Ind. Eng. Chem. Res. 2002, 41, 1129-1134
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APPLIED CHEMISTRY Esterification of Benzoic Acid with 2-Ethylhexanol in a Microwave Stirred-Tank Reactor G. Pipusˇ , I. Plazl, and T. Koloini* Faculty of Chemistry and Chemical Technology, University of Ljubljana, Asˇ kercˇ eva 5, P.O. Box 537, 1000 Ljubljana, Slovenia
A microwave applicator with a 2.45-GHz magnetron generator, which provides 460 W of output power, and waveguides to a stirred-tank reactor with a full volume of 500 mL was designed. The microwave reactor operates at high pressure and high temperature. Esterification of benzoic acid with 2-ethylhexanol was chosen as the model reaction. The esterification was homogeneously catalyzed with sulfuric acid and para-toluene sulfonic acid. As heterogeneous catalysts, Cs2.5H0.5PW12O40, sulfated ZrO2, Fe2(SO4)3, and montmorillonite KSF were used. For the same operating conditions, the rate of esterification was concluded to be the same in both the microwave reactor and conventional experimental setups. The results show that homogeneous catalysts are more effective than heterogeneous catalysts. Fe2(SO4)3 was the most effective catalyst among the solid catalysts tested. The conversions in the microwave stirred-tank reactor were in agreement with the conversions predicted on the basis of the kinetic parameters obtained under conventional heating. High-pressure and high-temperature operating conditions allowed the reaction rate of esterification to be greatly increased. Introduction Microwave dielectric heating not only is a wellestablished procedure for the domestic preparation of meals but also is widely used industrially for the processing of food and industrial materials. Microwave applications have been designed for the heating of rubber, wood, paper, and agricultural products. More recently, microwave dielectric heating has attracted the attention of chemists.1,2 Since then, the chemical applications of microwave heating have been extended to almost all areas of chemistry.3 Microwave heating has been applied to batch and continuous flow reactors placed in domestic microwave ovens. However, the electric field in domestic microwave ovens has a very complex pattern and is usually nonuniform.4 This can cause uneven heating of materials and the formation of hot spots. Microwave heating is widely applied in organic synthesis, as it provides the advantage of possible rate enhancements due to thermal effects, direct heating of materials, and rapid startup. Microwave-assisted organic chemistry have consequently been the subject of excellent reviews.5,6 However, the literature in this area is mostly experimental, with the reactions being carried out in small beakers or in small-volume PTFE vessels. The influence of microwave irradiation on the reaction kinetics of the acid-catalyzed esterification of 2,4,6trimethilbenzoic acid in I-PrOH was investigated by * Corresponding author: Prof. Dr. Tine Koloini, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, P.O. Box 537, 1000 Ljubljana, Slovenia. Tel.: +386 1 2419 500. Fax: +386 1 2419 530. E-mail: tine.koloini@ uni-lj.si.
Raner and Strauss.7 The rate of esterification was concluded to be the same in both the microwave reactor and oil bath experiments. Chemat and co-workers studied the esterification of stearic acid with butanol under homogeneous and heterogeneous conditions.8 They found that, under heterogeneous reaction conditions, a microwave-heated reaction mixture gave a higher yield than a conventionally heated reaction mixture. The effect was greatest for the catalyst Fe2(SO4)3 and smallest for montmorillonite KSF. However, there was no microwave effect on the rate of esterification under homogeneous reaction conditions. These increases in reaction rate were attributed to hot spots on the surface of the solid catalysts. The aim of our work was to construct a microwave reactor that would allow for the heating of a larger volume under controlled conditions. A specially designed microwave applicator was developed, in which the microwaves are delivered by a system of waveguides to the stirred-tank reactor. The applicator operates in single mode so that the electric field is uniform and more controlled operating conditions can be established. The microwave stirred-tank reactor of 500-mL volume operates at elevated pressure. Measurements of temperature have proved to be problematic in the presence of the strong electromagnetic field. One way of ensuring accurate measurements of the temperature is with a fiber optic probe, but this approach is valid only up to 250 °C. Another possibility is using a specially designed thermocouple immersed in an absorbing liquid.9 The esterification of benzoic acid with 2-ethylhexanol was chosen as the model reaction to be studied in the microwave stirred-tank reactor. The esterification was homogeneously catalyzed with sulfuric acid and with
10.1021/ie010107a CCC: $22.00 © 2002 American Chemical Society Published on Web 02/05/2002
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Figure 1. Microwaves delivered by waveguides; the stirred-tank reactor.
para-toluene sulfonic acid (PTSA). The use of solid catalysts is often preferred for liquid-phase reactions to the use of homogeneous catalysts, because it is easier to separate them and they do not pollute. Cs2.5H0.5PW12O40, sulfated ZrO2, Fe2(SO4)3, and montmorillonite KSF were used as heterogeneous catalysts. It has been reported10 that sulfated zirconia among solid superacid catalysts has the highest activity for esterification of phthalic anhydride with 2-ethylhexanol. Another interesting solid acidic catalyst is the Cs salt of H3PW12O40, Cs2.5H0.5PW12O40 (abbreviated as Cs2.5). It has been demonstrated11 that many reactions are effectively catalyzed by Cs2.5 even in the presence of water.12 Cs2.5 exhibited the highest catalytic activity among the solid oxide catalysts for the esterification of acrylic acid with 1-butanol.13 Experimental Section Our experiments were carried out in a specially designed microwave applicator (Figure 1) that operated in single mode. The microwave applicator consisted of a 2450-MHz magnetron with a full power of 460 W, a circulator used to protect the generator against reflected power, an impedance-matching waveguide section with four tuning screws, and a directional cross-guide coupler with a matching low-power waveguide load for monitoring the incident power. The reactor was made of a steel tube (inner diameter ) 94 mm, height ) 120 mm). Microwaves were delivered to the reactor by a system of waveguides through a window on the reactor side. The window was made of the special polymer Stycast 500, which is transparent to microwaves. A PTFE plug was inserted in the waveguide next to the reactor to isolate the microwave applicator from the reactor, because the reactor operated at elevated pressure. The reactor was equipped with a pressure sensor and a mechanical stirrer with a Rushton turbine, which operated at 200 rpm. A glass vessel (inner diameter ) 90 mm, volume ) 500 mL) containing the reaction mixture was placed inside the steel reactor. Four baffles were added to the glass vessel to allow for better mixing of the reaction mixture. The temperature was measured on-line with a NiCr-Ni thermocouple and recorded on a PC. A glass tube, which was connected to the valve, was placed in the reactor for sampling. The reaction mixture heated with microwaves reached 200 °C in less than 10 min. The boiling point of 2-ethyl hexanol (p.a., Merck, Darmstat, Germany) is 184 °C, and the applied pressure allowed the reaction mixture
Table 1. Activation Energies and Preexponential Factors for the Esterification of Benzoic Acid with 2-Ethylhexanol
catalyst H2SO4 PTSA Fe2(SO4)3 montmorillonite KSF Cs2.5H0.5PW12O40 sulfated ZrO2
concentration preexponential activation of catalyst factor energy (kJ (wt %) (s-1) mol-1 K-1) 2.2 0.225 2.45 2.9 0.74 0.74
1.98 × 105 5.69 × 103 1.48 × 108 7.95 × 104 1.19 × 103 2.91 × 105
63.2 59.7 92.7 74.1 64.4 87.0
to be heated above its boiling point. The final temperature of the reaction mixture was between 230 and 250 °C. The applied pressure was 5 bar at the beginning. During the experiments, the pressure increased to 12 bar because of the high temperatures in the reactor. Samples were collected at different time intervals. When samples were taken, the temperature decreased because of the slight pressure drop. The samples were immediately cooled to stop the reaction and were later analyzed on a GC. After 20 min of microwave irradiation, the microwave generator was turned off, and the temperature of the reaction mixture immediately started to decrease. All experiments were performed with a 10-fold excess of 2-ethylhexanol with respect to benzoic acid (p.a., Kemika, Zagreb, Croatia) to shift the esterification toward the products. Sulfuric acid (acidity ) 20.4 mmol/ g; p.a., Kemika, Zagreb, Croatia) and PTSA (acidity ) 5.26 mmol/g) were added as homogeneous catalysts. Sulfated ZrO2 (acidity ) 0.2 mmol/g, surface area ) 93 m2/g), Fe2(SO4)3, and montmorillonite KSF were used as heterogeneous catalysts. Cs2.5H0.5PW12O40 (acidity ) 0.15 mmol/g, surface area ) 107-116 m2/g) was prepared14 by titrating a solution of H3PW12O40 (p.a., Merck, Darmstat, Germany) with a solution of Cs2CO3 and then drying the product at 300 °C for 2 h. The concentrations of the catalysts are given in Table 1. In the conventional heating experiments, a 200-mL glass vessel with reflux equipment and a magnetic stirrer was placed in a thermostated water bath. The temperature was maintained to within (0.5 °C and monitored with a thermometer. The experiments were performed in the temperature range from 50 to 90 °C at atmospheric pressure. The speed of the magnetic stirrer was high enough to eliminate external mass transfer resistance. The heterogeneous mixtures were agitated using a mechanical direct driver stirrer with a 3-cm round-shaped Teflon blade at 400 rpm in a 200mL round-bottomed flask.
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Figure 2. Homogeneously and heterogeneously catalyzed esterification under conventional heating.
Figure 3. Plot of 1/C vs time at 80 °C under conventional heating.
Kinetics of Esterification
When alcohol is used in large excess, the reaction rate is essentially independent of the concentration of alcohol and eq 1 can be changed to
Benzoic acid reacts with 2-ethylhexanol to form 2-ethylhexyl benzoate and water
kf
C6H5COOH + CH3(CH2)3CH(C2H5)CH2OH {\ } k b
C6H5COOCH2CH(C2H5)CH2CH2CH2CH3 + H2O It was necessary to experimentally determine the kinetic parameters of the esterification of benzoic acid with 2-ethylhexanol because the kinetics of this particular esterification had not been previously studied. The kinetic parameters of esterification were determined in a glass vessel placed in a thermostated bath and equipped with reflux equipment and a magnetic stirrer. 2-Ethylhexanol was used in large excess to shift the esterification toward the reaction products. The esterification of benzoic acid with 2-ethylhexanol in the temperature range used in our experiments at atmospheric pressure is a relatively slow reaction (Figure 2), and several days were needed to reach equilibrium. Water, which is a product of esterification, is not soluble in 2-etylhexanol and forms a separate phase during esterification. Therefore, water was constantly removed, so that the esterification of benzoic acid with 2-ethylhexanol was shifted toward the products. We applied the same assumptions to the esterification of benzoic acid with 2-ethylhexanol that we used for the kinetics of the esterification of benzoic acid with ethanol.15 We assumed that the esterification of benzoic acid with 2-ethylhexanol is first-order with respect to 2-ethylhexanol and second-order with respect to benzoic acid. The rate of disappearance of benzoic acid can thus be given by
-
dCB ) kfCEHCB2 dt
dCB ) kCB2 dt
(2)
The straight lines in Figure 3, where 1/C - 1/C0 is plotted versus time, clearly show that esterification is second-order with respect to benzoic acid for the homogeneously and heterogeneously catalyzed reactions. The reaction rate constant is also dependent on the temperature and can be described by the Arrhenius law
k ) k0 exp(-Ea/RT)
(3)
where k0 is the preexponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin. The activation energies and preexponential factors for homogeneously and heterogeneously catalyzed esterifications are given in Table 1. The Arrhenius plot for heterogeneously catalyzed esterification under conventional heating is presented in Figure 4. It was reported16 that the activation energy is 56 kJ/ (mol K) for the esterification of acrylic acid with 2-ethylhexanol catalyzed by sulfuric acid. It can be seen from Table 1 that the activation energies are similar for the esterifications catalyzed by sulfuric acid and PTSA. The reaction rate is dependent on the concentration of H+, which is included in the preexponential factor k0. Although the concentrations of PTSA and sulfuric acid are the same, the reaction rate is higher for sulfuric acid, because of its higher specific acidic amount. The activation energy for the esterification catalyzed with Cs2.5 is similar to the activation energy of the homogeneously catalyzed reaction. However, the activation energies were higher when montmorillonite KSF, sulfated ZrO2, and Fe2(SO4)3 were used as solid catalysts.
(1)
where CB is the concentration of benzoic acid, CEH is the concentration of 2-ethylhexanol, and kf is the rate constant for the forward reaction.
Results and Discussion The design of our microwave applicator allowed a high level of microwave energy to be introduced into the stirred-tank reactor. Almost all of the available 460 W
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Figure 6. Temperature profile and calculated heat flow for water in the microwave stirred-tank reactor. Figure 4. Arrhenius plot for heterogeneously catalyzed esterification under conventional heating.
available 460 W of microwave power was converted to heat. The calculated heat flow at 150 °C was about 350 W (Figure 6), and the calculated heat loss at the same temperature was about 100 W. Thus, in total, about 450 W of microwave power was absorbed at 150 °C, which is approximately the same as the amount absorbed at room temperature. The absorbed heat of microwaves per unit volume was determined using the equation given by Metaxas and Meredith17
Q˙ MW ) 2πf0κ′′E2
Figure 5. Temperature profile of the reaction mixture and water in the microwave stirred-tank reactor.
was absorbed in the reactor, and the reflected power was minimal. The heating of 420 g of water showed that the heating rate was higher at the beginning of the experiments (Figure 5) and that it was the same at different applied pressures. The heating rate of the water decreased with increasing temperature (Figure 5) as a result of heat losses to the surroundings and heating of the reactor wall. The final temperature of the water was dependent only on the applied pressure. The water reached 100 °C in less than 5 min. The heat flow can be calculated from the heating rate according to
Q˙ ) mcP(∆T/∆t)
(4)
where m is the mass of water and cP is the specific heat. When the microwaves were turned off, the temperature of the water immediately started to decrease. Heat losses can be calculated by the same equations from the cooling rate of water. At the beginning, all of the
(5)
where E is the root-mean-square (rms) value of the electric field intensity, f is the frequency (2450 MHz), 0 is the dielectric constant of free space [8.85 A s/(V m)], and κ′′ is the relative loss factor for the dielectric being heated. The loss factor at a given frequency is a function of the composition of the material and its temperature. For water,17 it varies at 2450 MHz as 320/T in the temperature range 25-75 °C. Therefore, it is expected that, at higher temperatures, less microwave power will be absorbed. However, the results showed that almost the same power of microwaves was absorbed over the entire temperature range used in our experiments. A comparison of the heating of water with the heating of the reaction mixture, which consisted mainly of 2-ethylhexanol, dissolved benzoic acid, catalyst, and reaction products, shows (Figure 5) that the heating rate of reaction mixture is higher than the heating rate of water. The estimated specific heat18 of 2-ethylhexanol is 2.12 kJ/(kg K) at 20 °C, which is significantly smaller than the specific heat of water. According to eq 4, the heating rate is faster if the specific heat is smaller at the same absorbed microwave power. The absorbed powers of microwaves calculated from the heating rates are similar for water and the reaction mixture in the temperature range used. The conversions and selectivities of the esterification of benzoic acid over liquid and solid catalysts are summarized in Table 2. The conversions were calculated according to the concentration of 2-ethylhexyl benzoate. The concentration of 2-ethylhexyl benzoate was similar to the consumption of benzoic acid, which was the limiting reactant.
Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1133 Table 2. Conversions and Selectivities of the Esterification of Benzoic Acid with 2-Ethylhexanol in the Microwave Stirred-Tank Reactor catalyst
concentration of catalyst (%)
time (min)
conversion (%)
selectivity (%)
temperature (°C)
H2SO4 H2SO4 H2SO4 H2SO4 + C PTSA PTSA + C Fe2(SO4)3 Fe2(SO4)3 montmorillonite KSF Cs2.5H0.5PW12O40 sulfated ZrO2
2.2 2.2 0.225 0.225 0.225 0.225 0.75 2.45 2.9 0.74 0.74
12 20 17 16.5 24 18.5 20 15 20.5 22 22
96.5 98.4 97.2 94.5 96.9 78.5 94.8 98.4 34.7 17.3 25.1
74.3 50.4 86.7 85.1 94.8 94.0 87.8 73.8 13.6 19.0 95.0
234 253 251 250 256 242 241 240 248 246 251
Figure 7. Experimental and calculated conversions for heterogeneously catalyzed esterification in the microwave stirred-tank reactor.
Figure 8. Experimental and calculated conversions for homogeneously catalyzed esterification in the microwave stirred-tank reactor.
The results show (Figures 7 and 8) that homogeneous catalysts are much more effective than solid catalysts because of the higher concentration of H+ in the reaction mixture. The final reaction temperatures achieved
under microwave conditions are presented in Table 2. Fe2(SO4)3 is the most effective solid catalyst used in this study. The conversions of the esterification were higher for montmorillonite KSF than for Cs2.5 and sulfated zirconia (Figure 7). However, the concentration of montmorillonite KSF was more than 3 times higher than the concentration of Cs2.5 and sulfated zirconia. The reaction rate of esterification increased linearly with the amount of catalyst. Higher concentrations of Fe2(SO4)3 and H2SO4 allowed for a significant increase in the rate of esterification. However, a close control of reaction conditions is required because increases in the catalyst concentration or temperature can cause the dehydration of alcohols to ethers and olefins, which are the main side reactions in the synthesis of esters. The selectivity of the reaction was estimated according to the consumption of 2-ethylhexanol and the yield of ester 2-ethylhexyl benzoate. About 95% selectivity was observed for sulfated zirconia and PTSA, but the conversions was much higher for PTSA. Fe2(SO4)3 and H2SO4 gave slightly lower selectivities. When higher amounts of both catalysts were used, a decrease in selectivity from about 86% to about 74% was observed, although the esterification reached similar conversions in much shorter times. A 96% conversion was reached in 12 min with a high concentration of sulfuric acid. Further heating increased the conversion to 98%, but selectivity dramatically decreased from 74% to almost 50%. It should be emphasized that the selectivities of Cs2.5 and montmorillonite KSF were also very low. The kinetic parameters were determined separately to compare microwave and conventional heating because it was impossible to reproduce exact temperature profiles in the microwave reactor under conventional heating. The conversions of esterification were calculated from the measured temperature profiles (Figure 5) and kinetic parameters obtained under conventional heating. The agreement between the calculated and experimental conversions shows that experimentally obtained conversions of esterification could be predicted using the kinetic parameters determined under conventional heating and that no effects of microwaves on the homogeneously and heterogeneously catalyzed reaction were observed. Activated carbon, which absorbs microwaves very strongly, was added to the reaction mixture to promote localized superheating under microwave irradiation. The results show that there is no increase in the temperature of the reaction mixture because of the added activated carbon. The conversions of esterification when carbon was added to the reaction mixture are in good agreement (Figure 8) with the conversions predicted on the basis of the measured temperature and
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with the conversions of esterification without any carbon. The activated carbon had no effect on the conversion and selectivity of esterification. Conclusion A microwave stirred-tank reactor with a volume of 500 mL that operates at elevated pressure was constructed. The results showed that the design of the microwave applicator enabled almost all of the available microwave power to be absorbed in the stirred-tank reactor irrespective of the changes in dielectric properties. The use of a microwave generator with greater power could generate higher temperatures in the stirredtank reactor. No special microwave effects were observed on the homogeneously and heterogeneously catalyzed esterification of benzoic acid with 2-ethylhexanol even in the presence of activated carbon. Therefore, conversions of esterification can be successfully predicted using the measured temperature of the reaction mixture and kinetic parameters obtained under conventional heating. The use of microwave irradiation made it possible to heat the reaction mixture to a high temperature in a very short time. These operating conditions allowed the reaction time of esterification of benzoic acid with 2-ethylhexanol to be greatly decreased. Acknowledgment Grant J2-7508-0103 of the Ministry of Science and Technology of Slovenia supported the research. The authors thank S. Gajsˇek for his technical assistance. Literature Cited (1) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; Laberge, L.; Rousell, J. The Use of Microwave Ovens for Rapid Organic Synthesis. Tetrahedron Lett. 1986, 27, 279. (2) Gedye, R.; Smith, F.; Westaway, K. C. The Rapid Synthesis of Organic Compounds in Microwave Ovens. Can. J. Chem. 1988, 66, 17. (3) Mingos, D. M. P.; Baghurst, D. R. Applications of Microwave Dielectric Heating Effects to Synthetic Problems in Chemistry. Chem. Soc. Rev. 1991, 20.
(4) Plazl, I.; Pipus, G.; Koloini, T. Microwave Heating of the Continuous Tubular Flow Reactor in a Nonuniform Electric Field. AIChE J. 1997, 43, 754. (5) Strauss, C. R.; Trainor, R. W. Invited ReviewsDevelopments in Microwave-Assisted Organic Chemistry. Aust. J. Chem. 1995, 48, 1665. (6) Caddick, S. Microwave-Assisted Organic Reactions. Tetrahedron 1995, 51, 10403. (7) Raner, K. D.; Strauss, C. R. Influence of Microwaves on the Rate of Esterification of 2,4,6-Trimethylbenzoic Acid with 2-Propanol. J. Org. Chem. 1992, 57, 6234. (8) Chemat, F.; Poux, M.; Galema, S. A. Esterification of Stearic Acid by Isomeric Forms of Butanol in a Microwave Oven under Homogeneous and Heterogeneous Reaction Conditions. J. Chem. Soc., Perkin Trans. 1997, 2, 2371. (9) Plazl, I.; Leskovsek S.; Koloini, T. Hydrolysis of Sucrose by Conventional and Microwave Heating in a Stirred Tank Reactor. Chem. Eng. J. 1995, 59, 253. (10) Thorat, T. S.; Yadav, V. M.; Yadav, G. D. Esterification of Phthalic Anhydride with 2-Ethylhexanol by Solid Superacidic Catalysts. Appl. Catal. A: Gen. 1992, 90, 73. (11) Izumi, Y. Hydration/Hydrolysis by solid Acids. Catal. Today 1997, 33, 371. (12) Okuhara, T.; Kimura, M.; Kawai, T.; Xu, Z.; Nakato, T. Organic Reactions in Excess Water Catalyzed by Solid Acid. Catal. Today 1998. 45, 73. (13) Chen, X.; Xu, Z.; Okuhara, Y. Liqiud-Phase Esterification of Acrylic Acid with 1-Butanol Catalyzed by Solid Acid Catalysts. Appl. Catal. A: Gen. 1999, 180, 261. (14) Tatematsu, S.; Hibi, T.; Okuhara, T.; Misuno, M. Preparation and Catalytic Activity of CsxH3-xPW12O40. Chem. Lett. 1984, 865. (15) Pipus, G.; Plazl, I.; Koloini, T. Esterification of Benzoic Acid in a Microwave Tubular Flow Reactor. Chem. Eng. J. 2000, 76, 239. (16) Nowak, P. Kinetics of the Liquid-Phase Esterification of Acrylic Acid with n-Octanol and 2-Ethylhexanol Catalyzed by Sulfuric Acid. React. Catal. Lett. 1999, 66, 376. (17) Metaxas, A. C.; Meredith, R. J. Industrial Microwave Heating; IEE Power Engineering Series 4; Peter Peregrinus: London, 1988. (18) Perry’s Chemical Engineer’s Handbook, 6th ed.; McGrawHill Book Company: New York, 1984.
Received for review February 5, 2001 Revised manuscript received August 14, 2001 Accepted December 10, 2001 IE010107A
