I n d . Eng. Chem. Res. 1990,29, 1091-1095
1091
KINETICS AND CATALYSIS
Supercritical Thermal Decomposition of Cellulose: Experiments and Modeling Motonobu Goto Kumamoto University, Kumamoto 860, Japan
Oner Hortacsu Bogazici University, 80815 Bebek, Istanbul, Turkey
Ben J. McCoy* Department of Chemical Engineering, University of California, Davis, California 95616
T h e rates of thermal decomposition of cellulose from wood pulp in the presence of supercritical tert-butyl alcohol were measured dynamically in a continuous-flow system. The decomposition rate increased markedly with temperature. A kinetic model is presented for the supercritical thermal decomposition process. A three-reaction model, which consists of consecutive reactions to produce cellulose derivatives accompanying competitive char formation from the intermediate product, satisfactorily represents the data. The kinetic parameters for the reaction estimated from the model analysis were compared with pyrolysis results under vacuum. The rate of char formation for the supercritical process was typically smaller than that of pyrolysis. Supercritical fluid extraction has received increasing attention in a variety of fields due to the following features: (1)supercritical fluids provide high solubility and improved mass-transfer rates, and (2) operation can be manipulated by changing the pressure or temperature. Supercritical fluid extraction, or solvolysis, of lignocellulosic materials from biomass is similar to pyrolysis, which is used to convert biomass to useful chemicals or fluid fuels. Developments in pyrolysis studies were recently reviewed by Shafizadeh (1982) and Anta1 (1982). Pyrolysis proceeds through a series of complex concurrent and consecutive reactions, yielding char as well as a variety of volatile products (Hajaligol et al., 1982). The char formation reduces the yield of volatiles and causes poor heat transfer (Shafizadeh, 1982). These problems can be ameliorated by employing supercritical fluid extraction, that is, thermal decomposition with supercritical fluid (Koll and Metzger, 1978). When the pyrolysis is carried out with supercritical fluid, the intermediate reaction products that produce char are readily extracted due to the high solubility in supercritical fluid. Because cellulose comprises about 50% of most biomass materials, its pyrolytic properties have attracted special attention. Koll and Metzger (1978) and Koll et al. (1983) compared the supercritical acetone extraction of cellulose and its pyrolysis. By using supercritical fluid, char formation was reduced to 290, while pyrolysis produced 34% (under nitrogen) or 18% (under vacuum) char. The main products of the supercritical acetone extraction were anhydrosugars, and a higher yield of glucosan was obtained than for the pyrolysis. Hajaligol et al. (1982), to reduce pyrolytic cellulose char formation under helium, found it necessary to avoid secondary reaction of tar products on the char surface. In our previous paper (Goto et al., 19901, we applied supercritical tert-butyl alcohol (P,= 3.97 MPa, T,= 506.2 K) to the extraction of lignin from wood. Analysis of the
dynamic extraction revealed that the overall extraction rate was controlled by the degradation reaction, and mass transfer had a negligible influence on the process. Our present objectives are (1)to measure the supercritical fluid solvolysis rate of cellulose, (2) to develop a model for the reactive extraction process, and (3) to compare the results with conventional pyrolysis. We studied the thermal degradation and extraction of cellulose from wood pulp, which had been treated to remove lignin and hemicellulose. The extraction rate was measured dynamically with the fixed-bed reactor used in our previous work (Triday and Smith, 1988; Goto et al., 1990). A three-reaction model with first-order kinetics, that is, two consecutive reactions with competitive char formation, satisfactorily described the experimental observations.
Experimental Method The solvolysis of cellulose was carried out dynamically in a fixed bed contacted with flowing supercritical tertbutyl alcohol. In each run, the mass flow rate and pressure are held constant while the temperature is first increased at a constant rate (0.14 K/s) and then held constant at a chosen level until the end of the run. The apparatus and the operation procedure are identical with our previous work (Goto et al., 1990). The reactor was a 0.45-m-long stainless steel tube, of 0.029-m i.d. The flow rate of tert-butyl alcohol was 1.67 X lo-' m3/s (or 10.0 mL/min), measured at 298 K and 1.0 bar. The pressure in the reactor was 6.76 MPa (P,= 1.50),and the temperature was in the range 493-573 K (T,= 0.97-1.13). The concentration of the extracted cellulose derivatives was monitored continuously with a flow-through spectrophotometer (PerkinElmer Lambda 4B) at wavelength of 280 nm. No gas formation was observed. Cellulose samples (acetate pulp) were wood pulp composed of 96% cellulose (Acetanier-P, ITT Rayonier Inc.) from which lignin and hemicelluloses had been extracted
0888-5885/90/ 2629-1091$02.50/0 0 1990 American Chemical Society
1092 Ind. Eng. Chem. Res., Vol. 29, No. 7. 1990
previously; the average degree of polymerization (DP) was 2630. The size of the particles in the sample was 3 mm2 with 1.5-mm thickness. Samples of 0.05-0.18 g, dried for 24 h at 368 K, were placed on a screen in the reactor in one layer. The bed void fraction, q,, was calculated to be the volume fraction excluding the pulp volume from the bed, whose length was equal to the thickness of the particles. Since the dead space in the tubing between the reactor and the detector for our apparatus contributed only to the time delay (Triday and Smith, 1988), the measured absorbance was corrected for this delay.
Reaction-Extraction Model Cellulose is a macromolecule composed of linearly linked &( ~-+4)-D-glucopyranose units. The degree of polymerization and the crystallinity vary with the type of biomass. Although the mechanism of the pyrolysis of cellulose has not yet been definitively established, a number of studies have addressed the modeling of pyrolysis. The pyrolytic properties are controlled by the chemical composition of the major components, namely cellulose, hemicelluloses, and lignin, and some minor components. For the pyrolysis of cellulose under vacuum, Bradbury et al. (1979) suggested the following reaction scheme: cellulose
-
-
The initial reaction leads to the formation of an active cellulose, which subsequently decomposes by two competitive first-order reactions, one yielding anhydrosugars and the other char and a gaseous fraction. Broido and Nelson (1975) proposed a similar reaction scheme in which a sequence of weight-loss steps lead to the char formation. They explained that volatiles and char are produced by depolymerization and dehydration, respectively. The reaction step from cellulose to active cellulose may be a reduction in the DP, because the DP of cellulose undergoes a rapid drop to about 200 without weight loss during the early stages of pyrolysis (Antal, 1982). Our previous paper (Goto et al., 1990) revealed that the solvolysis of lignin from wood chips was controlled by the degradation reacton and that the mass-transport resistances were insignificant. Since the degradation of cellulose is slower than that of lignin under the experimental conditions and the pulp is much more porous than wood chips, we considered in the present work that the degradation reactions are likewise dominant. A single first-order reaction model is inadequate to fit the experimental data because of the following reasons. In our experiments, the concentration peak exited after the temperature had reached the highest value. The single reaction cannot represent this behavior with first-order kinetics, even after dead time corrections are made, because the reactant monotonously decreases with time. Furthermore, the experiments showed that a solid carbonaceous residue remained in the reactor even after complete cellulose extraction. We considered this residue to be similar to char in pyrolysis. Thus, we developed a three-reaction model, that is, a consecutive reaction with competitive char formation, as suggested for the pyrolysis of eq 1, cellulose -A (solid)
active cellulose B (solid)
.+
cellulose derivatives P (solution) char S (solid)
(2)
We assumed every reaction obeys first-order kinetics.
X
OExtracted amount of cellulose (sample weight loss) in 100-min run. *Extracted amount of cellulose (sample weight loss) in 300min run.
The degradation reactions are represented by the following equations: dCA/dt = -klCA
(3)
dC,/dt = k,CA - (k, + ks)cB
(4)
dCp/dt = k,CB
(5)
dC,/dt = k$B
(6)
where the reaction constants are given by
kj = ko,i exp(-E,,i/R,T)
active cellulose + volatiles char + gases
-
Table I. Solvolysis Run Conditions (P,= 1.50, q = 1.67 10-7 mJ/s) run T, K T, 103mo,kg yield,' % 0.1725 5.45 0.97 8 493 0.1788 11.4 4 513 1.01 1.03 0.1754 16.8 2 523 0.1780 72.0 1.08 1 548 0.1016 77.4 1.08 3 548 0.0672 77.2 7 558 1.10 0.0574 80.5 6 573 1.13 0.0627 9 573 1.13 80.3b
(i = 1-3)
(7)
As the three concentrations CA, CB and Cs are for components that are solid and fixed in the reactor, only the concentration of cellulose derivatives, Cp, can be monitored in the effluent from the reactor. Since a thin bed was used, the reactor can be assumed to be a differental reactor. The mass balance in the reactpr is represented in terms of the average concentration, Cp, dCp/dt
+ (2u/t&)Cp
= [(l- +,)/tb]
dCp/dt
(8)
where the rate dCp/dt is given by eq 5. From the mass balance around the differential reactor, the rate of extraction, Q , per unit initial mass, mo,of pulp is m,R = 2qCp
+ tbV dCp/dt
= V(l - tb) dCp/dt (9)
Results Experimental Runs. Table I lists the conditions for the experimental runs. The yield for the run is defined as the mass extracted from the sample, measured by the weight loss during the 100-min run. The yield increased significantly with the temperature. The calibration curve, which converts the absorbance in the effluent solvent to the concentration, was obtained as follows. The absorbance continuously measured during a run was integrated and plotted against the weight loss of the sample for runs at various temperatures. Since all the data for various degrees of extraction fall on a single line independent of the temperature, we regarded that the absorbance has a linear relationship with weight loss of the cellulose or concentration of cellulose derivatives in the solvent. The obtained linear relationship was used as a calibration curve. Figure 1 shows the global reaction-extraction rate calculated from the experimental data by eq 9 for various temperatures at a reduced pressure of 1.50. Figure l a displays the data for higher temperatures (T, 1 1.03) and Figure l b for lower temperatures (T, I 1.03). The data at T , = 1.03 are drawn in both parts of the figure. The extraction rate decreases significantly when the reduced temperature is below 1.03. The large temperature effect supports the hypothesis that the intrinsic reaction plays
Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990 1093 1.2
Table 11. Estimated Kinetic Parameters
1.1
run 1 .o m
5
L
m
0.9
r
L
c
n m
0.8 0.7
0
1000
'
2000
3000 t
1.0e-4I
.
5000
4000
06 6000
[SI
, . , . , . , . , .
1 .o 6 Oe-5
rt
0.9
m
1
4 Oe-5
0.8
-4 n
2.0e-5
0.7 nc
O.Oe+O
0
1000
2000
3000 t
4000
600yd"
5000
[SI
Figure 1. Effect of temperature on experimental extraction curves for (a, top) higher temperatures and (b, bottom) lower temperatures: (-)experimental, (---) predicted, (--) temperature.
80
-
5 l60
"
o
0
l
j; 40
201
0 09
,
, a B ,
1 .o
, 1 .l
Tr
,
0.97 1.01 1.03 1.08 1.08 1.10 6 1.13 9 1.13 2, 4, 8" 0.97-1.03 1, 6, 7, gb 1.08-1.13
10-% l/s 5.09 5.15 4.99 4.80 5.17 4.91 4.84 5.05 4.85 5.04
Ea,l,
Ka,29
kJ/mol 128 130 131 124 124 125 125 122 130 123
kJ/mol 85.3 82.3 97.3 125 124 123 125 122 85.2 125
&,3,
kJ/mol 130 127 130 133 131 129 132 129 129 131
Parameters were estimated simultaneously for the grouped data of runs 2, 4, and 8. bParameters were estimated simultaneously for the grouped data of runs 1, 6, 7, and 9.
1 1.2
1.1
8.0e-5
8 4 2 1 3 7
T,
1 1.2
1-1
Figure 2. Effect of temperature on the yield of cellulose derivatives in 100 min.
a significant role in the overall extraction process, as in the case of lignin extraction (Goto et al., 1990). The extraction was completed in 100 min a t the highest temperature (TI = 1.13), and the extraction rate drops to 9. To confirm complete extraction, the extraction was continued to 300 min at the same conditions (run 9). Since the yield in the 300-min run agreed well with that in the 100-min rin (run 6) as indicated in Table I, the extraction was regarded to be complete in 100 min at TI= 1.13. After the extraction, the remaining residue in the reactor was black solid, presumed to be char as in the case of pyrolysis. Thus, a part of the cellulose decomposed to the derivatives and was extracted into the supercritical solvent, while the remainder of the cellulose became char. The yield of 80% means that up to 20% of the cellulose remained as char. However, the extraction for the run a t temperatures lower than TI= 1.08 was not completed in 100 min. Hence, we cannot evaluate the amount of char from the yield in Table I. The yields a t the various temperatures were plotted against temperature in Figure 2. The yields for higher
temperatures, TI 2 1.08, were much higher than those for lower temperatures, TI I1.03. It is interesting that the temperature where the yield increases dramatically is higher than the critical temperature of tert-butyl alcohol. The extraction rate reached its maximum well after the temperature reached the plateau value. If the reaction is a single reaction with first-order kinetics, the maximum reaction rate should have appeared earlier because the reactant decreases with time. Thus, the reaction cannot be represented by a single first-order reaction. The best way to explain this behavior is with kinetics including consecutive reactions. Estimation of Rate Constants. The rate constants for the three reaction steps in eq 2 are assumed to have identical preexponential factors, a simplification also applied in the extraction of lignin (Goto et al., 1990) and in the cracking of wood pyrolysis tars (Boronson et al., 1989). Thus, the experimentally determined parameters for the three-reaction model are the preexponential factor, k,, and activation energies Ea,l,Ea,2,and Ea,3.These four parameters were evaluated by minimizing the sum of squares of the difference between the experimental and calculated extraction rates for three runs. Table I1 displays the resultant values at various temperatures. The table includes the results estimated for each run and for several simultaneous runs with a set of parameters.
Discussion As shown in Table 11, the estimated preexponential factors and activated energies Ea,1and Ea,3are nearly constant. The Ea,zactivation energies are different for higher (T, 2 1.08) and lower (TI I1.03) temperatures. The values of E,,2 for the lower temperatures vary somewhat because it is difficult to measure the concentration accurately at these low reaction rates. The values of Ea,zare nearly constant for higher temperatures. The four parameters were also estimated with the data of three simultaneous runs for lower temperatures (runs 2,4, and 8). The parameters for higher temperatures were estimated in the same manner as with the grouped data of four runs (runs 1, 6, 7, and 9). These estimated parameters are shown at the bottom of Table 11. The broken lines in Figure 1 indicate the predicted curves using the parameters so estimated. Every experimental data point agreed well with the predicted curve calculated by this method. When the extraction rate for lower temperatures was predicted with the parameters obtained for higher temperatures, the agreement between experimental and predicted curves was poor. Therefore, the nature of the reaction appears to be different at higher and lower temperatures. The kinetic parameters obtained for the pyrolysis of cellulose powder (Watman CF11) in the temperature range
1094 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
-
Table 111. Kinetic Parameters for the Pyrolysis (Bradbury et al., 1979) reaction 1 2 3 1.32 X 10" 2.83 X l O I 9 3.17 X lo1* ko, l / s 153 243 198 E,, kJ/mol
temperature
C
C 0
P
08
5 .c0 m
I
L
r
I-
0.4
08
0
Table 1V. Comparison of Reaction Rate Constants between Pyrolysis and This work T, K k1, 11s k2, 11s k3, 11s kdk, this work 473 1.31 X 7.71 X 10" 1.68 X 10" 4.60 3.97 523 2.61 X 10"' 1.61 X lo-' 4.06 X 573 3.09 X 1.99 X 5.64 X 10"' 3.52 4.31 X 1.67 X 0.258 pyrolysis 473 4.13 X 523 1.52 X 5.31 X lo4 6.90 X 10" 0.769 573 1.99 x 10-3 2.82 x 10-4 1.49 x 10-4 1.90
532-614 K under vacuum by Bradbury et al. (1979) are shown in Table I11 for the reaction scheme of eq 1. The activation energies of the pyrolysis are higher than those of this work. The distinctive feature of supercritical decomposition is that the activation energy of k , has a lower value than that of k,, while the situation is reversed for pyrolysis. To compare these parameters, the reaction rate constants at the temperatures 473, 523, and 573 K are listed in Table IV. The difference in the magnitude of the ratio k , / k , is apparently due to the fact the char formation increases with temperature for the supercritical process while it decreases for pyrolysis. Thus, the char formation of the supercritical process is less than that for pyrolysis, especially for lower temperatures. The amount for char formation depends on the environment, because pyrolysis under vacuum produced less char (18%) than that under nitrogen (34%) a t 573 K (Shafizadeh, 1982). Under vacuum, the intermediate product is removed easily from the solid reactant before it converts to char by the further successive reaction. Hajaligol et al. (1982) also were able to obtain low char yields when secondary reaction of the tar on char surfaces was avoided. In the case of supercritical extraction, the decomposed intermediate components are extracted quickly from the solid due to the high solvent power of supercritical fluid. Figure 3 shows the fraction of each component as a function of time predicted by the model with the parameters and the temperature history for T, = 1.08 (run 6). After the temperature reached a value sufficiently high for the reaction, cellulose decomposes exponentially and the intermediate product, active cellulose, increases initially and then decreases due to the successive reactions. The final products, cellulose derivatives and char, are produced as a result of competitive reaction. The ratio of cellulose derivatives to the char depends on the ratio of the reaction rate constants of these reactions. When comparing our results for cellulose decomposition with that of lignin, we note that the reaction rate of lignin decomposition was 1.8 X lo-, l / s at 523 K (Goto et al., 1990), while that of cellulose decomposition was 2.61 X lod and 1.61 X lo-* l / s as shown in Table IV. The decomposition of lignin is about 10 times faster than that of cellulose. Therefore, in cellulose extraction it is valid to assume that the whole decomposition and extraction process is controlled by the reaction process and the mass-transfer resistance is negligible. The mass-transfer resistance was not important even in the case of lignin decomposition. A two-parallel-reaction model was employed to explain the tailing part of the extraction curve of lignin. In contrast, a consecutive reaction model is needed to explain the extraction behavior of cellulose. The
-
cellulose derivatives
06
C
.-0
-
07
char
0 L
600
1000
2000
3000 t
4000
5000
6000
[SI
Figure 3. Predicted histories of each component during the reaction ( T , = 1.08).
highest extraction rate is observed at a time when the temperature had reached its highest value. Koll and Metzger (1978) achieved a cellulose degradation of 98% with supercritical acetone at 613 K. The microcrystalline cellulose used in that work is subject to more weight loss than the less crystalline cellulose used in the present experiments (Anta1 et al., 1980); that is, microcrystalline cellulose produces less char. Conclusion The thermal decomposition and extraction with supercritical tert-butyl alcohol of cellulose from wood pulp was investigated dynamically. The extraction rate increased with temperature. A first-order three-reaction model consisting of consecutive reaction with competitive char formation was applied to the reaction-extraction process. The predicted extraction rates for both models agreed well with the experimental data. Acknowledgment The financial support of National Science Foundation Grant CBT-8700092 is gratefully acknowledged. We are grateful to Prof. J. M. Smith for helpful discussions. Nomenclature CI = concentration of component I, either A, B, P, or S, denoted in eq 2, kg/m3 = arithmetic average of the concentration of feed and effluent for the differential reactor, kg/m3 Ea,i= activation energy, kJ/mol ki = reaction rate constant, s-l ko,, = preexponential factor, s-l L = length of the bed of particles, m mo = initial mass of wood pulp, kg P, = critical pressure, Pa PI = reduced pressure, PIP, q = flow rate, m3/s R, = gas constant, kJ/(mol.K) T,= critical temperature, K TI = reduced temperature, T I T , t = time, s u = superficial fluid velocity, m/s V = volume of reactor bed, m3
cp
Subscripts A = cellulose B = activated cellulose (intermediate product) P = cellulose derivatives S = char Greek Letters = bed void fraction R = global reaction-extraction rate, kg/(kgs) tb
Ind. Eng. Chem. Res. 1990,29, 1095-1103 Registry No. Cellulose, 9004-34-6; tert-butyl alcohol, 75-65-0.
Literature Cited Antal, M. J., Jr. Biomass Pyrolysis: A Review of the Literature Part l-Carbohydrate Pyrolysis. In Advances in Solar Energy; Boer, K. W., Duffie, J. A., Eds.; American Solar Energy Society, Inc.: 1982. Antal, M. J., Jr.; Friedman, H. L.; Rogers, F. E. Kinetics of Cellulose Pyrolysis in Nitrogen and Steam. Combust. Sci. Technol. 1980, 21, 141-152. Boronson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product Yields and Kinetics from the Vapor Phase Cracking of Wood Pyrolysis Tars. AZChE J. 1989,35, 120-128. Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. A Kinetic Model for Pyrolysis of Cellulose. J. Appl. Polym. Sci. 1979,23,3271-3280. Broido, A.; Nelson, M. A. Char Yield on Pyrolysis of Cellulose. Combust. Flame 1975,24, 263-268. Broido, A,; Javier-son, A. C.; Barrall, E. M., 11. Molecular Weight Decrease in the Early Pyrolysis of Crystalline and Amorphous Cellulose. J. Appl. Polym. Sci. 1973, 17, 3627-3635.
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Goto, M.; Smith, J. M.; McCoy, B. J. Kinetics and Masa Transfer for Supercritical Fluid Extraction of Wood. Znd. Eng. Chem. Res. 1990,29, 282-289. Hajaligol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product Compositions and Kinetics for Rapid Pyrolysis of Cellulose. Ind. Eng. Chem. Process. Des. Dev. 1982, 21, 457-465. Koll, P.; Metzger, J. 0. Thermal Degradation of Cellulose and Chitin in Supercritical Acetone. Angew. Chem. Znt. Ed. Engl. 1978,17, 754-755. Koll, P.; Bronstrup, B.; Metzger, J. 0. Liquefaction of Biomass with Supercritical Fluids in High Pressure/High Temperature Flow Reactor. In Chemical Engineering a t Supercritical Fluid Conditions; Paulaitis, M. E., Penninger, J. M. L., Gray, R. D., Jr., P., Davidson, P., Ed.; Ann Arbor Science: Ann Arbor, MI, 1983. Shafizadeh, F. Introduction to Pyrolysis of Biomass. J. Anal. Appl. Pyrol. 1982, 3, 283-305. Triday, J.; Smith, J. M. Dynamic Behavior of Supercritical Extraction of Kerogen from Shale. AZChE J. 1988, 34, 658-668. Received for review November 17, 1989 Revised manuscript received February 26, 1990 Accepted March 5, 1990
Kinetics of the Oxidation of Benzyl Alcohol by Hypochlorite Ion in the Presence of Phase-Transfer Catalyst Jing-Shan Do and Tse-Chuan Chou* Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, ROC 70102
T h e kinetics of the oxidation of benzyl alcohol by hypochlorite ion in the presence of tetrabutylammonium chloride as phase-transfer catalyst was studied. The results show that the oxidation of benzyl alcohol in two immiscible aqueous/dichloromethane systems was reaction controlled and occurred in the organic phase when the stirring rate was larger than 500 rpm. The experimental results also reveal that the reaction orders of both tetrabutylammonium ion pair and benzyl alcohol in the organic phase are both equal to one. The activation energy of the oxidation of benzyl alcohol is 10.4 kcal/mol. The extraction constant of tetrabutylammonium hypochlorite ion pair was obtained. The equilibrium constants, K1-K4,are evaluated to be equal to 0.05 M-l, 17.4,0.004 M-l, and 87.7, respectively. Introduction Inorganic cations and anions are frequently used as catalysts (Davies, 1970; Chou and Lin, 1983), oxidants (Stevens et al., 1980; Nwaukwa and Keehn, 1982a-c; Do and Chou, 1989; Mehrotra and Amis, 1974; Sala et al., 1977; Jow and Chou, 1988), and reducing agents (Mandal et al., 1987, 1988; Ward et al., 1988) in processes for producing organic chemicals. Usually, the solubility of inorganic cations, such as metal ions, and anions, such as C10-, in most organic solvents and solutions is very small. Reactions between the inorganic species and organic substances located in two immiscible phases, respectively, of a mixture are often inhibited because of the inability of reagents to meet. There are several methods to improve the solubility of inorganic ions in nonpolar organic solvents (Chou and h e , 1985; Hwang and Chou, 1987; Kuo and Chou, 1987a,b; Starks and Liotta, 1978; Dehmlow and Dehmlow, 1980). Metal ions, for example, are immobilized or exchanged on solid polymer supports (Chou and Lee, 1985; Hwang and Chou, 1987; Kuo and Chou, 1987a,b). Theoretically, the solubility of the immobilized metal ions on the solid particles in the organic solvent can be very large. Phasetransfer catalysts have been widely applied to overcome the heterogeneity problem-many inorganic oxidants also can be transferred into an organic phase by a phasetransfer catalyst (Dehmlow, 1977; Dockx, 1973; Starks,
* Author to whom correspondence should be addressed. 0888-5885/90/2629-1095$02.50/0
1987). Reaction rates, for example, oxidation, can be much improved (Starks and Liotta, 1978; Dehmlow and Dehmlow, 1980). However, the mechanism and kinetics of two-phase systems in the presence of phase-transfer catalysts are still unclear (Bar et al., 1984a,b). Permanganate employed as an oxidant to oxidize benzyl alcohol in the presence of a phase-transfer catalyst produced a 92% yield of benzoic acid (Herriott and Picker, 1974). No benzaldehyde was observed in this system. Aqueous sodium hypochlorite under phase-transfer-catalysis conditions was employed to promote the oxidation of hydroquinone and catechols (Ishii and Kishi, 1980). Using inorganic hypochlorite as oxidant in the presence of a phase-transfer catalyst was reported to proceed the oxidation of some polycyclic aromatic compounds to arene oxide (Krishnan et al., 1977). Oxidation of alcohols in the presence of phase-transfer catalysis by hypochlorite ion was reported by several research groups (Pletcher and Tait, 1978; Tabushi and Koga, 1979; Abramovici et al., 1985; Do and Chou, 1989). Several investigators (Regen, 1976; Schneider et al., 1982) reported using a triphase catalyst for the oxidation of primary and secondary alcohols. The reaction rate was very slow, and the yield of benzaldehyde was lower than that in the homogeneous phase-transfercatalysis system. In our previous work (Do and Chou, 1989), the oxidation of benzyl alcohol by hypochlorite anion, which was regenerated by anodic oxidation in the presence of quaternary ammonium salts as a phase-transfer catalyst, was reported. The quaternary ammonium salts 0 1990 American Chemical Society
