Langmuir 1997, 13, 6047-6052
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Synthesis of Alkylphenyl Ethers in Aqueous Surfactant Solutions by Micellar Phase-Transfer Catalysis. 1. Single-Phase Systems Cincin Siswanto, Turgut Battal, Oron E. Schuss, and James F. Rathman* Chemical Engineering Department, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210-1110 Received May 16, 1996. In Final Form: August 27, 1997X Phase-transfer catalysis and micellar catalysis are two conventional methods of promoting reactions between lipophilic and hydrophilic reactants. Phase-transfer catalysis employs organic solvents that may be undesirable for both economic and environmental reasons, while application of micellar catalysis is limited by the relatively low solubilization capacities of surfactant solutions. Micellar phase-transfer catalysis is a process that combines the best aspects of both conventional methods while avoiding some of the associated problems. Reaction systems consist of reactants, water, surfactant and a phase-transfer catalystsno organic solvent is used. The surfactant acts to solubilize and emulsify the lipophilic reactant, while the role of phase-transfer catalyst is to shuttle the hydrophilic reactant from the aqueous phase into the micellar environment where the reaction primarily takes place. Alkylation of phenol with 1-bromobutane was studied under phase-transfer, micellar, and micellar phase-transfer conditions in single-phase solutions at relatively high reactant loadings. Cationic (dodecyltrimethylammonium bromide), anionic (sodium dodecyl sulfate), and nonionic (dimethyldodecylamine oxide) surfactants were compared. Higher conversions with micellar phase-transfer catalysis over conventional micellar and phase-transfer catalysis were observed in nonionic and anionic surfactant systems. For cationic surfactant systems, no significant advantage was observed for micellar phase-transfer catalysis in comparison to conventional micellar catalysis. The effect of cationic surfactant concentration was studied and an optimum surfactant concentration was observed. Effects of initial reactant concentrations and two types of mixing were also studied. Mixing effects were significant, suggesting that mass transport rates of components between the aqueous and micellar pseudophases in these microheterogeneous systems at high reactant concentrations may affect reaction kinetics.
Introduction A wide range of solvents are employed in chemical synthesis processes in order to dissolve reactants, provide a suitable reaction environment, and facilitate transfer of materials. For processes in which one reactant is lipophilic and another is hydrophilic, high rates of reaction can be achieved only if the immiscibility of the reactants can be overcome. One strategy is to employ a solvent or solvent mixture in which both reactants are miscible so that the system is single-phase and rates are not limited by interphase transport. An alternate strategy is to dissolve each reactant in an appropriate solvent, without regard to whether the solvents themselves are miscible, and then add a phase-transfer catalyst to promote the transport of one reactant into the other phase. The primary disadvantage of both strategies is that they generally require the use of volatile organic solvents that are undesirable due to environmental concerns. A third approach is to perform these reactions in aqueous surfactant solutions, eliminating the need for organic solvents. Research over the past 40 years has demonstrated that reaction rates, yields, and selectivities can be significantly altered in micellar surfactant solutions; still, despite many studies in this area, relatively few industries employ this technology, in part due to the fact that few studies have addressed issues such as high reactant loading. For industrial applications, high reaction rates must be achieved, selectivity of the desired product must be attained, and methods of separating products and surfactants must be identified. High production rates require maximizing reactant loadings in these systems. * Author to whom correspondence should be addressed: e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, October 15, 1997.
S0743-7463(96)00487-8 CCC: $14.00
The reaction system investigated in this work is the preparation of unsymmetrical phenolic ethers via alkylation with alkyl halides1 (the Williamson reaction). The specific reaction of interest is the synthesis of butyl phenyl ether in aqueous sodium hydroxide solution.
The formation of symmetrical ethers may possibly be significant, e.g., dibutyl ether may be produced by
CH3(CH2)3Br + -OH h CH3(CH2)3OH + BrCH3(CH2)3OH + -OH h CH3(CH2)3O- + H2O CH3(CH2)3O- + CH3(CH2)3Br h CH3(CH2)3O(CH2)3CH3 + BrSince primary alcohols are weaker acids in aqueous solution than water is itself, formation of the alkoxide from an alcohol does not occur in aqueous sodium hydroxide solutions, even at high NaOH concentrations. For this reason, formation of symmetrical ethers are (1) Jursic, B. Tetrahedron 1988, 44, 6677.
© 1997 American Chemical Society
6048 Langmuir, Vol. 13, No. 23, 1997
generally assumed to be negligible;2 however, the apparent pKa of an alcohol solubilized in a micelle may be different than that in aqueous solution, so this assumption is questionable in aggregated systems. Furthermore, even if dialkyl ether formation is indeed negligible, conversion of the alkyl halide to alcohol may occur at high NaOH concentrations and represents an undesired side reaction in processes designed to synthesize alkyl phenyl ethers. The objective was to compare reaction rates and conversions observed under three catalytic schemes: (1) phase-transfer catalysis (PTC); (2) micellar catalysis (MC); (3) micellar phase-transfer catalysis (MPTC). MPTC is an integration of conventional micellar catalysis and phase-transfer catalysis. Each of these schemes is described in more detail below. MPTC combines the best aspects of PTC and MC, while avoiding some of their associated problems and limitations. Phase-Transfer Catalysis. Phase-transfer catalysis is widely used in industrial processes for reactions involving water-soluble and water-insoluble reactants.3-7 Reaction systems are two-phase, initially consisting of an organic solvent such as hexane containing the lipophilc reactant and an aqueous solution of the hydrophilic reactant. The role of the phase-transfer catalyst is to form a complex with the hydrophilic reactant in the aqueous phase and shuttle it into the oil phase. The reaction occurs primarily within the bulk oil phase, rather than at the liquid/liquid interface, and so much higher rates are observed compared to the same system without catalyst. Phase-transfer catalysts may also be designed to complex the lipophilic reactant and shuttle it into the aqueous phase. Numerous phase-transfer catalysts have been identified. Quaternary ammonium compounds such as tetrabutylammonium bromide (TBAB) are the most common due to their low toxicity and proven effectiveness for a wide range of reactions.3,8,9 In the aqueous phase, the TBA+ cation forms an ion pair with a water-soluble reactive anion; the ion pair is more soluble in the organic phase than in water, so the reactant is transported into the organic phase where the desired reaction proceeds, releasing TBA+, which then partitions back into the aqueous phase as an ion pair with the leaving group of the reaction. Although compounds such as TBA+ may exhibit some surface active behavior, they do not form micelles and are not surfactants in the classical sense. Micellar Catalysis. Previous work by numerous researchers has established a basic theoretical understanding of the mechanism of micellar catalysis, and has also provided a basis for predicting the behavior of general classes of reactions with different types of surfactants. Several excellent summaries have been published.10-15 Given an appropriate surfactant for a particular reaction, micellar solubilization and electrostatic forces act to locally (2) Freedman, H. H.; Dubois, R. A. Tetrahedron Lett. 1975, No. 38, pp 3251-3254. (3) Brandstrom, A. Adv. Phys. Org. Chem. 1977, 15, 267. (4) Antoine, J. P.; de Aquirre, L.; Janssesn, F.; Thyrion, F. Bull. Soc. Chim. Fr. 1980, 207. (5) Dehmlow, E. V.; Dehmlow, S. S. Phase-Transfer Catalysis; VCH Weinheim: New York, 1993. (6) Starks, C. M.; Liotta, C. L.; Halpern, M. Phase-Transfer Catalysis; Chapman and Hall: New York, 1994. (7) Herriott, A. W.; Picker, D. J. Am. Chem. Soc. 1975, 97, 2345. (8) Dockx, J. Synthesis 1973 441. (9) Starks, C. M. J. Am. Chem. Soc. 1971, 93, 195. (10) Fendler, J.; Fendler, E. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (11) Cordes, E., Ed. Reaction Kinetics in Micelles; Plenum Press: New York, 1973. (12) Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; p 509. (13) Bunton, C. A. Catal. Rev. Sci. Eng. 1979, 20, 1.
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concentrate both lipophilic and hydrophilic reactants near micelles, often resulting in dramatic increases in reaction rates, depending quantitatively on the binding constants of the reactants, the micellar reaction rate constant, and the surfactant concentration. Several studies have demonstrated that improved selectivity can also be achieved in micellar systems, even to perform stereospecific catalysis.16-18 Reaction conditions used in most of these studies are quite different than those typically encountered in industrial processes. For example, in order to simplify the kinetic treatment of data, fundamental investigations of bimolecular reactions in surfactant solutions have usually been performed with an excess of one reactant and very dilute concentrations of the other reactant, so that pseudofirst-order conditions could be assumed and effects of reactants and/or products on the surfactant critical micelle concentration (cmc) and micelle structure could be neglected. In an industrial process, on the other hand, it is often desirable to use stoichiometric amounts of reactants to minimize the quantity of unreacted material that must be subsequently removed from a product stream. It is also desirable to maximize reactant concentrations to obtain high rates of production, providing the required selectivity can be attained. The difficulties in extrapolating results from fundamental research to an industrial scale are not trivial. In conventional micellar catalysis, a single-phase reaction medium is obtained by exploiting the previously noted ability of aqueous surfactant solutions to solubilize materials that are normally insoluble in water. For the reaction studied here, the lipophilic reactant (1-bromobutane) is solubilized by micelles in aqueous solutions also containing sodium hydroxide, phenol, and surfactant at concentration much greater than its cmc. For this particular reaction, a cationic surfactant is appropriate because the reactive phenolate anion is concentrated or “bound” near the micelle by the positive charge on the micelle surface. Thus, surfactant micelles act to locally concentrate both reactants within the solution, one by solubilization due to the hydrophobic effect, the other by counterion binding due to electrostatic forces. Most attempts to model micellar catalysis data have been based on the pseudophase separation model (PSM) of surfactant solutions, in which micelles and the surrounding continuous solvent are treated thermodynamically as separate phases.19 Although a micellar solution is certainly microheterogeneous, it is macroscopically a single, thermodynamically stable phase. The power of the PSM is that classical phase equilibria thermodynamics can be applied to micellar solutions; the term “pseudophase” serves as a reminder that this approach, although extremely useful, is only an approximation. According to PSM, micellar catalysis can be mathematically modeled by considering reactions in the micellar and continuous pseudophases and the partitioning of various components between phases. Competitive counterion binding is most commonly described using the pseudophase ion exchange (14) Bunton, C. A.; Nome, F.; Quina, F.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357-364. (15) Bunton, C. A. In Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M., Kalyanasundaram, K., Eds.; Surfactant Science Series; Marcel Dekker, Inc.: New York, 1991; Vol. 38, Chapter 2. (16) Bunton, C. A.; Robinison, L.; Stam, M. F. Tetrahedron Lett. 1971, 121. (17) Brown, J. M.; Bunton, C. A. J. Chem. Soc., Chem. Commun. 1974, 969. (18) Ihara, Y. J. Chem. Soc., Chem. Commun. 1978, 984. (19) Shinoda, K. In Colloidal Surfactants; Shinoda, K., Tamamushi, B., Nakagawa, T., Isemura, T., Eds.; Academic Press: New York, 1963; Chapter 1.
Micellar Phase-Transfer Catalysis
model12,20 or Coulombic models based on the nonlinear Poisson-Boltzmann equation.14,21 The partition coefficient needed to describe the distribution of the lipophilic reactant between the aqueous and micellar pseudophases can often be determined experimentally or estimated by fitting the kinetic models. Micellar Phase-Transfer Catalysis. One advantage of micellar catalysis over PTC is that surfactant micelles act to concentrate both reactants, thereby achieving the required reaction rates without the need for environmentally hazardous organic solvents. Biodegradable, nontoxic surfactants can be selected and used at fairly low concentrations. Disadvantages of conventional micellar catalysis include the competitive binding between reactive and nonreactive counterions, which can greatly reduce the rate of reaction, and the fact that the surfactant must be ionic and opposite in charge compared to the watersoluble reactant. For example, cationic surfactants are used when the hydrophilic reactant is anionic despite the fact that the solubility of the lipophilic reactant may be much lower in cationic micelles than in anionic or nonionic micelles. Also, some of the most extensively studied cationic surfactants in the micellar catalysis literature are either known to be toxic or have not been approved for use or release into biological systems. Micellar phase-transfer catalysis is a process, first proposed by Van der Horst et al.22 as a method for preparing derivatives to improve their analysis by chromatography, that provides a novel way of synthesizing materials in aqueous surfactant solutions. As in conventional PTC, a quaternary ammonium cation (e.g., tetrabutylammonium, TBA+) is used as a phase-transfer catalyst to form an ion pair with the anionic reactant. As in conventional micellar catalysis, the lipophilic reactant is solubilized by surfactant micelles. In MPTC, the TBA+phenolate complex is only sparingly water soluble and so is solubilized into the micelle, thereby facilitating the desired reaction; i.e., the phase-transfer catalyst shuttles the hydrophilic reactant into the micellar pseudophase for reaction with the solubilized lipophile. MPTC offers several advantages over both conventional PTC and MC: (1) the reaction system is aqueous, no organic solvent is required, thereby reducing the environmental impact of the process; (2) the system provides intimate mixing of the reactants, which are concentrated in micelles; (3) since electrostatic binding of one of the reactants is not required, competitive counterion binding is not an issue; biodegradable, nontoxic surfactants can be used and chosen so as to maximize solubility of the reactants. Cationic, nonionic, and even anionic surfactants may be considered. MPTC can therefore be employed using a much wider range of surfactants than conventional micellar catalysis. Experimental Section Materials. Dimethyldodecylamine oxide (DDAO, 30% in water), dodecyltrimethylammonium bromide (DTAB, 98%), tetrabutylammonium bromide (TBAB, 99%), and 1-bromobutane (98%) were purchased from Fluka Chemical Corp. Butyl phenyl ether (99%) and sodium dodecyl sulfate (SDS, 97%) were purchased from Aldrich Chemical Co., Inc. Phenol (89% aqueous solution) was bought from Mallinckrodt. Sodium hydroxide pellets of 98.5% purity were obtained from Jenneile Enterprises. Distilled deionized water from a Barnstead MP-1 still was used in all solutions. (20) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: NewYork, 1984; Vol. 2, p 1015. (21) Gunnarsson, G.; Johnsson, B.; Wennerstrom, H. J. Phys. Chem. 1980, 84, 3114. (22) Van der Horst, F. A. L.; Post, M. H.; Holthuis, J. J. M.; Brinkman, U. A. Th. Chromatographia 1989, 28, 267.
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Figure 1. Effect of surfactant (DTAB) concentration on synthesis of butyl phenyl ether at 55 °C by conventional micellar catalysis at high reactant loadings. Initial solutions contained 0.15 mol/kg phenol, 0.15 mol/kg 1-bromobutane, and 1.0 mol/ kg NaOH. Experimental setup is type II. Methods. Samples taken from the reaction vessel were diluted with 0.10 mol/kg NaOH solution to a final phenol concentration in the range 0.0007-0.0020 mol/kg. A quartz sample cell with 1 cm path length was used, and UV spectra were collected using a Hewlett-Packard Model 8453 diode array spectrophotometer. Appropriate blank and calibration measurements were performed. Concentrations of phenol and the butyl phenyl ether product were calculated simultaneously using standard peak fitting algorithms. Each reaction solution initially consisted of 50 g of total material. All reactions were performed at 55 ( 1 °C and progress was followed for 7-8 h. Temperature was controlled using a heating mantle, and vessels were closed to prevent loss by evaporation. Two types of reaction setups were studied: Type I: Solution contained in a 200 mL Erlenmeyer flask and stirred continuously using a 0.8 by 3.8 cm cylindrical magnetic stir bar rotating at 1600 rpm. Type II: Solution contained in a 200 mL round-bottom flask and stirred continuously using a mechanical mixer equipped with a 1.9 by 6.0 cm PTFE stirring blade (Kontes, part 789030-0021) rotating at 350 rpm.
Results and Discussion The effect of cationic surfactant concentration on micellar catalysis was studied in the alkylation of 0.15 mol/kg of phenol and 0.15 mol/kg of 1-bromobutane. The cationic surfactant (DTAB) concentration was varied from 0.10 to 1.20 mol/kg. Cationic (DTAB), nonionic (DDAO), and anionic (SDS) surfactant systems were then investigated with the addition of 0.05 mol/kg phase-transfer catalyst (TBAB) to explore the effect of surfactant type on micellar phase-transfer catalysis. The effects of initial reactant concentration and mixing were also studied. The effect of cationic surfactant (DTAB) concentration on the alkylation of phenol with 1-bromobutane by conventional micellar catalysis is shown in Figure 1. This system is similar to other nucleophilic substitutions presented in numerous prior studies on micellar catalysis; however, the stoichiometric reactant concentrations used here were higher than those generally employed in earlier work. Solutions were single-phase throughout the reaction and initially contained 0.15 mol/kg of both phenol and 1-bromobutane, representing a total reactant loading of 3.4 wt %. As shown in Figure 1, all reactions exhibit a plateau in the phenol conversion; once this plateau is reached, further conversion occurs very slowly. An optimum surfactant concentration is observed; for this
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Figure 2. Reciprocal plot of data from Figure 1. Lines represent linear regression fit to linear region observed for each data series as t f 0. Note that the slope (k2) is highest for the 0.50 mol/kg data, even though at longer times the experimentally observed conversion is greatest for 0.30 mol/kg. Table 1. Apparent Second-Order Rate Constants from Data in Figure 1 mol/kg DTAB
k2 (kg mol-1 min-1)
mol/kg DTAB
k2 (kg mol-1 min-1)
0.10 0.30 0.50
0.044 0.059 0.065
0.70 0.90 1.20
0.055 0.052 0.041
particular reaction system, highest conversions were observed for solutions containing 0.30 mol/kg DTAB. The apparent second-order rate constant (k2) for this reaction is defined by
rate ) k2[Pht][BBt]
(1)
where subscript t denotes the total (bulk) concentrations of phenol (Ph) and 1-bromobutane (BB), respectively. These types of reactions are often studied using excess concentrations of the water-soluble reactant so that the reaction is pseudo first order with respect to the lipophilic reactant, in which case:
rate ) kψ[BBt]
(2)
Given k2 or kψ as a function of surfactant concentration, application of the pseudophase separation model (PSM) allows calculation of bimolecular rate constants for the reaction in the aqueous and micellar pseudophases. For the reactions in Figure 1, in which stoichiometric quantities of reactants were used, eq 1 is appropriate and k2 can be determined from a plot of the reciprocal of [Pht] as a function of time, as shown in Figure 2. These plots were linear only during the initial reaction time period, so values of k2 were determined by linear regression using data for t e 100 min and are listed in Table 1. These results demonstrate the difficulty in extending existing micellar catalysis models to systems with higher reactant loadings. For example, the highest initial rate is observed for the 0.50 mol/kg DTAB system; however, at longer times the conversion observed for the 0.30 mol/kg DTAB system is significantly higher. At long times, eq 1 predicts much higher conversions than what are actually observed for all systems except 0.30 mol/kg DTAB, for which the predicted result is lower than the experimental value. The most likely explanation for these results is the
Figure 3. Side reaction of 1-bromobutane in the absence of phenol. Initial solutions contained 0.5 mol/kg DDAO, 1.0 mol/ kg NaOH, and 0.15 mol/kg (b) or 0.60 mol/kg (O) 1-bromobutane.
previously discussed side reaction of the alkyl halide to form the corresponding alcohol and possibly the symmetrical ether. To determine whether the extent of the side reaction was significant, a series of reactions were studied for aqueous solutions containing only surfactant, 1-bromobutane, and sodium hydroxide. Results of two such experiments are shown in Figure 3; although no reaction of bromobutane was observed in the absence of NaOH, Figure 3 clearly shows that the side reaction is important in strongly alkaline solutions. As expected, the conversion increased with increasing NaOH/bromobutane ratio. The primary product of the side reaction was assumed to be butanol, since similar reactions for solutions initially containing surfactant, butanol, and sodium hydroxide were observed to be stable; formation of dibutyl ether was not observed. In the reaction systems shown in Figure 1, the side reaction affects the phenol reaction in several ways, the most obvious being that the phenol conversion is reduced because of loss of bromobutane via the side reaction. A possible secondary effect of butanol formation would be its incorporation into the micelles; in fact, the ether product itself is also lipophilic and so remains solubilized in the micelles. The presence of these compounds in the micelle may induce significant changes in the micellar environment over the course of the reaction, changes that, depending on a number of factors, may either inhibit or enhance further reaction. For example, their presence in the micelle might affect the orientation of solubilized lipophilic reactant (1-bromobutane), in some cases making it less reactive so that conversion plateau at levels much lower than desired. The effect of the phase-transfer catalyst (TBAB) on reaction rates and conversion is compared for different surfactant systems in Figure 4. Stoichiometric (equimolar) proportions of the reactants were employed in all solutions. In the absence of both surfactant and phasetransfer catalyst, this reaction did not occursthe conversion after 500 min was negligible. With TBAB but no surfactant, the initial rate was similar to that observed in the DTAB and DDAO surfactant systems; however, the phenol conversion plateaus at approximately 20% after 1 h. This system represents conventional phase-transfer catalysis: a two-phase system in which the phase-transfer catalyst forms an ion pair with the phenolate ion and shuttles it into the organic phase, in this case the neat lipophilic reagent itself, for reaction. Sampling solutions
Micellar Phase-Transfer Catalysis
Figure 4. Effect of surfactant type and tetrabutylammonium bromide (TBAB) on synthesis of butyl phenyl ether at 55 °C. Initial solutions contained 0.15 mol/kg phenol, 0.15 mol/kg 1-bromobutane, and 1.0 mol/kg NaOH. Filled symbols denote runs with 0.50 mol/kg surfactant and 0.05 mol/kg TBAB. Open symbols denote runs with 0.50 mol/kg surfactant and no TBAB. Dotted symbols represent results for no surfactant, 0.05 mol/ kg TBAB. Experimental setup is type I.
containing no surfactant was difficult since these systems were two-phase and poorly emulsified. All other systems discussed in this paper were single-phase, and reproducibility of the conversion profiles was good. Reactions in solutions containing surfactant but no phase-transfer catalyst (TBAB) proceed by conventional micellar catalysis, while reactions in solutions containing both surfactant and TBAB may also proceed by micellar phase-transfer catalysis. On comparison of the different surfactants in Figure 4, conversion was highest for the cationic surfactant (DTAB) system, lower for the nonionic surfactant (DDAO), and much lower for the anionic surfactant (SDS). In fact, the rate and conversion observed for SDS systems are significantly lower than in the absence of surfactant, indicating that SDS micelles actually inhibit the reaction. These results are as expected, since electrostatic interactions result in the reactive phenolate anion being concentrated near the surface of cationic surfactant micelles and excluded from anionic surfactant micelles. Addition of TBAB has a negligible effect on the cationic surfactant system in Figure 4 but does provide improved conversion in both DDAO and SDS systems. In solutions containing cationic surfactant micelles, the reactive phenolate anion is locally concentrated by electrostatic interactions (i.e., counterion binding) at the micelle surface. This effect is sufficient for catalytic purposes and no further advantage is observed upon addition of TBAB. For solutions containing DDAO, electrostatic interactions are not a factor, so the conversion is indeed improved by formation of the TBA+-phenolate ion pair, resulting in enhanced partitioning of the water-soluble reactant into the micellar pseudophase. The effect of TBAB is most pronounced for SDSsin this system, TBA+ also acts as an organic counterion that binds strongly at the anionic surfactant micelle interface, reducing the net charge on the micellar surface, but also in the process reducing the concentration of free TBA+ available for complexation of phenolate. The results observed for DDAO provide the best demonstration of the potential advantages of MPTC over
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Figure 5. Effect of initial reactants concentration and TBAB on synthesis of butyl phenyl ether at 55 °C. Initial solutions contained 0.50 mol/kg DDAO, and 1.0 mol/kg NaOH. Filled symbols denote runs with 0.05 mol/kg TBAB, and open symbols denote runs with no TBAB. Experimental setup is type I.
PTC or MC. At a given surfactant concentration above the cmc, solubilization of 1-bromobutane is significantly higher in DDAO solutions than in DTAB solutionssfor example, at 25 °C the solubilization capacity of 1.0 mol/kg DDAO is 1.8 mol/kg 1-bromobutane, compared to 1.2 mol/ kg in 1.0 mol/kg DTAB. An important objective of this work was to demonstrate that enhanced solubilization capacity can be taken advantage of by using micellar phase-transfer catalysis since, unlike conventional micellar catalysis, it is not necessary to use a cationic surfactant for a reaction involving an anionic reactant. The effect of initial reactant concentrations on micellar phase-transfer catalysis in the DDAO system is illustrated in Figure 5. Although the general shapes of the reaction profiles are similar, rates and conversion were significantly higher at 0.60 mol/kg reactant loading compared to 0.15 mol/kg. Further, the benefits of micellar phase-transfer catalysis over conventional micellar catalysis are also more pronounced at higher loading. The conversion in DDAO/ TBAB mixtures is much higher than either DDAO or TBAB alone; this result clearly demonstrates the usefulness of micellar phase-transfer catalysis as a means of improving performance in nonionic surfactant systems. As described in the Experimental Section, two types of mixing were studied and several interesting differences were observed. Figures 4 and 5 present data obtained using the type I experimental setup. Figure 6 illustrates results obtained when the experiments shown in Figure 5 were repeated using the type II setup. When using the mechanical mixer instead of the magnetic stirrer, there is no longer a large effect of reactant loadingsdifference between 0.15 and 0.60 mol/kg reactants loading is only slight. The effectiveness of MPTC compared to MC is also less pronouncedsrates and conversion with MPTC (i.e., with TBAB present) were still significantly higher early in the reaction, but reactions reach roughly the same plateau conversion, so that no advantage is observed for MPTC at extended reaction times. Noting that the mixing in both setups was quite vigorous (1600 rpm for type I and 350 rpm for type II), either of these mixing modes would be sufficient for simple single-phase, noncolloidal solutions to ensure complete mixing, and so no significant differences
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Siswanto et al. Table 2. Comparison of Results Obtained Using Reaction Vessels with Different Geometries and Mixing Modesa
surfactant DDAO
DTAB
phenol conversion type I setup
phenol conversion type II setup
mol/kg TBAB
mol/kg reactants
100 min
300 min
100 min
300 min
0 0.05 0 0.05 0 0.05
0.15 0.15 0.60 0.60 0.15 0.15
24% 25 31 39 33 33
34% 38 54 64 58 58
30% 40 22 29 49 48
61% 60 61 63 69 68
a Reactions at 55 °C with 0.50 mol/kg surfactant and equimolar amounts of phenol and 1-bromobutane.
Figure 6. Same conditions as Figure 5, except with type II experimental setup.
would be expected. Although single-phase, the solutions studied here are microheterogeneous, and this may be key to understanding the observed differences between the two types of mixing. One possible explanation is that the mixing resulted in localized temperature increases that increased reaction rates and that this effect was more pronounced in the type II setup. Although these solutions were somewhat viscous, this explanation seems unlikely since the temperature was controlled during the reaction and no increase in temperature was observed. The fact that the reaction rates for this particular reaction are much slower than the rate at which localized heat effects would be expected to dissipate also argues against this explanation. More likely, this is a consequence of the microheterogeneous nature of these systems: although the bulk fluid flow was certainly not laminar for either type I or II mixing modes, transport of materials between aqueous and micellar pseudophases may in fact be diffusion controlled. Colloidal hydrodynamics are known to be important in many phenomena but to date have not been reported in micellar catalysis, in which transport rates of species into and out of the micelles are generally assumed to be much faster than the reaction rates. This assumption has been shown to be valid for micellar systems at very low reactant loading;23 however, results presented here suggest that the assumption may not necessarily be valid at high reactant loading. (23) Almgren, M.; Linse, P.; Van der Auweraer, M.; De Schryver, F. C.; Gelade, E.; Croonen, Y. J. Phys. Chem. 1984, 88, 289.
Table 2 summarizes the effects of TBAB, reactant loading, and effect of mixing for the DTAB and DDAO surfactant systems. For DDAO systems, type II experiments resulted in slightly higher conversions, and the effect of TBAB is apparent at 100 minsno effect is observed at 300 min. For DTAB surfactant systems, results with and without TBAB are identical and higher conversions are observed using the Type II setup. The second paper of this series will discuss this same reaction in systems at even higher initial reactant concentrations, in which the systems are two-phase emulsions. Conclusions This paper demonstrates for the first time the application of micellar phase-transfer catalysis as a means of attaining higher reaction rates and conversions than observed in conventional micellar catalysis. Systems containing a phase-transfer catalyst (TBAB) and a nonionic (DDAO) or anionic surfactant (SDS) exhibited higher conversions than either TBAB-only or surfactant-only systems. This synergism shows that reactions normally performed by conventional micellar catalysis using a cationic surfactant can alternatively be performed using other types of surfactants, so that surfactants can be selected based on their solubilization capacity for the lipophilic reactant. Two types of experimental geometries and mixing modes were studied and differences were noted that suggest mass transport of species between the aqueous and micellar pseudophases may be important, at least for systems containing relatively high initial concentrations of reactants. Acknowledgment. This work was supported by the Emission Reduction Research Center (ERRC) and the National Science Foundation (CT-9308592). LA960487I
