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J. Phys. Chem. B 2001, 105, 4510-4513
Pressure Tuning of Reaction Equilibrium of Esterification of Acetic Acid with Ethanol in Compressed CO2 Zhenshan Hou, Buxing Han,* Xiaogang Zhang, Haifei Zhang, and Zhimin Liu Center for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China ReceiVed: October 23, 2000; In Final Form: January 18, 2001
The esterification of acetic acid and ethanol in compressed CO2 was studied at 333.2 K and at pressures up to 16 MPa, and the original molar ratio of the components was CO2:CH3COOH:CH3CH2OH:p-toluenesulfonic acid (catalyst) ) 90:5:5:0.05. The phase behavior and the isothermal compressibility (kT) of reaction system was also determined under the reaction conditions. The conversion increases with increasing pressure in the two-phase region and reaches a maximum in the critical region of the reaction system where the system just becomes one phase. Then the conversion or apparent equilibrium constant Kx decreases with pressure after the pressure is higher than the critical value. In the supercritical region where the system is one phase, Kx is closely related to the isothermal compressibility and degree of inhomogeneity of the fluid, which is often referred to as the local density and/or local composition enhancement. At the higher pressures the “clustering” in the reaction system is not significant and the Kx is close to that of the reaction in the absence of CO2.
Introduction Supercritical fluids (SCFs) have already been used in some industry processes,1-5 such as the food and fragrance industries. Over the past decade there has been a growing interest in using SCFs as the reaction media. It is well-known that a small change in the pressure near the critical point of a fluid causes a significant change in density-dependent properties such as the solubility parameter, mass transfer, viscosity, and dielectric constant. There are some unique advantages for conducting chemical reactions in SCFs. For example, reaction rates, yields, and selectivity can be adjusted by varying pressure without the need for a harsh chemical change; SCFs (such as CO2, H2O) can be used to replace environmentally undesirable solvents or avoid undesirable byproducts; Mass transfer is improved for heterogeneous reactions; and simultaneous separation and reaction may be accomplished for some reactions. It is not surprised that in recent years the use of SCFs, especially supercritical (SC) CO2 and H2O, as solvents for chemical reaction media is receiving much attention. Up to now, many papers have been published, especially in the last 10 years, and this topic has been reviewed recently.6-14 In a supercritical mixture, the solvent, cosolvent, and solute differ in structure, size, and polarity and is more compressible compared with a liquid mixture. It has large free volume so that the attractive forces can move molecules into energetically favorable locations. As a result, the local density of SC solvent and the cosolvent around the solute molecules differ from the bulk, which is often referred to as local density enhancement and/or local composition enhancement. Many papers can be found in the literature that investigated the inhomogeneity in SCFs.15-17 How the local density and/or local composition enhancements in a reaction system affect thermodynamic and kinetic properties of the reaction is a very interesting area, and * Corresponding author. Tel: 86-10-62562821. Fax: 86-10-62559373. E-mail:
[email protected].
some investigations have been conducted on this. For example, Ellington and Brennecke studied the effect of local composition enhancement on the reaction rate of esterification of phthalic anhydride with methanol in SC CO2 at 313.2 and 323.2 K,18,19 and a 25-fold decrease in rate constant based on bulk concentration was observed when increasing the pressure from 9.75 to 16.65 MPa. Collins et al.20 studied the disproportionation of toluene over a ZSM-5 zeolite catalyst at conditions near the critical point of toluene. They found that the selectivity for the commercially desirable product, p-xylene, reaches its maximum near the critical region, which can be attributed to clustering near the critical point. Peck et al.21 studied the effect of pressure on the tautomeric equilibria of 2-hydroxypyridine and 2-pyridone in SC propane at 393 K and in 1,1-difluoroethane at 403 K at infinite dilution. They found that the equilibrium constant Kc in supercritical 1,1-difluoroethane increased 4-fold for a pressure change of 4 MPa. Lu et al.22 studied the keto-enol tautomeric equilibrium of 5,5-dimethyl-1,3-cyclohexane in SC CO2 at 308.15 K in the pressure range from 8 to 16 MPa. They reported that Kc decreased considerably in the pressure range from 8 to 12 MPa. Reaction systems are mixtures, and thus the phase behavior and critical point depend on both original composition and conversion. It is no doubt that the phase behaviors and critical parameters of the reaction systems are crucial for studying the reaction at supercritical conditions. The phase behaviors and critical parameters of many pure compounds and binary mixtures have been reported. Comparing to binary mixtures, experimental data for ternary and more complex mixtures are scarce.23-26 Usually the reported phase behaviors were not studied for reaction mixtures. We are very interested in the effect of phase behavior and intermolecular interaction in reaction systems on the thermodynamic properties of the chemical reactions, which is very important for both pure and applied sciences and has not been well studied. Reversible reactions have many advantages for exploring this. In this work, we studied the phase
10.1021/jp003903n CCC: $20.00 © 2001 American Chemical Society Published on Web 04/25/2001
Pressure Tuning of Reaction Equilibrium
Figure 1. Schematic diagram of the apparatus for measuring the phase behavior, density, and critical point: (1) gas bank, (2) high-pressure pump, (3) sample bomb, (4) high-pressure volume-variable view cell, (5) water bath, (6) magnetic stirrer.
behavior of the reaction system for esterification of acetic acid with ethanol in the presence of compressed CO2, and the equilibrium constant of the reaction in different phase regions was studied. Experimental Section Materials. Vieville et al.27 found that p-xylenesulfonic acid and cation-exchange resins were good catalysts for the esterification of oleic acid with methanol. In this work we used p-TSA as the catalyst. Acetic acid, ethanol, and p-toluenesulfonic acid (p-TSA, the catalyst) were A. R. grade reagents produced by Beijing Chemical Plant. The purity of CO2 was better than 99.95% as supplied by Beijing Analytical Instrument Factory. All of the chemicals were used without further purification. Apparatus and Procedures for Phase Behavior. Figure 1 shows the schematic diagram of the apparatus for measuring the phase behavior and critical points of the reaction system. It consisted mainly of a high-pressure view cell, a constant temperature water bath, a high-pressure pump, a pressure gauge, and a magnetic stirrer. The apparatus was similar to that described previously.28 The high-pressure view cell was composed of a stainless steel body, a stainless steel piston, and two glass windows. The volume of the cell could be changed in the range from 20 to 50 cm3 by moving the piston. The cell was immersed in a constant temperature water bath which was controlled using a HaakeD8 controller, and the temperature was measured by accurate mercury thermometers with an accuracy of better than (0.1 K. The pressure gauge was composed of a pressure transducer (FOXBORO/ICT) and an indicator; it was accurate to (0.025 MPa in the pressure range from 0 to 20 MPa. The experimental procedure was similar to that described elsewhere.28 In a typical experiment, the air in the view cell was replaced by CO2. The desired amount of acetic acid with the catalyst (p-TSA) and ethanol were charged into the cell. The suitable amount of CO2 was then introduced into the cell with a CO2 sample bomb of 41 mL. The temperature of the water bath was controlled at a suitable value. The system pressure was adjusted by moving the piston in the cell. The system was equilibrated for at least 4 h after the pressure had been constant, which was enough for phase equilibrium and reaction equilibrium to be reached (see the following sections). The phase behavior could be seen through the windows of the cell. The volume of the system was known by the position of the piston, which was calibrated before the experiments. The
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Figure 2. The plot of density and compressibility (kT) versus pressure for the reaction systems at 333.2 K at equilibrium condition.
density of the mixture was calculated on the basis of the masses of the components and the volume of the system. Apparatus and Procedures for Reaction. The reactions were carried out in a stainless steel reactor of 30 mL. The reactor was first purged with CO2. The desired amount of acetic acid with catalyst and ethanol were loaded into the reactor. The suitable amount of CO2 was then introduced into the reactor with a CO2 sample bomb. The reactor was then placed in a constant temperature water bath of 333.2 K to allow the reaction to take place. After the desired reaction time, the reactor was quenched in cool water and then placed in a freezer of 250 K for at least 1 h. Then the CO2 in the reactor was released slowly. Experiments showed that the reactants and the products entrained by the released CO2 were negligible. The product was analyzed by GC (GC112, Shanghai Analytical Instrument factory) with a FID detector, and then the ethanol conversion and apparent equilibrium constant Kx was easily calculated on the basis of the molar ratio of CH3COOCH2CH3 and CH3CH2OH. Results and Discussions Phase Behavior. The critical temperature and pressure of a mixture are functions of the composition. The phase behavior of the reaction system was studied at different temperatures and pressures with the original molar ratio of CO2:CH3COOH:CH3CH2OH:p-TSA ) 90:5:5:0.05. Strong critical opalescence was observed at 333.2 K and 10.33 MPa, which is the critical point of the reaction system under equilibrium conditions. The dependence of density on pressure at 333.2 K is illustrated in Figure 2. The uncertainty of the density data in the figure is less than (0.2%. There are two phases in the system, as the pressure is lower than the critical pressure (10.33 MPa) and the system becomes homogeneous at the higher pressures. It should be emphasized that the density is the “apparent” density when two phases exist in the system because the density and composition of the liquid and the vapor phases are different. The isothermal compressibility (kT) is an important characteristic parameter of fluids and is defined as following
1 ∂V 1 ∂F kT ) ) V ∂P T F ∂P T
( )
( )
(1)
where V and F denote the molar volume and density of the fluid, respectively. kT values calculated using the density data in Figure 2 and eq 1 are also shown in the figure, which shows that the kT reaches a maximum in the critical region of the reaction mixture. Reaction. The reaction can be expressed by reaction 1, which takes place in the presence of compressed CO2 and it is a
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Hou et al.
Figure 3. The plot of ethanol conversion versus reaction time at 333.2 K.
Figure 4. ln Kx for acetic acid-ethanol esterification versus pressure.
reversible reaction
CH3COOH + CH3CH2OH S CH3COOCH2CH3 + H2O (reaction 1) The conversion of ethanol is defined as the ratio of the reacted and the original moles of ethanol. Figure 3 shows the variation of ethanol conversion with reaction time at some selected pressures. It is estimated that the uncertainty of the conversion is less than (2.0%. It can be seen that the reaction equilibrium can be reached in 3 h under supercritical conditions and can be reached in 6 h under other conditions. It should be pointed out that the density-pressure relation in Figure 2 cannot exactly represent that in the reaction system when the reaction equilibrium is not reached, because the data in Figure 2 was obtained under chemical equilibrium conditions. However, we believe that the difference should not be considerable, because the pressure change was not noticeable when the equilibration time was longer than 1 h during the phase behavior measurement. Thermodynamic equilibrium constant Ka is independent of pressure, which is defined as following
Ka ) γCXCγDXD/γAXAγBXB ) KγKx
(2)
Kγ ) γCγD/γAγB
(3)
Kx ) XCXD/XAXB
(4)
and
where XA, XB, XC, and XD are mole fractions of CH3COOH, CH3CH2OH, CH3COOCH2CH3, and H2O at equilibrium condition, respectively. γA, γB, γC, and γD are activity coefficients. Kx is the apparent equilibrium constant. Apparent equilibrium constant Kx is directly related with the conversion of ethanol or acetic acid. Figure 4 illustrates the effect of pressure on Kx. The values of Kx were calculated using
the conversion at a reaction time of 6 h. It can be seen that at the lower pressures (in the two-phase region) Kx increases with pressure and reaches a maximum in the critical region where the system becomes one phase. In the two-phase region the reaction takes place in both phases, and Kx should be considered as “average”, and we cannot give a detailed discussion concerning this pressure range because we did not determine the Kx in the two phases separately. The reaction system is homogeneous at pressures higher than critical pressure. The Kx drops down sharply with increasing pressure after passing the critical point. In other words, Kγ increases dramatically with increasing pressure because Ka is independent of pressure. Kγ is directly related to the activity coefficients of the components or the intermolecular interaction in the reaction system. In SCF mixtures there exist density inhomogeneities, which are often referred to as local density and/or local composition enhancement or “clustering”. In the reaction system studied in this work, the clustering is very complex, CO2 may aggregate with the reactants and the products, and the concentration of a reactant or a product around others may be higher than that of the bulk. Many features of SCFs or SCF solutions originate from the fact that the local density or local composition enhancements are very sensitive to pressure and temperature in the highly compressible region. In recent years, researchers have paid much attention to this, many excellent papers have been published, and there have been some review papers on the topic15,16,29 in the literature. A study on the clustering in very complex systems such as that in this work was not found, and researchers have paid much attention to dilution solutions. However, on the basis of the results published we can deduce that the degree of clustering should decrease with increasing pressure when the density of a fluid is higher than the critical density. Combining the results in Figures 2 and 4 and the analysis above we deduce that in the one-phase region Kx increases with the increased degree of clustering, i.e., Kγ decreases with the increasing degree of clustering. The figures also show that Kx or Kγ is directly related with the isothermal compressibility of the reaction system. Kx increases as the isothermal compressibility is increased. At the high pressures the clustering is not significant and the Kx or Kγ is close to that of the reaction in the absence of CO2, as can be seen from Figure 4. Acknowledgment. This work was financially supported by National Key Basic Research Project (G2000048010) and National Natural Science Foundation of China (20073056, 29725308). The authors are also very grateful to the reviewers for their valuable suggestions. References and Notes (1) McHugh, M. A.; Ktukonis, V. J. Supercritical Fluid Extraction, 2nd ed.; Butterworth-Heinemann: Boston, 1994. (2) Brennecke, J. F. Chem. Ind. 1996, 831. (3) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Nature 1996, 383, 313. (4) Subramaniam, B.; Rajewski, R. A.; Snavely, W. K. J. Pharm. Sci. 1997, 86, 885. (5) Peker, H.; Srinivasan, M. P.; Smith, J. M.; McCoy, B. J. AIChE J. 1992, 38, 761. (6) Savage, P. E.; Gopalan. S.; Mizan. T. I.; Martino, C. J.; Brock, E. C. AIChE J. 1995, 41, 1723. (7) Ikariya, T. Principles and DeVelopments of Supercirtical Fluids Reactions; CMC: Tokyo, 1998. (8) Eckert, C. A.; Chandler, K. The 4th Int. Symp. Supercrit. Fluids; Sendai, Japan, 1997; p 799. (9) Brennecke, J. F.; Chateauneuf, J. E. Chem. ReV. 1999, 99, 433. (10) Baiker, A. Chem. ReV. 1999, 99, 453. (11) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. ReV. 1999, 99, 475.
Pressure Tuning of Reaction Equilibrium (12) Darr, J. A.; Poliakoff, M. Chem. ReV. 1999, 99, 495. (13) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Chem. ReV. 1999, 99, 543. (14) Savage, P. E. Chem. ReV. 1999, 99, 603. (15) Kajimoto, O. Chem. ReV. 1999, 99, 355. (16) Tucker, S. C. Chem. ReV. 1999, 99, 391. (17) Maddox, M. M.; Goodyear, G.; Tucker, S. C. J Phys Chem B 2000, 104, 6248. (18) Ellington, J. B.; Park, K. M.; Brennecke, J. F. Ind. Eng. Chem. Res. 1994, 33, 965. (19) Ellington, J. B.; Brennecke, J. F. J. Chem. Soc., Chem. Commun. 1993, 1094. (20) Collins, N. A.; Debenedetti, P. G.; Sundaresan, S. AIChE J. 1998, 34, 1211.
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4513 (21) Peck, D. G.; Mehta, A. J.; Johnston, K. P. J. Phys. Chem. 1989, 93, 4297. (22) Lu, J.; Han, B. X.; Yan, H. K. J. Supercrit. Fluids 1999, 15, 135. (23) Kordikowski, A.; Schneider, G. M. Fluid Phase Equilib. 1993, 90, 149. (24) Gauter, K.; Peters, C. J. Fluid Phase Equilib. 1998, 150, 501. (25) Kordikowski, A.; Robertson, D. G.; Poliakoff, M. J. Phys. Chem. B 1997, 101, 5853. (26) Lemert, R. M.; Johnston, K. P. Fluid Phase Equilib. 1989, 45, 265. (27) Vieville, C.; Mouloungui, Z.; Gaset, A. Ind. Eng. Chem. Res. 1993, 32, 2065. (28) Zhang, H. F.; Liu, Z. M.; Han, B. X. J. Supercrit. Fluids 2000, 18, 185. (29) Tucker, S. C.; Maddox, M. W. J. Phys. Chem. B 1998, 102, 2437.
