Heterogeneous Catalytic Hydrogenation in Supercritical Fluids

Ind. Eng. Chem. Res. 2008, 47, 4561–4585

4561

Heterogeneous Catalytic Hydrogenation in Supercritical Fluids: Potential and Limitations Tsunetake Seki,† Jan-Dierk Grunwaldt, and Alfons Baiker* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, CH-8093 Zurich, Switzerland

Heterogeneous catalytic hydrogenation of organic compounds in supercritical fluids (SCFs) is reviewed, covering the work published up to 2007. The potential as well as limitations of the application of SCFs as solvents are examined based on the existing knowledge. SCFs, particularly CO2, offer some attractive opportunities as substitutes of classical organic solvents, but their beneficial application requires careful weighing of economical and ecological factors. 1. Introduction In spite of the required high-pressure conditions, supercritical fluids (SCFs) have gained importance as “green” alternatives to classical organic solvents particularly for the hydrogenation of organic compounds over heterogeneous catalysts.1 Attractive features of the application of SCFs as solvents include the following: (i) the hydrogenation rates in SCFs are often much higher compared to those in liquid organic solvents owing to the great miscibility of gaseous hydrogen and organic substrates in SCFs (i.e., formation of single phase or “expanded” liquid) and the favorable mass transport properties (high density, low kinematic viscosity, high diffusivity), (ii) the easy and complete separation of the solvent (SCF) from the products by simple expansion of the reaction mixture, (iii) the potential of reduction of volatile organic compound (VOC) emissions due to fugitive losses, and (iv) the possible tuning of density related properties such as polarity and dissolution power. Note that reactions in “supercritical” fluids mostly only refer to conditions that lie above the critical parameters (Pc, Tc) of a pure fluid component which does not infer that a “single” phase is present. In reaction mixtures, often a biphasic mixture is present which changes its composition as the reaction proceeds. Such mixtures composed of a dense solvent phase and expanded liquid swollen by dissolving a large quantity of solvent molecules also exhibit similar advantageous properties which facilitate the variation of density, polarity, and other physical properties over a wider range than with pure SCFs.2 Both homogeneous3 and heterogeneous catalysts have been tested in various hydrogenations in SCFs, but heterogeneous catalysts are much more suitable for large-scale chemical productions because they can be used in fixed-bed continuous-flow reactor systems. The technical interest in this reaction technology is reflected by a number of patents that have been filed during the past years4 and industrial demonstration plants for continuous hydrogenations using SCFs for chemical production processes that have been brought to operation by Hoffmann-La Roche, the University of Nottingham/Thomas Swan group,1e and the University of Göttingen/ Schering AG group.5 * To whom all correspondence should be addressed. Fax: +41 44 632 11 63. Tel.: +41 44 632 31 53. E-mail: [email protected]. † Research Fellow of the Japan Society for the Promotion of Science.

In the present review, we focus on the heterogeneous catalytic hydrogenation of organic compounds in supercritical fluids. CO and CO2 hydrogenation in SCFs are out of the scope, and the reader is referred to the pertinent literature.6,7 Research reported up to 2007 is accounted for, and examples are classified according to the operation mode (continuous or batch) and the type of catalyst employed. Finally, the potential and limitations of the application of SCFs in heterogeneous catalytic hydrogenation of organic compounds are examined based on the existing knowledge. 2. Hydrogenations in Continuous-Flow Reactors The research examples are classified according to the type of metal catalyst applied, because the kind of metal determines chemoselectivity in hydrogenations not only under classical conditions (i.e., gas-phase hydrogenations and hydrogenations in organic solvents) but also in SCFs. Note that in certain cases a change in the chemoselectivity of metals has been observed when the reaction conditions were changed from conventional to SCFs conditions. Work on the use of group-7 to group-9 metal catalysts is scarce. A few studies on continuous hydrogenations were performed with Ru- and Rh-based catalysts. 2.1. Group-8 Metal Catalysts. 2.1.1. Ru/SiO2. The catalytic cyclization of levulinic acid to γ-valerolactone in the presence of H2 is considered to be a promising process in view of green sustainable chemistry, because the substrate is obtained by the acid-catalyzed dehydration of hexose sugars which are renewable biomass. Bourne et al. developed a unique continuousflow system for this reaction, which involves an efficient, reduced energy separation/purification process of the product downstream (Table 1, I, A).8 The schematic view of the system is depicted in Scheme 1, where high-pressure CO2 plays an important role for reactant-product separation. Even though γ-valerolactone is miscible with H2O in the absence of CO2, they are separated completely in the presence of high-pressure CO2. Thus water-free γ-valerolactone could be obtained via back pressure regulator with easily removable CO2 and H2, leaving the aqueous phase containing unreacted levulinic acid. The authors concluded that this separation technique can be widely used, particularly for the reactions which require water as

10.1021/ie071649g CCC: $40.75  2008 American Chemical Society Published on Web 06/17/2008

CO2

CO2

cyclohexene

propane

acetophenone

70-130

propane

CO2

40-250

CO2

cyclohexene; 1-octene; 1-octyne; cyclohexanol; THF; benzaldehyde, propionaldehyde; acetophenone, cyclohexanone; isophorone; m-cresol; furan nitrobenzene; N-benzylidenemethylamine; 2-butanone oxime; cyclohexene fatty acid methyl esters (rapeseed oil)

unsaturated ketone + isomersg

40-400

CO2

dimethyl itaconate

70

150 or 200

50-180

30-100

40-100

CO2

dimethyl itaconate

180–200

T (°C)

CO2

solvent

levulinic acid

substrate(s)

P (MPa), total

A. Ru/SiO2 10

6-12

10-20

20

flow rate of substrate ) 0.24 g mol h-1; flow rate of H2 ) 90 × 10-6 Nm3 min-1

D. Pd/C 13.6

C. Pd/SiO2-Al2O3 10 flow rate of substrate ) 0.05 or 0.1 mL min-1; substrate:H2 ) 1:4

B. Pd/Al2O3

10

substrate concentration ) 2.4-15 wt %; partial pressure of H2 ) 0.20-2.3 MPa flow rate of substrate ) 10-60 g min-1; flow rate of H2 ) 1.9-11.4 NL min-1

6-12

A. Pd/aminopolysiloxane 4-20

flow rate of substrate ) 0.3-2.0 mL min-1; substrate:H2 ) 1:2.0-9.0

flow rate of substrate ) 0.5-20.0 mL min-1; substrate:H2 ) 1:1.0-8.0

III. group-10 metal catalysts

flow rate of substrate ) 0.15 mL min-1 (2.5 M solution in i PrOH); substrate:H2 ) 1:2.5

B. [Rh(Josiphos 001)(COD)]+BF4--PTA-alumina

flow rate of substrate ) 0.25 mL min-1 (2.5 M solution in i PrOH); substrate:H2 ) 1:4

A. [Rh(S,S-Skewphos)(NBD)]+BF4--PTA-alumina

II. group-9 metal catalysts

flow rate of substrate ) 0.3 mL min-1 (levulinic acid-H2O solution, 75% w/w levulinic acid); substrate:H2 ) 1:1.5-4.5

I. group-8 metal catalysts

note for feeding amount of substrate and H2

v

v

v

v

v

ra

Table 1. Heterogeneous Catalytic Hydrogenations in Supercritical Fluids with Continuous-Flow Reactors (Work Published up to 2007)

v

Sb

22

16e

8

t c (h)

1

2

2f

2

phase under optimized conditionsd

14

13

12

11

10

9b

9a

8

ref(s)

4562 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

CO2

CO2

dehydroisophytol

phenylacetylene + styrenei

see Table 2 substrate, 0-5.3 mol%; H2, 20 mol%

CO2

CO2 or propane 210-250

propane

fatty acid methyl esters (soybean oil) methylated sunflower oil

I. Pt/C 3.0-14.0

15

A. copper chromite and CuO-MnO2-Al2O3 20-25

IV. group-11 metal catalysts

substrate, 2 mL; H2, 0.1-8 MPa

v

vo

v

vj

f

vVn

vk

V

V

Sb

2

12j

2-3

4-6

tc (h)

1

2

1

1l

1

1

2 (based on the data of ref 12)

phase under optimized conditionsd

25

24

23

22

20

17

16

15b

15a

ref(s)

a Change of reaction rate in the supercritical region (above critical point of pure solvent) with respect to subcritical conditions (v increase, V decrease). b Change of selectivity in the supercritical region (above critical point of pure solvent) with respect to subcritical conditions (v increase, V decrease, f no drastic change). c Catalyst lifetime. The catalytic systems remained stable toward continuous operation for at least the indicated times with minimal degradation of performance. d Number of phases that afforded the best result is shown. (1) homogeneous single phase; (2) two phases composed of expanded liquid and dense solvent. e For the hydrogenation of m-cresol. f Description on phase behavior was given only for cyclohexane-H2-CO2 system10b and isophorone-H2-CO2 system.1e g The substrate had a structure of R1sCHdCdCHsCOsR2, where R1 and R2 were not reported. The substrate contained three of its isomers. h Organic modifiers were added to increase the selectivity of the product, isophytol. i Styrene was added to render the system close to the industrial conditions. j For the hydrogenation of cyclohexene and one-pot synthesis of 2-ethylhexanal from crotonaldehyde. k For the one-pot synthesis of 2-ethylhexanal from crotonaldehyde. l Phase behavior was visually observed only for the cyclohexene-H2-CO2 system. m Cinchonidine was added to increase the ee of the product, (R)-ethyl lactate. n The ee increased with low H2 concentration, while decreased with high H2 concentration. o Above critical point of pure CO2, the hydrogenation took place higher in biphasic system than in homogeneous single phase system.

280

50

H. cinchonidine-modified Pt/Al2O3 4.0-12.6 substrate:modifier ) 2500:1; flow rate of substrate ) 1.0 or 4.3 mL min-1; substrate:H2 ) 1:2-20

R-pinene

30-140

ethane

G. Pd/Amberlyst 15 4-18 flow rate of substrate ) 0.3-0.5 mL min-1; substrate:H2 ) 1:1.8-4.0

ethyl pyruvate + cinchonidinem

60

v

v

F. Pd-immobilized microchannel 9 50 (channel: 60) substrate, 0.20 mmol; H2, 0.9 MPa

5-25

alloy 12.7

ra

v

E. amorphous Pd81Si19 substrate:modifier ) 2000:1; flow rate of substrate ) 38 g h-1; flow rate of H2 ) 20 NL h-1 flow rate of substrate ) 33.5-38 g h-1; flow rate of H2 ) 4 or 20 NL h-1 phenylacetylene:styrene ) 1:9; flow rate of substrates ) 10 g h-1; phenylacetylene:H2 )1:10-80

P (MPa), total

5-19

55-85

40-120

86

T (°C)

CO2

solvent

note for feeding amount of substrate and H2

cyclohexene; benzaldehyde; crotonaldehyde; trans-2-hexenal

2,4-diphenyl-4-methyl-1-pentene; CO2 ethyl 10-undecenoate; 10-undecen-1-ol; 3-phenyl-2-propyn-1-ol; 4-phenyl-3-buten-2-one; 3-benzyloxy-1-propynylbenzene

CO2

dehydroisophytol +organic modifierh

substrate(s)

Table 1. Continued

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4563

4564 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Scheme 1. Schematic View of the Continuous-Flow Reactor System with Downstream Separator for the Catalytic Conversion of Levulinic Acid (Denoted LA) to γ-Valerolactone (Denoted GVL)

Scheme 3. Selective Hydrogenation of Isophorone to Dihydroisophoronea

a The following conditions led to quantitative conversion: substrate flow rate, 0.5–2.0 mL min-1; fluid flow rate, 0.75 L min-1; isophorone:H2 ) 1:2.0; total pressure, 12 MPa; reactor temperature, 140–200 °C.

Scheme 2. Asymmetric Hydrogenation of Dimethyl Itaconatea

a The Skewphos ligand afforded the highest conversion (66%) and ee (63%) under the following conditions: CO2 flow rate (liquefied at -10 °C), 1.0 mL min-1; substrate flow rate, 0.25 mL min-1 (2.5 M solution in i PrOH); substrate:H2 ) 1:4; total pressure, 10 MPa; reactor temperature, 60 °C (see ref 9a). The Josiphos 001 ligand, on the other hand, afforded 83% ee under the following conditions: CO2 flow rate (liquefied at -10 °C), 0.5 mL min-1; substrate flow rate, 0.15 mL min-1 (2.5 M solution in i PrOH); substrate:H2 ) 1:2.5; total pressure, 16 MPa; reactor temperature, 55 °C (see ref 9b).

cosolvent and generate water as byproduct (the present reaction also). Note, however, that the insufficient ability of water to dissolve gases in turn may cause gas–liquid mass transport problems, although the solubility of H2 should be slightly improved in the presence of scCO2. 2.2. Group-9 Metal Catalysts. 2.2.1. [Rh(chiral-phosphine-ligand)(diene)]+BF4--PTA-Alumina. Stephenson et al. performed the continuous catalytic asymmetric hydrogenation of dimethyl itaconate to (R)-dimethyl 2-methylsuccinate in scCO2 (Table 1, II, A; Scheme 2).9a A traditional complex catalyst for enantioselective hydrogenations, [Rh(S,S-Skewphos)(NBD)]+BF4- (NBD ) norbornadiene), was immobilized onto an alumina support with a phosphotungstic acid linker and used for the reaction. Under the typical conditions (total pressure, 10 MPa; substrate:H2 ratio of 1:4), both the conversion and enantiomeric excess (ee) strongly depended on the temperature and showed a maximum at 60 °C corresponding to 66% conversion and 63% ee, while the respective values dropped to 10 and 5% at 40 °C and to 61 and 32% at 100 °C. The best ee achieved was equal to that observed with the catalyst in conventional solvents, indicating that enantioselectivity of the catalyst was not impaired by the use of scCO2. The long-term

experiments under optimal conditions showed that the catalyst is stable with minimal degradation of performance over a period of 8 h on stream. In addition, ICP-MS analysis showed that less than 1 ppm of the immobilized Rh complexes were leached, corroborating their stability during the reaction. Phase behavior of the reaction mixture was not reported. Later, the same group tested a number of chiral phosphine ligands for higher ee, using cyclooctadiene (COD) as the diene ligand (Table 1, II, B; Scheme 2).9b Among the ligands screened, Josiphos 001 with ferrocenyl backbone exhibited the highest ee which reached 83% at 55 °C and a total pressure of 16 MPa. This ee was higher than that observed with a homogeneous chiral Rh-diphosphine catalyst completely dissolved in scCO2.3b 2.3. Group-10 Metal Catalysts. 2.3.1. Pd/Aminopolysiloxane. Hitzler and Poliakoff performed the hydrogenation of various organic compounds in scCO2 and scC3H8 using Deloxan aminopolysiloxane-supported Pd, Pt, and Ru catalysts (Table 1, III, A).10 They employed 5- and 10-mL tubular reactors which were filled with 4 and 9 mL of the catalysts, respectively. The most important parameters with the flow apparatus were the reactor temperature and the concentration of H2, and it was possible to maximize the yield of any of the hydrogenation products by changing the parameters. The Pd catalyst was effective for the hydrogenation of CsC multiple bonds in alkenes, alkynes, and aromatic rings and of CdO bonds in aromatic aldehydes and ketones in scCO2. Under optimal conditions, quantitative conversion and high selectivity to the desired product could be achieved. The hydrogenation of nitrobenzene, oximes, and Schiff bases also proceeded smoothly over the catalyst to give the corresponding amines in excellent yields, although flammable scC3H8 must be used in these reactions to avoid the insoluble carbamic acid salt formations in scCO2 (see section 4.2). The Ru catalyst afforded better results (but still low product yields) for the reduction of aliphatic aldehydes and ketones to the corresponding alcohols compared to the Pd catalyst. As a commercially practical approach, the selective hydrogenation of CdC bond in isophorone could be achieved over the Pd catalyst to give dihydroisophorone in quantitative yield (Scheme 3). It is noteworthy that 30 mL min-1 of isophorone was successfully hydrogenated in a 100-mL reactor, because the product, dihydroisophorone, is an industrially important chemical as a solvent for vinyl resins, lacquers, varnishes, paints, and other coatings. This has been one of the most important examples that demonstrated the usefulness of scCO2 as a solvent for large-scale chemical synthesis.1e Phase behavior estimation of the cyclohexane-CO2-H2 system at 12 MPa and 0-270 °C employing Peng–Robinson equation of state indicated that lower H2 concentration and higher temperature enhance the miscibility and that at higher temperatures relatively small quantities of CO2 are sufficient to ensure the miscibility.10b Consequently, the homogeneous single phase at the entrance of the reactor which often can be

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4565 Scheme 4. Hydrogenation of Acetophenone and Subsequent Kinetic Resolution of the Corresponding Alcohol Product with Vinyl Acetatea

a These reactions were performed in series in continuous-flow scCO2, using a hydrogenation reactor and an enzyme reactor. The conversions and ee were obtained under the following conditions: CO2 flow rate, 1 mL min-1; acetophenone flow rate, 0.1 mL min-1; acetophenone:H2 ) 1:4.

realized with small amounts of CO2 will remain so during the hydrogenation, because hydrogenation is exothermic and consumes H2. Note, however, that industrially important isophorone hydrogenation could be performed with excellent selectivity and conversion with as much as 50% isophorone in the reaction stream. The stream was then composed of two phases including isophorone liquid swollen by dissolved CO2 (expanded liquid).1e Macher et al. studied continuous partial hydrogenation of methylated rapeseed oil (fatty acid methyl esters) in scC3H8 using 3% Pd supported on aminopolysiloxane (Table 1, III, A).11 The goal of this investigation was to obtain partially hydrogenated oil product with a low trans content (99% ee) at reasonable conversions. Unfortunately, the selectivity to 1-phenylethanol in the hydrogenation was not reported, although it might be not important for the outcome of the kinetic resolution. Note that the group had previously performed the same hydrogenation using a Deloxan aminopolysiloxane-supported 5% Pd catalyst.10 The catalyst then afforded ethylbenzene, 1-cyclohexylethanol, and ethylcyclohexane in addition to 1-phenylethanol, and the product selectivities could be controlled by changing reactor temperature. 2.3.4. Pd/C. Arunajatesan et al. performed continuous catalytic hydrogenation of cyclohexene to cyclohexane over Pd/C at 70 °C and 13.6 MPa, under which conditions the mixture forms a single homogeneous phase (Table 1, III, D).14 Excellent temperature control of the reactor (70 °C) due to the presence of dense CO2 with high heat capacity and stable catalyst activity (>80% conversion) as well as selectivity (ca. 100%) were realized throughout a 22 h run. It was crucial to use cyclohexene that had been pretreated with an alumina trap for removal of organic peroxides to achieve stable catalytic activity, because the peroxides may initiate oligomerization of cyclohexene leading to fouling and thus to deactivation of the catalyst. Such peroxides are formed from olefins by atmospheric oxygen and sunlight, and are usually involved in the commercial product.

4566 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Scheme 5. Selective Semihydrogenation of Dehydroisophytol to Isophytol

The authors reported that the content of the peroxides could be decreased from 180 ppm to less than 6 ppm by the pretreatment with the alumina trap. The formation of CO or palladium formate (Pd-OC(O)H), which can also cause deactivation, was not observed in this catalytic system. 2.3.5. Amorphous Pd81Si19 Alloy. Tschan et al. performed the selective semihydrogenation of dehydroisophytol to isophytol in scCO2 using a nonporous Pd81Si19 glassy alloy catalyst and organic modifiers that suppress the overhydrogenation of isophytol (Table 1, III, E; Scheme 5).15a The increase in the selectivity to isophytol is highly desired, because the semihydrogenation is a prominent step in the industrial synthesis of vitamins E and K. The organic modifiers possessing one or more than one heteroatom (N, O, and/or S) increased the selectivity compared to that observed for the reaction without a modifier, however, at the expense of a lower conversion. The rough tendencies in the correlation between the selectivity and the property of modifier were as follows: (i) a higher selectivity was observed with an increase in the number of heteroatoms, (ii) the presence of conjugated double bonds increased the selectivity, (iii) the selectivity depended strongly on the nature and position of the heteroatoms, (iv) the change in the structure of the alkyl moiety (e.g., straight-chain to branched-chain) affected the selectivity, and (v) the selectivity increased with higher modifier concentration. Among the modifiers tested, propyl disulfide (nPrS-SPrn) and isopropyl disulfide (iPrS-SPri) exhibited excellent performance; in the presence of propyl disulfide or isopropyl disulfide, isophytol could be obtained in 85.7 and 84.3% yield, respectively. The effect of the addition of lead was also examined for the catalyst with or without a modifier, but its main role was to decrease the activity, while the selectivity did not increase remarkably. In a later study, the authors investigated the effect of changing CO2 pressure, temperature, and H2 concentration on the selectivity and conversion (Table 1, III, E).15b Modifiers were not used in this investigation. Reaction results and parallel phase behavior studies showed that the single-phase system gave much higher yields of isophytol than the two-phase system composed of dehydroisophytol-rich phase and CO2-rich phase. The singlephase operation enhanced the miscibility of H2 with dehydroisophytol, leading to a higher conversion even with a lower concentration of H2. In addition, the smaller H2-concentration in the single-phase system contributed to the higher selectivity by suppressing the overhydrogenation of isophytol. It is also noteworthy that the single-phase operation could be performed at a lower reaction temperature of 55 °C, which is favorable for the suppression of byproduct oligomer formations. Thus, the best result (conversion 70%, selectivity 87%) could be obtained in the single-phase system at 55 °C and 20 MPa employing a CO2:H2:substrate ratio of 56:1:1.3. The lower H2consumption and the lower reaction temperature in the singlephase operation offer an economical benefit, although the high catalyst cost due to the large precious metal content cannot be neglected and maybe improved in future using another type of palladium catalyst. Styrene is a widely employed chemical as a monomer in the synthesis of a number of polymeric compounds. Industrially, it

is manufactured by ethylation of benzene and subsequent dehydrogenation of ethylbenzene. In the latter reaction, however, overdehydrogenation also takes place to produce phenylacetylene, which has detrimental effects on the molecular weight of the resultant polymer and on the polymerization rate. While unreacted ethylbenzene can be easily removed by distillation, phenylacetylene is not easily removable by conventional separation processes and has to be hydrogenated to styrene. Thus it is highly desirable to develop an efficient catalytic system for the semihydrogenation of phenylacetylene to styrene. Tschan et al. achieved a high conversion and a good selectivity for the semihydrogenation of phenylacetylene by applying the amorphous Pd81Si19 alloy catalyst and scCO2 as a solvent for the reaction (Table 1, III, E).16 A mixture containing phenylacetylene and styrene with a molar ratio of 1:9 was used as the reactant stream, which is closely related to the industrially interesting application of the purification of styrene. Typically the phenylacetyrene:H2 molar ratio of 1:10 was employed. The study of the influence of total pressure on the reaction at 55 °C revealed that the conversion increased noticeably above 8 MPa, reached a maximum at 13 MPa, and remained almost constant when the pressure was further increased, while the selectivity to styrene continued to decrease above 8 MPa. The optimal pressure affording styrene in the highest purity was ca. 13 MPa at 55 °C. Since the mixture formed a single homogeneous phase above 13.5 MPa at 55 °C, the above results indicate that the edge of the “sc” single-phase region is an ideal medium for the semihydrogenation. The influence of the reaction temperature at a constant total pressure of 12.7 MPa was investigated in the range 55–85 °C, showing that the conversion increased monotonously from 66 to 91% with increasing temperature and that the selectivity showed a slight maximum at ca. 65 °C. The mixture existed as a single phase at low temperatures and separated into a liquid phenylacetylenestyrene-rich phase and a dense CO2-rich phase at higher temperatures. The complex phase behaviors that change the concentration of phenylacetylene, styrene, and hydrogen in various phases as well as the difference in reaction rate at various temperatures account for the dependence of conversion, selectivity, and purity of styrene on the temperature. The effect of hydrogen concentration on the reaction was also investigated in the phenylacetylene:H2 molar ratio range of 1:10–80 at 55 °C and total pressure of 12.7 MPa. The selectivity decreased steadily with the increase in hydrogen concentration (79% at 1:10; 62% at 1:80), while the conversion increased slightly (90% at 1:10; 95% at 1:80). The large decrease in the selectivity to styrene brought about the monotonous decrease in the purity of styrene with increasing hydrogen concentration. These data indicate that a higher hydrogen concentration favors overhydrogenation to ethylbenzene and thus the hydrogen excess must be kept as low as possible to achieve a high purity of styrene. In the report, a detailed diagram of binary fluid-phase of phenylacetylene and CO2 was also shown, which can serve as a good guide for understanding the multicomponent system and shows that the presence of hydrogen mainly lowers the overall density.

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4567 Scheme 6. Continuous “One-Pot” Synthesis of 2-Ethylhexanal from Crotonaldehydea

a The following conditions led to the best results (conversion, 98%; selectivity to 2-ethylhexanal, 67%; selectivity to butyraldehyde, 32%): substrate flow rate, 0.3 mL min-1; flow rate of expanded gaseous CO2, 1 L min-1; crotonaldehyde:H2 ) 1:4.0.

2.3.6. Pd-Immobilized Microchannel. Kobayashi and coworkers developed an effective microfluidic system for hydrogenation in scCO2 (Table 1, III, F).17 The microchannel reactor possessed a channel 200 µm in width, 100 µm in depth, and 40 cm in length on a glass plate (3 cm × 7 cm). Amino groups were first immobilized onto the silica surface inside the reactor, followed by their treatment with microencapsulated palladium. Subsequent cross-linking of the polymer at 150 °C gave the Pd-immobilized microchannel. The olefinic CdC and CtC bonds of the compounds listed in Table 1, III, F, were hydrogenated quite smoothly and selectively to give the corresponding products in nearly quantitative yields within 1 s residence time. These interesting results were attributed to the elimination of mass transport limitation of gaseous hydrogen by the use of scCO2 medium and to the very large interfacial area per unit of volume, i.e., 10 000-50 000 m2 m-3. It is remarkable that benzyloxy group, which is widely employed as a protective group of alcohols, did not undergo hydrogenolysis in this reaction system. The authors stressed that the concept established in this research may in future be applied to largescale synthesis by using a number of channels in parallel. 2.3.7. Pd/Amberlyst 15. Amberlyst 15 is a commercially available strongly acidic cation exchange resin composed of styrene-divinylbenzene matrix with sulfonic acid groups. The synthesis and use of Amberlyst 15-supported transition-metals as “bifunctional” catalysts were first reported by Laufer and Hoelderich in their study of hydroxylation of benzene.18 Later, they showed that Pd-doped Amberlyst 15 catalyst (denoted Pd/ Amberlyst-15) is quite effective for the one-pot synthesis of a potential analgesic in ethanol solvent, which involves the dehydration and subsequent hydrogenation.19 However, the reaction had to be performed at a relatively high reaction temperature of 150 °C due to the inherent gas–liquid mass transport problem in the hydrogenation step, which caused unfavorable coke formation and the deterioration of the surface sulfonic acid groups. Seki et al., on the other hand, performed the continuous one-pot synthesis of 2-ethylhexanal from crotonaldehyde over 1% Pd/Amberlyst-15 in scCO2 (Table 1, III, G; Scheme 6).20 The reaction involves hydrogenation steps but can be performed at a relatively low reaction temperature of 60 °C owing to the great miscibility of gaseous hydrogen and crotonaldehyde in scCO2 solvent. Under optimized conditions (60 °C, 16 MPa total, crotonaldehyde:H2 ) 1:4.0), 2-ethylhexanal was formed in ∼70% selectivity at quantitative conversion of crotonaldehyde. Interestingly, the byproduct was only butyraldehyde, which is also industrially important and easily separated from 2-ethylhexanal by distillation. The clean conversion was attributed to the efficient metal-acid bifunctional catalysis and also to the striking difference in activity between CdC and CdO bond hydrogenation. Actually, for the cyclohexene hydrogenation to cyclohexane, the 1% Pd/Amberlyst-

15 catalyst exhibited high activity comparable to or even higher than conventional 1% Pd/C, whereas it showed an activity far below that of 1% Pd/C for the benzaldehyde hydrogenation to benzyl alcohol. In the hydrogenation of cyclohexene, the elimination of the gas–liquid interface was found to be crucial to achieve the highest catalytic performance of 1% Pd/ Amberlyst-15. On the basis of the XPS, XAFS, and DRIFT spectroscopy of adsorbed CO, it was proposed that the metalproton adduct, [Pdn-H]+, formed by proton transfer from sulfonic acid group to Pd particle, functions as “hybrid” site, promoting both hydrogenation and aldol condensation. 2.3.8. Cinchonidine-Modified Pt/Al2O3. The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate over cinchonidine (CD)-modified Pt/Al2O3 has been well studied using organic solvents such as acetic acid, toluene, and ethanol.21 The use of supercritical fluids as solvents for the reaction was first attempted by Minder et al.47 in 1995 using a batch reactor (see section 3.3.5). Later, Wandeler et al. reported the results collected employing a continuous-flow reactor system (Table 1, III, H).22 Two kinds of fluids, namely, dense CO2 and C2H6, were tested and they were introduced successively in that order in one continuous experiment performed at 30 °C and 10 MPa with a solvent:ethyl pyruvate:CD:H2 ratio of 500:1:0.0004:10, where the substrate flow was 4.3 mmol min-1. Drastic increases in both conversion and enantiomeric excess (ee) were observed when the feeding solvent was changed from CO2 to C2H6, in accordance with the results by Minder et al. (see section 3.3.5). The poisoning by CO formed via the reverse water-gas shift reaction (CO2 + H2 f CO + H2O) was proposed to be at the origin of the poor results in scCO2. The effect of changing the ethyl pyruvate:H2 ratio and pressure revealed that the increase in conversion with the transition from a multiphase to a single homogeneous phase was more prominent at 1:2 than at 1:20, demonstrating that the reaction rate depends on the concentration of gaseous hydrogen. On the other hand, the ee value was in the range 55–75%, depending on the temperature and H2 concentration. The authors stressed that scC2H6 is superior to conventional toluene solvent due to easy product separation after the reaction as well as much higher reaction rate (turnover frequency (TOF) value 15 s-1 in scC2H6; 1.8 s-1 in toluene). The corresponding full account reported the relation between phase behavior and catalytic performance of cinchonidinemodified Pt/Al2O3 in detail, which clearly illustrates the limitations of describing multicomponent phase behavior with that of pure solvent.22b The total pressure required for the transition from gas–liquid two-phase system to homogeneous single phase system increased remarkably with higher H2 concentration as well as temperature. The phase diagram of a binary system was shown to be much more appropriate and useful compared to that of a “pure” fluid system for rationalizing the phase behavior of multicomponent systems. In the report also the effect of temperature on the catalysis was investigated in the range 35–140 °C at 10 MPa with a solvent:ethyl pyruvate:CD:H2 ratio of 200:1:0.0004:2 and 200:1:0.0004:20, respectively. The hydrogenation rate did not show strong dependence on temperature. In addition, the ee value decreased drastically with increasing temperature, indicating that the favorable adsorption mode of cinchonidine, which is also crucial to achieve high reaction rate, was changed at higher temperature. Increasing the density of ethane increased the ee for the reaction with low H2 concentration, but decreased it for that with high H2 concentration. This behavior for the ee was attributed to the superposition of the following effects which accompanied the increase of the density of ethane: enhanced stabilization of the favorably

4568 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Table 2. Reaction Conditions for the Continuous Hydrogenation of Fatty Acid Methyl Esters (Soybean Oil; Taken from ref 24) condition denotation a

I IIa IIIb IVb a

medium

pressure (MPa)

temp (°C)

H2 mole fraction

substrate flow (µL min-1)

residence time (s)

scCO2 scC3H8 scCO2 scC3H8

20 20 25 25

210-250 210-250 250 250

0.10-0.25 0.10-0.25 0.25 0.25

37.5 150 50 250

5.5 5.5 9 9

Both Cu-Cr catalyst and Cr-free catalyst were used.

b

Only Cr-free catalyst was used.

adsorbed cinchonidine and increased surface H2 concentration due to better mass transport, which has a positive influence on the ee but accelerates the unfavorable hydrogenation of the cinchonidine anchoring moiety (quinoline ring). 2.3.9. Pt/C. Milewska et al. performed the hydrogenation of a-pinene to cis- and trans-pinane under high-pressure CO2 using 1% Pt/C as catalyst (Table 1, III, I).23 The circulation reactor system was composed of a sapphire-windowed cell and a tubular reactor enclosing the catalyst bed. The substrate in high-pressure CO2 in the cell was continuously withdrawn from the bottom of the cell by a pump, circulated through the catalyst bed, and sent back to the top of the cell. Thus when the mixture in the cell was biphasic, the lower liquid phase, namely, the expanded liquid in which a relatively large amount of CO2 dissolved, is introduced into the reactor. At a reaction temperature of 50 °C and a H2 pressure of 4 MPa, the mixture became a single homogeneous phase above a CO2 pressure of 12.0 MPa, below which it separated into liquid and vapor phase. The initial test reactions employing two kinds of Pt/C catalysts in both homogeneous and biphasic phase revealed that the catalyst possessing larger platinum particles exhibits higher activity for the reaction compared to that with a more uniform platinumdistribution. Thus, the performance of the “poor” metaldistribution catalyst was used for further investigations. The highest reaction rates were observed at a CO2 pressure of 7.5–9.6 MPa, indicating that the CO2 pressure just below that required to form a single homogeneous phase is most appropriate for the reaction. Under such conditions, the mass content of CO2 in the liquid substrate is 35 to more than 90% and the expanded liquid can dissolve a large quantity of gaseous hydrogen. This was also supported by the effect of changing H2 pressure on the reaction, which showed that the concentration of R-pinene around the catalyst is the main factor that affects the reaction rate, while mass transfer of hydrogen to the catalyst surface is sufficiently fast even under the biphasic conditions at a CO2 pressure of 7.5–9.6 MPa. Thus higher reaction rates could be achieved in the biphasic systems in which the catalyst was surrounded by the concentrated R-pinene. Another advantage of the biphasic system seems to be the relatively retarded H2 access to the catalyst surface that might suppress the poisonous CO formation via the reverse water-gas shift reaction (CO2 + H2 f CO + H2O). It is noteworthy that the same hydrogenation over Pd/C using a batch reactor also proceeded faster in the biphasic system than in homogeneous single phase (see section 3.3.2). The liquid substrate in such biphasic systems is swollen because it contains large amounts of CO2. 2.4. Group-11 Metal Catalysts. 2.4.1. Copper Catalysts. Andersson and co-workers performed the continuous hydrogenation of fatty acid methyl esters of soybean oil in scCO2 and scC3H8 using a copper chromite catalyst and a chromium-free, copper- and Al2O3-based catalyst (Table 1, IV, A).24 To compare the performances of the catalysts in the two different media, the reactions were performed under the four kinds of conditions summarized in Table 2. Under conditions I and II, the yield of the fatty alcohol increased with increasing temperature and/or hydrogen content regardless of the sort of catalyst and medium, while the other variables such as the substrate flow rate, pressure,

and residence time had little effect on the hydrogenation process. Performances of the chromium-free catalyst in the two different media were compared in detail under conditions III and IV in Table 2. A high conversion of 97.2% was achieved in scCO2 with a substrate flow rate of 50 µL min-1, while scC3H8 afforded the same conversion even with a much higher flow rate of 250 µL min-1. The higher production of the fatty alcohol per time in scC3H8, however, was offset by the lower selectivity in scC3H8 caused by the formation of significant amounts of C16 and C18 n-alkanes. Formation of such alkane byproducts was not observed in scCO2. In addition, from a practical point of view, CO2 is superior to propane due to its nontoxicity, nonflammability, and low cost. The hydrogenation method was subsequently coupled with an enzymatic-catalyzed transesterification, which yields the fatty alcohol products directly from soybean oil. In this two-step synthetic procedure, the pump for feeding fatty acid methyl esters was replaced with a continuous reactor system in which a multiple syringe pump system delivers appropriate amounts of CO2, soybean oil, and methanol into the reaction cell containing the Novozym SP 435 catalyst. Almost quantitative formation of C16 and C18 fatty acid methyl esters was observed in the transesterification step, and the subsequent hydrogenation also took place smoothly to give the fatty alcohol products in a high yield of 96.5%. The authors mentioned that the byproduct methanol in the hydrogenation step can be recycled back to the transesterification, although such a recycle process was not involved in the reaction system. van den Hark and Härröd investigated the hydrogenation of methylated sunflower oil to the corresponding fatty alcohols in scC3H8 using a fixed-bed reactor system consisting of two reactors (Table 1, IV, A).25 The substrate oil was first introduced into the first reactor containing 2% Pd/zeolite for the saturation, followed by the second reactor containing a Cu-based catalyst, in which the saturated substrate was converted into the corresponding fatty alcohols according to: CH3(CH2)16COOCH3 + 2H2 f CH3(CH2)16CH2OH + CH3OH At high substrate concentrations, a rapid drop of the reaction rate was observed, and the benefits of the propane addition were completely lost. This drop was attributed to a split of the supercritical reaction mixture into two phases, i.e., a substraterich phase and a hydrogen-rich phase. When this phase-split occurred using small catalyst particles (e32 µm), the pressuredrop over the catalyst bed increased sharply, because the formed liquid droplets blocked the void space in the porous catalyst bed. These two phenomena, i.e., decrease in reaction rate and pressure, could be used to deduce the product and substrate solubility in the reaction mixture. The product, i.e., fatty alcohols, showed the most unfavorable solubility among the components in the reaction mixture. The solubility increased with increasing pressure and decreased with increasing temperature and in the presence of hydrogen. Under the process conditions (15 MPa, 280 °C, and 20 mol % hydrogen), a single phase was observed up to 2 mol % (i.e., 15% by mass) substrate/ product. Besides the minimum pressure in the catalyst bed, substrate transport limitation was shown to be an important factor in process optimization. Thus, it was emphasized that

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4569

egg-shell catalysts or fine catalyst particles (100–300 µm) should preferably be used in the continuous supercritical reactors. In the former work, the authors performed the same reaction only with the Cu-based catalyst and investigated the effect of various reaction parameters on the catalytic behavior.26 3. Hydrogenations in Batch Reactors 3.1. Group-8 Metal Catalysts. 3.1.1. Ru/C. Sato et al. attempted the selective ring hydrogenation of phenylethanols to cyclohexylethanols (Table 3, I, A; Scheme 7).27 Typically, 7.0 mmol of 1- or 2-phenylethanol was treated in a 50-mL reactor with 3.0 MPa of H2 in 9.0 MPa of CO2 over 0.1 g of carbon- or alumina-supported 5% transition-metal catalyst at 50 °C for 30 min. For both 1- and 2-phenylethanol, Rh/C exhibited the highest turnover numbers (TONs;28 1215 and 581, respectively) and conversions (99.9 and 50.6%, respectively) among the catalysts tested but lower selectivities to the desired 1- and 2-cyclohexylethanol (55.1 and 54.8%, respectively) compared to the others owing to its high activity for dehydroxylation (see also section 3.2.1). Ru/C, on the other hand, showed much higher selectivities (86.5% for 1-cyclohexylethanol; 85.9% for 2-cyclohexylethanol) with reasonable conversions, indicating that this catalyst is most appropriate for the selective ring hydrogenations. The influence of CO2 and H2 pressure was investigated for the Ru/C-catalyzed 1-phenylethanol hydrogenation in the partial pressure ranges 5–20 and 1–6 MPa, respectively, which showed that increasing CO2 pressure at a constant H2 pressure of 3.0 MPa or increasing H2 pressure at a constant CO2 pressure of 9.0 MPa led to enhanced conversion and selectivity to 1-cyclohexylethanol. The same tendency was observed in the hydrogenation of 2-phenylethanol over Ru/C under otherwise similar reaction conditions. At 50 °C and 3 MPa of H2, it took 13 and 17 MPa of CO2 to completely dissolve 1- and 2-phenylethanol, respectively. Since the CO2 pressure effect on the conversion was not so striking, the authors proposed that the mass transfer of phenylethanols is not the rate-determining step. The pressure effect was speculated to be due to the enhanced removal of the products from the catalyst surface and/or the difference in the concentrations of adsorbed species or the electronic properties of metal particles that are influenced by CO2. The solubility of the products, however, was not reported. 3.2. Group-9 Metal Catalysts. 3.2.1. Rh/C. Charcoalsupported rhodium (Rh/C) is an old catalyst used for the hydrogenation of aromatic rings. Its activity is usually high and the reaction proceeds satisfactorily with low hydrogen pressures and at low temperatures (even at room temperature) in protic solvents. The catalytic performance, however, can be further improved by using scCO2 medium (Table 3, II, A). Shirai and co-workers investigated the hydrogenation of aromatic rings bearing a hydroxyl group in scCO2 with a 50mL stainless-steel autoclave using charcoal-supported 5% transition-metal catalysts.29 For the hydrogenation of phenol (20 mmol) conducted at 55 °C for 2 h with a H2 pressure of 10 MPa and a CO2 pressure of 10 MPa, the turnover number (TON28) increased in the order Pt/C < Pd/C < Ru/C < Rh/C. Rh/C (22.8 mg) could convert phenol into cyclohexanol completely at a higher temperature of 80 °C in 2 h. In addition, the catalyst could be reused at least three times without loss in the activity when the reaction was performed at 55 °C for 4 h; 100% conversion was achieved for each run with 10 and 90% selectivity for cyclohexanone and cyclohexanol, respectively. Cresols (18.5 mmol) were also converted to the corresponding methylcyclohexanones and methylcyclohexanols over Rh/C

(45.5 mg) at a H2 and CO2 pressure of 9 and 11 MPa, respectively, but the conversions and selectivities in 2 h depended strongly on the position of the methyl group. The conversion increased in the order p-cresol (47%) < o-cresol (88%) < m-cresol (99%), while the selectivity to methylcyclohexanol (69%) prevailed over that of methylcyclohexanone (31%) only for the hydrogenation of o-cresol. The Rh/C-scCO2 catalytic system was also successfully applied to the hydrogenation of naphthols at 50 °C, leading to the formation of tetrahydronaphthols and tetralones.30,31 The hydrogenated products are shown in the reaction pathway proposed on the basis of the product distribution (Scheme 8). Partially hydrogenated product, 5,6,7,8-tetrahydro-1-naphthol, was formed as major product with about 70% selectivity regardless of the H2 and CO2 pressure, while the conversion increased drastically with increase in either of the pressures. The authors observed that 1-naphthol completely dissolved at CO2 pressures higher than 10 MPa, but the solubility of the products was not reported. A unique stereoselectivity was observed in the hydrogenation of 4-tert-butylphenol over Rh/C in scCO2.32a The reaction can afford both cis- and trans-4-tert-butylcyclohexanol, but the cis/(cis + trans) ratio achieved in scCO2 was higher than that in conventional 2-propanol when the reaction was performed at 40 °C over 5 mg of the catalyst (5% Rh) with 2.00 mmol of the substrate, 2 MPa of H2, and 15 MPa of CO2. In addition, the ratio in scCO2 remained constant as the reaction proceeded, whereas in 2-propanol it gradually decreased. The cis-4-tert-butylcyclohexanol is formed by the direct hydrogenation of 4-tert-butylphenol and 4-tert-butylcyclohexanone, while trans-4-tert-butylcyclohexanol is formed through the flipping of 4-tert-butyltetrahydrophenol. The results imply that the scCO2 acts not only as solvent but also as inhibitor for the flipping, although the detailed role of the compressed CO2 was not elucidated. It is also interesting to note that the cis/(cis + trans) ratio depended on the CO2 pressure, showing a maximum at 10 MPa in the CO2 pressure range 0.1–25 MPa with a constant H2 pressure of 2 MPa. Effect of hydrochloric acid addition on the stereoselectivity was also examined, which showed that the cis ratio can be increased in the presence of the acid for both in scCO2 and in 2-propanol. The ratio, however, is higher in the scCO2-HCl system than in the 2-propanol-HCl system. In a subsequent full account, the same authors reported the results of the hydrogenation of 2- and 3-tert-butylphenol as well as 4-tert-butylphenol.32b Typically, the reaction in scCO2 was performed at 40 °C with 20 mg of the catalyst, 2.00 mmol of substrate, 2 MPa of H2, and 10 MPa of CO2. For comparison, the hydrogenation was also carried out in 10 mL of 2-propanol. In the hydrogenation of 2-tertbutylphenol, cis-2-tert-butylcyclohexanol was predominantly formed with a cis/(cis + trans) ratio of 0.96 in both scCO2 and 2-propanol. The reaction model based on the basic organic stereochemistry implied that the bulky tert-butyl group inhibits the adsorption of the intermediates leading to the trans product. On the other hand, the intermediate, 3-tertbutylcyclohexanone, in the hydrogenation of 3-tert-butylphenol can be desorbed and readsorbed from both sides of the ring due to less steric effect of the tert-butyl group, and thus the cis/(cis + trans) ratio decreased when the consecutive hydrogenation of 3-tert-butylcyclohexanone proceeded. The Rh/C catalyst treated with HCl was also tested for the 2and 3-tert-butylphenol. The main role of the added acid was the acceleration of the hydrogenation of the intermediates,

CO2

CO2

cinnamaldehyde; R-methyl-trans-cinnamaldehyde; crotonaldehyde (7.5)

cinnamaldehyde (7.5)

50 or 70 110

CO2 CO2

ethane

50

CO2

ethyl pyruvate (180)

50 50

CO2 CO2

R-pinene (7) R-methylstyrene (substarate:H2 ) 1:3.42) cinnamaldehyde (18.8); crotonaldehyde (35.6) isophorone (13 or 26) 1- and 2-naphthol; 2-methyl-1-naphthol (1.4)

CO2

100-250 50-200

CO2(+cosolventf) CO2(+cosolventg)

maleic anhydride (25) 4-oxoisophorone (50)

4-methoxycinnamic acid benzyl ester (0.186)

35

CO2

cyclohexene (100)

50

50

25-100

25-45

50

60 60 50

CO2 CO2 CO2

CO2

50 40 or 80

CO2 CO2

citral (6.5)

55-80

CO2

phenol (20); o-, m-, and p-cresol (18.5) 1- and 2-naphthol (1.4) 2-, 3-, and 4-tert-butylphenol (2.00) naphthalene (2.3) tetralin (2.34) biphenyl (2.3)

50

T (°C)

CO2

solvent

1- and 2-phenylethanol (7.0)

substrate(s) (amount/mmol)

P (MPa), solvent

A. Ru/C 5-20

A. Rh/C

10-22 0.1-26 0.1-25

8-18 0.1-25

10-20

12-28 6.1-32 3.1-28

11-21 2.1-27

16-30

8-23

P (MPa), total

9.2-21.2

4.0

1-6

7-22

10.0-18.0

F. Pt/Al2O3 4-17

G. Pt/SiO2 6.0-14.0

7-20

11-21

D. Pd/SBA-15 8.0-20.0

10-23 16.5-23

9.0-18.0

14 7.0-13.0

4.2-15 10.0-21.0

C. Pd/MCM-48 7-17

9.1-20.1 16-22

7.0-14.0

E. cinchonidine-modified Pt/Al2O3 1-14 6

1.2

1-6

1-6 0.5-1.5

1.0-6.0

A. Pd/Al2O3 0.7-2.8 2.1-12 0.96-3.2 8.3-19.3 B. Pd/C 2-6 8-12

III. group-10 metal catalysts

B. SBA-15-supported RhCl[P(m-C6H4SO3Na)3]3 5.0 11.0 16.0

2-9 6-12 3-9

1-6 2

6-10

II. group-9 metal catalysts

1-9

I. group-8 metal catalysts

P (MPa), H2

Table 3. Heterogeneous Catalytic Hydrogenations in Supercritical Fluids with Batch Reactors (Work Published up to 2007)

v

vi

v

v

v

v

v

v

v v

v

v

ra

f

vi

v

v

f

v vVh

V f

fd ve

v

Sb

1

1

42

2i

51

49

47

46

45

43 44

40 41

38 39

37

31,33,35 34, 35 36

30, 31 32

29

27

ref(s)

2

1 1

1 1 1

1 1d2e

1

number of phase under optimized conditionsc

4570 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

40-90 50

CO2 CO2

CO2 CO2

cinnamaldehyde (7.5) citral (6.5) 2-6 2-6

6.0-17.0 6-17

CO2 50

4 b

6.0-16.0

V. wall of SUS316 stainless steel reactor as catalyst

50 35-70

A. Ru-Pt/MCM-48

0-15 6.0-16.0

6-16 8-17

7-17

P (MPa), solvent

10-20

9-21 10-21

1.1-16 10.0-20.0

10-20 9-18

11-21

P (MPa), total

v

v v

v viVk

v v

v

ra

v

v v

v vi

v v

v

Sb

2

2 1

1 1i,2k

1,2j 1

2

number of phase under optimized conditionsc

60

58 59

56 57

53 55

52b, c

ref(s)

Change of reaction rate in the supercritical region with respect to subcritical conditions (v increase, V decrease). Change of selectivity in the supercritical region with respect to subcritical conditions (v increase, V decrease, f no drastic change). c Number of phases that afforded the best result is shown. (1) homogeneous single phase; (2) two phases composed of expanded liquid and dense solvent. d For the hydrogenation of 2-tert-butylphenol. e For the hydrogenation of 4-tert-butylphenol. f The cosolvents were ethylene glycol dimethyl ether and acetone. g The cosolvents were cyclohexane, methanol, methyl isobutyl ketone, acetone, acetonitrile, and propyl acetate. h The selectivity to 3,5,5-trimethyl-4-hydroxycyclohexanone increased, while that to 3,5,5-trimethylcyclohexane-1,4-dione decreased. i For the hydrogenation of cinnamaldehyde. j The highest conversion was achieved in a two-phase system, while a single homogeneous phase afforded the highest selectivity. k For the hydrogenation of benzaldehyde.

a

1.1 2.0-4.0

4.0 1.0

I. Pt/C

2-6

IV. bimetallic catalysts

35 50

30-70

CO2 CO2

2-butyne-1,4-diol (5)

P (MPa), H2

H. Pt/MCM-48

T (°C)

nitrobenzene (16.2) 2-, 3-, and 4-nitroanisole; 2-, 3-, and 4-nitrotoluene; 2,4-dinitrobenzene; 2,4-dinitrotoluene (5) 2-, 3-, and 4-chloronitrobenzene; 2,5-dichloronitrobenzene (5) benzaldehyde (3.0-5.3); cinnamaldehyde (0.25)

solvent

CO2

substrate(s) (amount/mmol)

cinnamaldehyde (7.5)

Table 3. Continued

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4571

4572 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Scheme 7. Reaction Pathways for the Hydrogenation of 1- and 2-Phenylethanol

Scheme 8. Reaction Scheme of 1-Naphthol Hydrogenation over Rh/C in scCO2

tert-butylcyclohexanones, to the corresponding cis-tertbutylcyclohexanols, increasing the selectivity to the cis product particularly at the initial stage of the reactions. The group of Shirai also investigated in detail the hydrogenation of aromatic hydrocarbons over Rh/C in scCO2, which is important in terms of storing hydrogen by producing cyclic saturated hydrocarbons and of the production of a high performance diesel fuel. They found that Rh/C promotes the hydrogenation of naphthalene to afford decalin in high selectivity in scCO2 even at a low reaction temperature of 60 °C.31,33 The hydrogenation was typically performed in a 50-mL reactor, using 2.3 mmol of naphthalene, 6 MPa of H2, 10 MPa of CO2, and 0.10 g of charcoal-supported 5% transition-metal catalysts. Under the conditions, naphthalene completely dissolved in the compressed CO2 medium. The transition-metals tested other than Rh were Ru, Pd, and Pt, but their activity and selectivity were considerably lower compared to Rh. The turnover number (TON28), conversion, and selectivity to decalin achieved with Rh/C in 30 min were 190, 46.9%, and 61.2%, respectively, all of which were highest among the catalysts screened. Decalin was formed in parallel with tetralin without any induction period for about 90 min, after which tetralin was gradually converted into decalin. The catalytic behavior of Rh/C was examined also in n-heptane, but the activity and selectivity to decalin were lower than those in scCO2, presumably due to the lower surface hydrogen concentration in n-heptane. The effect of changing H2 pressure at a constant CO2 pressure of 10 MPa showed that

the conversion and selectivity to decalin increased with increase in H2 pressure from 2 to 6 MPa, above which both of them stayed constant. The authors suggested that the catalyst surface would be saturated with hydrogen beyond 6 MPa of H2. The CO2 pressure was also changed in the range 10–22 MPa with a constant H2 pressure of 6 MPa, indicating that increasing CO2 pressure decreased the selectivity to decalin, while the conversion was not affected. The authors suggested that the catalyst surface was saturated with hydrogen, which led to the constant conversion, and that the desorption of the intermediate, tetralin, was enhanced as the CO2 pressure was increased, resulting in a decrease in the selectivity to the fully hydrogenated product. It is interesting to note that Pt/C itself exhibited almost no activity, but the mixture of Rh/C with Pt/C showed higher activity compared to Rh/C alone, implying that the dissociated hydrogen formed on platinum particles moved to rhodium particles by spillover, leading to the enrichment of hydrogen on the Rh/C catalyst surface. The above report did not refer to the selectivity to cis-decalin, which is more preferable than trans-decalin in terms of storing hydrogen because of the higher dehydrogenation rate. In addition, the cis-isomer is an intermediate for manufacturing 6,10-nylon and plasticizer. Shirai’s group later reported that cisdecalin can be obtained in high yield from tetralin over Rh/C by using scCO2 medium.34 The reaction was typically performed in a 50-mL reactor at 60 °C using 2.34 mmol of tetralin, 2 mg of 5% Rh/C, 6 MPa of H2, and 15 MPa of CO2. Under these conditions, tetralin was hydrogenated almost quantitatively in 150 min to give cis- and trans-decalin in 82 and 18% yield, respectively. The possible mechanism proposed by Shirai et al. is depicted in Scheme 9, which suggests that the desorption and subsequent readsorption in overturned manner of the intermediate, ∆1,9-octalin, leads to the formation of transdecalin. The isomerization of cis-decalin can also give transdecalin, but such a reaction was not observed under the conditions applied. It is thus expected that trans-decalin is formed when the hydrogenation rate is slow owing to the low concentration of hydrogen on the catalyst surface. Actually high selectivity to cis-decalin was observed in scCO2, which is greatly miscible with gaseous hydrogen, while the selectivity was lower in n-heptane. In addition, the higher concentration of surface hydrogen led to higher reaction rate in scCO2 than in n-heptane and under solventless conditions. The importance of the high concentration of surface hydrogen could be also seen in the enhanced selectivity to cis-decalin as well as the conversion

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4573 Scheme 9. Formation of trans-Decalin in Tetralin Hydrogenation

when the H2 pressure was increased from 6 to 12 MPa. The effect of CO2 pressure was investigated in the pressure range 0.1 to 26 MPa, which showed that increasing the pressure increased the conversion and the yield of cis-decalin but decreased the selectivity to cis-decalin. Tetralin is completely soluble above 10 MPa of CO2 at 60 °C and a H2 pressure of 6 MPa. Therefore, the increased diffusion of tetralin and enhanced removal of the products from the surface should be attributed to the enhanced conversion and yield, but the high CO2 density also promotes the desorption of ∆1,9-octalin, which seems to be related to the decrease in the selectivity to cis-tetralin (Scheme 9). The research group also studied the effect of reaction time, H2 pressure, and CO2 pressure on the cis-decalin selectivity in naphthalene hydrogenation.35 The tendency of the results was similar to that of the hydrogenation of tetralin and it again showed the importance of the high concentration of surface hydrogen to achieve the high selectivity to cis-decalin. Biphenyl also can be used as hydrogen storage compound, and its hydrogenation in scCO2 was investigated by Shirai’s group.36 The reaction was carried out in a 50-mL autoclave and the catalyst screening was performed at 50 °C using 20 mg of charcoal- or alumina-supported 5% metal catalyst, 2.3 mmol of biphenyl, 6 MPa of H2, and 10 MPa of CO2 for 30 min, which showed that Rh/C and Ru/C are suitable for the reaction. The conversion and turnover number (TON28) of Rh/C (74.7%, 1490) were higher than those of Ru/C (67.7%, 640), but the selectivity to the fully hydrogenated product, bicyclohexyl, was lower for Rh/C (39.3%) than for Ru/C (55.3%). Alternatively, the partially hydrogenated product, cyclohexylbenzene, was preferred over Rh/C. Extending the reaction time under the same conditions except for the decreased H2 pressure (3 MPa) and the increased CO2 pressure (15 MPa), however, led to the complete conversion to bicyclohexyl over both Ru/C and Rh/ C, indicating that the hydrogenation takes place by the consecutive hydrogenation via cyclohexylbenzene as well as the direct hydrogenation of biphenyl. The increase in H2 pressure in the range 3–9 MPa with 15 MPa of CO2 increased the conversion in 15 min for both Rh/C and Ru/C, presumably owing to the increase in hydrogen surface coverage but did not alter the selectivity to bicyclohexyl. The effect of changing CO2 pressure was also investigated in the range 5–25 MPa with a constant H2 pressure of 3 MPa. The selectivity of bicyclohexyl was almost constant in the CO2 pressure range, while the conversion increased with increase in CO2 pressure up to 15 MPa, above which it remained constant. The 2.3 mmol of biphenyl completely dissolved in 12–13 MPa of CO2 with 3 MPa of H2 at 50 °C. However, another experiment aimed at elucidating the effect of modifying the substrate amount on the conversion and selectivity demonstrated that the solubility was not a crucial factor determining the conversion. The authors thus proposed the following two reasons for the increase in the conversion in the CO2 pressure range 5–15 MPa: (i) increasing CO2 pressure increases the solubility of the products, enhancing their removal from the catalyst surface, and/or (ii) the concentration of the

Scheme 10. RhCl(TPPTS)3 (TPPTS ) P(m-C6H4SO3Na)3) Immobilized on Mesoporous Silica Material SBA-15

adsorbed species or the electronic state of metal particles was altered by the change of CO2 amount. The hydrogenation over Rh/C was tested also in organic solvents such as n-heptane and methanol, but the bicyclohexyl yields and TONs were lower than those obtained in scCO2. 3.2.2. SBA-15-Supported RhCl[P(m-C6H4SO3Na)3]3. Huang et al. immobilized the Wilkinson-type complex, RhCl(TPPTS)3 (TPPTS ) P(m-C6H4SO3Na)3), on mesoporous silica material SBA-15, and tested it as heterogeneous catalyst for the hydrogenation of CdC bonds (Table 3, II, B).37 The catalyst was obtained by treatment of Rh2(CO)4Cl2 with TPPTS in water, followed by the addition of SBA-15 and subsequent drying of the solid material. Fourier transform infrared (FTIR) measurements showed the disappearance of the CO ligands after the immobilization and also the change of the IR bands of the surface hydroxyl groups, implying that the catalyst has a surface structure as depicted in Scheme 10. Under both neat and scCO2 conditions, RhCl(TPPTS)3 exhibited hydrogenation activity only when it was supported on SBA-15. The hydrogenation of cyclohexene over the catalyst occurred much faster in scCO2 than under solventless conditions. 3.3. Group-10 Metal Catalysts. 3.3.1. Pd/Al2O3. Pillai and Sahle-Demessie reported the selective hydrogenation of maleic anhydride to γ-butyrolactone in scCO2 using 1% Pd/Al2O3 catalyst (Table 3, III, A).38a The catalyst was prepared by wet impregnation of alumina pellets with a solution of palladium chloride (0.02 M), followed by drying at 110 °C and subsequent calcination at 450 °C, and it was reduced at 450 °C under a flow of hydrogen before use. Typically, the reactions were performed for 2 h at 200 °C in a 500-mL stainless steel autoclave using 25 mmol of the substrate, 0.5 g of the catalyst, and 2.1 MPa of H2. In 10 mL of organic solvents such as polyethylene glycol, ethylene glycol dimethyl ether, and acetone, hydrogenation of only the CdC bond took place to afford succinic anhydride and the yields of γ-butyrolactone were quite low (i.e., 2–7%). In contrast, however, the use of scCO2 medium at a CO2 pressure of 12 MPa resulted in the selective formation of γ-butyrolactone in higher than 80% yield. The reaction was also conducted at a lower reaction temperature or at a lower CO2

4574 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Scheme 11. Hydrogenation of 4-Oxoisophorone

pressure, but these conditions led to a decrease in yield and selectivity. Due to the temperature limitation of the high-pressure view cell, it could not be identified whether the reaction medium was monophasic or biphasic under the best conditions at 200 °C. However, the phase behavior calculation of a mixture of CO2, H2, and maleic anhydride using the Peng-Robinson equation indicated that the mixture is homogeneous under the conditions applied. In a succeeding full account, the authors reported some additional data.38b They found that the product yield in ethylene glycol dimethyl ether or acetone can be increased when they are used with scCO2. However, the yields achieved in such scCO2-cosolvent systems were still far lower compared to that in scCO2 alone. Deactivation of the catalyst was also investigated, which showed that the selectivity to γ-butyrolactone began to decrease gradually after reusing the catalyst three times. CO chemisorption measurements indicated that the metal surface area decreased during reuse, implying that the active sites for the production of γ-butyrolactone are the same as those for the CO chemisorption and that the CO formation on the metal surface is one of the causes for the deactivation. Pillai and Sahle-Demessie tested the same 1% Pd/Al2O3 catalyst also in the hydrogenation of 4-oxoisophorone (denoted A) in scCO2 (Table 3, III, A; Scheme 11).39 The reaction was typically carried out at 100 °C for 2 h using a 500-mL reactor, 50 mmol of the substrate, 0.50 g of the catalyst, 1.7 MPa of H2, 13.3 MPa of CO2, and, if required, 50 mL of cosolvent. The reaction mixture was stirred with a spinning dynamic basket in which the catalyst was loaded. The effect of CO2 pressure variations at a constant H2 pressure of 1.7 MPa showed that the conversion improved with increasing the pressure and reached a maximum (100%) at 13.3 MPa. Further increase of CO2 pressure, however, resulted in monotonous decrease of the conversion. Since the mixture became single homogeneous phase at 13.3 MPa of CO2 or more, the results indicate that the reaction was mass transport-controlled up to a CO2 pressure of 13.3 MPa, above which it became kinetically controlled in which dilution effect decreased the hydrogenation rate. The hydrogenation was carried out also in conventional polar and apolar organic solvents under similar conditions except for the absence of CO2. The conversion obtained in apolar cyclohexane in 2 h was 100% which was equal to that given by 13.3 MPa of CO2, while other polar organic solvents afforded lower conversions. The polarity of the medium also affected the product selectivities. In apolar scCO2 and cyclohexane, 3,5,5-trimethylcyclohexane-1,4-dione (denoted B) and 3,5,5-trimethyl-4-hydroxycyclohexanone (denoted C) were mainly formed, whereas protic and other polar organic solvents such as methanol, acetonitrile, and propyl acetate yielded B and 3,5,5-trimethyl-4-hydroxy-1cyclohexanon-2-ene (denoted D) as major products. Note, however, that the above selectivity behavior in scCO2 was

observed only when the hydrogen pressure and the total pressure were 1.7 MPa or more and 15 MPa, respectively. The scCO2-cosolvent systems were also tested in the hydrogenation, revealing that cyclohexane gave the highest conversion as cosolvent among the organic cosolvents tested. However, the use of scCO2-cosolvent systems usually led to lower conversions compared to those obtained with pure scCO2 and the corresponding organic solvents. The reusability of 1% Pd/Al2O3 catalyst was examined in both scCO2 and methanol, which showed that the extent of deactivation was much lower in scCO2 than in methanol. Although monotonous decrease in metal dispersion with increasing the recycle number was observed for both catalysts spent in scCO2 and methanol, the decrease was more pronounced for the catalyst spent in methanol than in scCO2. The authors did not refer to the reason of this behavior. Probably, palladium leaching could be suppressed in apolar scCO2 medium, although the palladium content in the spent catalysts was not reported. 3.3.2. Pd/C. Chouchi et al. performed the hydrogenation of R-pinene to pinane over 10% Pd/C in scCO2 to investigate the relation between phase behavior and catalytic performance (Table 3, III, B).40 The hydrogenation was performed using a 40-mL reactor, 1 g of the substrate, 20 mg of the catalyst, and 8–12 MPa of CO2. Hydrogen was added to the reactor until the total pressure reached 14 MPa. The highest reaction rate was observed at CO2 pressures of 8.0–9.0 MPa, and the rate decreased drastically with further increment of CO2 pressure. Since the phase transition from two phases to single phase occurred at 9.5–10.0 MPa, the biphasic system favored the hydrogenation. The authors speculated that the limiting step for the catalysis is the access of substrate, not hydrogen, to the catalyst surface under the conditions applied. The catalyst thus must be surrounded by the concentrated liquid substrate which, however, contains large amounts of CO2. This can be best achieved with a CO2-expanded liquid which is still able to dissolve large amounts of hydrogen. Another work on the use of CO2-expanded substrate was performed by Phiong et al., who investigated the hydrogenation of R-methylstyrene to cumene (Table 3, III, B).41 Zhao et al. successfully performed the selective hydrogenation of R,β-unsaturated aldehydes to the corresponding saturated aldehydes in scCO2 using a commercially available 10% Pd/C catalyst (Table 3, III, B).42 The catalyst was prereduced at 200 °C for 3 h under a hydrogen flow before use. Representative results are given in Table 4. Even though the selectivities to the saturated aldehydes in scCO2 were equal to those obtained in toluene and under solvent-free conditions, the reactions in scCO2 took place much faster than those in the other media. 1-Propanol solvent was also tested but afforded a considerable amount of the diacetal as a byproduct, which decreased the selectivity. Visual inspection showed that less than 3% of cinnamaldehyde dissolved in scCO2 under the reaction conditions, indicating that the reactions occurred mainly in the liquid phase even under scCO2 conditions. Thus the complete miscibility of H2 in scCO2 and the dissolution of a large amount of CO2 in the liquid substrate led to an expanded liquid, which increased the H2 concentration in the vicinity of the solid catalyst, resulting in higher reaction rate in scCO2 compared to that in the other media. The effect of H2 and CO2 pressure on the hydrogenation of cinnamaldehyde was investigated, showing that the reaction rate increased with higher H2 and/or CO2 pressure, while the pressures did not affect the selectivity. It was demonstrated that the 10% Pd/C catalyst could be reused at least three times without loss in activity and selectivity.

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4575 Table 4. Hydrogenation of r,β-Unsaturated Aldehydes over 10% Pd/C in Various Reaction Media (Data Taken from ref 42)a selectivityc (%) reactant

solvent

time (min)

conversion (%)

TOFb (s-1)

HCAL

HCOL

COL

PhCHdCHCHO PhCHdCHCHO PhCHdCHCHO PhCHdCHCHO

CO2 (8.0 MPa) toluene 1-propanol none

60 60 60 60

100 18 75 64

3.1 0.6 2.3 1.9

87 88 71 87

13 12 22 13

0 0 0 0

CH3CHdCHCHO CH3CHdCHCHO CH3CHdCHCHO CH3CHdCHCHO CH3CHdCHCHO

CO2 (8.0 MPa) CO2 (8.0 MPa) toluene 1-propanol none

10 30 30 10 10

78 100 48 65 45

27.1 11.6 5.6 22.6 15.7

butanal

butanol

butenol

100 100 100 73 100

0 0 0 0 0

0 0 0 0 0

a Catalyst, 10% Pd/C, 0.01 g; PhCHdCHCHO, 18.8 mmol; CH3CHdCHCHO, 35.6 mmol; temperature, 50 °C; H2 pressure, 4.0 MPa; organic solvent, 15 mL. b TOF: moles of substrate reacted per mole of exposed surface Pd atoms per second. c HCAL, PhCH2CH2CHO; HCOL, PhCH2CH2CH2OH; COL, PhCHdCHCH2OH.

Sato et al. conducted the hydrogenation of isophorone in scCO2 using different supported noble metal (5%)-based catalysts (Table 3, III, B).43 This hydrogenation in scCO2 was already performed by Poliakoff and co-workers who used a continuous-flow reactor system (see section 2.3.1), but the behavior of catalysts other than Deloxan aminopolysiloxanesupported Pd was not reported. To compare the performance of the catalysts, the reaction was first performed at 50 °C for 30 min using a 50-mL reactor, 10 mg of a catalyst, 26 mmol of isophorone, 1 MPa of H2, and 9 MPa of CO2. Under these conditions, the turnover number (TON28) of the catalysts increased in the order Rh/Al2O3 < Pd/Al2O3 < Pt/Al2O3, Pd/C < Pt/C < Rh/C. This order did not depend on the difference in metal dispersion, and thus electronic state of the metals and geometry of the active sites were proposed as the factors influencing the order. In contrast to the moderate to low TONs, Pd/C and Pd/Al2O3 yielded dihydroisophorone at highest selectivity of 100%. The exclusive formation of dihydroisophorone over the supported palladium catalysts is quite attractive, because it is difficult to isolate dihydroisophorone from the other byproducts by distillation due to their close boiling points. Increasing hydrogen pressure (1–6 MPa) or CO2 pressure (9.1–20.1 MPa) for the Pd/C-catalyzed reaction at 70 °C led to higher conversion, but the selectivity was hardly affected by the change of the pressures, being almost 100% at any pressure. Unfortunately, phase behavior changes induced by the pressure changes were not reported. The reusability of Pd/C and Pd/ Al2O3 was examined under 9 MPa of CO2 at 70 and 50 °C, respectively, using 20 mg of catalyst, 13 mmol of isophorone, and 3 MPa of hydrogen. The high selectivities of these catalysts were maintained even after three times reuse, although their activities steadily decreased with increasing reuse time. Mine et al. reported the selective hydrogenation of 1-naphthol to 1-tetralone over Pd/C catalyst (Table 3, III, B).44 The reaction was performed at 110 °C with a 50-mL reactor, employing 0.5 MPa of H2, 16 MPa of CO2, 1.4 mmol of the substrate, and 75 mg of the catalyst containing 5% Pd. Under these conditions, the conversion reached 98% in 60 min and 1-tetralone was formed in 71% yield accompanied by the formation of 1,2,3,4tetrahydro-1-naphthol and 5,6,7,8-tetrahydro-1-naphthol. The amounts of the fully ring-hydrogenated products and the dehydroxylated products were almost negligible. This is in contrast to the results in n-heptane that afforded a higher hydrogenation rate than in scCO2 but gave the fully ringhydrogenated products and the dehydroxylated products in higher amounts. Methanol was also tested as solvent, but the hydrogenation did not proceed in this protic solvent. Thus the scCO2 medium not only promotes the hydrogenation smoothly

Scheme 12. Hydrogenation of Citral

but suppresses the undesirable overhydrogenations. The effect of changing the H2 pressure was examined in the pressure range 0.5–1.5 MPa with a constant CO2 pressure of 16 MPa by comparing the conversion and selectivity obtained after 10 min, which showed that the increase in the H2 pressure resulted in higher conversion but decreased the selectivity to 1-tetralone. The effect of increasing the CO2 pressure was also investigated up to 22 at 1 MPa of H2, but the activity and selectivity did not change with the CO2 pressure. The selective catalytic system using scCO2 was also successfully applicable to 2-naphthol and 2-methyl-1-naphthol, affording the corresponding tetralones in highest selectivity, whereas in n-heptane the amounts of the tetralones decreased after the maximum at 50–60 min because of the overhydrogenations. 3.3.3. Pd/MCM-48. The hydrogenation of citral to dihydrocitronellal is important in fragrance chemistry. However, the two CdC bonds and one CdO bond in citral can afford several byproducts as depicted in Scheme 12. To achieve a high selectivity to dihydrocitronellal, the two CdC bonds must be hydrogenated smoothly, while the CdO bond hydrogenation must be suppressed. Chatterjee et al. found that this can be realized by using mesoporous silica MCM-48-supported Pd catalyst and scCO2 medium (Table 3, III, C).45 The catalyst was prepared by impregnation from the aqueous solution of PdCl2, followed by drying at 80 °C and subsequent calcination at 300 °C. The reaction was typically performed at 50 °C for 2 h using a 50-mL reactor, 6.5 mmol of citral, 0.1 g of the catalyst (1% Pd), 4 MPa of H2, and 12.0 MPa of CO2. Under these conditions, 99.8% conversion and 100.0% selectivity to dihydrocitronellal could be achieved in 2 h, while these values, particularly the selectivity, achieved in the same reaction time decreased drastically in organic solvents such as benzene,

4576 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

methanol, and hexane. The other supports, SiO2, Al2O3, and C, were found to be also effective under the scCO2 conditions, but the results were slightly inferior to those obtained with Pd/ MCM-48. The effect of changing CO2 pressure at constant H2 pressure of 4 MPa was investigated in the pressure range 7–17 MPa, showing that the conversion increased with higher CO2 pressure up to 10 MPa, above which it remained constant (100%). The conversion, however, decreased at 17 MPa, probably due to the dilution effect. The selectivity to dihydrocitronellal, on the other hand, increased from 69.5 to 100% as the CO2 pressure was increased from 7.0 to 10 MPa and 100% selectivity was maintained in the CO2 pressure range 10–17 MPa. The H2 pressure was also varied in the pressure range 1–6 MPa at a constant CO2 pressure of 12 MPa. The selectivity did not change with the H2 pressure and stayed constant at ca. 100%, while the conversion increased with increase in H2 pressure; the conversion was 100% in the H2 pressure range 4–6 MPa. The authors suggested on the basis of DFT calculations that the high selectivity to dihydrocitronellal in scCO2 is due to the low dielectric constant (ca. 1.5) of the medium, which is favorable for the CdC bond hydrogenation in terms of both steric and electronic factors. It is noteworthy that the catalyst did not show any change in conversion and selectivity even after four successive reaction cycles. 3.3.4. Pd/SBA-15. Lee et al. tested mesoporous silica SBA15-supported palladium in the hydrogenation of 4-methoxycinnamic acid benzyl ester in scCO2 (Table 3, III, D).46 The scCO2 was used not only in the hydrogenation but also in the catalyst preparation. First, palladium(II) hexafluoroacetylacetonate (Pd(hfac)2), which is highly soluble in scCO2, was treated with SBA-15 in THF, followed by evaporation of the solvent and drying. The resultant solid was placed in scCO2 (60 °C, 10 MPa) to disperse the complex over the internal surface of SBA-15. After depressurization, the recovered material was washed with THF, calcined at 400 °C, and subsequently reduced with 4% H2/Ar at 300 °C to afford the SBA-15-supported palladium catalyst (denoted Pd/SBA-15) with a palladium content of 1.61 wt %. TEM images of the catalyst showed that the ordered mesoporous network of parent SBA-15 was retained even after the high-pressure treatment. In addition, they revealed the presence of well-dispersed Pd nanoparticles (∼6 nm) inside the mesoporous channels of SBA-15, which was realized by good mass transfer of Pd(hfac)2 in the porous network by virtue of scCO2 as well as high solubility of the complex in the medium. The hydrogenation in scCO2 was carried out at 45 °C for 60 min with a 10-mL reactor using 0.186 mmol of 4-methoxycinnamic acid benzyl ester, the catalyst with a molar ratio of substrate:Pd ) 50:1, and 1.2 MPa of H2. The CO2 pressure was varied in the range 8–20 MPa to study its effect on the conversion and selectivity; the substrate completely dissolved under the pressures applied. The conversion increased with higher CO2 pressure, showing a maximum at 15 MPa (100%), above which it decreased presumably because of the dilution effect. Thus under the optimal conditions with 15 MPa of CO2, the conversion reached 100% in a short time span of 10 min giving 3-(4-methoxyphenyl)propanoic acid benzyl ester and undesirable 4-methoxycinnamic acid in 95 and 5% selectivity, respectively. The selectivity did not vary with a change in CO2 pressure, but it should be noted that the selectivity to 3-(4methoxyphenyl)propanoic acid benzyl ester with Pd/SBA-15 was much higher than that achieved with Pd/C and Pd/Al2O3. This is because the latter two catalysts promoted the elimination of benzyl group as well as the CdC bond hydrogenation, giving 4-methoxycinnamic acid in high yields. The authors stressed

Scheme 13. Enantioselective Hydrogenation of Ethyl Pyruvate to (R)-Ethyl Lactate

that the remarkable suppression of the benzyl group elimination is attractive because the group is widely used as a protecting group of hydroxyl- and carboxylic groups. Unfortunately, the palladium particles on SBA-15 were not tightly immobilized and consequently a considerable amount of palladium was leached during the hydrogenation; the Pd content decreased from 1.6 to 0.2 wt % after the reaction. On the other hand, the reaction conditions did not affect the parent mesoporous structure of Pd/ SBA-15. 3.3.5. Cinchonidine-Modified Pt/Al2O3. Minder et al. carried out the enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate in scC2H6 using a 500-mL batch reactor (Table 3, III, E; Scheme 13).47 Typically, 20 mL of the substrate, 0.45 g of 5% Pt/Al2O3 modified with 0.088 g of cinchonidine, and 7 MPa of hydrogen were allowed to react in 121 g of ethane (6 MPa, 50 °C). Under these conditions, the conversion reached more than 95% in 10 min, and the product could be obtained with 74% ee. The reaction in scC2H6 proceeded much faster than that in ethanol and toluene due to the high miscibility of H2 in scC2H6, and the ee of the product formed in scC2H6 was slightly higher than that in toluene and much higher than that in ethanol. Variation of the ee as a function of reaction temperature revealed that there was almost no difference between scC2H6 and toluene, and the ee value dropped remarkably above 67 °C. In scC2H6, increase in catalyst/reactant weight ratio had a small positive effect on the ee, while in ethanol, increasing the ratio led to a drastic decrease in the ee. One of the reasons for this may be the great difference in gas–liquid transfer resistance at a higher catalyst/reactant ratio. The scC2H6 can be used even at a high catalyst/reactant ratio, rendering the reaction with a continuous-flow reactor system possible as already described in section 2.3.8. Unfortunately, scCO2, which is much more attractive than scC2H6 due to its nonflammable and nontoxic nature as well as for economic reasons, could not be used as efficient solvent for the hydrogenation, because of the formation of CO (reverse water-gas shift reaction) which is well-known as a poison for the reduction of carbonyl groups as evidenced by FTIR. However, the intensity of the vibrational bands due to adsorbed CO did not remarkably increase when the prereduction atmosphere was changed from a pure H2 flow to a CO2/H2 flow; under pure H2 flow, CO is formed from CO2 that had been adsorbed during storage of the catalyst in air. Thus, the authors concluded that not only CO but also other poisonous species that are less sensitive toward the IR analysis were formed in the presence of CO2, biasing the activity of Pt/Al2O3. In fact, a more recent study revealed the formation of a number of different species by hydrogenation of CO2 on such noble metal surfaces in scCO2.48 3.3.6. Pt/Al2O3. Bhanage et al. performed the hydrogenation of R,β-unsaturated aldehydes in scCO2 with a 50-mL reactor using 1% Pt/Al2O3 catalyst (Table 3, III, F).49 In contrast to 10% Pd/C,42 1% Pt/Al2O3 selectively afforded the corresponding unsaturated alcohol as major product. Representative results are shown in Table 5, which should be compared with those given in Table 4. The authors found that the conversion and selectivity to the unsaturated alcohols become much higher in scCO2 than in conventional ethanol and that the Pt/Al2O3 catalyst can be

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4577 Table 5. Hydrogenation of r,β-Unsaturated Aldehydes over 1% Pt/Al2O3 in ScCO2 and Ethanol (Data Taken from ref 49) pressure (MPa) selectivitya (%) reactant

conversion (%) H2 CO2 total in scCO2

PhCHdCHCHO PhCHdC(CH3)CHO CH3CHdCHCHO

40.3 79.9 59.2

SAL

UOL

18 18 18

6.6 2.6 22.0

92.6 96.0 70.8

4 4 4

10.1 21.0 33.3

77.7 79.0 45.8

b

4 4 4

14 14 14

in ethanolc PhCHdCHCHO PhCHdC(CH3)CHO CH3CHdCHCHO

29.5 42.3 41.8

4 4 4

a SAL, saturated aldehyde; UOL, unsaturated alcohol. b Substrate, 7.5 mmol; catalyst, 0.5 g; reaction temperature, 50 °C; reaction time, 2 h. c Substrate, 3.75 mmol; catalyst, 0.25 g; reaction temperature, 50 °C; reaction time, 2 h.

reused at least twice for the hydrogenation of cinnamaldehyde without any loss of the activity and selectivity. This stands in contrast to the results observed for the enantioselective hydrogenation of ethyl pyruvate where Pt/Al2O3 did not function in scCO2 due to the formation of poisoning species on the surface of Pt through CO2 reduction.22,47 The effect of changing H2 and/or CO2 pressure on the activity and selectivity to the unsaturated alcohol was examined at 50 °C in the total pressure range 7 to 22 MPa for the hydrogenation of cinnamaldehyde. Increasing H2 and/or CO2 pressure led to the increase of both conversion and selectivity. It was suggested that increase in the dielectric constant of CO2 and decrease in the electronic density of platinum particles at higher CO2 pressure account for the improved selectivity at higher CO2 pressure. The former may activate the more polar CdO group rather than the nonpolar CdC group, and the latter should enhance the adsorption of the CdO group on the platinum surface. However, note that the dielectric constant of CO2 increases only marginally with pressure. According to the data given by Obriot et al., the dielectric constants of CO2 at 50 °C are 1.04 at 4.0 MPa, 1.40 at 14.0 MPa, and 1.44 at 17.0 MPa, respectively.50 It is unlikely that such a small change in the dielectric constant can induce the selectivity increases observed. 3.3.7. Pt/SiO2. Zhao and co-workers reported the influence of electronic state and dispersion of platinum particles on the conversion and selectivity in the hydrogenation of cinnamaldehyde over Pt/SiO2 in scCO2 (Table 3, III, G).51 The Pt/SiO2 catalysts were prepared from [Pt(NH3)4]Cl2 and a porous silica gel by ion-exchange to achieve metal contents of 1 and 2 wt %. The hydrogenation was performed at 50 °C for 2 h employing 0.1 or 0.2 g of the catalyst, 7.5 mmol of the substrate, 4.0 MPa of H2, and 6.0–14.0 MPa of CO2, which afforded the conversion in the range 11.2–27.0% and the selectivity to cinnamyl alcohol in the range 33.0–88.1%. The reaction results and the XPS studies of Pt/SiO2 reduced with H2 under different conditions (300–800 °C, 2–4 h) showed that the selectivity for cinnamyl alcohol increased with increasing Pt0/Pt2+ ratio. The reason for this behavior was not elucidated, but the authors suggested that hydrogen dissociated on the more reduced and electronically less positive metallic platinum has higher nucleophilicity and thus attacks the carbonyl carbon of cinnamaldehyde more smoothly. For the catalyst with small Pt0/Pt2+ ratio, the selectivity to cinnamyl alcohol also depended on the degree of platinum dispersion, and higher degree of platinum dispersion led to a higher selectivity. 3.3.8. Pt/MCM-48. Chatterjee and co-workers used Pt/ MCM-48 in the hydrogenation of cinnamaldehyde to cinnamyl alcohol (Table 3, III, H).52b The catalyst was synthesized not

by the conventional impregnation method but under hydrothermal conditions using chloroplatinic acid as platinum source.52a The Pt source was added to an aqueous solution of cetyltrimethylammonium bromide as surfactant, followed by the addition of tetraethylorthosilicate as silica source to give the gel, which was heated at 140 °C in an autoclave and subsequently calcined at 550 °C to afford the “in situ” synthesized catalyst. The hydrogenation was performed at 50 °C for 2 h using 7.5 mmol of cinnamaldehyde and 0.1 g of the catalyst. The effect of changing the CO2 pressure was examined in the range 0–12.5 MPa with a constant H2 pressure of 4.0 MPa, showing that the conversion increased with higher CO2 pressure. The selectivity to cinnamyl alcohol, on the other hand, showed a maximum at 10.0 MPa. The authors proposed that the high compressibility of scCO2 around 10.0 MPa renders the local dielectric constant larger than that in the bulk area and that the increased solvent polarity promotes the hydrogenation of the CdO bond. The H2 pressure was also changed in the range 2.0–6.0 MPa with a constant CO2 pressure of 10.0 MPa, which revealed that increasing the pressure increased the conversion presumably due to the increased availability of hydrogen on the surface. The selectivity showed a maximum at a H2 pressure of 4.0 MPa. The effect of changing the reaction temperature was investigated in the range 30–70 °C with 4.0 MPa of H2 and 10.0 MPa of CO2. Although the conversion increased monotonously with increase in the temperature, the highest selectivity to cinnamyl alcohol was obtained at 50 °C. Summarizing the results, the optimal conditions were 4.0 MPa of H2, 10.0 MPa of CO2, and a reaction temperature of 50 °C, which afforded the target cinnamyl alcohol in 96.6% selectivity at 30.8% conversion. The phase behavior was not reported. The same authors later used the Pt/MCM-48 catalyst prepared by impregnation (denoted Pt-MCM-48 (im)) and compared its catalytic behavior for the same hydrogenation with that of the above in situ synthesized catalyst (denoted Pt-MCM-48 (is)).52c For the impregnation, the calcined MCM-48 was treated with 0.2 N HCl solution of [Pt(NH3)4]2+ or H2PtCl6, followed by filtrating, washing, drying at 80 °C, and calcination at 500 °C. Although the Pt-loaded MCM-48 catalysts prepared by the two different methods had a mesoporous structure similar to parent MCM-48, there was a large difference in the size of platinum particles between the catalysts. The Pt-MCM-48 (is) showed a fairly narrow particle-size distribution in the range 2–6 nm, while the Pt-MCM-48 (im) contained predominantly much larger particles in the range 20–30 nm. In addition, the electronic state of the platinum particles was also different as revealed by XPS analysis showing that Pt-MCM-48 (im) had a larger Pt0/Pt2+ ratio compared to Pt-MCM-48 (is). Their catalytic behaviors for the hydrogenation were also considerably different. The reaction was performed using a 50-mL reactor, 7.5 mmol of cinnamaldehyde, 0.1 g of catalyst, 2–6 MPa of H2, and 7–17 MPa of CO2. The major product formed over Pt-MCM-48 (is) was cinnamyl alcohol regardless of the pressure of H2 and CO2, whereas Pt-MCM-48 (im) mainly afforded hydrocinnamaldehyde. The authors attributed the difference in catalytic behavior, i.e., much higher conversion and selectivity to cinnamyl alcohol with Pt-MCM-48 (is) than with Pt-MCM-48 (im), to the difference in particle size and electronic state. Although there have been no sufficient evidence about the reaction mechanism, the CdO group with an electron deficient carbon atom should be more easily hydrogenated by hydrogen adsorbed on Pt0 rather than Pt2+. The smaller particles on Pt-MCM-48 (is), on the other hand, might be favorable for the CdO bond hydrogenation that is sensitive to the structure of platinum particles, particularly

4578 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Scheme 14. Possible Reaction Pathways in the Hydrogenation of Nitrobenzene

when silica is used as a support. The authors also proposed that the larger Pt particles on Pt-MCM-48 (im) caused multiple pore blockages by reactant and product molecules, leading to lower activity and selectivity. Both Pt-MCM-48 (is) and (im) could be recycled at least four times for the hydrogenation. 3.3.9. Pt/C. The Pt/C catalyst has been successfully applied for the selective hydrogenation of nitro group on aromatic rings in scCO2 to give the corresponding arylamines. The nitro group hydrogenations seem to be free from the site-block problem caused by carbon monoxide in situ formed by reverse water-gas shift reaction. Zhao et al. performed the hydrogenation of nitrobenzene in scCO2 using commercially available 5% Pt/C catalyst prereduced in a stream of hydrogen under different conditions (Table 3, III, I).53a The products that can form in the hydrogenation of nitrobenzene are depicted in Scheme 14. Increasing the reduction temperature of Pt/C promoted the sintering of platinum particles and the catalysts with different Pt particle sizes exhibited different catalytic behaviors in different solvents. In the reaction performed in a 50-mL reactor using 16.2 mmol of nitrobenzene, 10 mg of Pt/C, 4 MPa of H2, and 14 MPa of CO2, the conversion decreased with increasing Pt particle size, while the same change in the particle size did not affect the selectivity. In ethanol solvent, on the other hand, both the conversion and selectivity monotonously decreased with increasing particle size. Plots of turnover frequency (TOF54) in scCO2 and ethanol against the degree of Pt dispersion showed that the TOF in scCO2 did not depend on the Pt dispersion, whereas increasing the degree of Pt dispersion decreased the TOF in ethanol. The authors thus concluded that the hydrogenation in ethanol is structure sensitive, while it appears structure insensitive in scCO2. The effect of changing the CO2 pressure at a constant H2 pressure of 4 MPa was investigated using the Pt/C catalysts prereduced at 300 and 750 °C, which showed that the conversion increased with increasing CO2 pressure up to 10 MPa, above which it decreased presumably due to the dilution effect. The selectivity to aniline was almost constant in the CO2 pressure range 6–10 MPa, but increased at 14 MPa, particularly for the catalyst prereduced at 750 °C. The solubility of nitrobenzene under these conditions was also reported, revealing that the reaction mixture consisted of three phases, i.e., gas (CO2 rich), liquid (nitrobenzene), and solid (catalyst) at 10 MPa or below and that the liquid phase disappeared at 14 MPa. Hence, it seems that a CO2expanded substrate phase is beneficial in this case. The same authors later tested various types of hydrogenation catalyst including Pt/C in the hydrogenation of nitrobenzene in scCO2 (Table 3, III, I).53b Typically, the reaction was performed in a 50-mL reactor using 16.2 mmol of nitrobenzene, 10 mg of 5% metal catalyst prereduced at 300 °C with H2, 4.0 MPa of H2, and 14.0 MPa of CO2. Under these conditions, the hydrogenation activity increased in the order Ru, Rh < Pd < Pt, while the best support for Pt was charcoal, followed by SiO2 and Al2O3. With the 5% Pt/C catalyst, nitrobenzene was

quantitatively hydrogenated to aniline in 50 min. It is also noteworthy that the 5% Pd/C catalyst also gave the same results, although the conversion after 10 min was lower than that with 5% Pt/C. The effect of changing the CO2 pressure on the activity and selectivity of Pd/C and Pt/C was investigated in the CO2 pressure range 6–16 MPa with a constant H2 pressure of 4.0 MPa. For both catalysts, the conversion increased with increasing CO2 pressure up to 12 MPa, above which it decreased. The selectivity to aniline afforded by 5% Pd/C, on the other hand, reached 100% at 8, 14, and 16 MPa. The 5% Pt/C also showed similar behavior; the selectivity exhibited local maximum at 8 and 16 MPa. Since nitrobenzene and hydrogen became, according to parallel phase behavior studies, completely miscible with CO2 above a CO2 pressure of 12 MPa, the high conversion and selectivity at 14 MPa might be attributed to the elimination of the gas (H2)-liquid (nitrobenzene) mass transfer resistance. The metal dispersion also strongly influenced the turnover frequency (TOF54). In the former report,53a the authors reported the variation of TOF as a function of metal dispersion in the relatively small range (0.1) are used. The Pd/C catalyst also exhibited a similar single line of negative slope but showed no deviation even in the lower dispersion range. The hydrogenations over Pt/C and Pd/C were also performed in ethanol solvent (10 mL), but the conversion and selectivity to aniline were lower than those achieved in scCO2. Zhao et al. also reported the hydrogenation of a series of substituted aromatic nitro compounds in scCO2 using 5% Pt/C catalyst (Table 3, III, I).55 Typically, the reaction was carried out for 10 min at 50 °C using a 50-mL reactor, 5 mmol of substrate, 5 mg of the catalyst, 1.0 MPa of H2, and 15 MPa of CO2, under which conditions all the substrates and hydrogen were completely miscible with CO2. The reaction was also performed in 20 mL of ethanol using 2 mmol of substrate and 2 mg of the catalyst for comparison. The hydrogenation of nitroanisoles occurred with 100% selectivity in scCO2 and the conversion increased in the order 4-nitroanisole < 3-nitroanisole < 2-nitroanisole. Similar results were obtained in ethanol, but the conversion was slightly higher. Nitrotoluenes were also hydrogenated in both scCO2 and ethanol. The conversion increased in the order 2-nitrotoluene < 3-nitrotoluene < 4-nitrotoluene in scCO2 and ethanol, while the selectivities were higher in scCO2 than in ethanol except for 4-nitrotoluene that was formed in 100% selectivity in both media. The hydrogenation of dinitro compounds, 2,4-dinitrobenzene and 2,4-dinitrotoluene, were also investigated, showing that the mono hydrogenation and the hydrogenation at 4-position were favored in scCO2, while the double hydrogenated products and other

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4579

byproducts were formed with higher selectivities in ethanol. Summarizing the results, the total conversion in scCO2 was lower than those in ethanol, but the use of scCO2 led to higher selectivity. It is interesting to note that the addition of 15 MPa of N2 to the system with ethanol solvent caused a decrease in conversion but an increase in selectivity. Thus the hydrogenations were affected not only by the physical properties of scCO2 such as the solubilizing power, diffusivity, and viscosity, but also by the high-pressure condition. The effect of CO2 pressure was investigated at a constant H2 pressure of 1.0 MPa for 3-nitrotoluene and 2,4-dinitrotoluene, revealing that increasing the pressure increased the conversions up to 12 and 15 MPa, respectively, while above these pressures the conversion changes became marginal. The authors proposed that the reactivity of nitro group and surface metal particles are changed when changing CO2 pressure, which affects the conversion and selectivity. However, additional studies are required to prove this speculation. Ichikawa et al. reported the chemoselective hydrogenation of halogenated nitrobenzenes to halogenated anilines over Pt/C in scCO2 (Table 3, III, I).56 2-Chloronitrobenzene was chosen as model substrate and its hydrogenation was investigated in detail. Typically the hydrogenation was performed at 40 °C using a 50-mL reactor, 5 mmol of the substrate, 2 mg of the catalyst, 1.1 MPa of H2, and 10 MPa of CO2. Under these conditions, 2-chloronitrobenzene was completely consumed over 1% Pt/C in 150 min to give 2-chloroaniline and aniline in 99.7 and 0.3% selectivity, respectively. Visual observation of the phase behaviors indicated that 2-chlorobenzene, 2-chloroaniline, H2, and CO2 formed a single homogeneous phase under the conditions applied, which was found to be necessary to achieve the highest yield of 2-chloroaniline. When the reaction was performed without CO2, the selectivity to 2-chloroaniline decreased to 95.6% in 300 min. In addition, the dechlorination of 2-chloroaniline to aniline did not appreciably proceed even at 90 °C when the reaction was performed with 10 MPa of CO2. Thus the use of compressed CO2 is crucial for achieving the high selectivity to 2-chloroaniline. In situ diffuse reflectance FTIR spectroscopy under atmospheric conditions showed the presence of adsorbed CO after treatment of Pt/C with H2 and CO2. Moreover, increasing the Pt loading, which decreases the kinked and stepped surfaces on Pt metal, resulted in a decrease in the yield of aniline in the hydrogenation of 2-chloroaniline without CO2 at 90 °C. It was therefore concluded that the in situ generated CO by the reaction of CO2 with H2 (reverse water-gas shift reaction) is adsorbed on the kinked and stepped surfaces and thereby suppresses the undesirable dechlorination. Thus, the undesired conversion of 2-chloronitrobenzene to aniline could be noticeably suppressed in scCO2. We also should note that the increase in the selectivity to 2-chloroaniline also leads to the suppression of the formation of HCl that is a problematic corrosive component in large-scale productions. Similarly, 3-chloro- and 4-chloronitrobenzene were hydrogenated to the corresponding chloroanilines with 100% conversion at >99% selectivity. For 2,5-dichloronitrobenzene, a higher reaction temperature of 60 °C was required. Summarizing the results, scCO2 acted not only as a solvent to enhance the miscibility of the reaction mixture but also as a promoter for the selective hydrogenations due to the selective poisoning of sites active in the undesired reaction by CO formed via reduction of CO2. The Pt/C catalyst was successfully applied also to the hydrogenation of benzaldehyde and cinnamaldehyde in scCO2 by Zhao et al., although their goal was to elucidate the CO2

pressure effect on conversion and selectivity, not to optimize the reaction conditions (Table 3, III, I).57 The reaction performed at 50 °C for 2 h using a 50-mL reactor, 5 mmol of benzaldehyde, 10 mg of 5% Pt/C catalyst, 4.0 MPa of H2, and CO2 pressure in the range 6.0–16.0 MPa showed that increase in CO2 pressure led to a monotonous decrease in the conversion, while the selectivity to benzyl alcohol was always 100%. The phase transition from two phases (i.e., dense CO2 and liquid) to single homogeneous phase was observed around 10 MPa. On the other hand, the conversion in the hydrogenation of cinnamaldehyde conducted with 0.25 mmol of the substrate and 0.5 mg of the catalyst showed a maximum at a CO2 pressure of 10.5 MPa, where the phase transition occurred. The selectivity to cinnamyl alcohol was slightly improved at higher CO2 pressures. Highpressure FTIR investigations revealed that the red-shift of V(CdO) occurred as the CO2 pressure was increased for both aldehydes and that the shift was much more prominent for cinnamaldehyde than for benzaldehyde. A large red-shift of 1740 to 1695 cm-1 was observed for cinnamaldehyde when CO2 pressure was increased from 0 to 10.5 MPa, whereas the corresponding red-shift of benzaldehyde was 1723 to 1715 cm-1. In addition, the difference in the interaction with CO2 between the two aldehydes was also indicated by change of the V(CdO) band of CO2. The authors thus concluded that the CdO bond of cinnamaldehyde becomes much more reactive for the hydrogenation compared to that of benzaldehyde with increasing CO2 pressure. For the cinnamaldehyde hydrogenation, this CdO bond activation might prevail over the dilution effect of the substrate up to 10.5 MPa, whereas the rate of benzaldehyde hydrogenation was always determined by the dilution effect under the conditions applied. 3.4. Bimetallic Catalysts. 3.4.1. Ru-Pt/MCM-48. Chatterjee et al. used MCM-48-supported bimetallic Ru-Pt catalyst for the hydrogenation of cinnamaldehyde in scCO2 (Table 3, IV, A).58 The metals were loaded on MCM-48 not by the conventional impregnation method but under hydrothermal conditions using chloroplatinic acid and ruthenium acetylacetonate as platinum and ruthenium source, respectively, which is similar to the method employed for the synthesis of Pt/MCM48 (is) (see section 3.3.8). The XRD patterns of the Ru-Pt/ MCM-48 revealed that the parent mesoporous structure of MCM-48 remained unaltered after loading the metals. The TEM images also showed the unchanged mesoporosity of the Ru-Pt/ MCM-48 and demonstrated the presence of large and small metallic particles that exists on the outer and internal surface, respectively. XPS indicated that the oxidation number of the surface ruthenium was +3, while that of the platinum was zero. However, the authors later referred to the presence of Pt2+ as well as metallic Ru on Ru-Pt/MCM-48 (see the next paragraph with ref 59). The hydrogenation was typically carried out for 2 h at 50 °C using a 50-mL reactor, 7.5 mmol of cinnamaldehyde, and 0.1 g of the catalyst. Increasing the CO2 pressure in the range 0–7.5 MPa at a constant H2 pressure of 4.0 MPa increased the conversion and also the selectivity that reached 100.0% at 7.0 and 7.5 MPa. Further increase in the CO2 pressure decreased the conversion and selectivity. The pressure effect on the catalysis was not elucidated and might be caused by the combination of several factors. However, the dilution effect on cinnamaldehyde at higher CO2 pressures might be one of the factors that affected the results, because the solubility of cinnamaldehyde increased with CO2 pressure, and the substrate and H2 were completely miscible with 12.0 MPa of CO2. The H2 pressure was also varied at a constant CO2 pressure of 7.0 MPa, which showed that the conversion increased with higher

4580 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

H2 pressure in the range 2–6 MPa, while no drastic change in the selectivity to cinnamyl alcohol was observed (90–100% selectivity). Compared to Pt/MCM-48, the Ru-Pt/MCM-48 catalyst showed higher activity and selectivity to cinnamyl alcohol, indicating that the added ruthenium enhances the catalytic performance of platinum. The role of ruthenium was not elucidated, but the authors suggested that electron transfer from ruthenium to platinum occurs to render platinum much more active for the CdO bond hydrogenation compared to the case without ruthenium. It was previously demonstrated that a larger Pt0/Pt2+ ratio leads to higher selectivity to cinnamyl alcohol for Pt/SiO2 (see section 3.3.7). It is noteworthy that with the bimetallic catalyst, conversion and selectivity were similar to those of the first run for additional two cycles of recycling studies. The Ru-Pt/MCM-48 catalyst was also tested in the hydrogenation of citral and the catalytic behavior was compared with that of Pt/MCM-48 (Table 3, IV, A; Scheme 12).59 The detailed analysis of the XPS data showed that all the platinum of Pt/ MCM-48 existed as Pt0, while Ru-Pt/MCM-48 contained not only metallic Pt0 but also Pt2+, and that the bimetallic catalyst contained both oxidized and metallic ruthenium. The Ru-Pt/ MCM-48 and Pt/MCM-48 exhibited different product distribution. When the reaction was performed in a 50-mL reactor at 50 °C for 2 h using 0.1 g of Pt/MCM-48, 6.5 mmol of citral, 4.0 MPa of H2, and CO2 pressure in the range 8–17 MPa, citral was converted mainly to the CdO bond hydrogenated products, geraniol and nerol. On the other hand, the CdC bond hydrogenated product of citronellal was formed as the major product over Ru-Pt/MCM-48 under identical conditions. It is also interesting to compare the results with those obtained for the hydrogenation of cinnamaldehyde over Ru-Pt/MCM-48 (see the former paragraph with ref 58). The bimetallic catalyst selectively promotes the CdO bond hydrogenation for cinnamaldehyde, whereas it yields the CdC bond hydrogenated product for citral. Both the conversion and selectivity to citronellal with the bimetallic catalyst increased with higher CO2 pressure up to 12 MPa, above which they decreased. The pressure effect is not clear but was partially attributed to the dilution effect and the difference in activation volume of the reactions. Other parameters such as the H2 pressure and temperature were also changed to probe their effects on the bimetallic catalysis, revealing that increasing these parameters can lead to higher total selectivity to geraniol and nerol. The hydrogenation was performed also in n-hexane and under solvent-free conditions, but the conversion and selectivity to citronellal with Ru-Pt/ MCM-48 and to geraniol/nerol with Pt/MCM-48 were much lower compared to those obtained in scCO2. It is noteworthy that the bimetallic and monometallic catalyst could be used at least four times for the hydrogenation in scCO2 without any change in conversion and selectivity. There is definitely a large difference in catalytic behavior between the pure Pt catalyst and the Ru-Pt bimetallic catalyst as observed in this study. Unfortunately, the authors could not elucidate the effect of Ru on Pt catalysis and it remains to be explored. 3.5. Other Catalysts. 3.5.1. Wall of SUS316 Stainless Steel Reactor as Catalyst. Zhao et al. found that the hydrogenation of 2-butyne-1,4-diol to butane-1,4-diol proceeds selectively in a SUS316 stainless steel reactor without catalysts in scCO2 (Table 3, V; Scheme 15).60 The key is to use the SUS316 autoclave, because the conversion dropped remarkably when the reactor was changed to a glass tube autoclave and no reaction took place when the Teflon cell was used as a reactor. It was suggested that Ni, Cr, and Mn components contained in

Scheme 15. Hydrogenation of 2-Butyne-1,4-diol

the wall of SUS316 reactor acted as catalytically active species to promote the hydrogenation. The hydrogenation was performed at 50 °C in a 50-mL SUS316 reactor using 5 mmol of 2-butyne-1,4-diol (0.1 mmol per 1 mL reactor) and 4 MPa of H2. Increasing the CO2 pressure from 6.0 to 16.0 MPa increased the conversion and selectivity to the desired butane-1,4-diol, and the product was obtained in the highest selectivity of 84% at 100% conversion in 3 h when the CO2 pressure was 16.0 MPa. The conversion after the same reaction time under the same conditions was also 100% in ethanol and water, but the selectivity to butane-1,4-diol was much lower; the selectivity in ethanol and water were 77 and 13%, respectively. Nonpolar solvents such as hexane and toluene were also tested, but yielded butane-1,4-diol in quite low yields in 3 h. Visual inspection of the phase behavior indicated that 2-butyne-1,4-diol did not completely dissolve in scCO2 and was distributed in both the liquid and gaseous phases under the reaction conditions with 16.0 MPa of CO2. 4. Potential and Limitations of the Application of Supercritical Fluids as Solvents Compared to the great number of studies on the heterogeneous catalytic hydrogenation in organic solvents,61 far too little work has been done so far on the use of supercritical fluids. Also only a few spectroscopic studies under real operating conditions have been reported in this field.1g,48,56,62–64 Thus, we are still far from the possibility of predicting optimal reaction conditions for desired hydrogenations in SCFs. Nevertheless, the collected knowledge and experience provided by the research examples discussed in this review allow to extract a few general tendencies that are helpful to predict which catalyst, SCF, and conditions are suitable for certain hydrogenations. In the following sections, potential and limitations of heterogeneous catalytic hydrogenations in SCFs are discussed in terms of catalysts, SCFs, and reaction conditions. 4.1. Catalysts. Most heterogeneous catalysts that have been used in organic solvents can be used also in SCFs. The choice of supports as well as catalytic metals is crucial to achieve high conversion and selectivity. For most aliphatic and aromatic CdC bond hydrogenations in scCO2, charcoal support tends to afford better results compared to oxide supports such as Al2O3 and SiO2, as demonstrated by Shirai and co-workers.27,30,31,34b,36,43,44 Ordered mesoporous materials, MCM-48 and SBA-15, have been also tested as supports,37,45,46,52b,c,58,59 but so far possible advantages of using such materials in SCFs are not discernible. More work is needed here. However, it is expected that mass transfer of reactants and products in uniform mesopores occurs much more smoothly than in pores with irregular sizes containing “zeolitic” micropores, although the high diffusivity and low viscosity of SCFs might partly mitigate mass transfer problems encountered with smaller pores. It is also noteworthy that unique acid-metal bifunctional catalysis can be realized by using strongly acidic supports.20

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4581 Table 6. Critical Data of Supercritical Fluids Employed in Heterogeneous Catalytic Hydrogenations (Data Taken from ref 65) fluids carbon dioxide, CO2 ethane, C2H6 propane, C3H8

critical critical critical temperature (°C)a pressure (MPa)a density (kg m-3)b 30.9

7.375

468

32.2 96.6

4.884 4.250

203 217

a The number of digits given indicates the estimated accuracy of this quantity. b The values for the critical density cannot be assumed accurate to better than a few percent, although they are given to three decimal places.

Various catalytic metals have been tested for the hydrogenations in SCFs. The most active metal for the hydrogenation of olefinic CdC bonds in scCO2 is palladium, and the reaction can be performed without affecting the CdO bonds.10–12,14,17,20,40–43,45,46 This feature has led to successful selective CdC bond hydrogenation of R,β-unsaturated carbonyl compounds.10b,12,17,20,42,43,45,46 Platinum exhibits interesting catalytic behavior for hydrogenations in scCO2 and its activity strongly depends on the structure of substrates. Typically platinum is effective in scCO2 not only for the olefinic CdC bond hydrogenations23 but also for the selective hydrogenation of R,β-unsaturated aldehydes to unsaturated alcohols49,51,52b,c,57 and aryl nitro compounds to arylamines.53,55,56 However, in spite of its high activity for the hydrogenation of the CdO bond in R,β-unsaturated aldehydes in scCO2, platinum is ineffective for CdO bond hydrogenation of R-ketoesters like ethyl pyruvate in the same medium.22,47 The unique property of platinum in scCO2 has not been elucidated entirely, but several researchers observed the formation of carbon monoxide and its adsorption on platinum by IR spectroscopy.47,48,56 It has been suggested that the carbon monoxide is formed by the reverse water-gas shift reaction catalyzed by platinum: CO2 + H2 a CO + H2O The adsorbed CO can block the platinum sites active for both the desired and undesired reaction. An interesting example where adsorbed CO originating from the above reaction poisons sites active for an undesired reaction is the selective hydrogenation of halogenated nitrobenzenes to the corresponding halogenated anilines, in which the platinum sites active for the dechlorination were shown to be blocked by the in situ formed carbon monoxide.56 For the ring hydrogenation of aromatic compounds, rhodium, particularly on charcoal, exhibited the best catalytic performance among the transition-metals not only in organic solvents but also in scCO2.29–36 The performance of rhodium is usually inferior to that of palladium and platinum for the olefinic CdC bond hydrogenation and the hydrogenation of CdO bonds and nitro group, respectively. Research examples on the hydrogenation of multiple bonds other than aromatic rings with rhodium catalysts are scarce and the potential of rhodium as hydrogenation catalyst in SCFs therefore is still unclear. Unfortunately, Deloxan aminopolysiloxane-supported Pd, Pt, and Ru catalysts, which are very effective for the hydrogenation of a wide range of substrates in SCFs,1e,10 were withdrawn from commercial production. 4.2. Supercritical Fluids. Critical data of SCFs used for hydrogenations are given in Table 6.65 Among the SCFs, scCO2 is most attractive because of its nontoxicity, nonflammability, and low cost. In addition, scCO2 exhibits satisfactory solubilizing power for most low-molecular-weight apolar organic compounds

which are of interest as substrates for hydrogenations. Most hydrogenations actually have been performed in scCO2, as is obvious from Tables 1 and 3. The “ideal” scCO2 medium, however, has several inherent drawbacks. As described in section 4.1, some reactions do not take place in scCO2 owing to site blocking by carbon monoxide which is formed by the reverse water-gas shift reaction. In addition, researchers should note that carbamic acids and their salts are formed by the reaction of amino groups with CO2: A recent high-pressure 1H

NMR study of isopropyl amine (R ) Me2CH) in scCO2 showed that higher pressures shift the equilibrium toward the corresponding carbamic acid (right side in the upper reaction), while higher temperatures favor the free amine and CO2 (left side in the upper reaction).66 Even though carbamic acids decompose to the corresponding amines after releasing high-pressure CO2, their formations during the hydrogenations in scCO2 can lead to undesirable side reactions and to unfavorable phase behaviors owing to the low solubility of carbamic acid salts that are formed in the subsequent acid–base reaction with another amine molecule (the lower reaction). Schmid et al., for example, took a snapshot of the needle-shaped carbamic acid salt formed under the conditions of the formylation of morpholine.67 Their elemental analysis revealed that the salt formation occurred with a morpholine to CO2 ratio of 2:1. Supercritical hydrocarbons are free from such problems and have lower critical pressures compared to CO2, but these features may be offset by their high flammability and high cost. Another limitation of scCO2 as solvent is its low dielectric constant at any pressure and temperature, leading to the low solubility of polar substrates in the medium. In addition, we cannot expect good results in scCO2 for the hydrogenations that are accelerated in polar medium, although the elimination of gas–liquid mass transport limitation may partly compensate this deficiency. It is interesting to note that at near critical temperature the dielectric constant of scCHF3 can be varied in a much broader range compared to that of scCO2 by changing the pressure. The increased dielectric constant at higher pressure enhanced the ee value of the product in the asymmetric hydrogenation of tiglic acid with (S)-H8-BINAP-Ru complex.3a,68 The present authors are unaware of any published reports on the use of scCHF3 for heterogeneous catalytic hydrogenations, but we should note that scCHF3 can be a candidate as solvent for hydrogenations where the outcome strongly depends on the polarity of the medium. The SCFs required for the hydrogenations are usually provided from high-pressure gas cylinders using compressors. Necessity of such special apparatuses for high-pressure conditions can be a barrier to the implementation of hydrogenations in SCFs, particularly for nonspecialists and for those who work in laboratories where working space is narrow and thus only limited number of gas cylinders can be set for safety reasons. An interesting alternative is the one reported by Poliakoff and co-workers who developed practical continuous supercritical hydrogenation processes “without gases”, using the in situ decomposition of formic acid (HCO2H) and ethyl formate (HCO2Et).69 Formic acid is a source for both H2 and CO2, while the decomposition of ethyl formate is performed to generate C2H6 that dilutes hydrogen concentration. Thus formic acid and

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ethyl formate are separately fed into the first reactor containing 5% Pt catalyst to give the CO2-C2H6-H2 fluid, which is subsequently mixed with a substrate fluid and fed into the second hydrogenation reactor, giving the corresponding product downstream. During the catalytic decomposition, the reverse water-gas shift reaction also takes place to give carbon monoxide which poisons hydrogenation catalysts. However, by careful setting of the conditions, the CO concentration can be suppressed to levels that do not affect the performance of hydrogenation catalysts markedly. The other product, water, also deteriorates the performance of some hydrogenation catalysts by dissolving the active metals, but this could be solved by drying the gas mixture (e.g., insertion of a drying agent-packed column). 4.3. Reaction Conditions. Most heterogeneous catalytic hydrogenations are strongly influenced by H2 pressure as well as SCFs pressure and temperature. Typically, increasing H2 pressure increases the surface coverage of hydrogen and thus enhances the conversion and possibly also selectivity in kinetically controlled reactions. However, adding too much hydrogen leads to worse results because the presence of H2 lowers the density and thus solubilizing power of SCFs with the consequence that phase split occurs which in turn may generate mass transfer problems. The SCFs pressure, on the other hand, is a crucial parameter determining the phase behavior of the reaction system. Increasing the SCFs pressure increases the solubility of substrates and hydrogen and thus reduces or eliminates the gas–liquid mass transport resistance. Thus conversion and selectivity usually increase as the SCFs pressure is increased. However, there is a limitation for exploiting this effect, because at too high SCFs pressure the reaction may become kinetically controlled and dilution effects of the liquid substrate may lead to lower conversion. The term “dilution” is used here for the decrease in mole fraction of reactants (i.e., substrate and H2) accompanied by an increase in that of SCF solvents. The reactant molecules are surrounded by larger number of solvent molecules as the SCF pressure (density) is increased, just like in the classical diluted reactions performed with increased solvent/ reactant(s) molar ratio at atmospheric pressure conditions. Note, however, that the concentrations (unit: mole per volume) of the reactants in the single-phase supercritical hydrogenations do not change with the SCF pressures because of the constant fluid volume imposed by the inner reactor volume, which is different from the classical diluted conditions where the liquid volume is increased. Even though reaction kinetics of hydrogenations in supercritical media has not been fully established yet, the gas-phase Brønsted-Bjerrum equation introduced by Eckert and Boudart may be used for the simple kinetic explanation of the dilution effect.70 Decreases in the fugacity coefficients of substrate and H2 are supposed to occur with increasing SCF pressure (density) due to the severer constraints imposed on the reactant molecules by the increased number of surrounding solvent molecules, leading to lower reaction rates. However, fundamental studies on the role of fugacities in supercritical hydrogenation kinetics are needed. An adequate model of the transition state and a valid equation of the state of the mixture are necessary prerequisites for this task. Furthermore, fundamental work which examines supercritical fluid-solid interfaces is required to gain a comprehensive understanding of surface kinetics under supercritical conditions. Prediction of the phase behavior is still in its infancy1e and prediction of the influence of the SCFs pressure on reaction rate requires information about phase behavior and mass transfer as well as reaction kinetics. Phase behavior as well as interphase and intraparticle mass transfer may change as the reaction

proceeds due to changes of the concentration of the different reaction components.67 This information is normally not available and consequently reactions in SCFs are still a very empirical field. In supercritical single phase where gas–liquid mass transport resistance is eliminated, superposition of the dilution effect of the liquid substrate and the difference in activation volume71 among the reactions that can occur in the same reaction mixture might be related to the results. Every reaction has its own activation volume which changes with reaction conditions, and a negative activation volume results in an increase in reaction rate with increasing pressure. Savage et al.71c and Eckert et al.70b gave comprehensive descriptions on the relation between kinetic constant and activation volume. Although homogeneous single phase conditions seem beneficial to achieve high catalytic performance for most hydrogenations (see Tables 1 and 3), some hydrogenations proceed faster in two-phase systems containing an expanded liquid.1e,23,40 The expanded liquid-substrate contains large amounts of CO2 and thus can dissolve sufficient amounts of hydrogen for the reaction to take place under appropriate conditions. Consequently, for the hydrogenations where the access of substrate to the catalyst is the limiting step, a biphasic system may be advantageous to achieve a higher reaction rate. This choice seems to be essential, for instance, when we use a small amount of heterogeneous catalyst in a large batch reactor. For a comprehensive review on the reaction engineering employing expanded liquids, the reader is referred to the recent excellent review of Jessop and Subramaniam.2b Due to the great miscibility of hydrogen and substrates in SCFs, most reactions can be performed below 100 °C with high conversion and selectivity. This is attractive not only from an economical point of view but also in terms of suppression of undesired side reactions such as coke formation which is often observed in conventional gas-phase hydrogenations operated at much higher temperatures. Note, however, that even in the narrow temperature range (critical temperature to 100 °C), increasing temperature sometimes greatly increases not only the desired hydrogenation rates but also the rates of undesirable side reactions.9,59 In addition, at the same total pressure, the SCFs density drastically changes with the change in temperature near the critical points. Thus the reaction temperature also must be carefully selected for beneficial use of SCFs in catalytic hydrogenation. 5. Conclusions and Outlook Research examples on heterogeneous catalytic hydrogenations in supercritical solvents published till 2007 have been surveyed and potential and limitations of their application were discussed. Most classical hydrogenation catalysts can be used also in scCO2 and supercritical hydrocarbons with improved activity and, in some cases, enhanced selectivity. For most hydrogenations, highest reaction rates were observed in supercritical homogeneous single phase, whereas some reactions occurred faster in two-phase systems composed of dense solvent and expanded liquid (substrate). Thus while elimination of gas–liquid mass transport limitation is mostly beneficial, it is not a prerequisite for achieving high global reaction rates. Adsorption and desorption steps of reactants and products can be accelerated in supercritical solvents and thereby a shift from adsorption- or desorption-controlled surface processes to surface reactioncontrolled processes can occur.72 Furthermore, the effect of supercritical conditions on interphase and intraparticle mass transfer,62 clustering,73 and changes in the activation volumes70b,71 have to be considered for predicting the global catalytic behavior

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of a specific reaction system in a supercritical fluid. This prediction is in practice normally not possible due to a lack of knowledge of the influence of the supercritical conditions on the individual steps of the catalytic surface reaction and due to the fact that the phase behavior may change as the reaction proceeds. Nevertheless, the experimental investigations gathered in this review provide some important guidelines which have to be considered when SCFs are applied as solvents in hydrogenation reactions. From a practical point of view, CO2 is ideal as solvent due to its nontoxicity, nonflammability, and low price. We have to note, however, that CO2 reacts with amines to form reactive carbamic acids and insoluble carbamic acid salts. In addition, carbon monoxide formed by the reverse water-gas shift reaction can block sites active for hydrogenations, although in some cases CO poisons selectively only sites active for undesired side reactions and increases the selectivity to desired hydrogenation products. Supercritical hydrocarbons can be good alternative media for the hydrogenations where CO2 cannot be used, although they are flammable and more expensive. There has been virtually no work yet on the use of other supercritical media such as scCHF3 and supercritical alcohols for heterogeneous catalytic hydrogenations. These polar media may afford interesting results which cannot be observed in apolar supercritical fluids. Work on the use of supercritical fluid-cosolvent systems is also scarce and must be explored. Another field where further efforts seem promising is the design of optimal reactor configurations for the application of SCFs as solvents. Up today most researchers used conventional batch slurry reactors and continuous fixed-bed reactors which were not specifically designed for SCF application. Finally, efforts of transforming the fundamental knowledge obtained in laboratory into practical technology are highly desired. Representative work done in this direction involves the development of continuous-flow processes using gaseous hydrogen by the University of Nottingham/Thomas Swan team1e and Hoffmann-La Roche.5 The University of Göttingen/Schering AG group, on the other hand, built a pilot plant for the reduction of 1,4-androstadiene-3,17-dione to estradione in supercritical tetralin, where tetralin is not only a solvent but also a hydrogen donor.5 Advantages obtained by using SCFs such as high reaction rates, unique product selectivities, easy and complete separation of the solvent (SCF) from the reaction mixture, and high reusability of SCFs can afford considerable economical and environmental benefits that can overcompensate the higher costs generated by the need of high-pressure conditions. However, it is also clear that injudicious use of SCFs in catalytic hydrogenation can even reduce the overall process efficiency and sustainability of catalytic hydrogenation processes. Acknowledgment T.S. thanks Prof. Dr. K. Domen and Ms. M. Tomita (The University of Tokyo) for supporting his activity as a Research Fellow of the Japan Society for the Promotion of Science. This work is partially supported by the grant-in-aid from the Japan Society for the Promotion of Science (project code 070300000755). A.B. thanks the Swiss Federal Office of Energy (SFOE) for financial support. Literature Cited (1) For reviews of heterogeneous catalysis in supercritical fluids, see: (a) Subramaniam, B.; McHugh, M. A. Reactions in supercritical fluids-a review. Ind. Eng. Chem. Process Des. DeV. 1986, 25, 1. (b) Baiker, A. Supercritical fluids in heterogeneous catalysis. Chem. ReV. 1999, 99, 453.

(c) Hyde, J. R.; Licence, P.; Carter, D.; Poliakoff, M. Continuous catalytic reactions in supercritical fluids. Appl. Catal., A 2001, 222, 119. (d) Grunwaldt, J.-D.; Wandeler, R.; Baiker, A. Supercritical fluids in catalysis: opportunities of in situ spectroscopic studies and monitoring phase behavior. Catal. ReV.-Sci. Eng. 2003, 45, 1. (e) Licence, P.; Ke, J.; Sokolova, M.; Ross, S. K.; Poliakoff, M. Chemical reactions in supercritical carbon dioxide: from laboratory to commercial plant. Green Chem. 2003, 5, 99. (f) Beckman, E. J. Supercritical and near-critical CO2 in green chemical synthesis and processing. J. Supercrit. Fluids 2004, 28, 121. (g) Grunwaldt, J.-D.; Baiker, A. In situ spectroscopic investigation of heterogeneous catalysts and reaction media at high pressure. Phys. Chem. Chem. Phys. 2005, 7, 3526. (2) (a) Musie, G.; Wei, M.; Subramaniam, B.; Busch, D. H. Catalytic oxidations in carbon dioxide-based reaction media, including novel CO2expanded phases. Coord. Chem. ReV. 2001, 219–221, 789. (b) Jessop, P. G.; Subramaniam, B. Gas-expanded liquids. Chem. ReV. 2007, 107, 2666. (3) For work on homogeneous catalytic hydrogenations in SCFs, see: (a) Jessop, P. G.; Ikariya, T.; Noyori, R. Homogeneous catalysis in supercritical fluids. Chem. ReV. 1999, 99, 475, and references therein. (b) Lange, S.; Brinkmann, A.; Trautner, P.; Woelk, K.; Bargon, J.; Leitner, W. Mechanistic aspects of dihydrogen activation and transfer during asymmetric hydrogenation in supercritical carbon dioxide. Chirality 2000, 12, 450. (c) Flores, R.; Lopez-Castillo, Z. K.; Kani, I.; Fackler, J. P., Jr.; Akgerman, A. Kinetics of the homogeneous catalytic hydrogenation of olefins in supercritical carbon dioxide using a fluoroacrylate copolymer grafted rhodium catalyst. Ind. Eng. Chem. Res. 2003, 42, 6720. (d) Zhao, F.; Ikushima, Y.; Chatterjee, M.; Sato, O.; Arai, M. Hydrogenation of an R,β-unsaturated aldehyde catalyzed with ruthenium complexes with different fluorinated phosphine compounds in supercritical carbon dioxide and conventional organic solvents. J. Supercrit. Fluids 2003, 27, 65. (e) Berthod, M.; Mignani, G.; Lemaire, M. New perfluoroalkylated BINAP usable as a ligand in homogeneous and supercritical carbon dioxide asymmetric hydrogenation. Tetrahedron: Asymm. 2004, 15, 1121. (4) (a) Tacke, T.; Wieland, S.; Panster, P.; Bankmann, M.; Brand, R.; Magerlein, H. Hardening of unsaturated fats, fatty acids, or fatty acid esters. U.S. Patent 5,734,070, Degussa AG, 1998. (b) Jansen, M.; Rehren, C. Catalytic hydrogenation using amorphous metal alloy and a solvent under near-critical or super-critical conditions. U.S. Patent 6,002,047, Roche Vitamins, Inc., 1999. (c) van Amerongen, M. P.; Lievense, L. C. Stanol ester composition and production thereof. U.S. Patent 6,031,118, Lipton, Division of Conopco, Inc., 2000. (d) Poliakoff, M.; Swan, T. M.; Tacke, T.; Hitzler, M. G.; Ross, S. K.; Wieland, S. Supercritical hydrogenation. U.S. Patent 6,156,933, Degussa-Hüls AG; Thomas Swan & Co., Limited, 2000. (5) Jessop, P. G.; Leitner, W. Supercritical fluids as media for chemical reactions. In Chemical Synthesis Using Supercritical Fluids; Jessop, P. G., Leitner, W., Eds.; Wiley-VCH: Weinheim, Germany, 1999; p 1. (6) For recent work on catalytic hydrogenation of CO in SCFs (supercritical Fischer–Tropsch synthesis), see: (a) Elbashir, N. O.; Dutta, P.; Manivannan, A.; Seehra, M. S.; Roberts, C. B. Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer–Tropsch synthesis. Appl. Catal., A 2005, 285, 169. (b) Li, X.; Liu, X.; Liu, Z.-W.; Asami, K.; Fujimoto, K. Supercritical phase process for direct synthesis of middle iso-paraffins from modified Fischer–Tropsch reaction. Catal. Today 2005, 106, 154. (c) Elbashir, N. O.; Roberts, C. B. Enhanced incorporation of R-olefins in the Fischer–Tropsch synthesis chain-growth process over an alumina-supported cobalt catalyst in near-critical and supercritical hexane media. Ind. Eng. Chem. Res. 2005, 44, 505. (d) Bukur, D. B.; Lang, X.; Nowicki, L. Comparative study of an iron Fischer–Tropsch catalyst performance in stirred tank slurry and fixedbed reactors. Ind. Eng. Chem. Res. 2005, 44, 6038. (e) Linghu, W.; Li, X.; Asami, K.; Fujimoto, K. Process design and solvent recycle for the supercritical Fischer–Tropsch synthesis. Energy Fuels 2006, 20, 7. (7) For representative work on catalytic hydrogenation of CO2 in scCO2, see: (a) Jessop, P. G.; Ikariya, T.; Noyori, R. Homogeneous catalytic hydrogenation of supercritical carbon dioxide. Nature 1994, 368, 231. (b) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. Homogeneous catalysis in supercritical fluids: hydrogenation of supercritical carbon dioxide to formic acid, alkyl formates, and formamides. J. Am. Chem. Soc. 1996, 118, 344. (c) Kröcher, O.; Köppel, R. A.; Baiker, A. Sol-gel derived hybrid materials as heterogeneous catalysts for the synthesis of N,N-dimethylformamide from supercritical carbon dioxide. Chem. Commun. 1996, 1497. (d) Kröcher, O.; Köppel, R. A.; Fröba, M.; Baiker, A. Silica hybrid gel catalysts containing group(VIII) transition metal complexes: preparation, structural, and catalytic properties in the synthesis of N,N-dimethylformamide and methyl formate from supercritical carbon dioxide. J. Catal. 1998, 178, 284. (e) Baiker, A. Utilization of carbon dioxide in heterogeneous catalytic synthesis. Appl. Organometal. Chem. 2000, 14, 751, and references therein. (f) Jessop, P. G.; Joó, F.; Tai, C.-C. Recent advances in the homogeneous hydrogenation of carbon dioxide. Coord. Chem. ReV. 2004, 248, 2425, and references therein.

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ReceiVed for reView December 4, 2007 ReVised manuscript receiVed March 6, 2008 Accepted March 6, 2008 IE071649G