G,, catalyst prepared by ion exchange. This was attributed to the lower electron donation from graphite to metal when the particles are on the basal planes rather than at the extremities of the basal planes, either because electron conduction is lower in the direction perpendicular to the graphite plane or because oxygen- ated functional groups are present on graphite edges only.
A remarkable effect of novel carbon supports on the selectivity of ru- thenium catalysts was reported by Planeix et al. [SO]. Multilayer graphitic nanotubes prepared by discharge evaporation of graphite in helium were im- pregnated by ruthenium 2,5-pentadionate in toluene, dried, treated in nitrogen at 523 K and reduced in hydrogen. The metal was in the form of 3-7-nm particles on the external surface of the nanotubes. This ruthenium catalyst exhibited a very high selectivity to cinnamyl alcohol (92% up to SO% con- version). This high selectivity could be the result of both a steric effect on
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SELECTIVE HYDROGENATION OF CY,P-UNSATURATED ALDEHYDES 99
TABLE 10 Initial Selectivities in Cinnamaldehyde Hydrogenation and
Electronic Properties of Platinum
Catalyst Preparation KTma Scolb SHCALC s,,,," PtIC,, Ion exchange of active carbon 8.0 0 95 5 Pt/GC,I Deposition of colloids on graphite 7.2 54 44 2 WG, , Ion exchange of graphite 5.5 72 28 0
"Ratio of the adsorption coefficient of toluene and benzene. bCinnamyl alcohol. 'Hydrocinnamaldehyde. d3-Phenyl propanol. Source: Ref. 79.
large particles, such as that described for Pt/graphite catalysts (see Section II.C), or due to a support-induced electronic effect discussed in this section.
The importance of oxygenated functional groups on carbon supports was illustrated by Coloma et al. [81] in a study of the gas-phase crotonaldehyde hydrogenation on platinum catalysts containing different amounts of oxygen- ated surface groups. The catalyst Pt/AC was prepared by impregnation with Pt(NH,),Cl, of an activated carbon and hydrogen reduction. AC,, was ob- tained by oxidizing AC with 12 N Hz02, and ACoxs by treating ACox at 500°C under N2 to remove the less stable oxygen surface groups. The initial selec- tivities to crotyl alcohol were 5%, 26%, and 33% for Pt/AC, Pt/ACoXs, and Pt/AC,,, respectively. Therefore, the larger the amount of oxygenated surface groups on carbon, the higher the selectivity to unsaturated alcohol. Although no interpretation was given, it is likely that the oxygenated functional groups of the support act as electron-donor sites modifying the electronic properties of platinum in a way similar to that described above for graphite.
The structure of metal clusters encaged in zeolites, where most of the atoms are interacting with the anionic surface of the cage walls, is very sensitive to any change in the acid-base properties of the zeolite. The selec- tivity to unsaturated alcohol of ruthenium and platinum clusters encaged in Y-type zeolite was enhanced as the basicity of the zeolite was increased by changing the charge compensating cations [63,82]. Thus, in the hydrogenation of methylcrotonaldehyde (3-methylbutenal), the selectivities to unsaturated alcohol at 25% conversion were 10% and 35% on RuNa-Y and RuK-Y ze- olite, respectively, and 42% and 56% on PtNaY and PtKY [63]. The higher selectivity could be attributed to the lower activity for C-C bond hydro- genation due to a higher charge density on the metal, which is a consequence of the interaction with the more basic support. However, a possible activation of the C-0 bond by Kf cations acting as Lewis sites, which would also account for the selectivity improvement, was also considered.
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F. Effect of Reaction Products and Poisons
The selectivity to unsaturated alcohol decreases with conversion because of the hydrogenation of the C-C bond, which usually occurs above 80- 90% conversion in liquid-phase hydrogenation but at much lower conversions in gas-phase reactions (see, e.g., Ref. 61). In contrast, an increase of selec- tivity to unsaturated alcohol was reported in the case of cinnamaldehyde hydrogenation on Pt/C [38,50,83] and Ru/C [38,55] catalysts as the conver- sion was in the range 10-30%. Thus, the selectivity to cinnamyl alcohol of Pt/C catalyst increased from 0% initially, to 33% at 25% conversions (Table 1 and Fig. 5). As the main product formed at low conversion was hydrocin- namaldehyde, it was suggested [50] that part of these molecules remain ad- sorbed on the metal surface and modify the selectivity of the reaction. The improvement of selectivity to unsaturated alcohol could be due either to a steric ligand effect, whereby the adsorbed hydrocinnamaldehyde molecules sterically hinder a flat adsorption of the cinnamaldehyde molecules and force its adsorption via the C-0 group, or to an electronic ligand effect via an electron donation to platinum atoms which would lower the probability of C=C bond hydrogenation. This interpretation was corroborated by experi- ments showing that injection of hydrocinnamaldehyde at the beginning of the reaction decreased the reaction rate and increased the selectivity to unsatu- rated alcohol [84].
Galvagno et al. [55] suggested that the selectivity rise at low conversion in the case of the Ru/C catalyst could be due to the adsorption on the metal surface of carbon monoxide formed in small amounts by decarbonylation at the beginning of the reaction. This interpretation was based on earlier work by Vanderspurt [85] showing that the selectivity to unsaturated alcohol on rhenium catalysts increased upon exposure to a stream of hydrogen containing 1% carbon monoxide, which was attributed either to a poisoning of the sites responsible for C-0 hydrogenation or to an electronic modification of the metal sites induced by chemisorbed CO.
The effect of the presence of sulfur on the activity and selectivity of a Cu/Al,O, catalyst in the gas-phase hydrogenation of crotonaldehyde has been studied by Hutchings et al. [86]. In the absence of sulfur, 1-butanol was produced preferentially, whereas in the presence of suitable amounts of tio- phene, the reaction rate decreased and the product distribution shifted toward the formation of crotyl alcohol with a selectivity higher than 50% at low conversion. It was suggested that thiophene adsorption poisons the hydro- genation of C=C bonds more strongly than C=O bonds.
G. Effect of a Second Metal
Most of the reports on selective hydrogenation of a,@-unsaturated al- dehydes were devoted to catalytic systems where the activity and selectivity
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SELECTIVE HYDROGENATION OF (u,P-UNSATURATED ALDEHYDES 101
of platinum group metals were modified with a more electropositive metal. The nature of the association between the two metals and the valence state of the second metal are very diverse and often hardly known, because catalyst characterization was not properly achieved. We have tentatively classified the catalytic systems into three groups:
Catalysts where metallic promotors are added in ionic form to the liquid phase containing a platinum-group metal catalyst in suspen- sion. This process eventually results in the formation of bimetallic particles because under hydrogenation conditions metal cations can be reduced by hydrogen dissociated on the surface of metal particles. Catalysts involving bimetallic particles, usually prepared ex situ, where electropositive metal atoms are associated in the same particle with metal atoms of higher redox potential (usually platinum-group metals). Catalysts involving oxidized metal species at the interface between platinum-group metal particles and supports or migrating from oxide supports to the metal surface upon high-temperature reduction (SMSI-strong metal-support interaction state).
In any of the three catalytic systems considered, metal atoms of high redox potential A are promoted with electropositive metal atoms or oxidized metal species B. Two mechanisms could account for the promoting effect:
The electropositive metal B acts as an electron-donor ligand that increases the electron density on metal A, thus decreasing the bind- ing energies, particularly that of the C-C bond, and favors the hydrogenation of the C-0 with respect to the C-C bond. This effect has been studied in detail theoretically by Delbecq and Sautet [32] on model surface Pt80Fe20(lll), where the Fe atoms were lo- cated in the second layer. Calculations showed that the electropos- itive metal promotors acted as an electron-donating ligand increas- ing the electron density on the surface platinum atoms. The electron transfer from iron to platinum was also evidenced by x-ray absorp- tion edge spectroscopy [87]. This electronic ligand effect caused by a second atom is similar to that of electron-donating molecules or supports discussed in Section 1I.E. The electropositive metals, or oxidized metal species, on the surface of A act as electrophilic or Lewis sites for the adsorption and acti- vation of the C-0 bond via the lone electron pair of the oxygen atom as schematically described in Fig. 6 . Mechanism 2, which will be referred to as “electrophilic C-0 activation,’’ was the most frequently invoked to account for the promoting effect of electro- positive species.
Note that mechanisms 1 and 2 should favor selectivity enhancement-
1.
2.
3.
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2.
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102 GALLEZOT AND RICHARD
FIG. 6. Scheme of C=O bond activation by electropositive iron atoms on platinum surface. (From Ref. 88.)
the first by decreasing the activation of the C=C bond and the second in- creasing the activation of the C=O bond. However, the effect of mechanism 2 is predominant because large increases of the rate of C=O bond hydro- genation were observed in almost all the metal-promoted catalytic systems.
1. Metal Ions Additives
Metal ions additives were employed more than 75 years ago by Adams and co-workers [7-91 to improve the selectivity of platinum catalysts to un- saturated alcohols. Thus, Tuley and Adams [9] showed that both the rate of hydrogenation and the selectivity to cinnamyl alcohol was increased on plat- inum blacks or platinum oxide catalysts in the presence of iron chloride and zinc acetate. Adams and Garvey [S] found that these additives resulted in high yields of geraniol from citral. Later, Rylander et al. [89] reported that carbon-supported platinum catalysts gave high yields of unsaturated alcohols in the presence of iron chloride and zinc acetate, and that these additives had no beneficial effect on alumina-supported catalysts.
The effect of iron chloride additive on platinum catalysts can be ratio- nalized from the findings of Richard et al. [88]. Various amounts of FeCl, aqueous solutions were added to a slurry of a Pt/C catalyst in a solution of cinnamaldehyde in isopropanol at 100°C under 6 MPa of H,. Figure 7 shows
SELECTIVE HYDROGENATION OF a,P-UNSATURATED ALDEHYDES 103
that both the initial rates and the initial selectivities were improved with volcano-type behavior. After reaction, catalysts grains were cut into thin sec- tions with a ultramicrotome and analyzed by energy-dispersive x-ray spec- trometry (EDX) with a 1.5-nm spatial resolution in a field-emission-gun scanning transmission electron microscope (FEG-STEM). The individual par- ticles were bimetallic with an average composition slightly smaller than the overall sample composition. Therefore, almost all the Fez+ ions were reduced and iron adatoms deposited on the surface of the platinum particles. Because iron is much more electropositive than platinum, there is an electron transfer to platinum atoms and the iron atoms are electron deficient. These low-valent species on the surface can act as Lewis adsorption sites and activate the C=O bond according to the scheme given in Fig. 6. This mechanism ac- counts for the volcano-type activity curve of Fig. 7; there is an optimum of composition for FePt = 0.2, then further addition of iron inhibits hydrogen adsorption on the underneath platinum atoms. Note that the hydrogenation of the C=O bond could also be favored by the electron transfer from electro- positive iron to platinum that increases the hydridic character of chemisorbed hydrogen and thus the nucleophilic attack by H- species on the positively charged carbon of the polarized C-0 bond. The absence of a positive effect of iron chloride on the selectivity of alumina-supported catalysts [89], in contrast to a carbon-supported one (vide supra), could be due to the fact that iron would tend to be associated more with the oxide support than with platinum, so that mechanisms 1 and 2 could not occur.
The effects of various metal additives were studied by Galvagno et al. [90]. Table 11 gives the rate of cinnamaldehyde hydrogenation and the se- lectivity to cinnamyl alcohol before and after the addition of cobalt, iron, tin, and germanium chlorides to a slurry of Pthylon catalyst in ethanol solution of cinnamaldehyde at 343 K under hydrogen. The large increase of rates and selectivities was attributed to a polarization of the C-0 bond by the cations acting as Lewis sites.
Schroder and de Verdier [91] reported a positive effect of FeCl, additives
TABLE 11 Effect of Metal Chlorides on Cinnamaldehyde
to Pt/Al,O, catalysts on the selectivity to unsaturated alcohol in liquid-phase hydrogenation of cinnamaldehyde and 2-ethylhexenal. The addition of a small amount of oxygen to the feed gas decreased the selectivity but increased the activity; the effect of 0, was attributed to the decrease of the poisoning of platinum particles by CO issued from the decarbonylation of the aldehyde.
Hotta and Kubomatsu [92,93] hydrogenated 2-methylpentenal over Ra- ney cobalt in the presence of different additives. Upon addition of MnCl,, the selectivity toward the unsaturated alcohol increased. A similar behavior was observed upon addition of acetic acid [94]. The interaction between these Lewis or Bransted acidic modifiers of the metal surface and the adsorbed substrate was invoked to account for selectivity improvements.
2. Bimetallic Particles
In almost all the investigations carried out on bimetallic catalysts, the selectivity to unsaturated alcohol was improved when metal B (the promoter) was more electropositive than metal A . Thus, most of the systems involve a platinum group metal associated with a metal of the first transition row (often Fe or Co) or p-electron metals (often Ge or Sn). However, bimetallic parti- cles involving two platinum-group metals of different electronegativies (e.g., Pt-Ru) or two first row transition metals (e.g., Ni-Cr) have been employed.
Platinum-Based Catalysts. Beccat et al. [49] investigated the gas- phase hydrogenation of crotonaldehyde and methylcrotonaldehyde on the Pt,,Fe,,(lll) single-crystal face. From a quantitative LEED (low-energy elec- tron diffraction) analysis, it was concluded that the outer layer of the surface was covered only by Pt atoms; therefore, the iron atoms underneath were not accessible to reactant molecules. Table 12 gives the selectivity and activity data compared to those on the Pt(ll1) crystal face. Alloying platinum with iron produced an increase of the rate of formation of the unsaturated alcohols
bkl/mol. 'Selectivity defined as the ratio of the rate of unsaturated alcohol formation over the
overall reaction rate. Source: Ref. 49.
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SELECTIVE HYDROGENATION OF a,P-UNSATURATED ALDEHYDES 105
but to a much smaller extent than that measured on a supported Pt,,Fe,& catalyst [95] (vide infra). Indeed, because iron atoms are underneath the metal surface, the mechanism of C-0 activation by electropositive iron acting as Lewis sites cannot operate. The selectivity improvement can be interpreted from the calculation of Delbecq and Sautet [32] on a model PtFe(ll1) surface with no iron atoms on the surface that showed the di-a,=, adsorption mode is predominant with respect to di-u,--,.
Goupil et al. [95] have studied the cinnamaldehyde hydrogenation on Pt-Fe/C catalysts prepared by coimpregnation of an active charcoal with Fe(NO,), and H,PtCl, followed by H2 reduction at 673 K. Figure 8 shows that the rate of hydrogenation was higher by two orders of magnitude for an optimum ratio FePt = 0.2 that also corresponded to a maximum selectivity of 85% in cinnamyl alcohol. The voicano curve is similar to that obtained by adding iron salt to the reaction medium (Fig. 7, [SS]), but the rate enhance- ment was much larger and could be due to a more homogeneous surface composition of the catalyst prepared ex situ compared to that prepared by the redox process in the reaction medium.
Fouilloux [83] has also studied the hydrogenation of cinnamaldehyde on bimetallic Pt-Co catalysts prepared by impregnation of an active carbon with Co(NO,), and H,PtCl, in a mixture of benzene and ethanol. The im- prenated precursor was heated under nitrogen at 393 K and reduced under hydrogen at 683 K; it was checked that Pt-Co bimetallic particles were formed. The selectivity to cinnamyl alcohol at 70% conversion increased from 62% for pure platinum, to 90% for catalysts containing 20-60% of cobalt atoms, and the activity was maximum for a catalyst containing 20% cobalt atoms. The C=O activation by electropositive cobalt atoms was invoked to interpret the selectivity and activity enhancement.
Hydrogenation of cinnamaldehyde was carried out on Pt-Co colloidal particles protected by polyvinyl pyrrolidone (PVP) [75]. Table 13 gives the reaction data 'as a function of catalyst composition. The association of cobalt with platinum to form bimetallic particles homogeneous in size and compo- sition, particularly with a ratio Pt/Co = 3, dramatically increased the selec-
0 20 40 60
FIG. 8. Effect of iron content on the rate of cinnamaldehyde hydrogenation. (From Ref. 95.)
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106 GALLEZOT AND RICHARD
TABLE 13 Cinnamaldehyde Hydrogenation on PVP-Protected Pt and Pt-Co Catalysts
Conversion Selectivity Rate Catalyst (W ("/.I (mol/mol~,,,,,, h)
Note: Reaction conditions: EtOH: 50 ml; HZO: 10 ml; tetradecanol: 2.0 g; cinnamal- dehyde: 2.0 g; NaOH: 1 mg; metal (Co or Pt or Co + Pt): 2.57 X lo-' mmol; HZ pressure: 4.0 MPa; temperature: 333 K; time 5 h.
"Precipitate of unprotected Co colloids. Source: Ref. 75.
tivity, whereas a physical mixture of cobalt and Pt/PVP catalysts did not result in any selectivity improvement.
Bimetallic Pt-Ru/C catalysts, prepared by coimpregnation using acetone solution of H,PtCl, and RuCl,, were studied in cinnamaldehyde hydrogena- tion 1831. Two selectivity maxima were observed at Pt:Ru = 70:30 and for Ru in the range 50-80 at%. Giroir-Fendler et al. 1961 have studied the same reaction on Pt-Ru bimetallic catalysts prepared by coexchange of active char- coal or graphite supports with platinum and ruthenium amminocations. On the active carbon catalysts, two maxima of selectivity at 35 and 75 at% of ruthenium were also observed. The first maximum, which also corresponds to a rate maximum, has been interpreted by an electrophilic activation of the C=O by electropositive ruthenium atoms associated with platinum. It was verified by x-ray absorption spectroscopy at the Pt(L,,,) edge 1971 that there is an electron transfer from ruthenium to platinum.
Poltarzewski et al. [98] have studied the hydrogenation of cinnamalde- hyde and acrolein on bimetallic Pt-Sn catalysts supported on Nylon 66 pow- der ground at liquid-nitrogen temperature to obtain a surface area of - 1 m2/g. The powder was impregnated with an ethanol solution of H2PtC1, and SnCl,, dried, and reduced under H2 at 343 K. The selectivity to unsaturated alcohol increased from 0% to 75% in the case of cinnamaldehyde and from 30% to 60% in the case of acrolein as 15 at% of tin was loaded. This was attributed to effects 1 and 2 discussed at the beginning of Section II.G, namely the decrease of the C=C bond activation due to an electron-donating effect from tin to platinum, and the increase of C=O bond activation by electro- philic tin atoms. A maximum in the rate of hydrocinnamaldehyde hydroge- nation was also observed with Pt-Sn bimetallic catalysts containing 15% Sn atoms [99].
The promoting effect of tin on platinum supported on silica was studied
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SELECTIVE HYDROGENATION OF CX~P-UNSATURATED ALDEHYDES 107
by Marinelli et al. [loo] in the gas-phase hydrogenation of furfural. The experiments under isothermic conditions (170°C) resulted in selectivity higher than 80% in a large domain of tin concentration compared to less than 15% on pure platinum. The gas-phase hydrogenation of acrolein, crotonaldehyde, and methylcrotonaldehyde was also studied on Pt-Sn catalysts [23,101]. The increase of selectivity to unsaturated alcohol was also attributed to oxidized species activating the C=O bond, but it was suggested that these species are SnO, rather than low-valent tin.
Neri et al. [lo21 have studied the hydrogenation of citral on a Pt-Sn catalyst prepared by coimpregnating an activated carbon with aqueous solu- tion of H,PtCl, and SnC1, and reducing under H2 at 573 K. As the percentage of tin increased from 0% to 20% in the bimetallic catalysts, the activity passed through a maximum corresponding to a fivefold increase of activity. The selectivity to unsaturated alcohols (geraniol and nerol, the E and 2 forms of 3,7-dimethyl-2,6-octadieno17 respectively) increased from 65% to 90%. Elec- trophilic activation of the C-0 bond by cationic tin on the platinum surface was invoked.
Crotonaldehyde hydrogenation in the gas phase was studied by Coloma et al. El031 over bimetallic Pt-Sn catalysts supported on pregraphitized car- bon black. The catalysts were prepared by impregnation with aqueous solu- tions of H2PtC1,, then, after drying at 393 K, with acidic solution of SnCl,. The reduction was carried out by H, at 623 K. A detailed XPS study showed that both metallic and oxidized tin species were present. Both the activity and the selectivity to crotyl alcohol were improved by the presence of tin. The results were explained on the basis of a promoting effect of oxidized tin species on C-0 hydrogenation, and it was also suggested that the formation of Pt-Sn alloy may hinder the hydrogenation of the olefinic bond. In a sub- sequent study on the effect of the preparation mode of carbon-black-supported Pt-Sn catalysts [104], the selectivity to unsaturated alcohol was related to the amount of oxidized tin species.
Galvagno et al. [lo51 studied the effect of germanium promoter on the selectivity of platinum. Catalysts were prepared by coimpregnation of a ground nylon powder with H,PtCl, and GeC1, in ethanol solution followed by reduction with H, at room temperature. A 10-fold increase of the reaction rate of cinnamaldehyde hydrogenation and a jump of selectivity to cinnamyl alcohol from 2% to 64% were measured as only 3 at% germanium were added to platinum. The larger effect of germanium compared to tin is prob- ably due to a larger amount of oxidized species, particularly given the mild condition of reduction.
The gas-phase hydrogenation of crotonaldehyde over a series of silica- supported Pt-Ni catalysts was investigated by Raab and Lercher [106]. Al- though the rate of hydrogenation to butyraldehyde was a monotonic function of the nickel concentration, the rate to crotyl alcohol attained a maximum at -50 at% of nickel in the Pt-Ni alloy, which also corresponded to a maxi-
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108 GALLEZOT AND RICHARD
mum of selectivity. The enhancement in C-0 hydrogenation was attributed to nickel atoms present on the surface with a slightly positive charge.
Galvagno et al. [56,107- 1101 conducted a number of investigations on the hydrogenation of unsatu- rated a,P-unsaturated aldehydes on ruthenium catalysts promoted with tin. The selectivity to unsaturated alcohol was improved when ruthenium was associated with tin. In the case of cinnamaldehyde [108], catalysts were pre- pared by impregnation of an active carbon with aqueous solutions of RuC1, and SnCl, and reduction at 573 K under hydrogen. The turnover frequency calculated from CO titration of surface ruthenium and the selectivity to cin- namyl alcohol increased continuously up to 30 at% Sn and the selectivity was 90% up to 90% conversion. In the hydrogenation of citral [109], the selectivity to geraniol and nerol at 30% conversion increased from 35% on a Ru catalyst to 80% on Ru-Sn. In all these investigations, the selectivity improvement was attributed to oxidized tin on the surface of ruthenium acting as electrophilic species activating the C-0 bond.
Coq et al. [58] studied the gas-phase hydrogenation of acrolein, 2-meth- ylpropenal, 2-butenal, and 3-methylbutenal on alumina-supported bimetallic ruthenium catalysts promoted with various metals (Sn, Fe, Zn, Ge, Sb). The monometallic ruthenium catalyst was prepared by adsorption of acetylaceto- nate of ruthenium on the carrier in benzene solutions. After drying under nitrogen, the catalyst was reduced under hydrogen at 573 K. The bimetallic catalysts were prepared by controlled reaction under hydrogen of organo- metallic complexes such as Sn(C4H9)4 in n-heptane solution with the surface of Ru/Al,O, catalysts or by impregnation with acetylacetonate complexes such as Fe(acac),. The catalysts precursors were then reduced under hydrogen at 573 K. The metal promoters increased the rate of hydrogenation of acro- lein; however, except for a modest improvement of selectivity to ally1 alcohol induced by tin, all the other metals decreased the selectivity. The main effect of the metal promoters was to suppress the formation of acetone.
Rh-Sn bimetallic catalysts prepared by surface reaction of SnBu, on Rh/SiO, were very selective in citral hydrogenation to geraniol and nerol (96% selectivity at 100% conversion), as shown by Didillon et al. [66-681. In the first article [66], the promoting effect of tin on rhodium was attributed to the polarization of the C=O bond by low-valent tin acting as a Lewis site, but in subsequent articles [67,68], the steric effect of the organometallic ligands was invoked to account for the very high selectivity obtained on a nonselective metal. This is why these results were discussed in Section 1I.D.
As already mentioned in Section 1I.B deal- ing with metal specificities, metals of the first transition row are intrinsically weakly selective to unsaturated alcohols; however, as in the case of platinum- group metals, their selectivity can be improved by the presence of a more electropositive metal acting as a Lewis adsorption site for the adsorption and activation of the C=O bond.
The hydrogenation of citral on Raney nickel yielded mainly citronellol
Ruthenium and Rhodium-Based Catalysts.
Non Noble-Metal Catalysts.
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SELECTIVE HYDROGENATION OF CU,P-UNSATURATED ALDEHYDES 109
and some citronella1 but no geraniol or nerol [lll]. Using chromium-pro- moted Raney nickel in the presence of a strong base, Gradeff and Formica [112] achieved the preferential reduction of the carbonyl group of some y,6- unsaturated ketones. Court et al. [113,114], who studied citral hydrogenation on Raney nickel, found that with chromium or molybdenum promoters, small amounts of geraniol and nerol were formed.
Noller and Lin [20] studied the gas-phase hydrogenation of crotonal- dehyde on Ni-Cu/Al,O, catalysts prepared by coprecipitation of the Ni- and Cu-nitrates in ammonia solutions, calcination at 3OO0C, and hydrogen reduc- tion at 200°C or 430°C. The best selectivity to crotyl alcohol (54%) was obtained for the Cu0.33Ni0 66 composition. The effect of copper was interpreted in terms of the different strength of electron-pair donor (EPD) and electron- pair acceptor (EPA) of hydrogen bonded on nickel and on copper. Protonic hydrogen tends to polarize the C=O group, whereas hydridic hydrogen as- sociated with copper attacks the carbon atom of the C-0 group.
In the liquid-phase hydrogenation of crotonaldehyde on Ni-Cu/SiO, catalysts prepared by impregnation with ammino salts, Antunes Pereira da Silva and Dalmon [ 1151 observed a continuous increase of selectivity to crotyl alcohol while increasing the amount of copper and interpreted the increase with the ensemble theory.
A very complete study of the gas-phase hydrogenation of 2-butenal (cro- tonaldehyde) was carried out on unsupported bimetallic Co-Zn, Co-Cu, and Co-Mo catalysts prepared by partial thermal decomposition at 393 K (LT materials) or total decomposition at 493 K (HT materials) of bimetallic carbonyls and carboxylates [61]. Monometallic catalysts gave the highest yield in 2-butenol (28% yield at 82% conversion; see Section II.C), whereas bimetallic catalysts derived from organometallic clusters such as Mo2C01,, Cu2CoI2, and Zn,Co,, exhibited lower activities. This was attributed to the microstructure of the particles (e.g., in HT-Zn,Co,,, zinc atoms tend to cover the particle surface, thus decreasing the number of exposed active cobalt atoms). The unsaturated alcohol selectivity was improved in the case of HT- Cu2C01, compared to a HT-Co,Co,, catalyst. The promoting effect of the electropositive copper or molybdenum atoms was attributed to their bonding with the C-0 group of 2-butenal.
Silica-supported, bimetallic silver catalysts (Ag-Cd, Ag-Mn, Ag-La) prepared by coprecipitation of the nitrates in sodium hydroxide solutions and reduction under hydrogen at 623 K were employed in the gas-phase hydro- genation of crotonaldehyde [ 1161. Catalyst characterization showed that cad- mium was alloyed with silver, whereas manganese or lanthanum were in highly dispersed oxide form interacting with silver particles. A selectivity to crotyl alcohol as high as 84% at 65% conversion was measured on Ag-Cd/ SO, (Ag/Cd = 1.9); the surface polarity of the bimetallic sites was suggested to be responsible for the adsorption and hydrogenation of the C-0 bond.
Nagase et al. [117] have studied the hydrogenation of crotonaldehyde on a catalyst prepared by coprecipitation under the basic condition of silver
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110 GALLEZOT AND RICHARD
and manganese nitrates (20% Ag), the gel was dried and calcined at 500°C. Although the fresh catalyst was in oxidic form, it is likely that under the hydrogenation condition at 130-200°C silver was reduced, so that the work- ing catalyst was AglMnO, rather than silver-manganese oxide as suggested. The selectivity to 2-butenol was 71.8% at 85.1% conversion.
3. Effect of Oxidized Metal Species
Two kinds of electrophilic species are considered in this section: (1) oxidized metal species deposited on the catalyst during reduction with metal hydrides (more specifically, boron remaining on the catalyst after reduction with NaBH,) and (2) oxidized metal species at the interface between metal particles and support or migrating from the support to the metal surface during high-temperature treatments (SMSI effect).
Arai et al. [59] have studied cinnamaldehyde hydrogenation on Pt/A1203 catalysts prepared by impregnation with H,PtCl, and reduction either at 303 K with NaBH, solution or at 673 K under hydrogen. The latter catalyst was completely unselective to cinnamyl alcohol, whereas the former gave selec- tivities from 36% to 78% at 50% conversion, depending on the period of time during which the catalyst was contacted with the borohydrure solution. In another article, Arai et al. [60] studied the hydrogenation of crotonaldehyde and 3-methylcrotonaldehyde. In both cases, the selectivity of the catalysts reduced with NaBH, was much higher than those reduced under hydrogen at high temperatures. It was mentioned that significant amounts of boron remain on the catalyst; therefore, one can infer that oxidized boron species on plat- inum surface or at the interface with the support acted as electrophilic species activating the C-0 bond hydrogenation.
Chen et al. [118,119] prepared cobalt boride catalysts by reduction of cobalt acetate with sodium borohydride in aqueous or ethanolic solution. XPS studies showed that boron was under the form of borate or boron oxide, and elemental boron associated with cobalt with an electron donation from boron to cobalt. The selectivity data for cinnamaldehyde hydrogenation given in Table 14 indicate that boron-promoted catalysts have a high initial selectivity to unsaturated alcohol compared to Raney-cobalt catalyst. Boron-promoted cobalt catalysts were also more selective in crotonaldehyde hydrogenation. Catalysts were also prepared by coreduction with NaBH, of cobalt acetate and salts of various metals (Fe, Cu, Sn, Mo, Th) [119]. The presence of these metallic promoters produced a different change in the selectivity to cinnamyl alcohol; copper and tin tended to depress the selectivity because they sup- pressed the positive effect of boron, whereas thorium and molybdenum in oxidized form enhanced both activity and selectivity because of their inter- action with the electron pair of the oxygen atom of the carbonyl group.
Wismeijer et al. [120] found that Pt/TiO, catalysts heated at high tem- peratures under hydrogen (SMSI state) were more active in the hydrogenation of citronella1 to citronellol than Pt/TiO, in the normal state. These authors
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SELECTIVE HYDROGENATION OF (u,P-UNSATLJRATED ALDEHYDES 111
TABLE 14 Selectivity in Cinnamaldehyde Hydrogenation of Unsupported Cobalt Catalysts
Note: Reaction conditions: P = 811 Ha; T = 120°C; cinnamaldehyde/ethanol/Co = 5
"Cobalt boride prepared in water. bCobalt boride in ethanol solution. 'Raney cobalt. Source: Ref. 118.
m1/40 m1/2 mmol.
attributed the rate enhancement to hydrogen spillover phenomena due to a better contact of ruthenium with the support.
An extensive study of gas-phase crotonaldehyde hydrogenation was con- ducted by Vannice and Sen [19] on platinum catalysts supported on silica, alumina, and titania. On the two former supports, a 100% selectivity to bu- tyraldehyde was measured, whereas the selectivity and activity of Ti0,-sup- ported catalysts increased markedly with the temperature of reduction (Table
TABLE 15 Activity and Selectivity in Gas-Phase Crotonaldehyde Hydrogenation
Note: Reaction conditions: P = 1 atm; HJcrotonaldehyde = 23.7. "Activity in ~mol,,,,,,,,,/s gp,. bCatalyst reduced at 473 K. "Catalyst reduced at 773 K. Source: Ref. 19.
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15). The selectivity to crotyl alcohol was 37.2% on Pt/TiO, reduced at 773 K compared to 12.6% on the sample reduced at lower temperature, and a 60- fold increase of the TOF was measured. The activity of the high-temperature- reduced catalyst in the hydrogenation of crotyl alcohol to butanol was also 60 times larger. The higher activity for the C-0 bond hydrogenation was attributed to activation of the C-0 bond by either defect sites involving Ti3+ or Ti" species, created at the metal-support interface at high reduction tem- peratures, or to oxygen vacancies.
A similar investigation was conducted by Kaspar et al. [121] on osmium, iridium, rhodium, and ruthenium catalysts supported on TiO, and reduced at 473 K and 773 K. In the gas-phase hydrogenation of crotonaldehyde carried out in the pulse mode, it was found that the selectivity to crotyl alcohol was systematically higher as ruthenium and iridium catalysts were in the SMSI state. This was attributed to the effect of TiO, suboxides decorating the metal particles and favoring the activation and hydrogenation of the C-0 bond. Rhodium was totally unselective, and no improvement was observed in the SMSI state.
Coq et al. [122] found that the selectivity to cinnamyl alcohol of ruthe- nium catalysts was much higher on zirconia than on alumina support. The zirconia-supported catalysts were prepared by adsorption of Ru-acetylaceto- nate and reduction under hydrogen in the temperature range 573-973 K. It was suggested that the oxidized Zf" species, on the metal surface or at the metal-support interface, interact with the oxygen atom of the C=O bond, thus favoring its hydrogenation.
A similar explanation was invoked by da Silva et al. [123] to explain the high selectivity to cinnamyl alcohol observed with the Pt/TiO, catalyst (87%) as compared to the Pt/C catalyst (32%). Although the reduction tem- perature of the catalyst was lower, they suggested that a substoichiometric TiO, species formed during the reduction could be present and able to interact with the oxygen atom of the C-0 group.
Yoshitake and Iwasawa [29] carried out a detailed investigation of deu- teration of acrolein on platinum supported on niobium oxide. The catalysts prepared by impregnation were either reduced at low (393 K) or high (773 K) temperatures. The reaction of the former catalyst followed a conventional associative mechanism involving mainly adsorption and deuteration of the C=C bond of acrolein. In contrast, on the catalyst reduced at 773 K where islands of NbO, were formed on platinum surface (SMSI state), acrolein was adsorbed via a q&c-c~o) mode with the oxygen atom interacting with the Nb,O, islands and favored the formation of the allyl alcohol. A similar study was conducted by Yoshikate et al. [28] on Nb,O,-supported iridium catalyst in the normal and SMSI states. The interaction of the oxygen atom of ad- sorbed acrolein with the periphery of NbO, islands also favored the formation of allyl alcohol.
Shibata et al. [124] have studied the gas-phase hydrogenation of 2-bu- tenal and 2-methylpropenal on a catalyst obtained by oxidation of a gold-
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SELECTIVE HYDROGENATION OF CX,P-UNSATURATED ALDEHYDES 113
zirconium alloy with water at 573 K. This treatment resulted in the formation of metallic gold associated with zirconium oxide (30 m2/g). This catalyst dubbed AuZr(0) was much more active than Au/Zr02 catalyst prepared by impregnation of ZrO, with gold salt and hydrogen reduction at 673 K. It was also comparatively selective to unsaturated alcohol (58% of butenol at 16% conversion and 42% of 2-methylpropenol at 13.6% conversion). Although no interpretation was given, the role of ZrO, in the activation of the C=O bond can be anticipated.
H. Hydrogenation on Oxide Catalysts
Copper chromite catalysts were used of the hydrogenation of unsaturated carbonyl compounds, but their selectivity was generally low because of fur- ther hydrogenation to saturated products. Jenck and Germain [ 1251 studied the relative reactivities of several functional groups on copper chromite cat- alysts by means of competitive hydrogenation and observed the following reactivity series: C=O (aldehyde) > C=O (ketone) > C=C. This series holds for compounds with nonconjugated bonds where the unsaturated al- cohol formation is thus favored, but in conjugated unsaturated carbonyl com- pounds, the C-C reactivity is enhanced.
Hubaut et al. [126] have studied the gas-phase hydrogenation of methyl vinylketone, crotonaldehyde, and methacrolein on copper chromite catalysts at 40°C and atmospheric pressure. The selectivity to unsaturated alcohol was strongly dependent on the structure of the substrate. Thus, the initial selec- tivities were 0%, 15%, and 34% in the case of methyl vinylketone, croton- aldehyde, and methacrolein, respectively. The selectivity was controlled by steric hindrance and electronic effects induced by the methyl substituent. The hydrogenation of citral and cinnamaldehyde on copper chromite catalysts yielded a mixture of citronella1 and nerol, for the first, and dihydrocinnamic aldehyde and cinnamic alcohol, for the second, with selectivities to unsatu- rated alcohol of about 20% and lo%, respectively [127].
Ueshima and Shimasaki [ 1281 have studied the vapor-phase Merwein- Pondorf-Verley reaction for the hydrogenation of unsaturated aldehydes on MgO-B,O, catalysts at 200-270°C employing various alcohols as hydrogen- transfer agents. The best selectivity in the hydrogenation of acrolein were obtained with isopropanol (92.9% selectivity at 78.8% conversion).
I. Influence of Reaction Conditions
The effects of reaction conditions on the selectivity were rarely studied systematically for a given catalyst and reaction, particularly for reactions con- ducted in liquid phase. Therefore, only fragmentary information is available on the influence of reaction parameters such as prereduction of the catalyst, nature of the solvent, temperature, pressure, and concentration of reactant.
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The reaction rates, which are generally more affected than selectivities by reaction conditions, are often biased by diffusional limitation, particularly intraporous diffusion. The extent of diffusional limitation depends on the relative dimension of pores and molecules and on the functional groups pres- ent on supports, because they control parameters such as organophilicity and wettability. Because of the scarcity of comprehensive studies on the influence of reaction conditions, this section does not pretend to give a general account on the subject, and only a few investigations will be mentioned.
1. Effect of Catalyst Prereduction
Hydrogenation reactions can be conducted without pretreating the cat- alyst or after a prereduction which is usually carried out in the reactor. Cordier et al. [13,37] reported the selectivity data given in Table 16, showing that a pretreatment of the catalyst under 120 bars of hydrogen at 100°C greatly improved the selectivity of osmium, iridium, and platinum. The longer the duration of the pretreatment, the higher was the selectivity. Thus, as the pre- treatment at 100°C was carried out for 1 or 2 h, the selectivity of platinum rose from 80% to 88%. The effect of prereduction of bimetallic catalysts should be even more crucial, as it should affect the state of oxidation of electropositive metals involved in the C=O bond activation.
2. Solvent Effects
Rylander and Himelstein [ 1291 studied the effect of various solvents in cinnamaldehyde hydrogenation on a Pd/C catalyst. The selectivity to hydro- cinnamaldehyde was moderately affected; however, the activity was very low in hydrocarbons solvents and high in light alcohols and acetic acid. Similarly, Chen et al. El191 found little effect of the solvent in cinnamaldehyde hydro- genation on a cobalt-thorium catalyst reduced with NaBH, (Table 17).
The data given in Table 18 indicate that no solvent effect was observed in the reduction of 3-methyl-2-butenal (prenal) with sulfonated phosphine complexes of ruthenium in biphasic (aqueous-organic) systems [ 1301.
TABLE 16 Influence of Catalyst Prereducton on the Selectivity to Cinnamyl Alcohol of
Different Carbon-Supported Metal Catalysts
0 s Ir Pt Ru Rh Pd
Selectivity without
Selectivity with
prereduction (mol%) 63 65 50 66 20 0
prereduction (mol%) 97 90 88 61 11 0
Note: Reaction conditions: T = 60"C, P = 0.45 MPa, 90% ethanol, 10% water. Source: Ref. 13.
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SELECTIVE HYDROGENATION OF c&UNSATURATED ALDEHYDES 115
TABLE 17 Effect of Solvent on the Hydrogenation of Cinnamaldehyde
Note: Average activities, selectivities, and yields of cinnamyl alcohol after 90 min. Source: Ref. 119.
Neri et al. [I311 found both a marked effect of solvent and of the pres- ence of chloride on the support in the hydrogenation of citral on ruthenium catalysts. Table 19 gives the selectivities in ethanol and cyclohexane of silica- supported ruthenium catalysts prepared by dry impregnation with RuCl, or Ru(NO)(NO,), aqueous solutions and reduction under hydrogen at 623 K. Large amounts of citronellal acetal and of 3,7-dimethyl-octanal acetal were formed by reaction of ethanol with citronellal on the catalyst prepared from a RuCl, precursor. Formation of acetals is favored by the presence on the catalyst support of chloride ions from the precursor, whereas negligible amounts of acetals were formed on the catalyst issued from the Ru(NO)(NO,), precursor. In the presence of cyclohexane, no acetal was de- tected and isopulegol was formed in high yields by isomerization of citro- nellal; this could not occur in ethanol, because acetal formation prevents the isomerization of citronellal.
3. Eflect of Hydrogen Pressure
There are few examples of effect of hydrogen pressure on unsaturated aldehyde hydrogenation. Hydrogenations of crotonaldehyde in water at 293 K
TABLE 18 Solvent Effect in the Hydrogenation of Prenal on Ru-TPPTS in Biphasic Medium
Time Conversion Selectivity to prenal Organic solvent (min) (%I (%)
Note: Conditions: Atmospheric pressure. "Prepared by impregnation with RuC1,. "Prepared from Ru(NO)(NO,),. Source: Ref. 131.
in the presence of ruthenium, rhodium, and osmium catalysts were reported by Sokol'skii et al. [40,132]. Table 20 shows that the increase of hydrogen pressure from 0.1 to 2.02 MPa resulted in lower yields of crotyl alcohol on highly selective catalysts (0s-Al,O, and 0s-ZnO) and exerted practically no effects in the presence of borided ruthenium and Ru-Al,O, catalysts.
TABLE 20 Effect of Hydrogen Pressure on Hydrogenation of Crotonaldehyde
Hydrogen pressure SC=" Yield of crotyl Catalyst alcohol (mol%)
Os, borided 0.10 80 70 2.02 92 85
Os-Al2O3
0s-ZnO
Ru, borided
Ru-AlZO,
Ru-ZnO
0.10 2.02
0.10 2.02
0.10 2.02
0.10 2.02
0.10 2.02
80 68
97 76
11 9
78 68
96 87
8 6
13 11 18 23 22 39
18 25
Source: Ref. 40.
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TAB
LE 2
1 Se
lect
ed E
xam
ples
of
ol,P
-Uns
atur
ated
Ald
ehyd
e H
ydro
gena
tion
Cat
alyz
ed b
y Tr
ansi
tion
Met
al C
ompl
exes
P (H
,) T
Tim
e C
onve
rsio
n Se
lect
ivity
" Su
bstr
ate
Cat
alys
t (b
ar)
("C)
(h>
(%)
(%I
Ref
.
Citr
al
RuC
l,(PP
h,),
50
30
20
99
91
134
Citr
al
HR
uCl(P
Ph,),
+
5HC
l 99
98
13
4 C
roto
nald
ehyd
e ci
s-R
uCl,(
PTA
),b
1
80
3 88
10
0 13
8 C
inna
mal
dehy
de
Rh,
Cl,(
CO
), +
NEt
, 40
60
1
94
85
70
Cin
nam
alde
hyde
[I
r(O
CH
,)(co
d)],
30
100
2 98
96
14
1
Pren
al
RuH
,(TPP
TS)J
SiO
, 10
0 50
3
100
89
137
Ret
inal
(al
l tra
ns)
Ir 5
%/R
G 4
15-1
" 10
0 50
3.
5 10
0 97
.5
137
+ PC
yPh,
"Sel
ectiv
ity to u
nsat
urat
ed a
lcoh
ol.
bPTA
: 1,3,5-triaza-7-phosphaadamantane.
"Ir 5
% R
G 415-1 =
IrC
l,{(C
,HS)
P[(
CH
Z)9
SiO
~,~]
~.2N
[(C
H,)
,siO
~,~]
~.2S
iO~}
~.
d 53 c
c 4
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118 GALLEZOT AND RICHARD
111. HYDROGENATION WITH METAL COMPLEXES
This section is not intended to give a complete account of the literature data on hydrogenation of @-unsaturated aldehydes catalyzed by transition metal complexes or to deal with mechanistic aspects. However, a few ex- amples of selectivity data, which can be compared with those obtained on heterogeneous catalysts, will be given.
Most of the homogeneous catalytic systems were based on soluble com- plexes of metals of the second transition row such as ruthenium [130,133- 1391 on rhodium [70,130] or of the third transition row such as osmium [140] and iridium [137,141,142]. Most of these complexes involved phosphine ligands.
Table 21 gives a few data measured on hydrogenation reaction catalyzed by soluble complexes. Farnetti et al. [ 1411 reported that in the case of iridium, an increase of P/Ir ratio favors a higher selectivity toward the carbonyl-group reduction. They have also shown that to achieve a high selectivity, the cone angle of the phosphine, as defined by Tolman [143], must be greater than 140". In some cases, these reactions were conducted in the presence of carbon monoxide. Thus, Mizoroki et al. [70] evidenced a maximum of activity for a CO pressure of 20 bars, whereas the selectivity toward cinnamyl alcohol increased with the CO pressure.
Catalyst recycling was made possible by employing ruthenium com- plexes with sulfonated phosphines, especially tris(m-sulfopheny1)-phosphine trisodium salt (TPPTS) [130]. Because these complexes are soluble in water, hydrogenation reactions were carried out in biphasic medium, and the catalyst was recovered in the water phase. The reaction data given in Table 22 shows that excellent selectivity at 100% conversion can be obtained in three suc- cessive hydrogenations of 3-methyl-2-butenal (prenal).
In the case of aldehydes with longer carbon chains such as retinal, the biphasic catalytic system is limited by the low solubilities of the substrates. High yields were obtained by a supporting ruthenium aqueous-phase catalyst
TABLE 22 Hydrogenation of Prenal on Ru-TPPTS Catalyst
Time Conversion Selectivity Run (h) (%) (%I 1 1 100 96 2 0.5 99 97 3 0.5 99 97
Note: Reaction conditions: RuC1, = 0.1 mmol; TPPTS = 0.5 mmol; P = 20 bars; T = 35°C; 3-methyl-2-butenal = 20 mmol; water = 5 ml; toluene = 5 ml.
Source: Ref. 137.
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SELECTIVE HYDROGENATION OF a,@-UNSATURATED ALDEHYDES 119
on silica; however, polar solvents could not be employed because of the dissolution of the metal, and catalysts were poisoned in nonpolar media by adsorption of organic compounds. Better results were obtained with supported homogeneous phosphino-iridium catalysts (Table 21) [ 1371.
IV. CONCLUDING REMARKS
The review of the literature on selective hydrogenation of a,p-unsatu- rated aldehydes to unsaturated alcohol shows that this reaction, although ther- modynamically less favored than the hydrogenation to saturated aldehydes, can be achieved efficiently provided that the proper catalysts and appropriate reaction conditions are chosen. The following points were highlighted:
Metals may or may not be selective due to their specific electronic structure; however, their selectivity can be tuned by various factors which have been detailed in Sections 1I.C through 1I.G. These mod- ifications can produce drastic improvement of the selectivity toward the unsaturated alcohols. The most important effect is obtained as a platinum-group metal is associated in the same particle with a second, more electropositive metal or with oxidized metal species (see Section 1I.G). These bi- metallic systems can be prepared in very different ways and their nanostructure could be very diverse, but the common feature is that the oxidized species, in contact with or in the vicinity of the first metal, act as Lewis sites. These sites interact with the lone electron pair of the oxygen atom of the C=O group, polarizing the bond, and favoring its hydrogenation by hydridic hydrogen adsorbed on the first metal. The selectivity improvement is then mainly due to the increase of the rate of C-0 bond hydrogenation. Actually, the ac- tivity of bimetallic catalytic systems could be enhanced up to 100 times as in Pt,,-Fe,& compared to Pt/C (see Section II.G.2). Steric effects due to mutual constraints between the metal surface and the molecule (see Section I1.C) or imposed by the environment of the metal (e.g., cage effect in microporous solids forcing the al- dehyde molecule to adsorb end-on) could also produce large selec- tivity enhancement. In contrast to the previous case, the enhancement is essentially due to a decrease of C-C bond hydrogenation, because its adsorption on the metal surface is hindered by steric effects. Effects of electron-donating ligands, either adsorbed species or atoms of the underlying support, occur when most of the metal atoms are on the surface of particles (i.e., for a size smaller than 2 nm) and when these atoms are in close interaction with the surface ligands (see Sections I1.E and 1I.F). Alloying, even with the second metal underneath the surface, could also produce a similar electronic ligand
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120 GALLEZOT AND RICHARD
effect. The selectivity improvement is then, according to theoretical calculations, due to the decrease of C-C bond adsorption on the metal with the more filled d band because of a repulsive electronic interaction larger than that experienced by the C=O bond. There- fore, electronic ligand effects could produce a significant change of selectivity but only a moderate change in catalytic activity, if any. All the factors discussed above may act independently of each other, but their cumulative effects may concur to selectivity enhancement. The best yields in unsaturated alcohol that can be obtained with het- erogeneous catalysts are somewhat smaller than those given by ho- mogeneous catalysts. However, soluble complexes are more easily lost in solutions, even for homogeneous catalyst systems that allow catalyst recycling in biphasic medium.
Future investigations on selective hydrogenation of a,P-unsaturated al- dehydes will have to establish more accurately the relative importance of all the factors discussed in this review. This will require additional experiments on model metal surfaces, as well as on very homogeneous, well-characterized catalytic systems. Detailed kinetic studies, which are presently lacking, par- ticularly in the case of liquid-phase batch experiments, are needed to evaluate the effects of reaction conditions on the activities and selectivities of catalytic reactions, and to establish the possible effects of various diffusional limita- tions. As there are very few data available on catalyst aging in the course of liquid-phase reactions, experiments involving multiple recycling of catalysts should be carried out and postreaction characterization of catalysts should be conducted to check for any changes in their structure and morphology. Fi- nally, experiments involving the comparison of activity and selectivity data obtained for the same reaction conducted in the liquid phase and in the gas phase would be highly recommended.
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