Ind. Eng. Chem. Res. 1999, 38, 4133-4134
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Rebuttal to “Comments on ‘Kinetic Study for Synthesizing Dibenzyl Phthalate via Solid-Liquid Phase-Transfer Catalysis’ ” Hung-Ming Yang* and Huai-En Wu Department of Chemical Engineering, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan, Republic of China
Sir: We are grateful to D. Pilikauskas for his keen interest in our recent paper,1 in which we reported the investigations for the esterification of solid dipotassium phthalate (ArK2) and benzyl bromide (RX) in a solidliquid phase-transfer catalyzed system. As indicated in the preceding paper by Pilikauskas, this esterification reaction of dipotassium phthalate in solid-liquid phases is indeed complex. However, the catalytic behavior of the intermediate, di(tetra-n-butylammonium) phthalate (ArQ2), was realized using tetra-n-butylammonium bromide (QX) as the catalyst in the chlorobenzene solvent. The dependence of agitation speed on the concentration of ArQ2 revealed that the formation of ArQ2 was the key component in conducting the organicphase reaction. The nucleophilic attack of the anion Ar2on the organic substrate was found to conduct the reaction easier in the form of ArQ2 under anhydrous conditions. No significant enhancement on the product yield was observed when the molar ratio of RX to ArK2 was greater than their stoichiometric coefficients. The addition of potassium bromide affected the concentration of the active intermediate in the organic phase, which then influenced the overall reaction rate. A pseudo-second-order equation was applied to describe the reaction systems quite well. In the preceding paper, Pilipauskas proposes two aspects for further discussion: (1) How TBAB facilitates the solid-liquid phase-transfer process and (2) the nature of the rate enhancement by TBAB. He suggests that the reaction mechanism is like homogeneous solubilization and the nature of the rate enhancement by tetra-n-butylammonium bromide (TBAB) in this system can be attributed to an increased concentration of the anion, and not a change in the intrinsic rate constant. The reinterpretation of the data raised by Pilipauskas shows helpful explanations to our works, but we might not agree with some of his viewpoints for the following reasons. As mentioned in the paper,1 we have reported that less than 5% of the initial solid reactant ArK2 dissolved in chlorobenzene under the experimental conditions. By careful control of the dryness of the solid ArK2, the solubility of which in chlorobenzene was obtained as 0.0042 mol/L (1 g) was completely attributed to a shift to the left in the equilibrium reaction. We mentioned that the formation of ArQ2 was inhibited by larger quantities of KBr and limited to the equilibrium reaction. The reason is that the produced KBr during the reaction of ArK2 and TBAB might deposit on the surface of the solid ArK22,3 to retard the dissolution and the probability of contact of TBAB with ArK2. This leads to the fact that larger amounts of solid KBr in the organic solvent would reduce the effective surface area for the collision of soluble TBAB with ArK2. However, we agree that the effect of KBr is not merely a result of a change in the rate law. To further explore the sole behavior of KBr on the formation of ArQ2, the independent reactions of solid ArK2 and TBAB in the presence of different amounts of solid KBr without introducing the organic substrate were performed. The experimental results are shown in Figure 2. Without the addition of benzyl bromide, the profile of the concentration of ArQ2 was almost the same for the cases without and with 1 g of KBr. However, the steady-state concentration of ArQ2 decreases with increasing amounts of KBr added and shows a smaller value but the same tendency with those cases having the organic intrinsic reaction. This demonstrates that the effect of KBr on the concentration of
10.1021/ie9910672 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/14/1999
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
Figure 2. Variation of the concentration of ArQ2 for different amounts of KBr addition for the reaction of ArK2 with TBAB; chlorobenzene, 50 mL; ArK2, 0.005 mol; TBAB, 0.01 mol.
ArQ2 cannot be adequately described by eq 5 proposed by Pilipauskas. Regarding the mixing effect, the reaction and transfer of ArQ2 actually involve several steps. It includes the production of ArQ2 in the solid-liquid film near the surface of the solid particles, mass transfer of ArQ2 from the surface to the bulk organic phase, and then the reaction of ArQ2 with RX in the bulk organic phase. It is well-known that the general concentration profile of a component from the solid surface to the bulk phase can be easily deduced from film theory.4 The thickness of the layer between the solid and the bulk phase is subjected to the degree of agitation that influences the film mass-transfer coefficient. Hence, the agitation rate would affect the steady-state concentration of ArQ2 in the bulk phase. That cannot be explained simply by the equilibrium reaction if the magnitude of mass-transfer resistance cannot be neglected. The equilibrium concentration of ArQ2 was expected to exist around the surface of the solid particle due to the film reaction. The lesser degree of agitation leads to the thicker layer for mass transfer, and then the smaller steady-state concentration of ArQ2 in the bulk phase. It is noted that the produced KBr during the reaction might also deposit on the solid surface of ArK2 to reduce the effective surface area for conducting the ion-exchange reaction, and not merely influencing the rate of dissolution of ArK2. In addition, more vigorous mixing would generate a greater force to shear the solid surface, and can get rid of some deposited KBr to reduce its effect on the ionexchange reaction. Therefore, the steady-state concentration of ArQ2 is not considered simply as the equilibrium concentration, but results from the combination of several effects. Considering the esterification reaction conducted by the dissolved phthalate dianion with benzyl bromide in chlorobenzene without the catalyst, the corresponding
intrinsic reaction rate constant was estimated from the apparent rate constant ()0.0004 M-1 min-1) divided by the solubility of ArK2 ()0.0042 M) in chlorobenzene and was obtained as 0.095 M-2 min-1. The intrinsic reaction rate constant for using 0.01 mol of TBAB was estimated by the apparent rate constant ()0.0431 M-1 min-1) divided by the average concentration of ArQ2 ()0.027 M) in chlorobenzene and was obtained at about 1.57 M-2 min-1. It reveals that ArQ2 is much more reactive than the dissolved ArK2 with no doubt. Moreover, regarding the deviation of the linear relationship for the apparent rate constant kapp on larger amounts of TBAB, it was believed to come from the nonlinear variation of the steady-state concentration of ArQ2 when a greater amount of TBAB was used. With a higher reaction rate, much of the generated KBr would deposit on the solid surface to decrease the concentration of ArQ2 in the organic phase. This leads to the deviation of the linear relationship of kapp on the TBAB amounts. The steady-state concentration of ArQ2 in the reaction mixture was also influenced by the concentration of benzyl bromide. The apparent rate constant for the effect of benzyl bromide listed in Table 1 in the preceding paper by Pilipauskas was incorrect in the first row, which is the result without catalyst, not the run without benzyl bromide. However, the addition of benzyl bromide might shift the solubility of the catalytic intermediate in the organic phase and the corresponding intrinsic rate constant because of the substantial change in physical properties of the organic solvent, especially for large excess usage of benzyl bromide or solid reactant. A greater excess of solid reactant might induce the formation of solid aggregates under vigorous agitation. The phenomena for various molar ratios of reactants by keeping either the constant usage of solid reactant or constant organic substrate in a solid-liquid phase-transfer catalyzed system will be explored further. Acknowledgment We thank Dr. Daniel R. Pilipauskas for his interest in our paper. Literature Cited (1) Yang, H. M.; Wu, H. E. Kinetic Study for Synthesizing Dibenzyl Phthalate via Solid-Liquid Phase-Transfer Catalysis. Ind. Eng. Chem. Res. 1998, 37, 4536. (2) Yee, H. A.; Palmer, H. J. Solid-Liquid-Phase Transfer Catalysis. Chem. Eng. Prog. 1987, 83, 33. (3) Melville, J. B.; Goddard, J. D. A Solid-Liquid PhaseTransfer Catalysis in Rotating-Disk Flow. Ind. Eng. Chem. Res. 1988, 27, 551. (4) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; John Wiley & Sons: New York, 1960.
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