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Ind. Eng. Chem. Res. 2009, 48, 1376–1383
Kinetic Study of Synthesizing Dimethoxydiphenylmethane under Phase-Transfer Catalysis and Ultrasonic Irradiation Maw-Ling Wang*,† and Wei-Hung Chen‡ Department of EnVironmental Engineering, Hungkuang UniVersity, Shalu, Taichung County, 43302, Taiwan, Republic of China, and Department of Chemical Engineering, National Chung Cheng UniVersity, Ming-Hsiung, Chiayi County, 62101, Taiwan, Republic of China
The synthesis of dimethoxydiphenylmethane (DMODPM) from the reaction of methanol and dichlorodiphenylmethane (DCDPM) was successfully carried out in a liquid-liquid phase-transfer catalytic (LLPTC) reaction assisted by ultrasonic irradiation. Little of the desired product DMODPM was obtained from the reaction in the presence of only potassium hydroxide and quaternary ammonium salts. The production of the DMODPM product was greatly enhanced when the reaction system was irradiated by ultrasonic waves. Two sequential reactions in the organic-phase solution proceed to produce the desired product. However, the second rate of the organic-phase reaction was faster than the first. Therefore, the first intermediate product, chloromethoxydiphenylmethane (CMODPM), was not detected during or after the reaction. A kinetic model was developed based on the experimental data. A pseudo-steady-state hypothesis (PSSH) was employed to describe the characteristic behaviors of the kinetic data. The results were finally described by a pseudo-first-order rate law, from which the apparent rate constant (kapp,1) of the first organic-phase reaction was obtained. The effects of the reaction conditions on the conversion of DCDPM and the reaction rate were investigated in detail, including the agitation speed; the amounts of methanol, DCDPM, water, chlorobenzene, potassium hydroxide, and tetrabutylammonium bromide (TBAB); the ultrasonic power and frequency; the organic solvents; the quaternary ammonium salts; and the temperature. Rational explanations are provided for the experimental results. Introduction In principle, it is difficult to obtain a high yield of product or a large conversion from two or more reactants that are immiscible in solution. The two main reasons for this are that the contact area of these two reactants is small because of the limited interface, and that the activity of the reagents is low. Past efforts to enhance the conversion or to increase the reaction rate including the use of protic and aprotic solvents, high agitation speeds, and elevated reaction temperatures have produced limited results. In addition, such approaches entail side effects, which cause the reaction be carried out under anhydrous conditions in order to use aprotic solvents, produce byproducts as a result of side reactions at high temperature, and increase the difficulty in separating the product from solution. In contrast, phase-transfer catalysis (PTC)1-9 is now well recognized as an invaluable methodology for organic synthesis from two or more immiscible reactants, and its scope and application are the subjects of current research.10-13 Benzophenone dimethyl ketal is a specialty chemical with high added value, and its synthesis has received much attention in recent years. Currently, it is widely used for synthetic flavors, cosmetic reagents, photostabilizers in plastics, and pharmaceuticals. The conventional method for synthesizing ketals is to react a ketone and an alcohol under acid-catalyzed conditions. The main restriction in this process is the difficulty in forming a homogeneous phase for use of high-carbon ketones. Hence, the reactivity is low for the reaction of alcohols and ketones with high carbon numbers. In addition, the ketal compound is * To whom correspondence should be addressed. Tel.: +886-4-26318652ext 4160. Fax: +886-4-2652-9226. E-mail: chmmlw@ sunrise.hk.edu.tw. † Hungkuang University. ‡ National Chung Cheng University.
unstable in acidic solution, as the acid-catalyzed hydrolysis of the ketal takes place accompanying the forward reaction in synthesizing the ketal, so that only a 50% yield is obtained using the conventional acid-catalyzed conditions to produce the desired product. Recently, there has been much research to develop high-reactivity acid catalysts, although there has been limited success in obtaining ketals in high yield. In this work, dimethoxydiphenylmethane (DMODPM) (or benzophenone dimthyl ketal) was synthesized from the reaction of methanol and high-carbon-number dichlorodiphenylmethane (DCDPM) catalyzed by a quaternary ammonium salt and assisted by ultrasonic irradiation in a two-phase medium consisting of an aqueous solution of KOH and an organic solvent. The chemical effects of ultrasound have been attributed to intense local conditions generated by cavitation bubble dynamics in the ultrasonic field.14,15 Because of ultrasound in liquid-liquid phase-transfer catalysis (LLPTC) systems, cavitational collapse near the liquid-liquid interface disrupts the interface and impels jets of one liquid into the other, forming fine emulsions.16-28 This greatly increases the interfacial contact area across which transfer of species can take place and hence promotes the reaction. The reaction in this study was carried out in a basic solution, so that hydrolysis of the ketal product in acidic solution could be avoided. Moreover, the phasetransfer-catalyzed approach to the synthesis of DMODPM not only enhances the reaction and increases the yield, but also minimizes the byproducts. Based on the experimental data, a kinetic model was developed with which a pseudo-steady-state hypothesis (PSSH) was employed to describe the reaction. A pseudo-first-order rate law was used to describe the kinetic behaviors. The effects of the reaction conditions on the conversion of DCDPM, as well as the apparent rate constant (kapp,1) of the first reaction in the organic-phase solution, were investigated in detail. The results are explained satisfactorily
10.1021/ie8011616 CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1377
by considering the interaction between the reactants and the environmental species. Experimental Section 1. Materials. Dichlorodiphenylmethane (DCDPM); methanol (MeOH); potassium hydroxide; phase-transfer catalysts including benzyltriethylammonium bromide (BTEAB), tertraoctylammonium bromide (TOAB), tetrahexylammonium bromide (THAB), tetrabutylammonium bromide (TBAB), tetraethylammonium bromide (TEAB), and 4-(tributylammonium) propansultan (QSO3); organic solvents including methyl ethyl ketone (MEK), o-xylene, tributylamine (TBA), dibutyl ether, and chlorobenzene (C-benz); and other reagents used were all G.R.grade chemicals for synthesis. 2. Procedures. A. Synthesis of the Active Intermediate [MeOQ, CH3O-N(C4H9)4]. Methanol is soluble in the aqueous solution of KOH to produce potassium methanoxide (MeOK). Known quantities of TBAB (i.e., QBr) and MeOK were mixed in the aqueous solution to produce the active intermediate in the form of MeOQ (QOR). The product was then identified by its NMR spectrum (13C NMR, CDCl3, ppm), in which five peaks appeared with chemical shifts (δ) between 19.55-59.02 ppm shown. This confirmed that the reaction of potassium methanoxide (MeOK) and TBAB (or QBr) catalyst occurred in the aqueous solution to produce the active intermediate MeOQ. B. Identification of the Alkylation of DCDPM Product (DMODPM). Normally, one or two chlorides on the DCDPM molecule are available to be alkylated by one or two methanoxides. Therefore, there should be two substituted products, i.e., monochloro- and dichloro-substituted products. In general, the reactions are sequential substitutions. However, the second reaction is faster than the first, so that the monochloro-substituted product chloromethoxydiphenylmethane (CMODPM) was not observed during or after the reaction. Thus, only the product dimethoxydiphenylmethane (DMODPM) in which the two chlorides of DCDPM were substituted was found in the solution. In this work, the product was identified by both NMR spectroscopy and gas chromatography-mass spectrometry (GCMS). In the NMR analysis, the chemical shifts (δ) were 7.60-7.38 (d, 4H, sPh), 7.38-7.34 (t, 4H, sPh), 7.30-7.26 (t, 2H, sPh), and 3.20 (s, 6H, sCH3). The ratio of hydrogen a/b/c/d was 2:2:1:3. In the GC-MS analysis, the main charge/ mass ratio of the chemical formula was 228, which appeared as the M peak in the spectrum. The daughter ions after fragmentation were observed at 228, 197, 151, 105, and 77. C. Kinetics of Synthesizing the DMODPM Product. The reactor was a 150-mL four-necked Pyrex flask, permitting agitation of the solution, insertion of a thermometer, removal of samples, and feeding of the reactants. A reflux condenser was attached to the port of the reactor to recover the species from the gas phase. The reactor was submerged in a constanttemperature water bath with the temperature controlled to (0.1 °C. To start an experimental run, known quantities of methanol, toluene (internal standard), and potassium hydroxide (KOH) were dissolved in the organic-phase solution (chlorobenzene and water mixture) and introduced into the reactor. The liquid solution was stirred mechanically with a two-bladed paddle (5.5 cm) at 400 rpm. Then, a mixture of DCDPM, tetrabutylammonium bromide (TBAB), and chlorobenzene in the liquid phase was introduced into the reactor to initiate the reaction. During the reaction, a sample of 0.1 mL was withdrawn from the solution at a chosen time. The sample was immediately poured into chlorobenzene at 4 °C for dilution and retardation of the reaction and then analyzed by GC.
The product DMODPM for identification was purified from the reaction solution by vacuum evaporation to strip off the organic solvent. It was then recrystallized from dichloromethane as white crystals. The product DMODPM and the reactants (DCDPM and methanol) were all identified by GC-MS and NMR and IR spectroscopies. Their concentrations (or contents) were analyzed with a model GC17A instrument (Shimadzu). The stationary phase was 100% poly(dimethylsiloxane). The carrier gas was N2 (30 mL/min). The column was db-1 type. The results obtained from the instrumental analyses are consistent with those reported in the literature. Reaction Mechanism and Kinetic Model In this work, the overall reaction of methanol and dichlorodiphenylmethane (DCDPM) catalyzed by tetrabutylammonium bromide (TBAB) in the presence of a KOH solution/organic solvent two-phase medium can be expressed as
According to our observations, methanol first dissolves and reacts with KOH to produce potassium methanoxide (CH3OK or MeOK) in the aqueous solution. Then, CH3OK further reacts with TBAB catalyst (QBr) to form tetrabutylammonium methanoxide (MeOQ or QOR), which is an active organic-soluble intermediate. This active intermediate (MeOQ) then reacts with DCDPM through two sequential reaction steps in the organic phase to produce the desired product, dimethoxydiphenylmethane (DMODPM). Thus, the reaction mechanism of the overall reaction is expressed as
where ROH, ROK, and QOR represent methanol, potassium methanoxide, and tetrabutylammonium methanoxide, respectively; CMODPM and DMODPM are the monochloro-substituted (chloromethoxydiphenylmethane) and dichloro-substituted (dimethoxydiphenylmethane) products, respectively; and QX is the quaternary ammonium salt, where X can be either chloride or bromide. k1 and k2 are the two intrinsic rate constants of the organic-phase reactions. For a two-phase phase-transfer catalytic reaction, the rate is usually determined by four steps, i.e., (a) the ionic aqueous-phase reaction, (b) the organic-phase reaction, (c) the mass transfer of species QOR (active intermediate) from the aqueous phase to the organic phase, and (d) the mass transfer of species of the regenerated catalyst QBr from the organic phase to the aqueous phase. The mass transfers of species from the aqueous phase to the organic phase and vice versa are all fast. The ionic aqueous-phase reaction is also very fast. Therefore, it is obvious that the organic-phase reaction, which is usually slow, is the rate-determining step. The ionic reaction in aqueous solution is fast. Also, MeOQ formed from the reaction of potassium methoxide and tetrabutylammonium bromide (TBAB) is an organic-soluble compound. The transfer of MeOQ from the aqueous phase to the organic
1378 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
phase is also fast. Therefore, the two sequential reactions in the organic phase are the rate-determining steps for the whole reaction. From the GC spectrum of the reaction solution, only DMODPM product was observed, and no CMODPM was observed. This fact indicates that the second reaction is faster than the first one. Following the Bodenstein steady-state assumption, the production rate of CMODPM equals the consumption rate of CMODPM in the reaction solution (from the reaction mechanism). Once CMODPM is produced, it reacts with QOR very quickly to produce the final product DMODPM in the second reaction of the organic phase. Thus, the first reaction in the organic phase is the rate-determining step. Also, CMODPM was not observed (or detected). Thus, the rate of the change of CMODPM with respect to time was set to be zero, as shown in eq 1. Consequently, the first reaction in the organic phase is the rate-determining step. Thus, we have d[CMODPM]o )0 dt
(1)
where the subscript o denotes the species in the organic solution. The material balances for DCDPM, CMODPM, and DMODPM in the organic-phase solution are -
d[DCDPM]o ) k1[DCDPM]o[QOR]o dt
(2)
d[CMODPM]o ) k1[DCDPM]o[QOR]o dt k2[CMODPM]o[QOR]o (3) d[DMODPM]o ) k2[QOR]o[CMODPM]o dt Combining eqs 1 and 3 yields [CMODPM]o )
k1 [DCDPM]o k2
(4)
(5)
From eqs 2, 4, and 5, we obtain -
d[DCDPM]o d[DMODPM]o ) dt dt
(6)
This result indicates that the consumption rate of DCDPM equals the production rate DMODPM in the organic phase. No other byproducts were observed during or after the reaction. Therefore, by integrating the equation after combining eqs 2, 5, and 6, we have -ln(1 - X) ) kapp,1t
(7)
where kapp,1 is the apparent rate constant and X is the conversion of DCDPM, i.e. X)
[DMODPM]o [DCDPM]o,i - [DCDPM]o ) [DCDPM]o,i [DCDPM]o,i kapp,1)k1[QOR]o
(8) (9)
The subscript i represents the initial conditions of the species. The rate at which the reactant DCDPM is consumed can be calculated from eq 2, and the rate at which the final product DMODPM is produced can be calculated from eq 4. If we apply the pseudo-steady-state approach, the result obtained from eq 2 is the same as the result obtained from eq 4. Therefore, it is important to obtain k1 and [QOR]o. From our derivation shown in eq 9, kapp,1 ) k1[QOR]o. Therefore, the rate of the final product DCDPM from eq 2 can be calculated.
Figure 1. Effect of the agitation speed on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 300 W, 40 kHz.
As shown in eq 7, it is obvious that the reaction follows a pseudo-first-order rate law. The kapp,1 values were obtained by plotting the experimental data for -ln(1 - X) vs time (t). Thus, the reaction rate was calculated from eq 2. Results and Discussion In the absence of KOH and phase-transfer catalyst, the reaction of methanol and dichlorodiphenylmethane (DCDPM) was observed. However, the products were distributed and were not expected to be the desired ones. The DMODPM product was obtained in a small quantity when KOH was added to the reaction solution of methanol and dichlorodiphenylmethane. The desired DMODPM product was obtained in a minor quantity for reaction times longer than 12 h, and the reaction was also enhanced by addition of phase-transfer catalyst. However, the reaction rate was still low. To enhance the reaction rate greatly, ultrasonic irradiation was used to stimulate the reaction. The results obtained under various conditions are discussed in the following sections. 1. Effect of the Agitation Speed. In general, the function of agitation is to provide a well-mixed solution and to increase the interfacial area between the two phases. In general, the interfacial area between two phases is affected by the amount of water, as well as by agitation and the use of ultrasound. Therefore, it is important to study the effect of the agitation speed on the reaction. The effect of the agitation speed on the conversion of DCDPM is shown in Figure 1. The experimental data follow a pseudo-first-order rate law. As shown in Figure 1, the reaction is still enhanced by ultrasonic irradiation even when the reaction solution is not agitated by the stirrer. Therefore, the agitation and the ultrasonic irradiation are synergistic in enhancing the reaction. The conversion is increased with increased agitation speed from 0 to 200 rpm. For agitation speeds greater than 200 rpm, the conversion is nearly independent of the agitation speed. This verifies that the reaction of MeOQ and DCDPM, which was carried out in the homogeneous organic-phase solution, was the rate-determining step; i.e., MeOK first reacts with QBr quickly to form MeOQ, which then transfers rapidly to the organic-phase solution, where it reacts with DCDPM in the organic phase. The purpose of stirring is to provide both good mixing for the formation of MeOQ and rapid mass transfer of MeOQ from the aqueous phase to the organic phase. Table 1 shows the dependence of
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1379 Table 1. Effects of the Reaction Conditions on the Apparent Rate Constants (kapp,1) for the Ultrasound-Assisted Phase-Transfer Catalytic Reaction of MeOH and DCDPMa agitation speed (rpm) kapp,1 × 103 (min-1)
0 8.1
50 9.2
100 10.0
200 10.5
300 10.4
400 10.3
amount of MeOH (g) kapp,1 × 103 (min-1)
0.3 0.7
0.6 1.1
2.0 4.0
3.0 10.3
4.0 12.3
4.5 13.3
amount of TBAB (g) kapp,1 × 103 (min-1)
0.0 2.1
0.025 2.7
0.5 3.7
0.09 5.3
0.13 6.6
0.228 10.3
amount of KOH (g) kapp,1 × 103 (min-1)
1.0 1.3
2.5 3.8
3.7 6.2
5.0 10.3
5.5 12.1
6.2 15.8
amount of H2O (mL) kapp,1 × 103 (min-1)
10 10.3
15 10.2
20 9.0
25 8.1
30 5.9
35 4.0
amount of DCDPM (mmol) kapp,1 × 103 (min-1)
0.35 2.1
0.7 5.0
1.35 7.7
2.0 9.1
2.7 10.3
4.4 12.0
amount of chlorbenzene (mL) kapp,1 × 103 (min-1)
20 12.2
30 11.7
40 10.3
50 7.4
60 5.5
70 4.2
ultrasonic power (W) kapp,1 × 103 (min-1)
100 4.0
200 7.1
300 10.3
400 12.4
500 14.2
600 15.2
ultrasonic frequency (kHz) kapp,1 × 103 (min-1)
50 10.8
40 10.3
35 9.8
33 8.9
28 8.2
20 7.1
PTC catalyst (0.7 mmol) kapp,1 × 103 (min-1)
TOAB 12.4
THAB 12.0
TBAB 10.3
BTEAB 6.7
TEAB 5.3
QSO3 2.7
organic solvent (40 mL) kapp,1 × 103 (min-1)
MEK 18.7
C-benz 10.3
dibutyl ether 6.8
o-xylene 5.8
benzene 5.1
TBA 4.9
temperature (°C) kapp × 103 (min-1)
35 0.65
40 1.31
45 1.81
50 4.02
55 6.05
60 10.06
5.0 13.8
6.0 14.2
a Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 300 W, 40 kHz.
the kapp,1 value on the agitation speed. It can be seen that the kapp,1 value was slightly affected by the agitation speed up to 200 rpm. With further increases in the agitation speed above 200 rpm, the kapp,1 value was almost not influenced by the agitation speed. In studying the kinetics, we found that no further improvement in the conversion of reactant could be obtained by increasing the agitation speed above 400 rpm in either the presence or absence of ultrasonic irradiation. To obtain the kinetic data that were not influenced by stirring, the agitation speed was kept at 400 rpm to investigate the effects of the other reaction conditions on the conversion of DCDPM and the reaction rate in the subsequent experiments. 2. Effect of the Amount of Methanol. In this work, methanol, which acts as the aqueous-phase reactant, was used in large excess relative to the amount of DCDPM. The effect of the amount of methanol on the conversion of DCDPM is shown in Figure 2. The experimental data also follow a pseudofirst-order rate law. The conversion increased greatly as the amount of methanol was increased from 0.3 to 3.0 g. However, the rate of the conversion decreased with larger amounts of methanol from 3.0 to 6.0 g. The dependence of the kapp,1 value on the amount of methanol is shown in Table 1. The tendency for the change of the kapp,1 value with the amount of methanol is the same as that of the conversion with the amount of methanol. 3. Effect of the Amount of TBAB Catalyst. As stated, the purpose of adding TBAB is to promote the reaction and increase the conversion of DCDPM. Also, the reaction of methanol and DCDPM in a two-phase medium consisting of an aqueous solution of KOH and an organic solvent takes place even in the absence of TBAB. However, the reaction is greatly increased when a catalytic amount of TBAB catalyst is added to the reaction solution. Figure 3 shows the effect of the amount of TBAB catalyst on the conversion. The experimental data follow
Figure 2. Effect of the amount of methanol on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
a pseudo-first-order rate law. The conversion increases with increasing amount of TBAB catalyst. The corresponding kapp,1 values vs the amount of TBAB catalyst, which exhibit a linear relation, are reported in Table 1. 4. Effect of the Amount of KOH. In principle, KOH affects the distribution of the TBAB catalyst (QBr) and the active intermediate (MeOQ) between two phases, as well as the solubility of methanoxide (CH3Os) in the organic phase. The reason is that KOH reacts with methanol to form CH3OK, and then CH3OK further reacts with quaternary ammonium salts (QBr) to produce an active intermediate (CH3OQ or QOR). It is obvious that the amount of KOH directly affects the amount of CH3OK and, hence, the amount of CH3OQ (QOR) produced
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Figure 3. Effect of the amount of TBAB catalyst on the conversion of DCDPM. Conditions: 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
Figure 4. Effect of the amount of KOH on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
in the aqueous phase. Therefore, the distribution of QOR between the two phases is strongly affected by the amount of CH3OK produced from the reaction of KOH and CH3OH. The hydration of MeOQ, generally, also decreases when the concentration of alkaline compound is increased. Therefore, the reaction rate (or the conversion) is increased by increasing the amount of KOH. Figure 4 shows the effects of the amount of KOH on the conversion of DCDPM. The experimental data follow a pseudo-first-order rate law, and as expected, the conversion of DCDPM increased with increasing the amounts of KOH. The corresponding kapp,1 values, which are affected by the amount of KOH, are included in Table 1. 5. Effect of the Amount of Water. In general, the concentration of KOH, the distributions of the catalyst (QBr) and active intermediate(QOR)betweenthetwophases,andtheorganic-aqueous interfacial contact area are all affected by the amount of water. The amount of water directly affects the concentration of KOH in the aqueous phase. As stated in the preceding section, the concentration of potassium hydroxide is am important factor in determining the concentration of MeOQ (or QOR) in the organic phase. Hence, the amount of water indirectly affects the reactivity of the reaction. Figure 5 shows the effect of the amount of water on the conversion. The experimental data
Figure 5. Effect of the amount of water on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
follow a pseudo-first-order rate law. The conversion decreases with increasing amount of water (10-35 mL). Several factors affect the conversion and the reaction rate because of the change of the amount of water. First, the solvation of MeOQ with water increases when the amount of water is increased, which is unfavorable for the reaction. Second, the activity of MeOQ decreases with increasing amount of water; i.e., the effect of dilution on the reaction is unfavorable. Third, the interfacial contact area is increased with increasing amount of water, which is favorable for the reaction. From the experimental data shown in Figure 5, one can see that the conversion is decreased with increasing amount of water. Thus, it is clear that solvation and dilution effects dominate the whole reaction. The corresponding kapp,1 values, which are dependent on the amount of water, are shown in Table 1. 6. Effect of the Amount of DCDPM. With a homogeneous reaction, the conversion is usually not affected by the amount of reactant in solution. However, the conversion is highly affected by the amount of reactant in a two-phase reaction. In this work, DCDPM, which has two chlorides to be displaced, acts as the organic-phase reactant. The effect of the concentration of DCDPM on the conversion of DCDPM for the two-phase catalytic reaction of MeOH and DCDPM is shown in Figure 6. The experimental data follow a pseudo-first-order rate law, and the conversion increases with increasing amount of DCDPM. It is clear that the dilution effect dominates the conversion due to the change of DCDPM amount; i.e., the concentration of DCDPM is increased by increasing the amount of DCDPM. Hence, the reaction is enhanced at higher concentrations of DCDPM in the organic-phase solution. The corresponding kapp,1 values vs the amount of DCDPM are reported in Table 1. 7. Effect of the Volume of Chlorobenzene. In this work, the reactant DCDPM reacts with MeOQ in two sequential steps, to produce the final desired product from the organic-phase solution. The volume of chlorobenzene directly influences the concentration of DCDPM in the organic phase. A higher concentration of DCDPM can be obtained either by using less organic solvent (chlorobenzene) or by using more DCDPM. The effect of the volume of chlorobenzene on the conversion of DCDPM is shown in Figure 7. The concentrations of DCDPM and other reagents in the organic phase decrease with increasing amount of chlorobenzene. This characteristic in using a smaller volume of chlorobenzene to obtain a large concentration of DCDPM is not exactly the same as that of the case of using a
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1381
Figure 6. Effect of the amount of DCDPM on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
Figure 8. Effect of the ultrasonic power on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 400 rpm, 60 °C, 40 kHz.
Figure 7. Effect of the volume of chlorobenzene on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
Figure 9. Effect of the ultrasonic frequency on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 400 rpm, 60 °C.
larger amount of DCDPM dissolved in the organic-phase solution of a fixed amount. The concentrations of DCDPM and MeOH are both affected by the amount of chlorobenzene. Therefore, the results obtained from changing the amount of chlorobenzene and from changing the amount of DCDPM differ, leading to different results. The corresponding kapp,1 values vs the amount of chlorobenzene are also shown in Table 1. 8. Effect of the Ultrasonic Power and Frequency. The power and frequency of the ultrasonic irradiation as well as the other reaction conditions are independent factors (or parameters). Of course, energy is introduced into the reaction solution by ultrasonic irradiation. However, the solution temperature is not increased suddenly when ultrasonic irradiation is applied. Figures 8 and 9 show the effects of the power and frequency of the ultrasonic irradiation on the conversion (or the reaction rate), respectively. Those experiments were carried out at 60 °C and 400 rpm. That is, the reaction was carried out at a constant temperature and a constant agitation speed. The effect of ultrasonic waves on the biphasic reaction was studied by keeping the temperature of the reaction solution and the agitation speed constant. The increase in temperature due to applying ultrasonic irradiation on the reaction solution is small. In studying the effect of ultrasonic power and frequency on the reaction rate, the
solution was immersed in an isothermal water tank. The heat generated by ultrasonic irradiation was small and was quickly removed from the constant-temperature water bath. The primary purpose of this work was to improve the twophase reaction of DCDPM and methanol. As stated, unexpected and undesired products are obtained from the reaction of DCDPM and MeOH in the absence of KOH after a long reaction time. In the presence of quaternary ammonium salts, the reaction of DCDPM and MeOH is still slow, even in the presence of a large amount of KOH. To enhance the reaction, ultrasonic irradiation is provided for the reaction of DCDPM and MeOH catalyzed by quaternary ammonium salts in the two-phase medium consisting of an aqueous solution of KOH and an organic solvent used in this work. Without the application of ultrasonic power to the reaction solution, the conversion of DCDPM is low. The reaction follows a pseudo-first-order rate law, and the conversion is increased with higher ultrasonic power and frequency, indicating that ultrasonic waves enhance the nucleophilic substitution. The chemical effects of the ultrasound can be attributed to intense local conditions generated by cavitational bubble dynamics, i.e., the nucleation, formation, disappearance, and coalescence of vapor or gas bubbles in the ultrasonic field.16-28 However, in the phase-transfer catalytic
1382 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
Figure 10. Effect of the quaternary ammonium salts on the conversion of DCDPM. Conditions: 0.7 mmol of PTC, 90 mmol of methanol, 2.75 mmol of DCDPM, 5 g of KOH, 40 mL of chlorobenzene, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
Figure 11. Effect of the organic solvent on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of organic solvent, 5 g of KOH, 10 mL of water, 0.1 g of toluene, 60 °C, 400 rpm, 300 W, 40 kHz.
reaction, rate enhancements are typically due to mechanical effects, mainly through an enhancement of mass transfer. The use of sonication techniques for chemical synthesis has also attracted considerable interest in recent years, because they can enhance the selectivity and reactivity, increase the chemical yields, and shorten the reaction time.14,15 In addition, there is no decomposition of phase-transfer catalysts under the experimental conditions. In this work, we found that the kapp,1 values with ultrasonic conditions (unconventional method) under the present experimental conditions are higher than those of the silent conditions (conventional method). 9. Effect of the Phase-Transfer Catalysts. Quaternary ammonium salts are generally used as phase-transfer catalysts to promote reaction rates. In this work, five quaternary ammonium salts, namely, TOAB, THAB, TBAB, BTEAB, and TEAB, were investigated to test their reactivity with the anion CH3O-, depending on its degree of solvation and on the structure of its countercation. Along with the quaternary ammonium salts, 4-(tributylammonium) propansultan (QSO3) was also used to test its catalytic reactivity toward the reaction, with the results shown in Figure 10. The experimental data using these six quaternary ammonium salts follow a pseudo-first order rate law. The order of reactivity of these quaternary ammonium salts and QSO3 is TOAB > THAB > TBAB >BTEAB > TEAB > QSO3, indicating that TOAB, with the highest carbon number, has the highest reactivity. The largest yield (100%) after 5 h of reaction was obtained using TOAB as the phase-transfer catalyst. This reactivity is favorable for using a quaternary ammonium salt with hydrophobic properties, because the organic-phase reactant dichlorodiphenylmethane (DCDPM) is highly hydrophobic. A quaternary ammonium salt with high hydrophobic properties, such as TOAB, is favorable for the reaction of DCDPM and the active intermediate (QOR). Choosing a highly hydrophobic quaternary ammonium salt such as TOAB is favorable for the reaction of the active intermediate (QOR) and DCDPM. The corresponding kapp,1 values for using these catalysts are included in Table 1. 10. Effects of Organic Solvents. In this work, methylethylketone (MEK), chlorobenzene (C-benz), dibutylether, oxylene, benzene, and tributylamine (TBA) were chosen as organic solvents to investigate their reactivities. As shown in Figure 11, the dielectric constants for these organic solvents are in the order MEK (18.4) > C-benz (5.7) > dibutylether (3.1) > o-xylene (2.6) > benzene (2.3) > TBA (2.2). The order
of the reactivity of the reactions in these six organic solvents is MEK > C-benz > dibutylether > o-xylene > benzene > TBA. The dielectric constants are usually used as the main index in choosing an appropriate organic solvent in a PTC system; i.e., the reaction rate increases with increasing dielectric constant of the organic solvent. The reason is that the present reaction system is a type of SN2 substitution. The organic solvent affects the activities of the nucleophilic reagent and the substrate. A strong dipole-dipole moment is formed when the nucleophilic reagent is dissolved in an organic solvent of high polarity. A strong dipole-dipole moment makes the compound QOR bond longer and decreases the bond energy. Under such circumstances, the activity of the nucleophilic reagent is increased. Similarly, a dipole-dipole moment between the organic solvent and the substrate also forms. Thus, the bond between chloride and diphenyl group becomes longer. Therefore, a larger bond space exists, which makes it easy for the nucleophilic reagent to attack for reaction; i.e., chloride atom is easily attacked, which is favorable for the reaction. That is, the distance between the chloride atom and the diphenyl group is large in an organic solvent of high polarity. The corresponding kapp,1 values for the reactions in these organic solvents are reported in Table 1. 11. Effect of the Temperature. The reaction was also investigated at six temperatures in the range of 35-60 °C. The results are shown in Figure 12. The experimental data show that the reaction follows a pseudo-first-order rate law. In the present system, agitating the solution, raising the temperature, and applying ultrasonic irradiation all supply energy to the reaction solution in an independent way. Therefore, in the presence of agitation and ultrasonic irradiation, the conversion (or the reaction rate) also increases with increasing temperature. It is clear that the reactivity is increased with increasing temperature along with the ultrasonic effect. The result is consistent with results obtained previously in studyies of different reaction systems.14,15 Hence, the reaction rate is higher at higher temperature, and as shown in Figure 12, the apparent rate constant is also higher as higher temperature. From an Arrhenius plot of -ln(kapp,1) vs 1/T, the activation energy for kapp,1 can be obtained as 24.8 kcal/mol. The corresponding kapp,1 values for the reaction at various temperatures in the studied range are reported in Table 1. In general, the determination of optimum conditions can be classified as a technical or economic issue in studying the reaction rate. First, one can find optimum conditions technically
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1383
Figure 12. Effect of the temperature on the conversion of DCDPM. Conditions: 0.228 g of TBAB, 90 mmol of methanol, 2.75 mmol of DCDPM, 40 mL of chlorobenzene, 10 mL of water, 0.1 g of toluene, 5 g of KOH, 400 rpm, 300 W, 40 kHz.
if the process has maximum or minimum characteristics (e.g., maximum conversion or yield). Otherwise, one should find an optimum value based on the economic situation, if an increase in the rate is observed with an improvement in the reaction conditions. Toward this end, the economic situation should be considered for the case of the increase in rate observed with increased concentrations of MeOH, PTC, KOH, water, and DCDPM, as well as increased ultrasonic power and frequency. Under this situation, an appropriate optimum criterion, e.g., optimum time or optimum cost, should be defined. Conclusion In this work, dimethoxydiphenylmethane (DMODPM) was successfully synthesized by the phase-transfer catalytic reaction of methanol and dichlorodiphenylmethane (DCDPM) assisted by ultrasonic irradiation in a two-phase medium consisting of an aqueous solution of KOH and an organic solvent. A kinetic model was constructed to satisfactorily account for the factors of the reaction. Two sequential reactions of an active intermediate (MeOQ) and dichlorodiphenylmethane (DCDPM) in the organic phase for the SN2 substitution were indicated. However, only the final DMODPM product was obtained. The second intrinsic rate constant (k2) in the organic phase is much larger than the first intrinsic rate constant (k1), so the first SN2 reaction is the rate-determining step. The reaction is influenced by the agitation speed. Because of the solvation and dilution, the conversion is decreased with increased amount of water. The largest yield (100% conversion after 5 h of reaction) was obtained using TOAB as the phase-transfer catalyst. The reaction rate also increased with increasing amounts of methanol, quaternary ammonium salts, KOH, and DCDPM; increasing temperature; and increasing ultrasonic power and frequency, and it decreased with increasing volume of chlorobenzene. Methylethylketone (MEK) was found to exhibit the highest reactivity among the six organic solvents tested. Acknowledgment The authors thank the National Science council for financial support under Grant NSC-95-2221-E-241-022. Literature Cited (1) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis, 3rd ed.; VCH: New York, 1993.
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ReceiVed for reView July 29, 2008 ReVised manuscript receiVed November 3, 2008 Accepted November 6, 2008 IE8011616
