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Ind. Eng. Chem. Res. 2003, 42, 6688-6696
Hydrogenation of 4-Oxoisophorone over a Pd/Al2O3 Catalyst under Supercritical CO2 Medium Unnikrishnan R. Pillai and Endalkachew Sahle-Demessie* National Risk Management Research Laboratory, Sustainable Technology Division, MS 443, United States Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
Hydrogenation of 4-oxoisophorone has been studied over a 1% Pd/Al2O3 impregnated catalyst in supercritical CO2 (sc-CO2) medium at different reaction conditions. The effect of temperature, pressure, and reaction medium on the conversion and product selectivity is discussed. Phase behavior studies have also been carried out to determine the conditions at which a supercritical phase is formed as a function of various experimental parameters as well as the miscibility behavior of different reactant mixtures. A comparison of the sc-CO2 medium reaction with the conventional liquid-phase hydrogenation in organic solvents as well as CO2-organic cosolvents has also been made. The reaction rates in sc-CO2 medium are found to be comparable to that in commonly employed organic solvents. Selectivities different from liquid-phase reactions are observed under supercritical conditions. Catalyst deactivation is found to be much lower in scCO2 medium when compared to that in the organic solvent medium. The reaction mechanism leading to different products is discussed. Introduction
Scheme 1
Ketoisophorone or 4-oxoisophorone (2,6,6-trimethylcyclohex-2-ne-1,4-dione, 1) is a cyclic enedione of tremendous industrial importance because of its use as a key intermediate in the synthesis of various carotenoids and flavoring agents.1-4 The catalytic hydrogenation and enzymatic reduction of this compound lead to different products, some of which are of immense industrial importance. For example, an S stereoisomer of 4-hydroxyisophorone (4), a volatile constituent of saffron, is an intermediate in the synthesis of pharmaceuticals and natural pigments as well as a tobaccoflavoring agent.5,6 Hydrogenation of ketoisophorone has been studied over a variety of catalysts such as Pt/Al2O3, Pd/Al2O3, Pt/Co, Pd/C, PtO2, Raney Nickel, Rh complex, and Baker’s yeast,7-10 leading to products such as the saturated diketone 2, the saturated alcohol 3, and the unsaturated alcohol 4 (Scheme 1). Selective hydrogenation of R,β-unsaturated ketones to R,β-unsaturated alcohols is a highly desirable reaction in the synthesis of fine chemicals. The selectivity of this reaction is one of the most challenging because it is very difficult to enhance the rate of hydrogenation of the CdO group while simultaneously suppressing the hydrogenation of the CdC bond. It is known that the classical hydrogenation catalysts show no or negligible selectivity toward the hydrogenation of the CdO bond; the selectivity of the metal catalysts can be improved by the addition of promoters such as Fe2+, Zn2+, Ni2+, Sn2+, Ge2+, and K+.11-14 Other effective ways are electron-donating ligand effects from the supports, steric constraints in the metal environment, strong metal-support interaction, selective poisoning, the presence of substituents on the CdC bond, and pressure, temperature, and solvent effects.15 Most of the studies on the selective hydrogenation are, however, focused on molecules such * To whom correspondence should be addressed. Tel.: 513-569-7739. Fax: 513-569-7677. E-mail:
[email protected]. 10.1021/ie030571a
as crotonaldehyde,16 cinnamaldehyde,17-21 and citral.22-25 The solvent effects on the selectivity of a reaction are well-known even though there is only a limited understanding about the mechanism. Variation of the dielectric constants of the solvents affects the product selectivity, and nonpolar solvents tend to prefer the hydrogenation of the CdO bond over that of CdC. Supercritical carbon dioxide (sc-CO2), which is aprotic in nature, could be beneficial in some cases. We have recently demonstrated unusual selectivity for maleic anhydride hydrogenation to γ-butyrolactone in sc-CO2 medium.26,27 In the past decade, sc-CO2 has been increasingly used as an environmentally friendly reaction medium in place of toxic and hazardous organic solvents. Complete miscibility of sc-CO2 with reactive gases such as H2, O2, and also a variety of organic compounds could reduce mass-transfer limitations of multiphase reactions such as hydrogenations and oxidations in this medium.28 Conventional solid-catalyzed hydrogenations of liquids or solid compounds dissolved in organic solvents are proportional to the hydrogen concentration and often controlled by the hydrogen transport rate, which requires high mixing and heating energy to get reasonable rates of reaction. Eliminating the vapor-liquid interface
This article not subject to U.S. Copyright. Published 2003 by the American Chemical Society Published on Web 11/20/2003
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can help the reaction to move from a transportcontrolled regime to a kinetically controlled regime and hence increase the rate or gain better control of the reaction. Another problem is the control of temperature because many hydrogenation reactions are exothermic in nature and can cause thermal runaway conditions. Heterogeneous hydrogenation also suffers from selectivity problems in addition to the difficult task of separating the products from the starting materials. Use of scCO2 as an alternative medium for hydrogenations promises to overcome some of the problems described above and can offer both process and environmental advantages. Hydrogenation, oxidation, alkylation, and hydroformylation reactions are reported to provide more satisfactory results in sc-CO2 medium compared to the reactions in organic solvents.28 Also, slight changes in temperature and pressure near the critical point can change the activity and selectivity significantly. Several recent publications have demonstrated the potential of sc-CO2 as an alternative reaction medium for a variety of synthetic transformations including production of fine chemicals and pharmaceuticals.28-31 However, relatively few studies have been reported on hydrogenation reactions in supercritical fluid media.26,27,32-37 Hydrogenations of a variety of liquid compounds in sc-CO2 medium over Deloxan aminopolysiloxane-supported noble-metal (Pd, Ru, and Pt) catalysts have been reported by Poliakoff and coworkers.36 sc-CO2 has also been used as a reaction medium for the fixed-bed hydrogenation of cyclohexene to cyclohexane over a Pd/C catalyst37 and the selective hydrogenation of R,β-unsaturated aldehyde (cinnamaldehyde) over a Pt/SiO2 catalyst.38 However, no study has been reported so far on the hydrogenation of 1 (ketoisophorone) in a supercritical fluid medium. This paper presents the details of 1 hydrogenation over 1% Pd/Al2O3 impregnated catalysts under sc-CO2 medium as well as under other reaction conditions such as CO2organic cosolvents and also under a conventional organic liquid medium. Supported Pd catalysts are highly efficient CdC hydrogenation catalysts but are not known to be effective as CdO group hydrogenation catalysts such as Pt or Rh.39 Nonetheless, selectivities different from those obtained in liquid-phase organic solvents are observed under supercritical conditions, suggesting the potential of the supercritical medium for partial or selective hydrogenations over simple metal catalysts. Experimental Section Catalyst Preparation. A 1% Pd/Al2O3 (w/w) catalyst was prepared by wet impregnation of alumina pellets sieved to 1400 µm (Aldrich) with a 0.1 M palladium chloride (Aldrich) solution. After the impregnation step, the catalyst was dried at 383 K overnight and then calcined in air at 723 K for 5 h. The calcined catalyst was subsequently reduced in flowing hydrogen at 723 K for 5 h and then cooled to room temperature under hydrogen flow before being loaded into the hydrogenation reactor. Catalyst Characterization. Metal surface area, percentage dispersion, and particle size of Pd in the fresh and spent catalyst were determined by CO chemisorption, at room temperature, using a Micromeritics Auto Chem II Unit (model 2920). A stoichiometry of Pd/ CO ) 1 and a Pd surface density of 1.27 × 1019 atoms m-2 were assumed for determining the metal surface area of the catalysts.40
Figure 1. Schematic diagram of the hydrogenation reactor under sc-CO2 medium.
Phase Behavior Studies. Phase behavior studies of binary and ternary mixtures of 1 in CO2 with or without organic solvents were conducted using a 103 mL stainless steel pressure view cell (Thar Technologies Inc., Pittsburgh, PA). The view cell is equipped with a mechanical stirrer, and studies were conducted at different temperatures ranging from 323 to 398 K and at various H2 and CO2 partial pressures. The experimental data help to understand the conditions at which a single phase is formed as well as to understand the miscibility behavior of various phases at different reaction conditions. Hydrogenation Studies. Isothermal hydrogenation studies of 1 were conducted in a 500 mL stainless steel batch reactor (Pressure Products Industries, Inc., Warminster, PA) using 50 mmol of the substrate and 0.50 g of catalyst and the desired amount of any organic solvent if required. The schematic diagram of the reactor is shown in Figure 1. The catalyst was loaded into a spinning dynamic basket. The reactor was then heated to the reaction temperature while being filled with known amounts of hydrogen and carbon dioxide to a set pressure. The temperature of the reactor was monitored and controlled using a proportional-integral-derivative controller. When the reactor reached the set temperature, a syringe pump (ISCO, LC-260), with the jacket kept at 4 °C, was used to pump liquid CO2 and bring the reactor to the required set pressure. Spinning the catalyst in a basket minimizes mass-transfer limitations and increases heat transfer from the catalyst. At the end of the reaction time, the reactor was cooled to room temperature and vented slowly. The products were collected and analyzed by a Hewlett-Packard 6890 gas chromatograph using a HP-5 5% phenylmethylsiloxane capillary column (30 m × 320 µm × 0.25 µm) and a quadropole mass filter equipped HP 5973 mass selective detector under a temperature-programmed heating from 313 to 473 K at 10 °C/min. Quantification of the products was obtained using a multipoint calibration curve. Results and Discussion Phase Behavior Studies. The phase behavior of 1 in CO2 and CO2-expanded solvents has been investigated as a function of temperature as well as H2 and CO2 partial pressures. Parts a-c of Figure 2 show the vapor-liquid equilibrium relationship as a function of H2 and CO2 pressures, temperature and the nature of the organic solvent. It is found that, at 1.7 MPa H2
6690 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Figure 2. (a) Vapor-liquid equilibrium for a 1-CO2-H2 mixture as a function of hydrogen partial pressure at 373 K. (b) Vaporliquid equilibrium for a 1-CO2-H2 mixture as a function of temperature at 1.7 MPa hydrogen partial pressure. (c) Vaporliquid equilibirum for a 1-CO2-H2-organic solvent mixture as a function of the nature of the organic solvent at 1.7 MPa hydrogen partial pressure and 373 K (MIBK ) methyl isobutyl ketone, MeCN ) acetonitrile, MeOH ) methanol).
partial pressure and 373 K, a single phase is formed at a total pressure of 15 MPa. Increasing the hydrogen partial pressure (Figure 2a) or the temperature (Figure 2b) increases the total pressure required for the formation of the single phase. The effect of the nature of the organic solvent on the phase behavior at 373 K and 1.7 MPa H2 partial pressure shows that the total pressure required to achieve a single phase increases in the order cyclohexane < acetone < methyl isobutyl ketone < acetonitrile < methanol (Figure 2c), which is in the order of the increasing polarity of the solvent. An increase in the polarity of the solvent decreases its miscibility with carbon dioxide, and hence a higher pressure is required to achieve complete miscibility. In other words, the higher the polarity of the organic solvent, the greater is the pressure needed to achieve the single phase. Nevertheless, the total pressure
required to achieve the single phase for the 1-H2-CO2 cosolvent system (Figure 2c) is less than that required for the 1-CO2-H2 system (15 MPa, Figure 2a). Hydrogenation Studies. The preliminary results of the hydrogenation of 1 over a 1% Pd/Al2O3 catalyst at 373 K and a H2 partial pressure of 1.7 MPa in the absence and in the presence of various reaction media are given in Table 1. It is found that the reaction is highly accelerated in the presence of CO2 medium as the conversion increases significantly from 32% in the case of reaction without any solvent (entry 1) to 100% conversion when CO2 is used as the reaction medium (entry 2). It is apparent from the phase behavior studies that the reaction mixture is a single phase at this condition. The reaction is also accelerated in the liquid phase in the presence of organic solvents such as cyclohexane (entry 3), methanol (entry 4), methyl isobutyl ketone (entry 5), acetone (entry 6), acetonitrile (entry 7), and propyl acetate (entry 8), with the maximum conversion being obtained in the presence of CO2 and cyclohexane media. This demonstrates the similarity of sc-CO2 medium to hydrocarbon solvents and also the potential of sc-CO2 to enhance the reactions such as hydrogenations. The high oxoisophorone conversion or the reaction rate in CO2 (entry 2) is attributed to the increased solubility of the reactants in sc-CO2 and improved mass-transfer coefficient as well as to the decreased diffusion coefficient. Conventional solidcatalyzed hydrogenations of liquids or solid compounds dissolved in organic solvents are proportional to the hydrogen concentration and are often controlled by the transport rate, which requires more energy to get a reasonable rate of reaction from both mixing and heating. The liquid-phase reaction also suffers from the additional requirement of removing the solvents from the reaction mixture. Another very interesting feature is the similarity of product selectivity obtained in CO2 and cyclohexane media (entries 2 and 3). Hydrogenation of 1 in protic and other polar organic solvents such as methanol, acetonitrile, and propyl acetate affords mainly products 2 (3,5,5-trimethylcyclohexane-1,4-dione) and 4 (3,5,5-trimethyl-4-hydroxy-1-cyclohexanon-2-ene) with small amounts of 3 (3,5,5-trimethyl-4-hydroxycyclohexanone), whereas the reactions in the presence of CO2 and cyclohexane media afford mainly products 2 and 3 with little or no formation of the unsaturated alcohol 4. No significant hydrogenation of the cosolvent used is detected. The effect of adding organic cosolvents to CO2 is also examined because they are often suggested to be advantageous in many cases.41-44 However, the 4-oxoiosphorone conversion is found to decline in CO2 cosolvent systems (entries 9-15) when compared to the reactions in CO2 (supercritical phase) or the organic solvent alone (liquid phase) at the conditions studied. One may attribute this low conversion in CO2 cosolvent to the total pressure used (15 MPa), which is well above the supercritical pressure of these mixtures (Figure 2c) at the conditions studied. However, this does not appear to be the reason for the low conversion of the CO2expanded solvents because the reaction in CO2-acetone does not show any improvement in the conversion even at the supercritical pressure of 11 MPa (Table 1, entry 13). The phase behavior studies suggest that the reaction in a CO2 cosolvent medium is also a single-phase reaction in which 1 is dissolved in the CO2-expanded solvent phase. One reason for the low conversion in the
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6691 Table 1. Hydrogenation of 1 over 1% Pd/Al2O3 Catalysta
a
Substrate ) 50 mmol, catalyst ) 0.50 g, H2 ) 1.7 MPa, temperature ) 373 K, cosolvent ) 50 mL, time ) 2 h.
Table 2. Effect of Temperature on 1 Hydrogenation over a 1% Pd/Al2O3 Catalysta
a
Substrate ) 50 mmol, catalyst ) 0.50 g, H2 ) 1.7 MPa, temperature ) 373 K, time ) 2 h, MeOH ) methanol ) 50 mL.
CO2 cosolvent medium when compared to the solvent medium alone may be the relative low solubility of the reactant hydrogen due to the saturation of the solvent with CO2 (solvent expands under CO2 pressure). Effect of Temperature and H2-CO2 Pressures on the Reaction. The effect of the reaction temperature on the hydrogenation of 1 (Table 2) shows an increase in the conversion with temperature for the solventless (entries 1-4), CO2 (entries 5-8), and the CO2-methanol cosolvent reactions (entries 9-12). An increase in the
temperature from 323 to 473 K increases the conversion for the solventless and CO2-methanol cosolvent reactions. However, temperatures above 373 K do not appear to have any influence on the 1 conversion in the CO2 medium (entries 5-8), even though the reaction rate may still be increasing with the temperature. Methanol is taken as the solvent of choice for the CO2 cosolvent studies because it is the most environmentally benign solvent among the solvents studied in addition to being a good solvent for the reaction (high conver-
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Figure 4. Effect of the reaction time on the conversion and product selectivity of 1 hydrogenation at 373 K and at a total pressure of 15 MPa.
Figure 3. (a) Effect of CO2 pressure on the conversion and product selectivity of 1 hydrogenation at 373 K and at 1.7 MPa H2 pressure. (b) Effect of H2 pressure on the conversion and product selectivity of 1 hydrogenation at 373 K and at a total pressure of 15 MPa.
sion). A 100% 1 conversion is achievable in the absence of any solvent at 473 K; however, the same could be achieved in CO2 medium at 373 K. The product distribution changes with temperature in all of the cases. The selectivity of product 2 increases and that of product 4 decreases with temperature for the solventless reaction (entries 1-4). There is no appreciable formation of product 3 under this condition. In the case of reaction in CO2 medium, 2 and 3 are the main products with little formation of product 4. The selectivity of product 2 increases and that of product 3 decreases with temperature (entries 5-8). Higher selectivity of product 2 and lower selectivity of product 3 with temperature may be attributed to the increased rate of hydrogenation of the CdC bond as the temperature increases. For the reactions in CO2-methanol cosolvent system, there is no appreciable change in the selectivity of 2, whereas product 3 increases and product 4 decreases with an increase in the temperature (entries 9-12). Conversion increases with an increase in the CO2 pressure from 8.3 to 13.3 MPa, and thereafter it decreases with a further increase in the CO2 pressure to 19.3 MPa (Figure 3a). The reaction system is a ternary phase at 8.3 MPa pressure (solid catalyst, liquid substrate, and CO2-H2 gas phases), whereas it is a binary phase (solid and a single vapor-liquid phase) at 13.3 MPa CO2 pressure (total pressure ) 15 MPa). A decrease in the number of phases increases the masstransfer rate, decreases the diffusion coefficients, and thereby increases the rate. Increasing the CO2 pressure further may reduce the concentration of one of the reactants, and this could be the reason the conversion falls with a further increase in the CO2 pressure to 19.3 MPa. On the other hand, selectivity for 2 decreases and
those of 3 and 4 increase with an increase in the CO2 pressure from 8.3 to 13.3 MPa. An increase in the hydrogen pressure from 0.96 to 1.7 MPa increases the conversion, and a further increase does not affect the conversion (Figure 3b). Low hydrogen pressure (0.96 MPa) seems to favor the saturated diketone 2 and the unsaturated alcohol 4, whereas high hydrogen pressure (g1.7 MPa) favors the saturated diketone 2 and the saturated alcohol 3. There is no appreciable effect of hydrogen partial pressure on the product distribution above a hydrogen partial pressure of 1.7 MPa (Figure 3b). Effect of the Reaction Time. The effect of the reaction time on the 4-oxoisophorone conversion and product selectivity is shown in Figure 4. An increase in the reaction time increases the conversion as expected. Low conversion or short reaction time affords product 2 selectively with no significant formation of products 3 and 4. An increase in the reaction time from 15 to 60 min decreases the selectivity of 2 and increases the selectivity of product 4. A further increase in the reaction time, however, diminishes the selectivity of product 4, with the subsequent increase in the selectivity of product 3 indicating that product 3 may be formed by the CdC bond hydrogenation of 4. The above changes in the product selectivity pattern suggest that the initial reaction or the more facile reaction is the hydrogenation of the CdC bond followed by the hydrogenation of the CdO bond. Reaction Kinetics. The rate of 1 hydrogenation in CO2 is studied at different temperatures and reaction times to understand the order and kinetics of the reaction. The linearity of the logarithm of the oxoisophorone concentration against time at different temperatures, shown in Figure 5, implies that the reaction is first order with respect to the ketone concentration. The estimated rate constants are 1.78 × 10-4, 2.78 × 10-4, and 3.62 × 10-4 s-1 at temperatures of 373, 398, and 413 °C, respectively. The temperature dependence of the logarithm of the reaction rate versus the inverse of the temperature is shown in Figure 6. The straight line represents the linear fit of the data points, and the estimated activation energy is 30.4 × 10-7 kJ mol-1. Catalyst Deactivation Studies. The catalyst deactivation during the hydrogenation was studied by carrying out the reaction using the same catalyst sample for five successive runs (Table 3). A comparison has also been made with the results of similar studies in methanol. Conversion generally decreases with an
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Figure 5. Plot of ln A/A0 (A0 ) initial concentration; A ) final concentration) against reaction time at (a) 373 K, (b) 398 K, and (c) 413 K for 1 in a CO2 medium (H2 ) 1.7 MPa, CO2 ) 13.3 MPa).
Figure 6. Dependence of the reaction rate constant, ln k, on the inverse of temperature for 1 in a CO2 medium (H2 ) 1.7 MPa, total pressure ) 15 MPa).
increase in the number of reaction cycles in both CO2 and methanol media by as high as 40%. However, the percentage loss of conversion is slightly less in the case of reactions in CO2 medium than in methanol solvent (columns 3 and 4). The loss in conversion or the activity of the catalyst could be redeemed reasonably well (∼80%) by a regeneration (calcination and reduction) of the catalyst (entry 6). The selectivity of 2 increases whereas that of 3 decreases with successive reaction
cycles in CO2 (columns 5 and 6). This further confirms the more facile nature of the hydrogenation of the CdC bond when compared to that of the CdO bond as mentioned earlier. The formation of product 4 is relatively higher with the increase in the number of reaction cycles (column 7). The product distribution, however, varies only slightly with the number of reaction cycles for the reaction in methanol solvent (columns 8-10). A comparison of the adsorption properties of the catalysts after successive reaction cycles also shows a relatively higher decrease in the metal surface area and percentage dispersion of the metal on the catalyst from the methanol reaction medium than on the catalyst from the CO2 reaction medium (Table 4). When the metal area of the catalyst decreases from 37.3 m2 (g of metal)-1 to 11.7 m2 (g of metal)-1 in the case of the Pd/Al2O3 catalyst from CO2 medium reaction (column 2), the same reduction occurs from 37.3 m2 (g of metal)-1 to 8.1 m2 (g of metal)-1 for the catalyst from a methanol medium reaction (column 5). It appears that the decrease in 4-oxoisophorone conversion is not proportional to the decrease in the metal area and dispersion of the catalyst, and this may be due to the structure-insensitive nature of many hydrogenation reactions.45 The adsorption properties of the catalyst thus reveal that the catalyst deactivation is more pronounced in a methanol medium. In other words, conducting the reaction in CO2 medium minimizes the catalyst deactivation, further proving the advantage of this medium as an attractive alternative medium for reactions such as hydrogenations.37 Catalyst deactivation during hydrogenation in sc-CO2 medium has been reported to be due to the formation of carbon monoxide or metal formate.33,34 On the contrary, some researchers have observed little deactivation after an extended period of tests.37 Reaction Mechanism. The above results indicate that 2 and 3 are the major products during the hydrogenation of 1 in CO2 medium irrespective of the temperature and CO2 pressures, whereas 2 and 4 are the major products at low hydrogen pressure (0.96 MPa). However, product 2 is always the dominant product in a CO2 medium. On the other hand, the main products of the hydrogenation for the solventless reaction as well as for the methanol and CO2-methanol cosolvent
Table 3. Catalyst Deactivation/Recycling Studies for the 1 Hydrogenation over a 1% Pd/Al2O3 Catalyst in sc-CO2 and Methanola
a
Substrate ) 50 mmol, catalyst ) 0.50 g, H2 ) 1.7 MPa, total pressure ) 15 MPa, temperature ) 373 K, time ) 2 h, R ) regenerated.
6694 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 4. Effect of the Number of Reaction Cycles on the Adsorption Properties of a 1% Pd/Al2O3 Catalyst adsorption properties for the catalyst in CO2 as a reaction medium
adsorption properties for the catalyst in methanol as a reaction medium
reaction cycle number
metal area [m2 (g of metal)-1]
dispersion (%)
particle size (nm)
metal area [m2 (g of metal)-1]
dispersion (%)
particle size (nm)
fresh catalyst run 1 run 2 run 5
37.3 31.3 17.3 11.7
8.4 7.0 3.9 2.6
13.4 15.9 28.8 43.0
37.3 29.5 14.2 8.1
8.4 6.6 3.2 1.8
13.4 16.9 35.1 61.5
a
Substrate ) 50 mmol, catalyst ) 0.50 g, H2 ) 1.7 MPa, total pressure ) 15 MPa, temperature ) 373 K, time ) 2 h.
systems are 4 and 2 irrespective of reaction variables, with the former being the predominant one. The appearance of product 3 is followed by a corresponding decrease in the selectivity of product 4. These results suggest a parallel hydrogenation sequence of 1 to 2 + 4 to 3, where 3 is mainly formed by the hydrogenation of the unsaturated alcohol 4. 1 is a multifunctional adsorbate and hence can bind to the catalyst surface in a variety of ways in which the CdC or CdO moieties are favorably aligned, leading to multiple surface reaction channels that control the activity and selectivity. Carbonyl compounds have a high solubility in sc-CO2 medium46-48 because of the Lewis acid (LA)-Lewis base (LB) interactions between CO2 (LA) and the CO2-philic carbonyl groups (LB). Raveendran and Wallen have shown that, in addition to the above LA-LB interactions, there also exists a cooperative weak C-H‚‚‚O hydrogen bond between the hydrogen atom attached to the carbonyl carbon or the R carbon of the carbonyl compound and one of the oxygen atoms of CO2.49,50 The extent of these interactions depends on various conditions such as pressure, temperature, and the presence of other cosolvents, which can affect the reaction rate as well as the product selectivity. Baiker and co-workers have reported a dramatic influence of the solvent polarity on ketoisophorone (1) hydrogenation over Pd/Al2O3 catalysts.7 They have shown a high selectivity for 4 during the hydrogenation of ketoisophorone over a Pd/Al2O3 catalyst in protic solvents similar to our observations. Protic solvents promote the hydrogenation of the CdO group perhaps due to the partial protonation of the carbonyl oxygen.51 This study has shown that hydrogenation of 1 over a Pd/Al2O3 catalyst is more selective for the reduction of the sterically hindered carbonyl group, leading to the unsaturated alcohol 4 in protic solvents such as methanol, whereas it is more selective for the reduction of the CdC bond, forming the cyclic diketone (2) in nonpolar solvents such as sc-CO2. No hydrogenation of the sterically unhindered carbonyl group is observed in any case. The selectivity differences in protic organic solvents and aprotic organic solvents as well as CO2 may also be related to the different nature of adsorption of the reactant and the intermediates on the catalyst surface in different reaction media. For example, 1 can be adsorbed on the catalyst surface through the CdC bond (side-on mode, most preferred) or the CdO bond (endon mode) affording different products. The presence of methanol helps in the partial protonation of the CdO group; promoting the carbonyl hydrogenation51 and adsorption of the reactant on the catalyst surface through the CdC bond leads to its hydrogenation, resulting in products 4 and 2, respectively. On the other hand, in aprotic solvents such as CO2, there may not
be any protonation of the CdO group to enhance its hydrogenation, and the preferential adsorption mode of reactant 1 on the catalyst surface may be through the CdC bond due to the earlier mentioned LA-LB interaction of the CdO bonds with CO2, 46-50 resulting in the formation of relatively more product 2 than product 4. Catalyst deactivation reduces the number of metal atoms available for adsorption, leading to a further fall in the less-favored CdO bond adsorption, and this results in a higher selectivity for 2 with an increase in the number of reaction cycles (Table 3, columns 5 and 6). A shorter reaction time and low hydrogen partial pressure limit the hydrogenation of 4 to 3, resulting in a higher selectivity of 4 (Figure 3b). In general, the selectivity depends on the solvent, and extremes in the dielectric constant give extremes in selectivity.52 sc-CO2 has a very low dielectric constant (∼1.5), and its physical properties vary over a wide range with pressure and temperature, resulting in changes in selectivity. Conclusions In summary, hydrogenation of 1 has been studied over a 1% Pd/Al2O3 impregnated catalyst in sc-CO2 medium at different conditions of temperatures, pressures, and reaction media. Phase behavior studies show that 1 forms a supercritical phase with a H2-CO2 mixture at 373 K and at a total pressure of 15 MPa with a H2 partial pressure of 1.7 MPa. Studies on the effect of CO2-organic cosolvent systems do not show any advantage at the conditions employed. Higher conversions comparable to that obtained in organic solvent media are achieved in sc-CO2 reaction medium. Selectivities different from liquid-phase reactions are observed under supercritical conditions. The possible reaction mechanism is proposed. Even though catalyst deactivation is observed in both organic solvent and sc-CO2 reaction media, the extent of deactivation is much lower for the reaction in sc-CO2. This study further demonstrates the utility of sc-CO2 as an attractive alternative reaction medium for hydrogenations, with various advantages including ecofriendliness, desired product selectivity, and lesser catalyst deactivation. Acknowledgment U.R.P. is a postgraduate research participant at the National Risk Management Research Laboratory, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency. Literature Cited (1) Mayer, H. Synthesis of optically active carotenoids and related compounds. Pure Appl. Chem. 1979, 51, 535.
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Received for review July 7, 2003 Revised manuscript received September 23, 2003 Accepted October 13, 2003 IE030571A
