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Supplement A3: The Chemistry of Double-Bonded Functional Groups. Edited by Saul Patai Copyright 1997 John Wiley & Sons, Ltd.

ISBN: 0-471-95956-1

CHAPTER 16

Heterogeneous catalytic hydrogenation

´ ´ ´ ´ ´

MIHALY BARTOK and ARPAD MOLNAR

Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, Jozsef´ Attila University, Dom´ ter´ 8, H-6720 Szeged, Hungary

Fax: (36)62-312-921; e-mail: amolnar@chem.u-szeged.hu

I. ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

844

II. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

844

III. GENERAL CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . .

845

A. Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

845

1.

CDC bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

845

2.

CDO bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

845

3.

CDN bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

845

B. Experimental Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

846

1.

Hydrogenation with the use of an external hydrogen source . . . . .

846

2.

Hydrogenation without external hydrogen . . . . . . . . . . . . . . . . .

846

C. Solvent Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

847

IV. HYDROGENATION OF CDC BONDS . . . . . . . . . . . . . . . . . . . . . . .

848

A. Hydrogenation of Monoalkenes . . . . . . . . . . . . . . . . . . . . . . . . . .

848

1.

Reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

848

2.

Mechanistic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

850

3.

Stereochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

853

4.

New catalysts for alkene hydrogenation . . . . . . . . . . . . . . . . . .

859

 

a. Metal atoms, clusters and colloidal particles . . . . . . . . . . . . . .

859

 

b. Amorphous metal alloy powders . . . . . . . . . . . . . . . . . . . . .

860

 

c. Rapidly quenched amorphous metal alloys . . . . . . . . . . . . . . .

861

5.

Surface modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

863

6.

Hydrogenation of alkenes over metal oxides . . . . . . . . . . . . . . .

864

7.

Alkene hydrogenation as probe reaction . . . . . . . . . . . . . . . . . .

865

B. Hydrogenation of Dienes and Polyenes . . . . . . . . . . . . . . . . . . . . .

867

1.

Conjugated dienes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

867

 

a. Open-chain dienes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

867

843

844

 

´

 

 

 

 

Mihaly´ Bartok´ and Arpad´ Molnar´

 

 

 

b. Cyclic dienes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

870

 

 

c. Sulfur poisoning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

872

 

2. Nonconjugated dienes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

872

 

3.

Polyenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

874

C. Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

874

V. HYDROGENATION OF CDO BONDS . . . . . . . . . . . . . . . . . . . . . .

875

A. Hydrogenation of Aldehydes . . . . . . . . . . . . . . . . . . . . . . . . . . . .

875

B. Hydrogenation of Ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

876

 

1.

Reactivity and mechanistic studies . . . . . . . . . . . . . . . . . . . . . .

876

 

2.

Stereochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

878

 

3.

Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

879

C. Hydrogenation of Unsaturated Carbonyl Compounds . . . . . . . . . . . .

880

 

1.

Hydrogenation of unsaturated aldehydes . . . . . . . . . . . . . . . . . .

880

 

 

a. Reactivity and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . .

880

 

 

b. Mechanistic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

885

 

2.

Hydrogenation of unsaturated ketones . . . . . . . . . . . . . . . . . . . .

887

VI. HYDROGENATION OF CDN BONDS . . . . . . . . . . . . . . . . . . . . . .

890

VII. ASYMMETRIC HYDROGENATIONS . . . . . . . . . . . . . . . . . . . . . . .

892

VIII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

896

 

 

 

 

 

 

 

 

 

 

I. ABBREVIATIONS

 

CPG

controlled pore glass

 

CVD

chemical vapor deposition

 

ee

enantiomeric excess

 

EPR

electron paramagnetic resonance

 

ESCA

electron spectroscopy for chemical analysis

 

EXAFS

extended X-ray absorption fine structure

 

fcc

face-centered cube

 

HTR

high temperature reduction

 

LTR

low temperature reduction

 

M

metal

 

MRNi

modified Raney Ni

 

PVP

poly(N-vinyl-2-pyrrolidone)

 

RDS

rate-determining step

 

SMSI

strong metal

 

support interaction

 

 

 

STO

single turnover

 

TA

tartaric acid

 

THF

tetrahydrofuran

 

TOF

turnover frequency

 

active center of metal surfaces

*chiral atom

II. INTRODUCTION

This series of monographs reported on the hydrogenation of double-bonded functional groups in 19871. A few years earlier a detailed treatment of the topic also appeared in a book, though with main emphasis on the stereochemistry2. The present review, consequently, covers the results of this field published during the last decade. In a few cases, however, earlier publications are also cited to give a more comprehensive treatment of certain topics. The literature coverage extends up to the middle of 1995, but relevant publications after this date have also been included.

16. Heterogeneous catalytic hydrogenation

845

As a usual practice of this series only the important developments are emphasized. Only the hydrogenation of compounds possessing carbon carbon, carbon oxygen and carbon nitrogen double bonds or the combination of these is covered. Compounds with additional functional groups are not treated with the exception of a few examples of great significance. Results obtained with the use of heterogenized homogeneous catalysts are not included.

III. GENERAL CHARACTERISTICS

A. Catalysts

1. CDC bond

Platinum metals, first of all Pt, Pd and Rh, are the characteristic and most frequently used alkene hydrogenation catalysts which are highly active under ambient conditions3 8. In contrast, elevated temperatures and pressures are required when the much cheaper nickel is employed. Platinum is the catalyst of choice when isomerization is to be avoided. Palladium, in turn, has a tendency to bring about extensive isomerization (double bond migration). Metals used in catalytic hydrogenations are either prepared in situ by reducing their oxides or hydroxides (metal ‘black’ catalysts, e.g. Adams platinum9) or they are dispersed on a high surface area inert support (carbon, alumina, silica). Such supported metal catalysts are characterized by a large surface area per unit weight which is highly beneficial, since hydrogenation over heterogeneous catalysts is a surface reaction.

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12 14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes.

Certain oxides, first of all zinc oxide17, as well as copper chromite18 (a mixed copper chromium oxide), are also active in the saturation of the CDC bond. Partial sulfur treatment under controlled conditions of metals or oxides can result in sulfided catalysts which exhibit specific activity and selectivity in hydrogenations19 21.

2. CDO bond

Platinum, ruthenium and rhodium are the most often used metals in the hydrogenation of the CDO bond8,22 26. Slightly increased temperatures and pressures are usually required. Palladium is ineffective in the hydrogenation of aliphatic aldehydes and ketones but can be used to reduce aromatic carbonyl compounds. When aromatic aldehydes are hydrogenated, however, special care must be taken to avoid the hydrogenolysis of the resulting benzyl alcohol. This side-reaction can also occur during the hydrogenation of substituted aromatic carbonyl compounds even over platinum. Metal salts (FeCl3 and SnCl2) were found to promote the platinum-catalyzed hydrogenation of the aromatic carbonyl group.

Nickel, either as a Raney catalyst or in the form of nickel boride, is also effective in the reduction of the CDO bond. An increase in the catalytic activity can be brought about by metal promoters (chromium and molybdenum). Copper chromite may also be used.

3. CDN bond

Various classes of compounds possessing a CDN bond can yield different products depending on the hydrogenation conditions. Selectivity can be markedly affected by the catalyst. Pt, Pd and Raney nickel are used most often27 31.

846

´

Mihaly´ Bartok´ and Arpad´ Molnar´

Examples on the use of these and other heterogeneous catalysts in the hydrogenation of double-bonded functional groups are found throughout this review.

B. Experimental Variables

1. Hydrogenation with the use of an external hydrogen source

The catalytic hydrogenation of the CDC bond with the most active platinum metal catalysts can usually be carried out at room temperature and atmospheric hydrogen pressure in the liquid phase. Elevated temperature and pressure must be used when less active catalysts are employed. Hydrogenation is most often carried out in a batch mode when all components of the reaction mixture (reactant, catalyst, solvent and additives) are placed in an appropriate reaction vessel32,33. Since this is a three-phase system (solid catalyst, gaseous hydrogen and reactant in the liquid phase) effective agitation is essential to overcome mass transport limitations and to achieve good and reproducible performance. Continuously operated systems33 are of significance in both research (small-scale flow and pulse reactors and gas recirculatory reactors) and commercial hydrogenation processes.

2. Hydrogenation without external hydrogen

A convenient and useful alternative to conventional catalytic hydrogenation not requiring an external hydrogen source is transfer hydrogenation. The process, named by Braude and Linstead34, involves hydrogen transfer from a suitable hydrogen donor molecule, in most cases an organic compound, to the molecule to be reduced. The method, carried out usually at reflux temperature in a simple apparatus, is particularly useful for the hydrogenation of the CDC bond.

Pd is the most active and most frequently used metal in transfer hydrogenations35,36. Cyclohexene, a cheap, readily available and reactive molecule, is the preferred donor compound which is successfully used in transfer hydrogenations in the gas phase37 39. Tetralin and monoterpenes, and, in general, hydroaromatic compounds are also used. Raney Ni, a cheap alternative to palladium, is usually applied with alcohols or amines as the donor.

Much experimental evidence indicates that Pd mediates the formation of a donor acceptor complex followed by a direct hydrogen transfer36. This process, however, was disproved in the disproportionation of 1,4-cyclohexadiene (a special case of transfer hydrogenation) over colloidal nickel40. Another possibility is a consecutive dehydrogenation hydrogenation process.

In addition to hydrogen transfer, other procedures also allow one to perform alkene hydrogenations without an external hydrogen source. Metal-assisted reductions with NaBH4 can be considered as heterogeneous catalytic hydrogenation. Finely divided metal precipitated from its salt by NaBH4 is believed to catalyze hydrogen addition with excess NaBH4 serving as the hydrogen source41 43.

A new procedure also carries out hydrogenation without added hydrogen. Triethoxysilane and 5 mol% of palladium acetate in a mixture of THF and water yields finely divided palladium dispersed on a polysiloxane matrix with concomitant hydrogen evolution44. Alkenes, present in this reaction mixture, are transformed to the corresponding saturated hydrocarbons in 100% yield at room temperature (equation 1).

RCH

D

CHR0

Pd(OAc)2, (EtO)3SiH

RCH2

 

CH2R0

(1)

2 !

THF H O (5:1),

Me propionate, RT

R,R0 D H, Bu, C8H17

16. Heterogeneous catalytic hydrogenation

847

Though it is not a practical procedure, hydrogenation with spilt-over hydrogen is worth mentioning. Silica and alumina areogels can be activated in the presence of a supported metal catalyst (usually Pt or Ni) with hydrogen45,46. These spillover-activated oxides then promote hydrogenation of ethylene46 48. The mechanism is similar to that on metal oxides (see Section IV.A.6), since the molecular identity of the reacting hydrogen was found to be retained47.

C. Solvent Effects

In liquid-phase hydrogenation catalyzed by heterogeneous catalysts, solvents are often necessary to dissolve the reactant and product molecules. Solvents, however, can affect the hydrogenation reaction itself in various ways49,50. In competitive hydrogenations, polar solvents facilitate the hydrogenation of nonpolar compounds. This is because a polar solvent solvates more effectively a polar compound, and, therefore, the less solvated nonpolar compound preferentially adsorbs on the catalyst surface. The reactivity of 1- hexene in the competitive hydrogenation in a mixture with 2-methyl-3-butene-1-ol in the presence of a 5% Pt silica catalyst varied with the structure of solvent alcohols51. The rate of hydrogenation of 1-hexene increased with both increasing chain length and branching. The relative reactivities (the rate of hydrogenation of 1-hexene divided by that of 2-methyl-3-butene-1-ol measured at 293 K and 1 atm H2) were 2.3, 11.1 and 7.7, respectively, in MeOH, 1-heptanol and t-BuOH.

The role of the support was also demonstrated in the competitive hydrogenation of cyclohexene and cyclohexanone in water cyclohexane mixtures52. Cyclohexene was exclusively hydrogenated on a hydrophobic Rh C, since this catalyst is covered by a cyclohexane layer, which preferentially dissolves cyclohexene. On a hydrophilic Rh silica cyclohexanone, present mainly in the water phase covering the catalyst, was hydrogenated.

Solvent molecules can compete with the alkene being hydrogenated for the same adsorption site, thereby affecting the outcome of the hydrogenation reaction. This was demonstrated in the hydrogenation of methylenecyclohexane and 1-methylcyclohexene over palladium53. The two compounds were shown to undergo hydrogenation at comparable rates in ethanol. When methylenecyclohexane, in turn, was hydrogenated in ethanol benzene, a large amount of 1-methylcyclohexene formed by isomerization was isolated. Apparently, the hydrogenation of the latter compound is severely retarded due to the competitive adsorption of benzene, whereas the adsorption of methylenecyclohexane is not affected.

Adsorbed solvent molecules can alter the characteristics of the active sites. A 3M site possessing three coordinative unsaturations is assumed to catalyze hydrogen addition (see in detail in Section IV.A.7). When a solvent molecule, however, adsorbs at a 3M site, this site is transformed into a 2M site (a site with two coordinative unsaturations) which catalyzes only isomerization. The extent of hydrogen addition is, therefore, expected to decrease, whereas that of isomerization is expected to increase. This was indeed demonstrated in the liquid-phase hydrogenation of 4-methyl-1-cyclohexene over a 8.06% Pt SiO2 catalyst54. Surprisingly, however, the same phenomenon was observed in the gas-phase hydrogenation of 1-butene on Pt and Pd supported on controlled pore glass (CPG) when the catalysts were pretreated with solvents55 (Table 1). The largest effects are observed in the presence of THF and methanol, whereas pentane has no effect indicating that it hardly interacts with the active sites. The changes are much smaller on Pd due to the strong isomerization activity of this metal.

Solvents can also affect selective hydrogenation of bifunctional compounds. As Table 2 shows, highly selective hydrogenation of the conjugated double bond of ˛-ionone was achieved in etheral solvents56 (equation 2). This was attributed to the inhibition of

848

´

Mihaly´ Bartok´ and Arpad´ Molnar´

TABLE 1. Effect of solvents on the performance of catalysts in the hydrogenation of 1-butene (pulse experiments at ambient temperature with catalysts first treated with solvent then saturated with hydrogen)

 

 

4.9% Pt-CPG

 

 

 

1.8% Pd-CPG

 

 

butanea

2-butenesb

1-butene

 

butanea

2-butenesb

1-butene

None

41 C 20

2 C 2

27

 

22 C 12

18 C 45

4

Pentane

39 C 17

6 C 4

33

 

22 C 12

17 C 45

4

THF

20 C 12

9 C 9

50

 

19 C 9

21 C 45

6

Methanol

17 C 10

23 C 20

30

 

16 C 12

17 C 53

1

a Figures correspond to direct saturation and two-step saturation (see Section IV.A.7). b cis C trans.

TABLE 2. Hydrogenation of ˛-ionone in various solvents (equation 2)

Solvent

Temp.

Conversion

 

Yield of products (%)

 

(K)

(%)

1

2

 

 

 

 

 

Dioxane

363

86

80

4

Toluene

383

86

12

50

Dibutyl ether

413

96

63

28

Decalin

433

96

17

61

Tetraglyme

453

81

88

0

 

 

 

 

 

 

isomerization by CuC ions through the adsorption of nucleophilic solvent molecules.

O

 

O

O

 

CuA l2 O3

+

(2)

 

1 atm H2

 

 

 

 

 

(1)

(2)

IV. HYDROGENATION OF C=C BONDS

Heterogeneous catalytic hydrogenation of alkenes discovered at the turn of the century57 is one of the most studied catalytic reactions. It is a versatile and useful technique in organic synthesis frequently giving high yields. High chemo-, regioor stereoselectivities can also be achieved provided the suitable experimental conditions are applied. Due to this great practical as well as theoretical significance, a large number of papers, books and review articles are available treating every aspect of the field1,3 7,58 67.

A. Hydrogenation of Monoalkenes

1. Reactivity

The reactivity of CDC bonds depends on the number and nature of substituents attached to the sp2 carbon atoms. Substituents affect the reactivity by affecting the rate constant of reaction and the adsorption properties.

Increasing substitution, in general, results in decreasing rate of hydrogenation, known as the Lebedev rule49,68,69. Terminal olefins, correspondingly, exhibit the highest reactivity, and the rate of hydrogenation decreases in the order RCHDCH2 > R2CDCH2 >

16. Heterogeneous catalytic hydrogenation

849

RCHDCHR > R2CDCHR > R2CDCR2. In the hydrogenation of 1-alkenes the rate decreases monotonously with the chain length49. The relative adsorptivity of terminal alkenes was shown to be the same, indicating that the chain length affects the rate constant. Since the polar effect of alkyl groups is identical, the difference in reactivity is caused by the steric hindrance of the alkyl chain. Of stereoisomeric compounds, cis isomers are hydrogenated in preference to the corresponding trans compounds49,69. The reactivity of cycloalkenes usually shows a maximum at cyclohexene69. Exceptions, however, are known. Cyclohexene, for example, exhibits extremely low reactivity on the P-2 Ni B catalyst (relative reactivities are 1, 0.01, 0.26 and 0.20, respectively, for cyclopentene, cyclohexene, cycloheptene and cyclooctene)70. Other data indicate monotonous decrease in the reactivity of C5 C8 cycloalkenes over Rh71 and Pt69.

New observations of the relative reactivity of various alkenes in gas-phase hydrogenation on Pt72 and Cu73 have been reported. These data are in agreement with previous observations. The reactivity of terminal alkenes over Rh catalysts74, in contrast, was shown to increase in the sequence 1-hexene < 1-heptene < 1-octene. An attempt was also made to quantify steric, polar and adsorption effects with the utilization of linear free energy relationships in the liquid-phase hydrogenation of alkenes49.

Since the rate of hydrogenation is sensitive to operating conditions (temperature, pressure, catalyst quantity, solvent and agitation), relative rates determined in competitive hydrogenation of binary mixtures are considered to be more reliable than measuring individual rates50. Relative reactivities thus measured are determined by the ratio of rate and adsorption constants.

Supported metal catalysts with molecular sieving properties75,76 are able to differentiate between alkenes with structures of differing steric demands in competitive hydrogenation. After an early report on the selective hydrogenation of propylene in the presence

of isobutylene over a Pt-zeolite A catalyst77 new examples have recently been published78 84.

Shape-selective hydrogenation was demonstrated over an appropriately reduced Pt- ZSM-5 catalyst81,82. Neither Pt-ZSM-5 reduced in hydrogen nor Pt Al2O3 displayed any selectivity (Table 3). Selective hydrogenation of various terminal alkenes was, however, achieved when Pt-ZSM-5 was reduced in the presence of a mixture of alkenes and hydrogen. The greatest selectivity was observed in the hydrogenation of a binary mixture of a straight-chain and a geminal dimethyl-substituted alkene82 (Table 4). Shape selectivity was attributed to the large difference in diffusivities of the two types of compounds82.

Significant shape selectivity was exhibited by Pt catalysts modified with organosilicon compounds using CVD78 80. The treatment of Pt supported on silica, titania and zirconia did not result in significant improvements relative to the parent samples in the competitive hydrogenation of 1-nonene and trans-4-nonene78. The relative rate on Pt TiO2, for example, was 2.62, which changed to 2.80 when the catalyst was treated with (EtO)4Si. When zeolites were applied as supports, however, silane treatment afforded catalysts with excellent selectivity (Table 5)78 80. Catalysts treated with different silanes display

TABLE 3. Hydrogenation of alkene mixtures in the presence of Pt catalysts (548 K, flow reactor)

 

 

 

 

 

% Hydrogenation

 

 

 

 

1-hexene

4,4-dimethyl-1-hexene

 

 

 

 

 

0.5% Pt

 

Al2O3

27

35

 

1%

Pt-ZSM-5 reduced in H2 (573 K, 1 h)

29

39

1%

Pt-ZSM-5 reduced with alkenes C H2 (673 K, 1 h)

90

1

850

´

Mihaly´ Bartok´ and Arpad´ Molnar´

TABLE 4. Shape-selective hydrogenation of alkene mixtures in the presence of 1% Pt- ZSM-5 catalyst reduced in a mixture of alkenes and H2

 

Alkenes

Temperature

% Hydrogenation

straight-chain

branched

(K)

 

 

 

 

 

 

 

1-Pentene

4,4-Dimethyl-1-pentene

573

97

2

1-Heptene

4,4-Dimethyl-1-pentene

573

91

1

1-Hexene

6-Methyl-1-heptene

573

25

2

Styrene

2-Methylstyrene

698

50

2

 

 

 

 

 

TABLE 5. Selectivity in the competitive hydrogenation of 1-nonene and trans-4-nonene over Pt catalysts (150 mg of catalyst, 298 K, 1 atm H2, hexane)

 

 

 

 

cat

Relative rate

 

Initial rate (10

5

mol h

1 g 1)

 

1-nonene (r1)

trans-4-nonene (r2)

r1/r2

Pt-zeolite Aa

728

 

336

2.17

modified with Me2Si(EtO)2

192

 

30.6

6.3

modified with (EtO)4Si

47.7

 

 

2.63

18.14

modified with Ph2Si(MeO)2

159

 

 

9.33

17.0

modified with Ph2Si(EtO)2

149

 

 

7.33

20.3

Pt-zeolite Xa

647

 

173

3.73

Modified with (EtO)4Si

10.7

 

 

0.4

26.75

a 30 mg of catalyst.

TABLE 6. Reactivities of isomeric octenes relative to 1-octene in competitive hydrogenation (for reaction conditions see Table 5)

 

 

r1/r2a

 

Pt-zeolite A

Pt-zeolite A modified with (EtO)4Si

cis-2-Octene

1.49

3.33

trans-2-Octene

1.61

2.94

trans-3-Octene

1.95

6.31

trans-4-Octene

1.61

14.90

a Initial rate of 1-octene/initial rate of the corresponding octene.

strongly varying shape selectivity (Table 5). A comparison of the rates in the competitive hydrogenation of isomeric octenes indicates that both the position of the double bond and the steric structure have a significant effect on selectivity78,79 (Table 6).

XPS data suggest that the surface of the CVD catalysts is finely covered by a homogeneous silica layer except for Pt particles, where small holes are formed. Since hydrogenation exclusively occurs on Pt particles, selectivity is brought about by steric hindrance around the Pt site in the holes. Less hindered double bonds, consequently, are hydrogenated preferentially.

2. Mechanistic studies

An early mechanism proposed by Horiuti and Polanyi in 1934 to interpret the hydrogenation of the CDC bond85,86 over transition metals is still generally accepted and postulated in the majority of cases (equations 3 6). In its original form this mechanism

16. Heterogeneous catalytic hydrogenation

851

involves the dissociative adsorption of hydrogen molecule (equation 3) and the associative adsorption of alkene (equation 4). The 1,2-di- -adsorbed surface species thus formed reacts with adsorbed hydrogens in a stepwise manner, first forming an alkyl intermediate or half-hydrogenated state (equation 5), then the saturated product (equation 6). This corresponds to a reaction model in which the rate-determining surface reaction involves interaction between two adsorbed molecules or atoms (Langmuir Hinshelwood mechanism87). An alternative but rare possibility is the reaction of an adsorbate with a gas-phase species (Eley Rideal pathway88,89). In fact, this was only observed in the hydrogenation of cyclohexene over Cu(100)90 (see Section IV.A.3) and ethylene over Cu(111)91.

H2 +

2

 

2 H

 

 

 

(3)

C

C

+ 2

 

 

C C

 

(4)

C

C

+

H

2

C

C

(5)

 

 

 

 

 

 

 

H

 

C

C

+

H

2

C

C

(6)

 

 

H

 

 

 

H

H

 

A similar mechanism is well-established for transition-metal complexes92 94 including the observation of hydridoalkyl complexes95. Due to this close analogy between heterogeneous and homogeneous hydrogenation, step 3 may be viewed as migratory insertion of hydrogen and step 4 is the reductive elimination between hydrogen and the alkyl intermediate. Whereas this last step (equation 6) is virtually irreversible under hydrogenation conditions, both the formation and the reaction of the adsorbed alkene (equations 4 and 5) are reversible.

Various observations support the existence of this mechanism over heterogeneous metal catalysts. When the half-hydrogenated intermediate eliminates a hydrogen from another ˇ-carbon it reverts to an isomeric olefin (equation 7). This transformation accounts for the isomerization (double bond migration and cis trans isomerization) of alkenes occurring during hydrogenation63,67. When deuterium instead of hydrogen is used, the reversal of the half-hydrogenated intermediate results in deuterium exchange in alkenes96 100. It is also observed that deuterium addition does not result in a simple d2 saturated hydrocarbon6,67,101. Instead, d1, d2 and d3 products are usually formed in the ratio 1:2:1 independent of conversion60,102. This indicates that exchange and C H activation must be involved in the hydrogenation reaction.

H

 

 

2

(7)

RCH2 C C

R C C C

R CH C CH

H

H

 

 

852

´

Mihaly´ Bartok´ and Arpad´ Molnar´

The stereochemistry of hydrogenation, the long recognized predominantly cis addition103 105 (see Section IV.A.3) is also consistent with the stepwise addition of the two H atoms via the half-hydrogenated intermediate. H D exchange reaction of alkanes is also interpreted with the involvement of the surface alkyl intermediate106,107.

Further studies indicated that a considerable range of bonding configurations can be involved in hydrogenation. The -adsorbed intermediate 3 and the dissociatively adsorbed-vinyl (4) and -allyl (5) species were suggested to exist on metal surfaces67,108 112. The unusual properties of Pd are believed to be related to its ability to generate the - allyl ( 3-allyl) intermediate (6). 1,1,2- and 1,1,3-tri- -adsorbed species were shown to be involved by means of infrared spectroscopy in the adsorption of linear butenes on Ni113 and Co114. To underline the complexity of alkene hydrogenation recent papers have to be referred to where, in contrast with surface-bound hydrogen, bulk hydrogen was shown to be active in the hydrogenation of ethylene and cyclohexene on Ni catalysts115 117.

 

 

 

 

 

 

 

H

 

H

 

 

 

 

 

 

 

C

 

C

H2 C

 

CH2

HC

 

CH2

H2 C

CH2

H2 C

CH2

 

 

 

 

(3)

(4)

(5)

(6)

The most extensive mechanistic studies were carried out with ethylene over Pt(111) and Rh(111) single crystal faces under ultrahigh vacuum112,118,119. When exposed to ethylene the surface was shown to be instantly covered by an ethylidyne (7) overlayer120 122. Surface alkylidynes were also found to be present by means of IR spectroscopy upon adsorption of ethylene and terminal alkenes on supported metal catalysts110,123 125. Ethylidyne, however, is only a spectator, since its hydrogenation occurs at a rate six orders of magnitude lower than that of ethylene. The ethylidyne overlayer, in turn, can readily be compressed to open up new space for ethylene hydrogenation122,126. Reaction kinetics data were found to be consistent with the rapid equilibrium between molecularly adsorbed ethylene and surface ethyl species127. Strong evidence for the reversible formation of surface-bound ethyl groups on hydrogen-covered Fe(100) (migratory insertion of ethylene into a Fe H bond and ˇ-hydride elimination of ethyl group) was recently disclosed128. Similar results were found for C3 C6 alkenes96.

CH3 CH3

C CH

(7)(8)

Another reaction model originally proposed by Thomson and Webb129 assumes that H atoms are transferred from a strongly adsorbed hydrocarbon overlayer, represented as M Cx Hy , to the alkene weakly adsorbed on top of the overlayer. This suggestion, among others, is based upon the observations that the form of metal (powder, foil, film or dispersed metal) does not influence reaction rates significantly and the activation energy of ethylene hydrogenation is the same for different metals within a narrow limit. It was

Соседние файлы в папке Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups