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Ind. Eng. Chem. Res. 2008, 47, 6796–6800
RESEARCH NOTES Influence of Compressed Carbon Dioxide on Hydrogenation Reactions in Cyclohexane with a Pd/C Catalyst Jianmin Hao,†,‡ Chunyu Xi,† Haiyang Cheng,†,‡ Ruixia Liu,†,‡ Shuxia Cai,† Masahiko Arai,§ and Fengyu Zhao*,† State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China, Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100049, P. R. China, and DiVision of Chemical Process Engineering, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan
The potential of CO2-expanded liquid media for chemical reactions has been examined in this work, using cyclohexane as a solvent and Pd/C as a heterogeneous catalyst for hydrogenation of styrene, citral, and nitrobenzene with H2. The rate of hydrogenation reactions is increased, and the product selectivity is altered in the CO2-expanded cyclohexane phase. In the hydrogenation of citral, the selectivity to citronellal decreases with CO2 pressure, which changes from ∼80% in the neat cyclohexane to ∼65% at 16 MPa. The CO2 dissolved in the cyclohexane phase may accelerate the rate of further hydrogenation of citronellal. In the hydrogenation of nitrobenzene, the selectivity to aniline is large >90% in the absence of dense phase CO2 but it increases up to >95% on CO2 pressurization of the liquid reaction phase. 1. Introduction Supercritical carbon dioxide (scCO2) is a new green and effective reaction medium for organic syntheses and catalytic reactions.1-3 However, it is not always useful because of lower solubility of organometallic complex catalysts and organic substrates in scCO2. By contrast, CO2 can be soluble in conventional organic solvents and the quantity of CO2 dissolved can approach such a high value of 80 mol % in some cases. The dissolution of CO2 in an organic solvent will cause a significant expansion of the liquid phase, forming the so-called “CO2-expanded liquid”. The CO2-expanded liquid exhibits lower viscosity and faster diffusion compared with the original neat liquid. In addition, the dissolution of CO2 will enhance the solubility of other coexisting gases like H2 and O2 in the liquid phase.4-7 These features of the CO2-expanded liquid phases are beneficial for gas-liquid reactions such as hydrogenation reactions with H2, for which the reaction rate is often limited by low solubility of H2 into the liquid phases containing organic substrates and catalysts. Phiong et al.8 reported that the rate of R-methylstyrene hydrogenation was enhanced when the liquid substrate was compressed by CO2. Roberts et al. found the enhancement of reaction rate in hydrogenation of aromatic rings of polystyrene in CO2-expanded decahydronaphthalene.9 Moreover, it was reported that, in cinnamaldehyde hydrogenation, higher selectivity to cinnamyl alcohol was achieved in CO2expanded cinnamaldehyde phase (without additional solvent) than in a homogeneous scCO2 phase.10 For hydroformylation of 1-octene with a Rh(acac)(CO)2 catalyst, CO2-expanded acetone gave turnover numbers about 4-fold higher than those obtained in either neat acetone or compressed CO2.11 More recently, for Diels-Alder reaction in CO2-expanded acetonitrile, * To whom correspondence should be addressed. Tel. and Fax: +86431-85262410. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Hokkaido University.
Eckert et al. indicate that CO2 could act as a Lewis acid and accelerate the reactions between anthracene and 4-pheyl-1,2,4triazoline-3,5-dione.12 Those previous results show that the CO2expanded liquid phases are interesting media for chemical reactions and effective for accelerating reaction rates and changing product selectivity patterns.11-20 Very recently, Fujita et al. show that the pressurization with CO2 can enhance the rate of liquid phase Heck coupling reactions with a few selected substrates.21 During the course of our study on CO2-expanded liquid phases and reactions in them, the present work has been undertaken to examine the expansion of cyclohexane by CO2 pressurization and the potential of hydrogenation reactions of a few substrates (styrene, citral, nitrobenzene) using H2 and a Pd/C catalyst in the CO2-expanded cyclohexane medium. The previous studies have discussed the hydrogenation reactions of these substrates in homogeneous dense phase CO2 media.22-26 The present work will report and discuss the features of hydrogenation reactions in cyclohexane under compressed CO2 conditions and the roles of dense phase CO2 in the alternation of total conversion and product selectivity. 2. Experimental Details 2.1. Materials. Styrene (99.0%) and citral (95%) were purchased from Shanghai Shanpu Chemical and Shanghai Guoyao Group Chemical Reagent, respectively, and nitrobenzene (99.0%) and cyclohexane (99.0%), from Beijing Chemical Plant. These were used without further purification. Gases of CO2 (99.9%), H2 (99.999%), and N2 (99.9%) were obtained from Changchun Xinxing Gas and used as delivered. A commercial catalyst, 5 wt % Pd/C (Aldrich), was used for hydrogenation reactions, prior to which it was reduced by H2 stream at 573 K for 2 h. The size of supported Pd crystallites was measured by X-ray diffraction (XRD) line broadening and estimated to be 20.9 nm from the Scherrer equation.
10.1021/ie071453g CCC: $40.75 2008 American Chemical Society Published on Web 08/02/2008
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2.2. Phase Behavior Measurement. An 85 cm high pressure glass cell was used to examine the expansion of cyclohexane phase on pressurization with CO2. A certain volume of cyclohexane (V0) was put into the reactor and then CO2 gas was introduced into the reactor. When the desired CO2 pressure was obtained, the mixture was stirred for several minutes, after which the stirring was stopped and the state of the mixture was visually examined to determine the volume of the liquid phase (V). The degree of volume expansion of the liquid phase, V/V0, was measured at different CO2 pressures. 2.3. Hydrogenation Reactions. A 50 cm3 stainless steel autoclave was used to run hydrogenation reactions of styrene, citral, and nitrobenzene in cyclohexane with H2. Certain quantities of Pd/C catalyst, reactant, and solvent were charged into the reactor and the reactor was flushed with H2 three times to remove the air. Then, H2 and CO2 were introduced to the desired pressures. The reactions were conducted under continuous stirring for a certain time and the reactor was cooled down to room temperature and depressurized carefully by a backpressure regulator. The composition of reaction mixture was analyzed by a gas chromatograph (Shimadzu GC-14C, FID, capillary column, Rtx-Wax 30 m × 0.53 mm × 0.25 m) using a flame ionization detector (FID). The conversion was calculated from the moles of reactant consumed/total initial moles of the reactant loaded; the product selectivity was calculated from the moles of a certain product produced/total moles of all the products obtained. The experimental data were reproduced within errors 8 MPa, forming a single homogeneous gas phase. At a higher temperature of 353 K, the liquid volume also increases with CO2 pressure but less significantly up to 9 MPa, as compared to the behavior at 323 K. The extent of volume expansion is 95% at CO2 pressures examined. It seems that this selectivity is slightly improved at 4 MPa as compared to that in the neat cyclohexane (Figure 5). The minor products
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ReceiVed for reView October 27, 2007 ReVised manuscript receiVed June 24, 2008 Accepted July 12, 2008 IE071453G
