Synthesis of Novel Multisite Phase-Transfer Catalysts and Their

Department of EnVironmental Engineering, Hung Kuang UniVersity, ... Department of Chemical Engineering, National Chung Cheng UniVersity, Chiayi, Taiwa...
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Synthesis of Novel Multisite Phase-Transfer Catalysts and Their Applications to the Catalyzed Reaction of Bisphenol A and Allyl Bromide Maw-Ling Wang* Department of EnVironmental Engineering, Hung Kuang UniVersity, Taichung, Taiwan 433, Republic of China

Ze-Fa Lee Department of Chemical Engineering, National Chung Cheng UniVersity, Chiayi, Taiwan 621, Republic of China

In this work, an effective and easy-preparation multisite phase-transfer catalyst has been developed. The etherification of allyl bromide with bisphenol A catalyzed by this multisite phase-transfer catalyst was carried out in the presence of alkaline solution. The reaction parameters, including distribution coefficients, masstransfer coefficients, and Damkohler numbers, were used to describe the kinetics of the etherification reaction. The object of this work is to investigate the influence of the selection of reaction parameters on the kinetics of phase-transfer catalyzed etherification. A pseudo-first-order rate law is sufficiently applied to describe the kinetic behaviors of the reactions. The investigation considered effects of the parameters and conditions, including agitation speed, amount of organic solvent, amount of phase-transfer catalyst, amount of potassium hydroxide, catalyst structure, volume of water, inorganic salt, surfactant, cocatalyst, kind of organic solvent, and temperature, on the conversion of allyl bromide in etherification. The consequence of multisite phasetransfer catalyst on the reaction rate is contemplated. It is concluded that the phase-transfer catalytic reaction is recommended to be carried at a relatively low temperature in order to avoid the Claisen rearrangement of diallyl ether bisphenol A. Introduction The applications of phase-transfer catalysts have been discussed in many reports, mostly from the point of view of the scientific features and the potential of catalysis in this field.1-3 These catalysts are highly valuable in most chemical heterogeneous processes, including liquid-liquid, solid-liquid, and gas-liquid types of reaction, where more than one phase participates. The use of phase-transfer catalyst is continuously growing since the first scientific works, in the mid-1960s.4 Currently, phase-transfer catalysis (PTC) is widely used in industrial processes.5 Recently, the economic success of chemical and related industries depends highly on the development of new processes and retrofitting old processes which are environmentally benign.6,7 In general, ethers are one of the chemicals with high added value that are extensively used in various industries as the additives of petroleum chemicals and extractants.8 The phasetransfer catalysis technique has greatly simplified the Williamson ether synthesis, which is one of the most basic organic reactions.9-11 Moreover, multisite phase-transfer catalysts have greater catalytic activity.12 However, the syntheses of “multisite” PTCs have been previous less explored than the “single-site” PTCs in the past time. In recent publications, although more sophisticated catalysts have been advocated, the benefits associated with their usage are usually overestimated, especially when the difficulties associated with their syntheses are considered. The present work uses only one synthetic step to synthesize the novel multisite PTC based on the structure of the quaternary ammonium salt. By this process, several soluble multisite ammonium catalysts have been synthesized.13,14 Such catalysts * To whom all correspondence should be sent. E-mail: chmmlw@ sunrise.hk.edu.tw.

have an advantage over single-site catalysts in reactions involving divalent anions in that, generally, less multisite quaternary ammonium salt is required to obtain a high catalytic effect. It is often found that a relatively large amount of “singlesite” quaternary ammonium or phosphonium salt must be used as a phase-transfer catalyst in the substitution reaction, in order for it to proceed rapidly enough to produce the ether in an economically feasible time period. It is of interest, therefore, to develop phase-transfer catalysts which can be used in smaller proportions. A principle object of this work, therefore, is to provide novel and efficient bis-quaternary ammonium salts and investigate their kinetics. Diallyl ether bisphenol A can be used for many high-technology applications, including epoxidized adhesives for semiconductor chips, photoresists, tough impactresistant prepregs, moldings with high breaking toughness for fiber-reinforced structures, composites with high-heat water and chemical resistance, coating with high-heat water and chemical resistance, etc.15,16 The kinetics of synthesizing diallyl ether bisphenol A from the reaction of allyl bromide and bisphenol A in an alkaline solution/chlorobenzene two-phase medium are examined in detail. Experimental Section Materials. All reagents, 3-(N,N-dimethyloctylammonio)propanesulfonate (SB-8), 4,4′-bis(chloromethyl)-1,1′-biphenyl, 4,4′-isopropylidenediphenol (bisphenol A), acetonitrile, allyl bromide, benzene, benzyltriethylammonium chloride (BTEAC), biphenyl, chlorobenzene, cyclohexane, diallyldimethylammonium chloride (DADMAC), dibutyl ether, dimethyldodecylamine oxide (DDAO), dodecyltrimethylammonium bromide (DTAB), hexane, methanol, poly(ethylene glycol) 400 (PEG 400), potassium bromide, potassium chloride, potassium hydroxide, sodium bromide, sodium chloride, sodium dodecyl sulfate (SDS), tetrabutylammonium bromide (TBAB), tetra-

10.1021/ie058074c CCC: $33.50 © 2006 American Chemical Society Published on Web 06/02/2006

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butylammonium hydroxide (TBAOH), tetrabutylammonium iodide (TBAI), tetraheptylammonium chloride (THAC), toluene, triethylamine, and triphenyltin chloride were guaranteed G. R. grade chemicals. Procedures. (a) Synthesis of bis-Quaternary Ammonium Salts 4,4′-bis(Triethylmethylammonium)-1,1′-biphenyl Dichloride (4,4′B(TEMA)-1,1′-BP DC). A mixture of 12.56 g (0.05 mol) of 4,4′-bis(chloromethyl)-1,1′-biphenyl, 10.12 g (0.10 mol) of triethylamine, and 70 mL of acetonitrile was placed in a 150mL three-necked round-bottomed Pyrex flask. The mixture solution was stirred continuously using a mechanical mixer equipped with a poly(tetrafluoroethylene) (PTFE) half-moon blade agitating at 600 rpm. The reaction was carried out at 70 °C for 24 h and was gently refluxed. Then, the crude product was evaporated in a vacuum evaporator. A white solid of 4,4′bis(triethylmethylammonium)-1,1′-biphenyl dichloride was obtained. The bis-quaternary ammonium salts were stored in CaCl2 desiccators. The products were identified with elemental analysis and melting point. The measured melting point of multisite phase-transfer catalysts is 178-180 °C. Elemental analysis of carbon, hydrogen, and nitrogen was performed with an Elementar vario EL elemental analyzer. The results obtained from elemental analysis were as follows: Cfound, 70.10%; Ccalcd, 71.57%; Hfound, 10.06%; Hcalcd, 9.90%; Nfound, 5.23%; and Ncalcd, 5.21%. (b) Synthesis of Diallyl Ether Bisphenol A and Its Purification. Measured quantities of bisphenol A (15 mmol), allyl bromide (300 mmol), cyclohexane (50 mL), tetrabutylammonium bromide (0.3224 g, 1 mmol), water (20 mL), and potassium hydroxide (16.8 g) were charged in the reactor of a 250-mL three-necked Pyrex flask. The mixed solution was stirred continuously at 600 rpm agitation speed using a Yamato LR400C stirrer equipped with a PTFE stirring blade. During this time, the reactor was submerged in a constant-temperature water bath, with a HAAKE refrigerating circulator F6-C40, in which the temperature was maintained at 30 °C and controlled to within 0.1 °C. After 2.5 h of reaction, the two-phase solution was separated and the organic portion of the solution was washed five times with an alkaline solution to remove the TBAB catalyst using a separatory funnel. Vacuum evaporation was carried out by using a Yamato RE-440 equipped with a BM400 water bath. As the organic solvent, cyclohexane, was evaporated, a residual syrupy liquid was the product, diallyl ether bisphenol A (DAEBPA). The product was identified by mass spectrum for molecular weight and NMR (1H NMR and 13C NMR) for functional groups. (c) Kinetics for the Catalyzed Reaction of Bisphenol A and Allyl Bromide. The reactor was a 150-mL three-necked Pyrex flask. Predetermined quantities of 4,4′-isopropylidenediphenol (bisphenol A), allyl bromide, potassium hydroxide, biphenyl (as internal standard), organic solvent, water, and phase-transfer catalyst were introduced into the reactor. The reactor was kept in an isothermal water bath at a constant known temperature and mechanically agitated with an electric motor. For a kinetic run, a sample (∼0.3 mL) was withdrawn from the mixed solution at a predetermined time. Then, the withdrawn sample was immediately added to 5 mL of cool methanol to quench the reaction. High-performance liquid chromatography (HPLC) was carried out by using a Shimadzu SPD-10AVP with analyzed software glass vp 5.0 with UV wavelength 204 nm. An RP-18e (5 µm) column (Applied Merck Co.) was used to separate the components and to analyze the withdrawn sample experimentally.

Reaction Kinetics and Mechanism In this work, the reaction of allyl bromide with bisphenol A was selected as a model reaction to study the catalytic reactivity of a multisite phase-transfer catalyst in an organic-alkaline mixture. The overall reaction for this present system can be expressed as

For that, a rational mechanism is proposed as follows:

The mechanism was formulated on the basis of Stark’s extraction model.1,2 Thus, for this instance, the reactive anion is produced in a base-initiated reaction by proton extraction from the substrate. An organic-soluble active catalyst, OQ(Ph)2QO, was produced from the aqueous solution by reacting the catalyst X+Q(Ph)2Q+X- (i.e., X-(Et)3N+H2C(Ph)2CH2N+(Et)3X-) and dipotassium 4,4′-isopropylidenediphenoxide (K+O-(Ph)C(CH3)2(Ph)O-K+). The concentration of OQ(Ph)2QO in the organic phase is maintained at a constant value using a large excess of bisphenol A. Then, OQ(Ph)2QO, which is transferred from the aqueous phase into the organic phase, reacts with allyl bromide. During the reaction, the product OQ(Ph)C(CH3)2(Ph)OC3H5 was not detected. This result indicates that k2 is much higher than k1. Then, OQ(Ph)C(CH3)2(Ph)OC3H5 further reacts with allyl bromide to produce diallyl ether bisphenol A. Subsequently, X-Q+(Ph)2Q+X- transfers from the organic phase to the aqueous phase and regenerates O-Q+(Ph)2Q+O- in the aqueous phase. On the basis of experimental data, no byproducts were observed. In addition, an independent experiment was carried out in which bisphenol A was hardly dissolved in pure water and in the organic solvent. Since the ion exchange in the aqueous phase is all rapid, the etherification in the organic phase is the rate-determining step for the whole reaction system. The conversion can be expressed as

-ln(1 - X) ) kappt

(1)

where X is defined as the conversion of C3H5Br, i.e.,

X)1-

[C3H5Br]o [C3H5Br]o,i

(2)

in which [C3H5Br]o,i denotes the initial concentration of allyl bromide in the organic phase. A derivation of the above results is given in Appendix A. Results and Discussion In this work, the product of diallyl ether bisphenol A was synthesized from the reaction of allyl bromide and bisphenol

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Figure 1. Effect of the agitation speed on the apparent rate constant, kapp; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 30 °C.

A by phase-transfer catalysis in an alkaline aqueous solution/ organic solvent two-phase medium. Two sequential substitution reactions were carried out in the organic phase, although only the final product diallyl ether bisphenol A was observed in the organic phase. The product was successfully separated from the reaction solution. The effects of the operating conditions on the reaction rate are discussed and summarized below. 1. Effect of the Agitation Speed. To ascertain the influence of interfacial mass-transfer resistance for the transfer of reactants to the reaction phase, the speed of agitation was varied in the range of 0-1200 rpm. Experiments with different stirrer speeds were done to verify if interfacial mass-transfer limitation occurs, and the influence of the agitation speed on the reactivity is shown in Figure 1. It can be seen that there is weak reactivity when no stirring is applied, whereas the reactivity is no longer masked by interfacial mass-transfer limitations at an agitation speed of 800 rpm or higher. The figure also shows the rate of reaction with a 6-fold variation as the agitation speed varies from 0 to 1200 rpm. Thus, the experimental results indicate that the conversion increases as the agitation speed is increased from 0 to 800 rpm, although the conversion remains almost constant when the agitation speed is >800 rpm. Therefore, it can be concluded that the interfacial mass-transfer resistance is negligible when the agitation speed is >800 rpm. To eliminate the effect of the interfacial mass-transfer resistance on the reaction, the agitation speed was fixed at 1000 rpm in the following experiments. The mass-transfer rates of the active catalysts between two phases are difficult to realize because of the difficult identification of the active catalyst during the reaction.17 2. Effect of the Amount of Chlorobenzene. Here, chlorobenzene was chosen as the organic solvent in this etherification reaction, and the volume of organic solvent was varied in range of 10-120 mL. Typical results for the apparent rate constant kapp are shown in Figure 2. In general, the volume of organic solvent affects the area of the interface between the organic phase and the aqueous phase and the concentration of the reactant and the active catalyst in the organic phase. The interfacial area between two phases is increased and the concentration of the reactant and the active catalyst is decreased with the increase in the volume of chlorobenzene. These two effects conflict in their influences on the conversion or the reaction rate, as can be seen in Figure 2, where the reaction rate decreases with the volume of chlorobenzene. It is clear that

Figure 2. Effect of the amount of chlorobenzene on the apparent rate constant, kapp; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm, 30 °C.

Figure 3. Plot of -ln(1 - X) of allyl bromide vs time with different catalysts; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of catalyst, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm, 30 °C. The error bar represents one standard deviation. Each error bar represents the standard deviation of the data for four repeating experimental runs.

the dilution effect of the concentration of reactant and active catalyst, rather than the interfacial area, dominates the reaction. The effect of interfacial area between the organic phase and the aqueous phase would be insensitive. It is clear that the experimental condition was set at high agitation speed, so the effect of interfacial area would no longer dominate the reaction rate. In other words, the dilution effect substantially surpasses the effect of interfacial area. 3. Effect of Phase-Transfer Catalyst. In this work, the newly synthesized 4,4′-bis(triethylmethylammonium)-1,1′-biphenyl dichloride (4,4′-B(TEMA)-1,1′-BP DC) was employed as the phase-transfer catalyst. Its reactivity was compared with those of the conventional phase-transfer catalysts, such as benzyltriethylammonium chloride (BTEAC), diallyldimethylammonium chloride (DADMAC), poly(ethylene glycol) 400 (PEG 400), 3-(N,N-dimethyloctylammonio)propanesulfonate (SB-8), tetrabutylammonium iodide (TBAI), tetrabutylammonium hydroxide (TBAOH), and tetraheptylammonium chloride (THAC). The results are given in Figure 3, indicating that the reaction follows a pseudo-first-order rate law. The order of the relative catalytic activity of different catalysts in the etherification of allyl bromide with bisphenol A is as follows: 4,4′-B(TEMA)-1,1′-BP DC >

Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4921 Table 1. Effect of the Novel Multisite Phase-Transfer Catalyst and the Conventional Phase-Transfer Catalysts on the Apparent Rate Constant, kappa catalyst

kapp (103 min-1)

BTEAC DADMAC PEG 400 SB-8 TBAI TBAOH THAC 4,4′-B(TEMA)-1,1′-BP DC

3.56 5.89 4.21 2.95 6.66 6.75 6.83 7.53

a Reaction conditions: 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of catalyst, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm, 30 °C.

THAC h TBAOH h TBAI > DADMAC > PEG 400 > BTEAC > SB-8. SB-8, which is a zwitterionic detergent, employed as a phase-transfer catalyst, shows the lowest catalytic activity in this etherification system, although it has good reactivity for certain reactions.18 A higher total carbon number in the alkyl groups gives a higher reaction rate, since the lipophilicity and extraction capability of the catalysts results in the catalytic effect being increased. Nevertheless, the total carbon number of DADMAC is less than that of BTEAC, but its reactivity is higher than that of BTEAC. This is due to the more lipophilic groups, allyl groups, which cause the higher extraction capability. Compared with TBAI and TBAOH, it can be seen that the anion affects the reactivity insensitively. The oxygen atom of PEG 400 would chelate with the cation and draw it into the organic phase. In addition, the anions accompany the chelating complex compound. As in the above account, the poly(ethylene glycol) also plays a role of phase-transfer catalyst. Even though its catalytic reactivity is not good, in some cases, it may be chosen as a phase-transfer catalyst because of its cost and availability. It is noteworthy that the multisite phase-transfer catalyst possesses a higher reactivity than a single-site catalyst, although the total carbon number of 4,4′-B(TEMA)-1,1′-BP DC is less than that of THAC. This could be because the large cation used as a phase-transfer catalyst does not form strong ion pairs with the nucleophile in the organic phase; so the anion behaves as a “naked” ion. In applying eq 1, the apparent rate constants for all phase-transfer catalysts which were employed in this etherification are given in Table 1. 4. Effect of the Amount of Phase-Transfer Catalyst. The effect of the amount of phase-transfer catalyst was varied over a range of 0-10 mmol. As shown in Figure 4, the apparent rate constant increases initially with an increase of the catalyst concentration. Nevertheless, the apparent rate constant would not continue to increase when the catalyst loading exceeded 4 mmol. This is because the active catalysts in the organic phase reached the saturated concentration so that adding more catalyst would not increase the apparent rate constant. Also, as shown in Figure 4, the apparent rate constant using 4 mmol increases almost 5-fold compared with that of the absence of the catalysts. Even with vigorous mixing, such systems show little tendency to react, since the nucleophile and reactant remain separated in the water and organic phases, respectively. Therefore, the multisite phase-transfer catalyst indeed greatly facilitates the etherification reaction. 5. Effect of the Amount of Potassium Hydroxide. In principle, the addition of potassium hydroxide is important for the base initial reaction to form bisphenoxide. The distribution of the active catalyst OQ(Ph)2QO between two phases and the hydration of OQ(Ph)2QO in the organic phase are influenced

Figure 4. Effect of the amount of 4,4′-B(TEMA)-1,1′-BP DC catalyst on the apparent rate constant, kapp; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm, 30 °C.

Figure 5. Plot of -ln(1 - X) of allyl bromide vs time with different amounts of potassium hydroxide; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 40 mL of water, 1000 rpm, 30 °C.

by potassium hydroxide. Thus, the effect of potassium hydroxide, which is added to the participate reaction, is investigated in this work. As shown in Figure 5, the reaction rate dose not monotonically increase or decrease with the increase in the amount of potassium hydroxide in the aqueous phase. An optimal value of the amount of potassium hydroxide to obtain a maximum value of kapp is presented. If the amount of potassium hydroxide is 20 g. This phenomenon can be attributed to the interaction of the two factors. First, the pH value of the aqueous phase is raised with adding potassium hydroxide. The solvation effect of quaternary ammonium cation is weak in the base condition, so the further addition of potassium hydroxide would reduce the hydration number of OQ(Ph)2QO and correspondingly increase the activity of OQ(Ph)2QO. Second, the solubility of bisphenol A increases in alkaline solution. Bisphenol A was practically insoluble in pure water, but it was soluble in a dilute alkaline solution. Nevertheless, the solubility of bisphenol A would decrease with a further increase in a large amount of potassium hydroxide because of the salting-out effect. When the addition

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Figure 6. Effect of the volume of water on the apparent rate constant kapp; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 1000 rpm, 30 °C.

of potassium hydroxide is >20 g, the bisphenol A would be salted out to form a white gel. The etherification would change from a liquid-liquid PTC to a liquid-solid PTC system, which would decrease the rate of the etherification reaction. Therefore, the reaction is enhanced by the addition of an appropriate amount of potassium hydroxide, but it would be retarded by using a large amount of potassium hydroxide. In the absence of potassium hydroxide, the etherification reaction would be sluggish. 6. Effect of the Amount of Water. In phase-transfer catalytic systems, water is found to have a profound effect in determining the activation of the reactant anion and the partition of the active catalyst. The effect of adding water on the reaction was also explored, and the experimental results are shown in Figure 6. A maximum apparent rate constant was obtained with an optimal amount of water. When the amount of water is 40 mL. The etherification would be sluggish in the absence of water or with minimal water. When minimal water was present in this system, the almost-naked bisphenoxide was transferred into the organic phase. However, the minimal water would not adequately dissolve the bisphenol A in the aqueous phase. Therefore, a liquid-liquid two-phase system was formed by adding more water in the range of 0-40 mL. However, when more water (40-100 mL) was introduced, the concentration of the active catalyst in the organic phase would be decreased and the overall reaction rate would be decreased. Hence, the volume of water would have an optimal value. 7. Effect of the Organic Solvents. In principle, there are several alternatives to affect the rate of the PTC reaction, from which the complicated process is considered as controlled by chemical reaction. Thus, a solvent will affect mainly the apparent rate constant kapp. The choice of organic solvent is often crucial for good extraction efficiency and high nucleophilic reactivity. Therefore, the etherification reaction of allyl bromide with bisphenol A was carried out in a variety of solvents, viz., acetonitrile, benzene, cyclohexane, dibutyl ether, hexane, toluene, and chlorobenzene. Their influences on the reactivity with results were shown in Figure 7. In general, the phase-transfer catalysts must have the ability to transfer the reactive anion into the organic, phase reacting with organic-phase reactant. Fur-

Figure 7. Plot of -ln(1 - X) of allyl bromide vs time with different organic solvents; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of organic solvent, 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm, 30 °C. The error bar represents one standard deviation. Each error bar represents the standard deviation of the data for four repeating experimental runs. Table 2. Effect of the Organic Solvents on the Organic-Phase Apparent Rate Constant, kappa organic solvent



ENT

kapp (103 min-1)

acetonitrile benzene cyclohexane dibutyl ether hexane toluene chlorobenzene

37.5 2.3 2.0 2.8 0.5 2.4 5.6

0.460 0.111 0.006 0.071 0.009 0.099 0.188

9.09 5.39 5.73 4.85 7.13 6.23 7.60

a Reaction conditions: 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of organic solvent, 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm, 30 °C.

thermore, if the anion is being held more tightly by the cation, then this would hinder the reaction. The effect of solvent on the rate of an SN2 reaction has previously been conceptually described.19 Strong solvation of the anion (including hydration) reduces the reactivity of the anion. Furthermore, the phasetransfer catalyst reaction is intrinsically limited by the reaction rate, i.e., the nonpolar solvent can promote the rate-determining step in the organic phase by reducing the extent of solvation (including hydration) of the reactant anion and increasing the concentration of the quaternary ammonium cation in the organic phase. Nevertheless, the low polarity of the solvent would provide an environment in which the catalyst may be unable to ionize both sites, and thus, the reactivity of the multisite phasetransfer catalyst is low. Therefore, the impact of the organic solvents cannot be ignored. Moreover, the organic solvents also affect the distribution of active catalyst between the organic phase and the aqueous phase. In summary, the ways in which the organic solvent affects the reactivity are complicated. The corresponding pseudo-first-order constants are given in Table 2, which shows that the order of the relative activities of these organic solvents is as follows: acetonitrile > chlorobenzene > hexane > toluene > cyclohexane > benzene > dibutyl ether. It could be found that the reactivity is not simply monotonically increasing with dielectric constants or normalized Reichardt’s solvatochromic parameters.20 8. Effect of the Inorganic Salt. In the past reports, an inorganic salt was usually added to enhance the reaction. These inorganic salts affect not only the concentration of the

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Figure 8. Plot of conversion of allyl bromide vs time with different inorganic salts added; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 0.1 mol of inorganic salt, 1000 rpm, 30 °C.

Figure 9. Plot of conversion of allyl bromide vs time with different surfactants; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 2 mmol of surfactant, 1000 rpm, 30 °C. The error bar represents one standard deviation. Each error bar represents the standard deviation of the data for four repeating experimental runs.

active catalyst in the organic phase but also the reaction environment. Figure 8 indicates the effect of reaction rate influenced by the addition of inorganic salt in the present etherification. However, the addition of inorganic salt shows no benefit on the reaction rate. Here, it can be seen that the addition of potassium chloride, potassium bromide, sodium chloride, and sodium bromide would slightly decrease the conversion of allyl bromide. It is conjectured that the saltingout effect would retard the concentration of the active catalyst in the organic phase. Hence, the reaction rate would decrease as the inorganic salt is added. 9. Effect of the Surfactant. The effect of the addition of different surfactants on the reaction rate was also investigated in this work. Cationic (dodecyltrimethylammonium bromide, DTAB), anionic (sodium dodecyl sulfate, SDS), and nonionic (dimethyldodecylamine oxide, DDAO) surfactants were used for comparison. Figure 9 presents the catalytic effects observed for the etherification of allyl bromide with bisphenol A in the two-phase reaction systems of phase-transfer catalyst alone and surfactant plus phase-transfer catalyst. The use of a two-phase

system with added phase-transfer catalyst and the use of a microemulsion are two alternative approaches to overcome reactant incompatibility problems in organic synthesis. Both routes have proved useful, but for entirely different reasons. In phase-transfer catalysis, the nucleophilic reactant is carried into the organic phase, where it becomes highly reactive. However, there is no transfer of reactant from one environment to another in the microemulsion approach. The success of the method relies on the very large oil-water interface at which the reaction occurs.21,22 Here, the phase-transfer catalyst and surfactant are added together in this etherification in order to combine the two approaches into one; i.e., to carry out a nucleophilic substitution reaction in a microemulsion in the presence of phase-transfer catalysts. It can be seen that the order of the reactivity is as follows: CTAC/PTC > SDS/PTC > DDAO/ PTC > PTC alone. Here, it can be found that the reactivity of DDAO/PTC is higher than that of PTC only. This is due to the fact that the nonionic surfactant would substantially extend the oil-water interfacial area. The etherification was, thus, carried out in the microemulsion. Furthermore, the reactivity of cationic or anionic surfactants is superior to that of nonionic surfactant. The surfactant plays multiple roles in these reaction systems: emulsifier, solubilizing agent, and possibly even phase-transfer catalyst. The presence of SDS would lead to a remarkable decrease in surface tension and enhanced solubilization power in the presence of phase-transfer catalyst. It is conjectured that this is due to electrostatic interaction, since the electrical repulsion will prevent the micellar contact. Here, essentially higher conversion can be achieved using any of these surfactants. Thus, this study shows that a substitution reaction performed in a phase-transfer catalytic system can be accelerated by addition of a surfactant. 10. Effect of the Cocatalyst. A previous paper reported that fluorination of alkyl halides or sulfonate with KF can be efficiently executed using a liquid-solid PTC methodology in the presence of additional cocatalysts, triorganotin halides.23 Here, the triphenyltin chloride was employed as cocatalyst in the etherification reaction with phase-transfer catalyst. The application of this process in the etherification reaction catalyzed by triorganotin chloride is shown in Scheme 1. The reacting anions are converted into lipophilic anions via complexation with multisite phase-transfer catalyst and cocatalyst so that it would be more lipophilic for entering the organic phase. After reaction, the multisite phase-transfer catalyst and cocatalyst would both return to the aqueous phase to regenerate the active catalyst. Figure 10 presents the conversion in three reaction systems: phase-transfer catalyst and cocatalyst, phase-transfer catalyst alone, and no phase-transfer catalyst and cocatalyst. It shows that the conversion in the absence of phase-transfer catalyst and cocatalyst reaction system is lower than that in the presence of phase-transfer catalyst and cocatalyst or phasetransfer catalyst alone. Moreover, it is also found that the conversion in the reaction system with phase-transfer catalyst and cocatalyst is greater than that of phase-transfer catalyst alone. Therefore, the etherification reaction can be accelerated by addition of the cocatalyst, triphenyltin chloride, in the phasetransfer catalytic system. In general, triphenyltin chloride will react with hydroxide ion. Then, hydroxide ion will be brought to the organic phase. However, alkoxide ion is a strong nucleophilic reagent in comparing the reactivities of alkoxide and hydroxide ion. Therefore, the schematic reaction diagram only shows that the organic substrate reacts with the alkoxide ion for simplicity, to avoid the complicated reaction.

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Scheme 1. Reaction Mechanism in the Presence of Phase-Transfer Catalyst and Cocatalyst

11. Effect of the Temperature. The etherification reaction was also performed at different temperatures, and there was a resulting increase in the temperature at which the reaction rate intensified, as expected. This result is due to the increased temperature facilitating the nucleophilic substitution reaction, thus enhancing the reaction rate. However, a high temperature usually favors the side reactions. At higher temperatures, the product diallyl ether bisphenol A would produce Claisen rearrangement, which is an intramolecular rearrangement,24,25 so a lower temperature is preferred. In addition, to avoid the evaporation of allyl bromide, the temperature must also not be too high, preferably under 30 °C. The results are depicted in an Arrhenius plot to calculate the apparent activation energy (see Figure 11). If the reaction was under the mass-transfer-control condition, the activation energy would be 8 kcal/mol.26 The apparent

Figure 10. Plot of conversion of allyl bromide vs time with cocatalyst; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′-B(TEMA)-1,1′BP DC, 20 g of potassium hydroxide, 40 mL of water, 1 mmol of cocatalyst, 1000 rpm, 30 °C.

activation energy ) 13 kcal/mol. This value supports the conclusion that there are no interfacial mass-transfer limitations and the etherification reaction is under the chemical reaction control with the high agitated speed. Conclusions The catalytic reactivity of multisite phase-transfer catalysts has been investigated in the reaction of allyl bromide with bisphenol A in an organic-alkaline two-phase medium. The reaction mechanism has been proposed, and a kinetic model accounting for the observed rate has been developed. The results indicate that the novel multisite phase-transfer catalyst is effective in the etherification. A larger amount of phase-transfer catalyst would promote the etherification, but when it exceeds the amount of phase-transfer catalyst added, it does not exhibit further promoting effects. When the agitation speed exceeds 800 rpm, the mass-transfer resistance at the alkaline/organic

Figure 11. Arrhenius plot for ln(kapp) vs 1/T; 15 mmol of allyl bromide, 25 mmol of bisphenol A, 3 mmol of internal standard (biphenyl), 20 mL of chlorobenzene, 1 equiv mmol of 4,4′-B(TEMA)-1,1′-BP DC, 20 g of potassium hydroxide, 40 mL of water, 1000 rpm.

Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4925

interface can be ignored. The amounts of potassium hydroxide and water also exhibit optimal values in enhancing the reaction. Acetonitrile is a better organic solvent in terms of obtaining higher conversion. The addition of inorganic salt decreases the reactivity of the etherification reaction for the salting-out effect. Systems containing a phase-transfer catalyst and a surfactant exhibited higher conversions than those of systems with phasetransfer catalyst alone. Etherification is decreased by adding a larger amount of chlorobenzene. Triphenyltin chloride was chosen as the cocatalyst in this etherification system, which shows quite sufficient catalytic reactivity. An appropriate temperature is recommended to prevent side reaction and the significant vaporization of allyl bromide.

denote the characteristics of the species in the bulk of organic and aqueous phases, respectively. Furthermore, k1 and k2 are the intrinsic rate constants of the two sequential reactions in the organic phase, while kaq,1 and kaq,2 are the intrinsic rate constants of the two ionic reactions in the aqueous phase. A is the interfacial area between two phases, and Q0 is the total catalyst. KOQ(Ph)2QO and KXQ(Ph)2QX are the mass-transfer coefficients of OQ(Ph)2QO and XQ(Ph)2QX between the two phases. MOQ(Ph)2QO and MXQ(Ph)2QX are the distribution coefficients of OQ(Ph)2QO and XQ(Ph)2QX between two phases, respectively, i.e.,

MOQ(Ph)2QO )

Acknowledgment The authors would like to thank the National Science Council for financial support under Grant No. NSC93-2811-E-241-001. Appendix A: Reaction Kinetics On the basis of the proposed mechanism and two-film theory, the material balances for the regenerated XQ(Ph)2QX and the active catalyst, OQ(Ph)2QO, in the organic and aqueous phase areas follows:

d[OQ(Ph)2QO]o ) dt

(

KOQ(Ph)2QOA [O-Q+(Ph)2Q+O-]a -

)

k1[C3H5Br]o[OQ(Ph)2QO]o (A-1) d[O-Q+(Ph)2Q+O-]a ) dt kaq,2[K+O-(Ph)C(CH3)2(Ph)O-K+]a[X-Q+(Ph)2Q+X-]a [OQ(Ph)2QO]o (A-2) KXQ(Ph)2QXAf [O-Q+(Ph)2Q+O-]a MOQ(Ph)2QO

)

d[XQ(Ph)2QX]o ) k2[C3H5Br]o[OQ(Ph)2QX]o dt KXQ(Ph)2QXA([XQ(Ph)2QX]o -

+

+

-

+

-

+

+

d[X-Q+(Ph)2Q+X-]a d[XQ(Ph)2QX]o ) 0, )0 dt dt

(

k1 Q0 ) 1+ + Vo k2 k

)

aq,2[K

+

O-(Ph)C(CH3)3(Ph)O-K+]a

(

-

kaq,2[K O (Ph)C(CH3)3(Ph)O-K+]a

-

Q0 ) Vo([OQ(Ph)2QO]o + [OQ(Ph)2QX]o + +

fMXQ(Ph)2QXk1[C3H5Br]o

+

k1[C3H5Br]o k1[C3H5Br]o 1 1 [OQ(Ph)2QO]o + + KXQ(Ph)2QXA f MOQ(Ph)2QO KOQ(Ph)2QOA

The total amount of multisite phase-transfer catalyst Q0 in the solution is +

(A-8)

Combining eqs A-1-A-5 and A-8, we obtain the following:

kaq,2[K Q (Ph)C(CH3)2(Ph)O K ]a[X Q (Ph)2Q X ]a (A-4)

-

(A-7)

)

[OQ(Ph)2QO]o (A-9)

The following Damkohler numbers, DaOQ(Ph)2QO and DaXQ(Ph)2QX, are defined as

-

-

[X-Q+(Ph)2Q+X-]a,s

fk1[C3H5Br]o

d[X Q (Ph)2Q X ]a ) KXQ(Ph)2QXAf([XQ(Ph)2QX]o dt MXQ(Ph)2QX[X-Q+(Ph)2Q+X-]a) +

[XQ(Ph)2QX]o,s

(A-6)

On the basis of the experimental observation, the concentrations of OQ(Ph)2QO and XQ(Ph)2QX in the organic and aqueous phases reach constant values at the beginning of the reaction.27,28 Therefore, a pseudo-steady-state hypothesis (PSSH) is applied, i.e.,

+

MXQ(Ph)2QX[X-Q+(Ph)2Q+X-]a) (A-3) -

[O-Q+(Ph)2Q+O-]a,s

d[O-Q+(Ph)2Q+O-]a d[OQ(Ph)2QO]o ) 0, )0 dt dt

[OQ(Ph)2QO]o MOQ(Ph)2QO

(

MXQ(Ph)2QX )

[OQ(Ph)2QO]o,s

-

[XQ(Ph)2QX]o) + Va([O Q (Ph)2Q O ]a + [X-Q+(Ph)2Q+X-]a) (A-5) where f is the volume ratio of the organic solution (Vo) to the aqueous solution (Va). Ph and Q denote -C6H4- and -CH2N(Et)3 groups, respectively. The subscripts, o and a,

DaOQ(Ph)2QO )

k1[C3H5Br]o KOQ(Ph)2QOA

(A-10)

DaXQ(Ph)2QX )

k1[C3H5Br]o KXQ(Ph)2QXA

(A-11)

R is defined as the ratio of the organic-phase reaction rate to the aqueous-phase reaction rate, i.e.,

R)

k1[C3H5Br]o kaq,2[K+O-(Ph)C(CH3)2(Ph)O-K+]a

Combining eqs A-9-A-11, we have the following:

(A-12)

4926

Q0 ) Vo

Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006

{(

1+

)

k1 1 + + (1 + fMXQ(Ph)2QX)(R) + k2 fMOQ(Ph)2QO

(1 + 1f )(Da

OQ(Ph)2QO

}

+ DaXQ(Ph)2QX) [OQ(Ph)2QO]o (A-13)

Several experiments were carried out to measure the OQ(Ph)2QO in the aqueous and organic phases. It is found that OQ(Ph)2QO keeps at a constant value after 1 min of reaction. This result indicates that the mass-transfer rates of OQ(Ph)2QO and XQ(Ph)2QX between the two phases are rapid compared to that of the organic-phase reaction. Furthermore, the rates of aqueous ion exchange are faster than those of the organic phase. On the basis of the experimental evidence, both the Damkohler numbers as well as R are small. Therefore, eq A-13 can be simplified to

[OQ(Ph)2QO]o )

fk2MOQ(Ph)2QO

Q0 k2 + (k1 + k2)fMOQ(Ph)2QO Vo

(A-14)

Moreover, material balances for the compounds in the reaction solution are as follows:

-

d[C3H5Br]o ) k1[C3H5Br]o[OQ(Ph)2QO]o + dt k2[C3H5Br]o[OQ(Ph)2QX]o (A-15)

d[OQ(Ph)2QX]o ) k1[C3H5Br]o[OQ(Ph)2QO]o dt k2[C3H5Br]o[OQ(Ph)2QX]o (A-16) d[H5C3O(Ph)C(CH3)2(Ph)OC3H5]o ) dt k2[C3H5Br]o[OQ(Ph)2QX]o (A-17) Combining eqs A-14 and A-16, and applying the PSSH approach, eq A-15 can be written as

-

d[C3H5Br]o ) kapp[C3H5Br]o dt

(A-18)

where kapp is the apparent rate constant and is defined as

kapp ) 2k1[OQ(Ph)2QO]o )

2fk1k2MOQ(Ph)2QO

Q0 k2 + (k1 + k2)fMOQ(Ph)2QO Vo (A-19)

Integrating eq A-18 yields

-ln(1 - X) ) kappt

(A-20)

where X is defined as the conversion of C3H5Br, i.e.,

X)1-

[C3H5Br]o [C3H5Br]o,i

(A-21)

in which [C3H5Br]o,i denotes the initial concentration of allyl bromide in the organic phase. Thus, the value of kapp can be obtained from experimental data in conjunction with eq A-20.

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Literature Cited (1) Starks, C. M.; Liotta, C. L.; Halpern, M. Phase-Transfer Catalysis: Fundamentals, Applications and Industrial PerspectiVes; Chapman and Hall Publications: New York, 1994.

ReceiVed for reView September 19, 2005 Accepted April 29, 2006 IE058074C