6384
Ind. Eng. Chem. Res. 2003, 42, 6384-6388
Aerobic Oxidation of Cyclohexanol to Cyclohexanone in Compressed CO2 and Liquid Solvents Yanhong Chang, Tao Jiang, Buxing Han,* Liang Gao, Rui Zhang, Zhimin Liu, and Weize Wu The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
The aerobic oxidation of cyclohexanol to cyclohexanone was conducted in compressed CO2 with a copper-based catalyst, and the effect of phase behavior and a small amount of cosolvents (toluene or fluorobenzene) on the conversion and selectivity of the reaction in supercritical (SC) CO2 was studied. To explore the advantages of the reaction in compressed CO2, the reaction was also performed in liquid toluene and fluorobenzene and in the absence of solvent for comparison. The results indicated that the selectivity of the reaction in compressed CO2 with and without cosolvents was much higher than that in the liquid solvents or without solvent. The conversion of cyclohexanol was improved effectively by adding a small amount of the cosolvents and the selectivity for the desired product was still very high. It was advantageous to carry out the reaction in CO2. Introduction The oxidation of primary and secondary alcohols, especially partial oxidation into aldehydes and ketones, is a ubiquitous transformation in organic chemistry.1-3 Different oxidants, such as hydrogen peroxide, tert-butyl hydroperoxide, chromium reagents, air, or oxygen, have been used in these reactions.4-8 In recent years, reactions in supercritical fluids (SCFs) have received much attention.9,10 Up to now, different reactions in SCFs have been studied, such as hydrogenations,11 oxidation reactions,12,13 and alkylation reactions.14 There are some unique advantages to conduct reactions in SCFs. For example, varying pressure or adding a small amount of cosolvent can tune reaction rates, yields, and selectivity without the need for harsh chemical changes. Some environmentally more acceptable supercritical (SC) solvents, such as CO2 and water, can be used to replace organic solvents. Study of phase behaviors and critical points is a long established field. Up to now, the phase behaviors of many pure compounds and mixtures have been determined. However, most of the investigations are restricted to the mixtures of less than three components. Moreover, nearly all of the studied mixtures are not related directly to the reaction systems. Reaction mixtures are different from those normally studied because they are usually more complex and the composition varies with conversion or reaction time. Therefore, the phase behaviors and critical parameters of reaction systems change with conversion. Recently, researchers have begun to study the phase behaviors of the reaction mixture.15,16 Metal-catalyzed oxidation of alcohols to carbonyl compounds using molecular oxygen as an oxidant represents a significant step in synthetic organic chemistry. Recently, aerobic oxidation of alcohols has been conducted in different media,17-19 including CO2expanded acetonitrile.20 Copper-based catalyst is regarded as one of the most efficient catalysts because it can oxidize a wide range of alcohols into the corresponding aldehydes and ketones in organic solvents under * To whom correspondence should be addressed. Tel: 8610-62562821. Fax: 86-10-62559373. E-mail:
[email protected].
mild conditions, and this catalyst is simple and readily available. In addition, the selectivity for the desired ketone is as high as that when using copper-based catalyst at relatively low reaction temperature. Compressed CO2 is a very attractive reaction medium for oxidation reactions because it is nonflammable. The oxidation of alcohols in compressed CO2 with a copperbased catalytic system is a cleaner process compared with organic solvents and may be an effective way to reduce the byproducts. In this work, we first investigated the phase behavior of the reaction mixture of aerobic oxidation of cyclohexanol in compressed CO2 using a copper chloride/phenanthroline catalytic system. The reaction was then conducted in compressed CO2 (with and without cosolvent) in the vapor-liquid equilibrium region and in the SC region of the reaction system. The reaction was also carried out in some liquid organic solvents and in the absence of solvent for comparison. The results showed that it was advantageous to carry out the reaction in compressed CO2. Experimental Section Materials. CO2 was supplied by Beijing Analytical Instrument Factory with a purity of 99.995%. Cyclohexanol, cyclohexanone, toluene, fluorobenzene, N,Ndimethylformamide (DMF), CuCl, K2CO3, diethylazodicarboxylate, and 1,10-phenanthroline were A.R. grade and produced by Beijing Chemical Plant. All the chemicals were used without further purification. Apparatus and Procedures To Measure the Phase Behavior. The apparatus and the experimental procedures were similar to those described previously.21 Figure 1 shows the schematic diagram of the apparatus for measuring the phase behavior. It consisted mainly of a high-pressure view cell, a constant temperature water bath, a high-pressure pump, a pressure gauge, a gas tank, and a magnetic stirrer. The high-pressure view cell is shown schematically in Figure 2. It was composed of a stainless steel body, a stainless steel piston, two quartz windows, two compact components, and the Teflon seals. The piston could be moved up and down by a screw. The cell could be used up to 20 MPa. The volume of the cell could be changed in the range from 20 to 50 cm3 by moving the piston.
10.1021/ie030040m CCC: $25.00 © 2003 American Chemical Society Published on Web 07/29/2003
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6385
Figure 3. Experimental setup for the oxidation reaction. Figure 1. Schematic diagram of the apparatus to measure phase behavior.
Figure 2. Schematic diagram of high-pressure volume-variable view cell.
The cell was immersed in a constant temperature water bath, which was controlled by a HAAKE D8 temperature controller, and the accuracy of the temperature measurent was (0.1 K. The pressure gauge was composed of a pressure transducer (FOXBORO/ICT, Model 93) and an indicator, which was accurate to (0.025 MPa in the pressure range of 0-20 MPa. In a typical experiment, the air in the view cell was first removed by a vacuum pump. A suitable amount of liquid chemicals were charged into the view cell. Then desired amount of N2 (N2 was used to replace O2 for phase behavior measurement) was added by controlling the nitrogen pressure, which had been calculated on the basis of the amount of the liquid chemicals in the views cell, the volume of the view cell, and the temperature of the view cell before adding the gas. To add the nitrogen, a sample bomb (30 mL) with a known amount of nitrogen was connected to the view cell, and the nitrogen was filled into the view cell slowly until the desired pressure was reached. The amount of nitrogen charged was also checked by weighing the sample bomb before and after filling the view cell, and the results of the two methods agreed well. The view cell was charged with CO2 using another sample bomb of 47 mL in a similar way as charging nitrogen. The amount of CO2 in the cell was known by weighing the sample bomb. The composition of the mixture could be calculated easily from the masses of the components in the view cell. It was estimated that the mole fractions of the components were accurate to (0.0002. The system pressure was adjusted by moving the piston down until a homogeneous phase was observed.
Scheme 1
The system was allowed to equilibrate for at least 60 min at the desired temperature. After equilibration, the pressure was decreased via moving the piston until the phase separation point was observed, and the corresponding pressure and volume were recorded. Then the experiments were conducted at another temperature by repeating the same procedure. The density of a mixture was easily known from the volume and the mass of the chemicals in the system. Apparatus and Procedures To Conduct the Reaction. A batch reactor was used, and its schematic diagram is shown in Figure 3. The reactor was made of stainless steel with an inner volume of 12.8 mL. The fluctuation of the temperature in the reactor was (0.2 K, which was controlled by a PID temperature controller made by Beijing Tianchen Electronic Company (model SX/A-1). In a typical experiment, CuCl and phenanthroline were loaded into the reactor. Cyclohexanol with or without cosolvent was then charged. The black complex formed was stirred at room temperature for 10 min. Diethylazodicarboxylate and solid K2CO3 were then added, and the mixture was stirred for 5 min. Suitable amounts of O2 and CO2 were introduced into the reactor using sample bombs, of which the procedures were similar to those of adding a gas for the phase behavior measurement discussed above. For all the experiments the molar ratio of CO2:O2:cyclohexanol was 94:2:4, and that of cyclohexanol:CuCl:phenanthroline:diethylazodicarboxylate:K2CO3 was 100:5:5:5:5. Once the reactants and the catalyst were charged, the reactor was heated to the desired temperature and the stirrer was started. After the reaction had proceeded for a desired time, the reactor was cooled, and then CO2 and O2 were released slowly, passing through the absorbing tube in the ice bath. Experiments showed that the amount of reactants and products entrained by the released gases was negligible. The liquid mixture in reactor was analyzed by gas chromatography (Agilent 4890D, Agilent Technologies Inc.) with a FID detector. Results and Discussions The oxidation reaction in our work could be expressed by Scheme 1. At the beginning, the reaction system contained three components, CO2 (solvent), cyclohexanol (C6H12O), and O2. During the reaction process there were five compo-
6386 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 4. Effect of temperature and conversion (Conv) on the phase separation density.
nents, CO2, O2, C6H12O, cyclohexanone (C6H10O), and H2O. Once the original molar ratio (before reaction) of CO2:C6H12O:O2 and the conversion of C6H12O (a) were defined, the composition of the reaction system was fixed. Therefore, the phase behavior of the reaction system could be determined by preparing the mixtures with the desired compositions. In this work, we studied the effect of phase behavior of the reaction system, cosolvent, and reaction time on conversion and selectivity of the reaction. The reaction was also carried out in some liquid organic solvents and in the absence of any solvent. Phase Behavior of the Reaction System. In this work N2 was used to replace O2 in the phase behavior measurement. The main reason was that the oxidation reaction might occur during the experiment in the presence of O2, which affected the phase behavior measurement. In addition, among the compounds, the physical properties of N2 were most similar to that of O2. Therefore, to a first approximation, we assumed that N2 could be used replace O2 for dilute solutions (the concentration of O2 was less than 2 mol % in this work). It had been reported that N2 was suitable replacement of O2 for phase behavior study.13,22,23 In this work, we determined the phase equilibrium data of the reaction system with the original molar ratio of CO2:C6H12O:N2 ) 94:4:2. The composition at a known conversion was calculated and the mixture was prepared from the pure chemicals and the phase behavior of the mixture was determined. Figure 4 shows the effect of temperature and conversion on phase separation density. A mixture was in the single-phase region as the density was higher than the phase separation density and changed into two phases at the lower densities. Only a small amount of byproducts was formed at the reaction conditions of this work as CO2 was used as solvent, which will be discussed in the following sections, so they were neglected in the study of the phase behavior. Effect of Reaction Time. The dependence of the conversion and selectivity on the reaction time with a fluid density of 0.77 g/mL is shown in Figure 5. The reaction fluid was in the single-phase region, as shown in Figure 4 (point A). It can be seen that cyclohexanone is a major product and the amount of byproduct is very small. In other words, the copper-based catalyst possessed very high selectivity for desired product at experimental conditions. As expected, the conversion increased with increasing reaction time. The selectivity decreased monotonically with reaction time, suggesting that the byproducts were formed by a consecutive
Figure 5. Effect of reaction time on the conversion and selectivity at at 333.2 K (F ) 0.77 g/mL). Table 1. Effect of Apparent Density on the Conversion and Selectivity at 333.2 K for 6 h point
apparent density (g/mL)
cyclohexanol conversion (%)
selectivity (%) cyclohexanone
A B C
0.77 0.71 0.63
7.51 8.20 10.8
98.9 97.7 94.0
oxidation of cyclohexanone. Analysis using GC-MS, IR, showed that the main byproduct was hexandioic acid under the conditions of this work. Effect of Phase Behavior of the Fluid on the Reaction. To study the effect of phase behavior of the reaction system on the aerobic oxidation of cyclohenanol, we conducted the reaction in a two-phase region and single-phase region of the reaction fluid by controlling the temperature and density of the reaction system. We chose different apparent densities, illustrated in Figure 4 by A, B, and C. Point A was in the single-phase region. Point B was close to the phase boundary, and point C was in the two-phase region. The reaction results are displayed in Table 1. The influence of phase behavior on the conversion and selectivity was noticeable. In the two-phase region of reaction mixture, there existed a CO2-rich vapor phase and a cyclohexanol-rich liquid phase. The catalyst contacted with the liquid phase, and the reaction took place mainly at the liquid/ catalyst surface. The conversion of cyclohexanol was higher in the two-phase region. This suggested that the oxidation reaction was favored at the liquid/catalyst surface. It may be attributed to the ability of polar liquid to stabilize the polar transition state(s), thereby lowing the activation energy and increasing the reaction rate.20,24 Cyclohexanone was the major product, as shown in Table 1; that is, the selectivity for cyclohexanone was very high when CO2 was used as the solvent. The selectivity in the two-phase region was relatively low since the conversion of cyclohexanol was higher. The other reason might be that the diffusion coefficient of the components in the liquid phase was smaller than those in the single-phase mixture because CO2 could enhance the diffusion,9 and the concentration of CO2 in the liquid phase was smaller. The product was in contact with the catalyst for a longer time, which resulted in further oxidation. Therefore, the selectivity was lower. Effect of Solvent on the Conversion and Selectivity. The oxidation reaction was also conducted in toluene, in fluorobenzene, and in the absence of solvent.
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6387 Table 2. Effect of Different Solvents on the Conversion and Selectivity at 333.2 K for 6 h solvent
alcohol conversion (%)
selectivity (%) desired product
no solvent toluene fluorobenzene CO2a CO2a,b CO2 + toluenea,c CO2 + fluorobenzenea,c
19.8 33.4 26.5 7.51 30.0 18.6 21.7
70.6 77.9 75.2 98.9 89.4 97.3 99.0
a Apparent density F ) 0.77 g/mL. b Reaction time 36 h. c The amount of cosolvent in CO2 is 0.5 mol %.
The reaction temperature, the molar ratio of the solvent: alcohol:O2, and the amount of catalyst were the same as those of the reaction with CO2 as solvent. The experimental results are presented in Table 2. The oxidation rate of cyclohexanol in organic solvents or without solvent was larger than that in CO2. It can be ascribed to the fact that the interaction between the reactants and the organic solvents is stronger than that between the reactants and CO2 because CO2 has a low dielectric constant and lacks other properties of the liquid media.24 To compare the selectivity at similar conversion, the reaction in SC CO2 was carried out for a longer time, and the result was also given in Table 2. Clearly, selectivity for the desired product in SC CO2 was much higher than those in the organic solvents or in the absence of solvent. The main reason was that the diffusivity of the components in SC CO2 was much higher than that in the organic solvents.25 The product could escape more easily from the surface of the catalyst, which avoided further oxidation effectively. Effect of Cosolvents on the Reaction. It is known that cosolvent plays a very important role in tuning the properties of SCFs,26-30 including tuning the kinetic and thermodynamic properties of reactions.28-30 In this work the effect of cosolvents fluorobenzene and toluene on the oxidation reaction in SC CO2 were investigated, and the results are also shown in Table 2. The conversion of cyclohexanol was improved effectively by adding a small amount of cosolvents and the selectivity for the desired products was still very high. The cosolvents could enhance the reaction rate because the reaction rate in the liquids was much faster. The concentration of a cosolvent around the solutes or reaction species can be much higher than that in the bulk.10,28-32 This can explain partially the fact that a small amount of cosolvent can improve the reaction rate significantly. It is interesting that the reaction rate in liquid toluene was faster than that in liquid fluorobenzene, while the fluorobenzene was a more effective cosolvent, as is shown in Table 2. To our knowledge, it was very difficult to give precise explanation for this phenomenon. We can only give a possible reason. SC CO2 has a very strong solvent power for fluorinated compounds,33,34 which may change the interaction of cosolvent and reactant, or cosolvent and catalyst. Conclusion The aerobic oxidation of cyclohexanol to cyclohexanone was conducted in SC CO2 with and without cosolvents toluene and fluorobenzene using copperbased catalyst. The reaction was also performed in liquid toluene, in fluorobenzene, or in the absence of
solvent for comparison. The results demonstrated that the selectivity in SC CO2 with and without cosolvents was much higher. It was advantageous to carry the reaction in CO2. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (20073056) and National Key Basic Research Project (G2000048010). Literature Cited (1) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981. (2) Ley, S. L.; Norman, J.; Griffith, W. P.; Marsden, S. P. Tetrapropylammonium Perruthenate Pr4N+RuO4-; TPAP: A Catalytic Oxidant for Organic Synthesis. Synthesis 1994, 639. (3) Fung, W. H.; Yu, W. Y.; Che, C. M. Chemoselective Oxidation Of Alcohols to Aldehydes and Ketones by tert-Butyl Hydroperoxide Catalyzed by A Ruthenium Complex of N,N′,N′′-Trimethyl1,4,7-Triazacyclononane. J. Org. Chem. 1998, 63, 2873. (4) Krohn, K.; Vinke, I.; Adam, H. Transition-Metal Catalyzed Oxidations. 7. Zirconium-Catalyzed Oxidation of Primary and Secondary Alcohols with Hydroperoxides. J. Org. Chem. 1996, 61, 1467. (5) Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; Springer: Berlin, 1984. (6) Yang, X. T.; Li, Y.; Huang, M. Y.; Jiang, Y. Y. Oxidation of Benzyl Alcohol with Dioxygen Catalyzed by An Inorganic PolymerPlatinum Complex. Polym. Adv. Technol. 1996, 7 (1), 47. (7) Mallat, T.; Bodnar, Z.; Hug, P.; Baiker, A. Selective Oxidation of Cinnamyl Alcohol to Cinnamaldehyde with Air Over BiPt/Alumina Catalysts. J. Catal. 1995, 153, 131. (8) Giannandrea, R.; Mastrorilli, P.; Nobile, C. F.; Suranna, G. P. Aerobic Oxidation of Aldehydes, Ketones, Sulfides, Alcohols and Alkanes Catalyzed by Polymerizable Beta-Ketoesterate Complexes of Iron (iii), Nickel (ii) and Cobalt (ii). J. Mol. Catal. 1994, 13, 27. (9) Jessop, P. G.; Leitner, W. Chemical Synthesis using Supercritical Fluids; Wiley-VCH: Weinheim, 1999. (10) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Supercritical Fluids as Solvents for Chemical and Materials Processing. Nature 1996, 383, 313. (11) Hitzler, M. G.; Poliakoff, M. Continuous Hydrogenation of Organic Compounds in Supercritical Fluids. Chem. Commun. 1997, 1667. (12) Loeker, F.; Leitner, W. Steel-Promoted Oxidation of Olefins in Supercritical Carbon Dioxide Using Dioxygen in the Presence of Aldehydes. Chem. Eur. J. 2000, 6, 2011. (13) Mukhopadhyay, M.; Srinivas, P. Influence of the Thermodynamic State on Cyclohexane Oxidation Kinetics in Carbon Dioxide Medium. Ind. Eng. Chem. Res. 1997, 36, 2066. (14) Hitzler, M. G.; Smail, F. R.; Ross, S. K. Poliakoff, M. Friedel-Crafts Alkylation in Supercritical Fluids: Continuous, Selective and Clean. Chem. Commun. 1998, 359. (15) Ke, J.; Han, B. X.; George, M. V.; Yan, H. K.; Poliakoff, M. How does the critical point change during a chemical reaction in supercritical fluids? A study of the hydroformylation of propene in supercritical CO2. J. Am. Chem. Soc. 2001, 123, 3661. (16) Stradi, B. A.; Stadtherr, M. A.; Brennecke, J. F. Multicomponent Phase Equilibrium Measurements and Modeling for the Allylic Epoxidation of Trans-2-hexen-1-ol to (2R,3R)-(+)-3Propyloxiranemethanol in High-Pressure Carbon Dioxide. J. Supercrit. Fluids 2001, 20, 1. (17) Ebitani, K.; Fujie, Y.; Kaneda, K.; Immobilization of a Ligand-Preserved Giant Palladium Cluster on a Metal Oxide Surface and its Nobel Heterogeneous Catalysis for Oxidation of Allylic Alcohols in the Presence of Molecular Oxygen. Langmuir 1999, 15, 3557. (18) Marko`, I. E.; Giles, P. R.; Tsukazaki, M.; Chelle`-Regnault, I.; Urch, C. J.; Brown, S. M.; Efficient Aerobic RutheniumCatalyzed Oxidation of Alcohols into Aldehydes and Ketones. J. Am. Chem. Soc. 1997, 119, 12661. (19) Marko`, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.; Urch, C. J. Cpper-Catalyzed Oxidation of Alcohols into Aldehydes and Ketones: An Efficient, Aerobic Alternative. Science 1996, 274, 2044.
6388 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 (20) Wei, M.; Musie, G. T.; Busch, D. H.; Subramaniam, B. CO2Expanded Solvents: Unique and Versatile Media for Performing Homogeneous Catalytic Oxidations. J. Am. Chem. Soc. 2002, 124, 2513. (21) Zhang, H. F.; Liu, Z. M.; Han, B. X. Critical Points and Phase Behavior of Toluene-CO2 and Toluene-H2-CO2 Mixtures in CO2-rich Region. J. Supercrit. Fluids 2000, 18, 185. (22) Jenzer, G.; Schneider, M. S.; Wandeler, R.; Mallat, T.; Baiker, A. Palladium-Catalyzed Oxidation of Octyl Alcohols in “Supercritical” Carbon Dioxide. J. Catal. 2001, 199, 141.J (23) Hou, Z. S.; Han, B. X.; Gao, L.; Liu, Z. M.; Yang, G. Y. Selective Oxidation of Cyclohexane in Compressed CO2 and in Liquid Solvents over MnAPO-5 Molecular Sieve. Green Chem. 2002, 4, 426. (24) Musie, G.; Wei, M.; Subramaniam, B.; Busch, D. H. Catalytic oxidations in carbon dioxide-base reaction media, including novel CO2-expanded phase. Coord. Chem. Rev. 2001, 219221, 789. (25) McHugh, M. A.; Ktukonis, V. J. Supercritical Fluid Extraction, 2nd ed.; Butterworth-Heinemann: Boston, 1994. (26) Andrew, D.; Des Islet, B. T.; Margaritis, A.; Weedom, A. C. Photo-Fries Rearrangement of Naphthyl Acetate in Supercritical Carbon Dioxide-Chemical Evidence for Solvent-Solute Clustering. J. Am. Chem. Soc. 1995, 117, 6132. (27) Zhang, X. G.; Han, B. X.; Hou, Z. S.; Zhang, J. L.; Liu, Z. M.; Jiang, T.; He, J.; Li, H. P. Why Cosolvent Can Enhance the Solubility of Solute in Supercritical Fluids? New Evidence and Opinion. Chem. Eur. J. 2002, 8, 5107.
(28) Ellington, J. B.; Park, K. M.; Brennecke, J. F. Effect of Local Composition Enhancements on The Esterification of Phthalic-Anhydride with Methanol in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1994, 33, 965. (29) Li, H. P.; Liu, J.; Zhang, H. F.; Han, B. X. Effect of CoSolvent and Pressure on the Thermal Decomposition of 2,2′-azobis(isobutyronitrile) in Supercritical CO2. J. Supercrit. Fluids 2001, 21, 227. (30) Eckert, C. A.; Chandler, K. Tuning Fluid Solvents for Chemical Reactions. J. Supercrit. Fluids 1998, 13, 187. (31) Tucker, S. C. Solvent Density Inhomogeneities in Supercritical Fluids. Chem. Rev. 1999, 99, 391. (32) Lu, J.; Han, B. X.; Yan, H. K. Thermodynamic Studies of Chemical Equilibrium in Supercritical Carbon Dioxide-Cosolvent Solutions Using UV-Vis Spectroscopy. J. Supercrit. Fluids 1999, 15, 135. (33) McFann, G. J.; Johnston, K. P.; Howdle, S. M. Solubilization in Nonionic Reverse Micelles in Carbon-Dioxide. AIChE J. 1994, 40 (3), 543. (34) Cooper, A. I.; Londono, J. D.; Wignall, G.; McClain, J. B.; Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M. A.; Burke, L. C.; Fre`chet, J. M. J.; DeSimone, J. M. Extraction of A Hydrophilic Compound From Water into Liquid CO2 Using Dendritic Surfactants. Nature 1997, 389, 368.
Received for review January 13, 2003 Revised manuscript received April 17, 2003 Accepted May 22, 2003 IE030040M
