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Synthesis of Alkylphenyl Ethers in Aqueous Surfactant Solutions by Micellar Phase-Transfer Catalysis. 2. Two-Phase Systems Turgut Battal, Cincin Siswanto, and James F. Rathman* Chemical Engineering Department, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210-1110 Received July 18, 1996. In Final Form: August 27, 1997X Alkylation of phenol with 1-bromobutane was conducted under phase-transfer catalysis (PTC), micellar catalysis (MC), and micellar phase-transfer catalysis (MPTC) conditions at high reactant loadings (e50 wt %) that exceeded the solubilization capacity of micellar solutions, so the systems were two-phase emulsions. Cationic (cetyltrimethylammonium chloride, CTAC), nonionic (N,N-dimethyldodecylamine N-oxide, DDAO), and anionic (sodium n-dodecyl sulfate, SDS) surfactants were compared. Systems containing a phasetransfer catalyst, tetrabutylammonium bromide (TBAB), and surfactant exhibited much higher conversions than either TBAB-only (PTC) or surfactant-only (MC) systems. Phenol conversions of ∼95% were attained after 60 min in CTAC, 250 min in DDAO, and 400 min in SDS. Although the apparent reaction rates decreased in order CTAC > DDAO > SDS, essentially the same high conversion can be achieved using any of these surfactants. An advantage of micellar phase-transfer catalysis is that the charge of the surfactant is less important than that in conventional micellar catalysis. Reactant concentrations were changed to study the effect of reactant loading on the reaction rates and conversions. Significantly higher reaction rates were attained at higher reactant loading, but the reactions reached the same final conversion. The solution alkalinity was also found to strongly affect this reaction. Increasing the initial NaOH concentration exerted a strong proportional effect on the observed rates of reaction.
Introduction paper,1
In a previous we demonstrated the application of micellar phase-transfer catalysis (MPTC) as a means of attaining higher reaction rates and conversions than those obtained by conventional micellar catalysis for single-phase systems. Effects of surfactant type, experimental geometries, and mixing modes were studied. While investigations of single-phase systems are invaluable for gaining an understanding of how various factors affect these reactions, systems containing considerably higher reactant concentrations are of more practical interest. Industrial synthesis processes commonly employ reactant loading of 10-50 wt %, levels that exceed the solubilization capacity of micellar solutions. Performing these reactions in aqueous micellar systems thus necessitates working with two-phase emulsions. The initial reaction solution consists of emulsified droplets of the lipophilic reactant dispersed in a continuous aqueous phase containing surfactant and the hydrophilic reactant. Emulsions are heterogeneous mixtures of two or more immiscible liquids, one of which is dispersed as droplets in the other. Emulsions are thermodynamically unstable but may be temporarily stabilized through the introduction of surfactants. Water/hydrocarbon emulsions may consist of oil droplets dispersed in a continuous aqueous phase (o/w) or droplets of the aqueous phase dispersed in the oil (w/o). Key factors that determine the emulsion type are the chemical nature of various components, the concentration of the surfactant(s) used, the volume fractions of oil and water, temperature, and (in some cases) the type of mixing employed. Although reactions in emulsions are complex from a modeling standpoint, they offer a reaction environment with several advantages that can be exploited. They provide a high interfacial area per unit volume so that * Author to whom correspondence should be addressed: e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Siswanto, C.; Battal, T.; Schuss, O. E.; Rathman, J. F. Langmuir 1997, 13, 6047.
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production rate of interfacial reactions may be appreciable, and limitations due to interphase mass transport are greatly reduced or eliminated. Second, rheological and heat transfer properties of emulsions are often superior from a processing standpoint to single-phase systems. Third, droplet size and coalescence can be controlled so that, for example, polymerization of liquid reactants to form solid particles of a desired size can be accomplished. The most common application in chemical manufacturing of a two-phase o/w emulsion as a reaction medium is emulsion polymerization. Polymer synthesis performed in a dispersing medium allows for improved control of particle size and heat dissipation. Prerequisite components for emulsion polymerization include the reactive monomer (30-60% by volume), a dispersing medium (usually water), an initiator (generally soluble in dispersing medium), and an emulsifying agent. During the initial stages of the reaction, polymerization primarily occurs in monomer-swollen micelles. Polymer-monomer particles are formed which then become the main site of polymerization. Toward the end of the reaction, the system is a latex dispersion of the polymers particles. The particle nucleation mechanism is important in determining the number of particles present in the system and the rate of polymerization. Three possible nucleation loci have been widely discussed in the literature: (1) micelles; (2) aqueous phase via homogeneous (or coagulate) nucleation; (3) monomer droplets. Emulsion polymerization continues to attract attention as a method for preparing polymers, agricultural products, novel materials, and pharmaceuticals. The role of surfactant aggregates in these systems is not completely understood, and further investigation is needed to develop appropriate quantitative models for these complex systems. The reaction investigated in this work is the preparation of unsymmetrical phenolic ethers via alkylation with alkyl halides2 (the Williamson reaction). The specific reaction of interest is the synthesis of butyl phenyl ether in aqueous (2) Jursic, B. Tetrahedron 1988, 44, 6677.
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Figure 1. Simplified schematic of the various reaction environments present in an oil-in-water emulsion containing normal micelles in the aqueous phase.
sodium hydroxide solution.
The objective was to compare reaction rates and conversions observed under three catalytic schemes in emulsion systems: (1) phase-transfer catalysis (PTC);3-10 (2) micellar catalysis (MC);11-18 (3) micellar phase-transfer catalysis (MPTC),1,19 which is an integration of conventional micellar catalysis and phase-transfer catalysis. A schematic of the reaction system is shown in Figure 1. The desired reaction may proceed at four different localities: (1) reaction of phenolate ion with 1-bromo(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) Tan, S. N.; Dryfe, R. A.; Girault, H. H. Helv. Chim. Acta 1994, 77, 231. (9) Dehmlow, E. V.; Fastabend, U. Gazz. Chim. Ital. 1996, 126, 53. (10) Bhattacharya, A. Ind. Eng. Chem. Res. 1996, 35, 645. (11) Fendler, J.; Fendler, E. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (12) Cordes, E., Ed. Reaction Kinetics in Micelles; Plenum Press: New York, 1973. (13) Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; p 509. (14) Bunton, C. A. Catal. Rev. Sci. Eng. 1979, 20, 1. (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) Broxton, T. J.; Wright, S. Aust. J. Chem. 1991, 44, 103. (17) Sirovsky, F. S. Russ. Chem. Bull. 1995, 44 (11), 2026. (18) Hori, K.; Kamimura, A.; Kimoto, J.; Gotoh, S.; Ihara, Y. J. Chem. Soc., Perkin Trans. 2 1994, 2053. (19) Van der Horst, F. A. L.; Post, M. H.; Holthuis, J. J. M.; Brinkman, U. A. Th. Chromatographia 1989, 28, 267.
butane at the liquid/liquid interface; (2) PTC, reaction of TBA+:phenolate complex with 1-bromobutane in the dispersed droplet phase; (3) MC, reaction of phenolate ion with 1-bromobutane solubilized in normal micelles in the aqueous phase; (4) MPTC, reaction of TBA+-phenolate complex with 1-bromobutane solubilized in micelles. In this study, alkylation of phenol with 1-bromobutane was conducted under PTC, MC, and MPTC conditions at high reactant loading, up to 50% (w/w), so that the solubilization capacity of micellar solutions was exceeded and solutions were initially two phase. Cationic, nonionic, and anionic surfactants were compared. The effects of surfactant, phase transfer catalyst, and sodium hydroxide concentrations were investigated. Experimental Section Materials. Dimethyldodecylamine oxide (DDAO, 30% in water), tetrabutylammonium bromide (TBAB, 99%), and 1-bromobutane (98%) were from Fluka Chemical Corp. Butyl phenyl ether (99%), sodium dodecyl sulfate (SDS, 97%), and cetyltrimethylammonium chloride (CTAC, 25% in water) were from Aldrich Chemical Co., Inc. Phenol (89% aqueous solution) was from Mallinckrodt. Sodium hydroxide pellets (98.5%) were from Jenneile Enterprises. Distilled deionized water from a Barnstead MP-1 still was used in all experiments. Methods. Reactions were performed by placing 75 g of the initial solution in a 200 mL round-bottom flask and stirring 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. Temperature was maintained at 60 °C using a heating mantle and water bath, and vessels were closed to prevent loss by evaporation. Samples (0.1-0.4 g) taken from the reaction vessel were diluted with 0.10 mol/kg NaOH solution to a final phenol concentration in the range 0.00010-0.00020 mol/kg. Progress was followed for 7-8 h. 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 with 1 nm resolution. Appropriate blank and calibration measurements were performed. Concentrations of phenol and the butyl phenyl ether product were calculated simultaneously using standard peak fitting algorithms.
Results and Discussion Figure 2 presents the catalytic effects observed for the alkylation of phenol with 1-bromobutane in various reaction systems: no surfactant and no phase-transfer
Micellar Phase-Transfer Catalysis
Figure 2. Effect of MPTC on synthesis of butyl phenyl ether at 60 °C in micellar CTAC solutions. All solutions initially contained 1.0 mol/kg phenol, 1.0 mol/kg 1-bromobutane, 6.0 mol/kg NaOH, and water. Concentrations (mol/kg) of CTAC and TBAB were as follows: (9) 0.1, 0.05; (0) 0.1, 0.0; ([) 0.0, 0.05; (]) 0.0, 0.0.
catalyst; phase-transfer catalyst alone; surfactant alone; and surfactant plus phase-transfer catalyst. All systems were two-phase emulsions and initially contained 1.0 mol/ kg of both phenol and 1-bromobutane, representing a total reactant loading of 25% (w/w). In the absence of both surfactant and phase-transfer catalyst, the reaction proceeded very slowly, with phenol conversion less than 10% observed after 400 min Reactions performed with TBAB but no surfactant proceed by conventional phase transfer catalysis; as shown, the phenol conversion reached 30% over the same time interval, indicating that the phasetransfer catalyst alone significantly increased the reaction rate, as expected. Reactions with surfactant but no TBAB proceed by conventional micellar catalysis; for the CTAC system in Figure 2, the conversion was only slightly higher at any given time than in the TBAB-only system. The surfactant plays multiple roles in these reaction systems: emulsifier, solubilizing agent, and possibly even as a phase-transfer catalyst. The critical micelle concentration (cmc) of CTAC in water at 60 °C is 0.0015 mol/kg; addition of NaOH and bromobutane both act to decrease the cmc, so at the high surfactant concentrations used here only a small fraction of surfactant is present in monomer form. For reactions performed using surfactant concentrations below 0.0010 mol/kg, reaction rates and conversions were not significantly different than the results shown in Figure 2 for no CTAC and no TBAB, despite the fact that sufficient surfactant was present to form a good emulsion of 1-bromobutane droplets dispersed in the aqueous medium. The fact that no catalytic effect was observed for CTAC at submicellar concentrations indicates that (1) reaction at the liquid/liquid interface has little contribution to the overall conversion, so that emulsification by a surfactant is, in and of itself, not sufficient to increase the net rate of production and (2) CTA+ monomers are not efficient phase-transfer catalysts. The most significant result in Figure 2 is that the phenol conversion is much higher in the system containing both CTAC and TBAB than either of these components by themselves. This strong synergistic effect clearly illustrates the advantage of micellar phase-transfer catalysis over conventional PTC or MC. For solutions containing both CTAC and TBAB, the observed enhance-
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Figure 3. Effect of MPTC on synthesis of butyl phenyl ether at 60 °C in micellar DDAO and SDS solutions. All solutions initially contained 1.0 mol/kg phenol, 1.0 mol/kg 1-bromobutane, 6.0 mol/kg NaOH, and water. Surfactant concentrations were 0.10 mol/kg DDAO (b, O) or SDS (2, 4). Empty symbols denote runs with no TBAB and filled symbols denote runs with 0.05 mol/kg TBAB.
ments in reaction rate and conversion are due primarily to the formation of the TBA+-phenolate ion complex. This complex is only sparingly water soluble and so is solubilized into the micelle pseudophase, thereby facilitating the desired reaction; i.e., the phase transfer catalyst shuttles the hydrophilic reactant into the micellar region for reaction with the solubilized lipophile. Figure 3 shows results obtained for the same reaction system in Figure 2 with nonionic (DDAO) and anionic (SDS) surfactants. The profile for the DDAO-only system was similar to the CTAC-only data in Figure 2, a somewhat surprising result since in single-phase systems (at much lower reactant loading) cationic micelles better catalysis than nonionic surfactants due to counterion binding of phenolate. One possible explanation is that, at the high phenol/surfactant ratio used here, solubilization of 1-bromobutane in micellar DDAO is higher than CTAC;1 higher solubilization of the lipophilic reactant in DDAO compensates for the lack of counterion binding in DDAO as compared to CTAC systems, so that the observed reaction rates for these two surfactants are similar. Conversions in the SDS-only system were significantly lower, as expected since anionic SDS micelles electrostatically repel the reactive phenolate anions, tending to inhibit the reaction. Addition of the phase-transfer catalyst TBAB to these surfactant systems again resulted in a remarkable synergism, with phenol conversions of ∼95% after 250 min in DDAO and after 400 min in SDS. Although the apparent reaction rates decrease in order CTAC > DDAO > SDS, essentially the same high conversion can be achieved using any of these surfactants. The catalysis observed for SDS is especially significant, since addition of TBAB effectively overcomes the electrostatic repulsion between phenolate and the micelle surface, resulting in much greater reaction rate. The fact that high conversion can be attained with SDS/TBAB emphasizes a key advantage of micellar phase-transfer catalysis, since the charge of the surfactant is less important than that in conventional micellar catalysis. This provides much greater flexibility in designing a process for a particular reaction and permits other factors such as surfactant toxicity, cost, solubilization capacity, and ease of separation to be considered when selecting the surfactant.
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Figure 4. Effect of MPTC on synthesis of butyl phenyl ether at 60 °C. All solutions initially contained 2.0 mol/kg phenol, 2.0 mol/kg 1-bromobutane, 6.0 mol/kg NaOH, and water. Surfactant concentrations were (9, 0) 0.20 mol/kg CTAC or (b, O) DDAO or (2, 4) SDS or ([, ]) no surfactant. Empty symbols denote runs without TBAB, and filled symbols denote runs with 0.10 mol/kg TBAB.
In a previous paper,1 we reported similar results for this reaction performed in single-phase solutions at lower reactant loading and higher surfactant concentrations. The primary difference between the single- and two-phase systems is that the observed advantage of MPTC over PTC and MC is much greater in the two-phase systems. The higher conversions attained in two-phase systems can be attributed to a number of factors. First, the undesired side reaction in which 1-bromobutane is converted to butanol was not observed in the two-phase reactions. In single-phase systems, this reaction resulted in maximum phenol conversions less than 70%; however, in two-phase systems, phenol conversions of 95% or greater were routinely obtained, indicating that the side reaction was not a problem. Second, the emulsified phase acts both as a continuous source of the lipophilic reactant and as a sink into which the butyl phenyl ether product preferentially partitions. In single-phase reactions, the product remains solubilized in the micelles and may thereby reduce the overall yield. To observe the effect of reacting loading on the reaction rate and conversion, reactant concentrations were doubled to 50% (w/w) while keeping the reactant/surfactant and reactant/catalyst concentration ratios constant. Results are shown in Figure 4. For surfactant-only and TBABonly systems, the rate and conversion in Figure 4 are higher than those in Figures 2 or 3. For surfactant/TBAB systems, the rates are significantly higher at higher reactant loading, but the reactions reach essentially the same high conversion (∼95%) as at 1.0 mol/kg loading. An interesting feature of the surfactant/TBAB micellar phase-transfer catalysis data shown in Figures 2, 3, and 4 is the acceleration of the reaction observed during the reaction, generally at some point when the conversion was between 20 and 60%, depending on the surfactant. The reason for this phenomenon is not understood, but the sharp increase in reaction rate could indicated an abrupt change in micelle shape and/or orientation of 1-bromobutane solubilized in micelles, or possibly an inversion from an o/w emulsion to a w/o emulsion. Conductivity measurements, commonly used to charac-
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Figure 5. Effect of surfactant and TBAB concentration on synthesis of butyl phenyl ether at 60 °C. All solutions contained 2.0 mol/kg phenol, 2.0 mol/kg 1-bromobutane, 6.0 mol/kg NaOH, and water. Concentrations (mol/kg) of CTAC and TBAB were as follows: (b) 0.16, 0.10; (4) 0.08, 0.10; (9) 0.08, 0.05; (]) 0.08, 0.00.
terize the continuous phase in emulsions,20 were made at various times during the reaction. Water-like values were observed at all times and no abrupt change was observed, suggesting that these system remain as o/w emulsions at all times. In addition to reactant loading, the reaction rates and conversions for MPTC systems are affected by concentrations of surfactant, phase-transfer catalyst, and NaOH. For the CTAC/TBAB system, compositional effects are shown in Figure 5. The initial reaction rate was similar for all conditions; however, significant differences were observed beyond the initial period. As before, in the absence of TBAB, the reaction proceeds by conventional micellar catalysis and conversion after 400 min was only 40%. For the 0.08 CTAC/0.05 TBAB system, an abrupt increase in reaction rate was observed at 320 min (conversion 55%), and the conversion eventually plateaus above 95%. Further increases in CTAC or TBAB concentration resulted in similar autoacceleration at earlier times; the slopes also become progressively steeper, indicating higher overall reaction rates. These results illustrate the ability to achieve high rates and conversions by addition of small amounts of phase-transfer catalyst to a micellar system. Increasing the TBAB concentration increases the concentration of the TBA+-phenolate ion pair solubilized in the micelles, so that the reaction rate increases accordingly. Increasing the surfactant concentration increases the effective volume available for solubilization of 1-bromobutane and the ion pair, again resulting in increased reaction rate. Many previous studies of micellar catalysis in single-phase systems, in which the initial reactant concentrations were well below the solubilization capacity of micelles, have shown that there is an optimum surfactant concentration at which the apparent overall reaction rate is maximum. The decrease in rate at higher surfactant concentrations is primarily a dilution effect, resulting from the distribution of reactant molecules over a larger micellar volume. No such maximum was observed in this study, nor was one expected, since the presence of emulsified lipophilic reactant ensures that the micelles will remain “saturated” throughout the initial period. (20) Ross, S.; Morrison, I. Colloidal Systems and Interfaces; John Wiley & Sons: New York, 1988; pp 283-285.
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drophobic interactions between phenol and the hydrophobic groups of the micellar surfactant and the fact that hydroxide counterions are bound much less efficiently than the other counterions present in this system (chloride, bromide, phenolate). It is also reasonable to expect that the high NaOH concentrations employed here have significant influence on micellar shape and size and mass transfer rates of ions and lipophilic molecules between bulk phases and between the aqueous and micellar pseudophases. Conclusions
Figure 6. Effect of initial NaOH concentration on synthesis of butyl phenyl ether at 60 °C. All solutions contained 2.0 mol/ kg phenol, 2.0 mol/kg 1-bromobutane, 0.20 mol/kg CTAC, 0.10 mol/kg TBAB, and water. NaOH concentrations (mol/kg) were (2) 6.0, (4) 5.0, (O) 4.0, and (b) 2.0.
As shown in Figure 6, the solution alkalinity also strongly influences this reaction. The minimum required amount of NaOH is determined by the phenol concentration, since the solution must remain sufficiently alkaline during the reaction to fully convert phenol to phenolate. Figure 6 presents results for initial NaOH/phenol molar ratios of 1.0, 2.0, 2.5, and 3.0. Increasing NaOH concentration clearly exerts a strong proportional effect on the observed rates of reaction. This effect is partly due to increasing ionic strength, which promotes the formation of the TBA+-phenolate ion pair. Also, although the pH of all solutions was well above the pKa of phenol for all systems studied, the molar phenol/phenolate ratio in the micelle pseudophase may be significantly greater than that in the aqueous pseudophase due to favorable hy-
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 those observed in conventional phase-transfer or micellar catalysis. Addition of the phase-transfer catalyst TBAB to surfactant systems results in a remarkable synergism, with phenol conversions of ∼95% after 60 min in CTAC, after 250 min in DDAO, and after 400 min in SDS. Although the apparent reaction rates decrease in order CTAC > DDAO > SDS, essentially the same high conversion can be achieved using any of these surfactants. An important observation in comparing the results presented in the previous work is that the conversions reached in the emulsion systems were much higher than those achieved in the single-phase reaction systems. The emulsified phase acts as a continuous source of the lipophilic reactant and also as a sink into which the ether product preferentially partitions; since the product does not then remain solubilized in the micelles, more reactant can be solubilized and the reaction proceeds to a higher percent conversion. Acknowledgment. This work was supported by the Emission Reduction Research Center (ERRC) and the National Science Foundation (CTS-9308592 and CTS9528627). LA960712W
