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Kinetics of Esterification of Phenylacetic Acid with p-Cresol over H-β Zeolite Catalyst under Microwave Irradiation B. M. Chandra Shekara,† C. Ravindra Reddy,† C. R. Madhuranthakam,‡ B. S. Jai Prakash,† and Y. S. Bhat*,† † ‡
Chemistry Research Laboratory, Bangalore Institute of Technology, K.R. Road, Bangalore 560 004, India Department of Chemical Engineering, University of Waterloo, University Avenue W, Waterloo, Ontario N2L 3G1, Canada ABSTRACT: Heterogeneous esterification of phenylacetic acid with p-cresol over H-β zeolite was studied without using any solvent. Experiments were designed, conducted, and analyzed to study the effect of two different types of heating on the reaction yield. Primarily, the comparison was made between using the microwave irradiation and conventional heating. Microwave irradiation was found to be superior to conventional heating in terms of yields of ester for short reaction periods. The effects of reaction parameters such as catalyst amount, mole ratio of reactants, reaction time, reaction temperature, and water removal were investigated under microwave heating to optimize ester yield. Conversion of phenylacetic acid was studied at different temperatures, and the kinetics of the reaction was modeled using the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model. The corresponding model parameters were estimated by using nonlinear least-squares method. The reaction parameters thus obtained were used to evolve simulated values of conversion. Experimental values were found to be in very good agreement with the simulated values as evident from regression coefficient value of 0.987.
1. INTRODUCTION Esterification is an industrially important reaction due to a wide range of applications of esters.1 Esterification of carboxylic acid with an alcohol proceeds slowly at low temperatures, in the absence of a catalyst. The rate of reaction can be increased by increasing the temperature with the aid of a catalyst. However, the conversion is governed by equilibrium. Thus the major challenge in production of ester is to establish the conditions to enhance reaction rate and to obtain higher conversion. Equilibrium can be shifted toward ester, either by constantly removing one of the products or by using one of the reactant in excess, usually alcohol.2 Use of microwave irradiation is one of the most popular means to increase the reaction rate and product yields. Microwaveenhanced chemistry is based on effective and rapid heating by direct coupling of microwave energy with the molecules that are present in the reaction mixture.3 The microwave acceleration has been observed in various types of esterification catalyzed by Brønsted acids, Lewis acids, and base catalysts.4 Most of the industrial esterification processes employ organic solvents and homogeneous catalysts like H2SO4, HCl, and toluene-4-sulfonic acid.5 However, the use of organic solvents and aforesaid homogeneous catalysts do not conform to green chemistry principles. Hence to make esterification as green as possible, it would be desirable to carry out the process with reusable solid acid catalyst under solvent free conditions. Several solid acids like clays,6 supported heteropoly acids,7 zeolites,8 and ion-exchange resins9 have been investigated as heterogeneous catalysts for esterification. H-β zeolite is an important solid acid widely used in organic synthesis due to its high acidity and three-dimensional large pore structure.10,11 It has been investigated as a catalyst in several esterification reactions.12-15 However, to the best of our knowledge no literature is available on the kinetics of esterification over H-β zeolite under microwave heating. p-Cresyl phenylacetate is an important perfumery grade ester having narcissus odor with a honey note. r 2011 American Chemical Society
It is widely used in floral soaps, jasmine, blossoms compositions, and beauty care products.16 Al3þ-montmorillonite17 and ion-exchange resins18 have been investigated as heterogeneous catalysts for the synthesis of p-cresyl phenylacetate under conventional heating with a solvent. In both the cases longer duration of time is required to obtain equilibrium conversion. This paper discusses the development of a clean and green process for the synthesis of p-cresyl phenylacetate with enhanced reaction rate under microwave heating using H-β zeolite as catalyst under solvent free condition.
2. EXPERIMENTAL SECTION 2.1. Catalysts and Chemicals. The ammonium form of beta zeolite (Si/Al = 30) was supplied by S€ud-Chemie India Ltd. It was converted into protonic form by calcination at 813 K in dry air for 8 h. All other chemicals used in this study were procured from SD fine chemicals, India. p-Cresol was distilled before use, and all other chemicals were used without further purification. 2.2. Experimental and Procedures. Esterification of p-cresol (PC) with phenylacetic acid (PAA) to give p-cresyl phenylacetate (PCPA) was studied under conventional as well as microwave heating. Reactions under microwave heating were conducted in a microwave reactor ‘START-S’, which feature built-in magnetic stirrer and software that enables the online control of temperature of the reaction mixture with the aid of Infrared sensor by regulation of microwave output. Reactions under conventional thermal heating were carried out in an oil bath heated over a rota mantle. Effect of reaction time on conversion Received: May 21, 2010 Accepted: February 10, 2011 Revised: January 20, 2011 Published: February 28, 2011 3829
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Figure 1. Effect of catalyst amount on conversion of PAA. Reaction conditions: Temperature 463 K, mole ratio (PC:PAA) 2:1, maximum power 1000 W.
was studied under conventional as well as microwave heating. Esterification under microwave heating was found to be faster with higher conversion than conventional heating. Hence various reaction parameters like effect of reaction time, temperature, mole ratio of reactants, and catalyst amount on the yield of ester were investigated under microwave heating. Effect of removal of water formed during the reaction on equilibrium conversion of PAA was also studied. Further, to obtain kinetic data, conversion of PAA was studied at different temperatures. All reactions were carried out in a 50 mL glass vessel with condenser mounted on top of it. A Teflon stirring bar was used for agitation. Reactor vessel was kept over Teflon block so that the reaction mixture is exactly in line with Infrared sensor which monitors the temperature. In a typical reaction, 5 mmol of PAA (molar mass-136.15 g/ mol, density-1.0809 g cm-3), 10 mmol of PC (molar mass108.13 g/mol, density-1.0347 g cm-3), and 100 mg of H-β zeolite were taken. The reaction was carried out at desired temperature without using solvent. After the reaction, 10 mL of toluene was added and filtered to separate the catalyst. Analysis of the reaction ingredients before and after the reaction was performed by gas chromatography (Chemito model GC1000, FID detector) using a BP 20 capillary column (30 m 0.32 mm). Water content of reaction mixture was estimated by Karl Fischer titration method using KF Moisture Titrator. To confirm the product, filtrate was washed with 5% sodium hydroxide solution in a separating funnel to remove unreacted starting compounds. The organic layer was dried over anhydrous sodium sulfate, and the solvent was distilled under reduced pressure. The product obtained was confirmed by IR, 1H NMR, and melting point.
3. RESULTS AND DISCUSSION 3.1. Esterification of p-Cresol and Phenylacetic Acid. 3.1.1. Effect of Catalyst Amount. A set of reactions was carried out under
otherwise similar conditions by varying the amount of H-β zeolite from 10 to 150 mg to check the effect of catalyst amount on the conversion of PAA. Results are shown in Figure 1. No conversion was observed in the absence of catalyst. With an increase in the amount of catalyst, the conversion of PAA increased up until 100 mg, beyond which the conversion reaches a steady state. An increase in the conversion with the amount of catalyst is apparently due to an increase in the number of
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Figure 2. Effect of reaction temperature on conversion of PAA. Reaction conditions: reaction time 10 min, catalyst amount 100 mg, mole ratio (PC:PAA) 2:1, maximum power 1000 W.
Figure 3. Effect of mole ratio of reactants on the yield of ester. Reaction conditions: reaction time 10 min, temperature 463 K, catalyst amount 100 mg, maximum power 1000 W.
catalytically active sites. Similar observations have been reported in the literature.19 3.1.2. Effect of Temperature. Figure 2 shows the influence of temperature on the conversion of PAA. No conversion was observed when reaction was conducted for 10 min at 393 K. Hence reactions were conducted at 403, 423, 443, and 463 K keeping other conditions constant. Conversion of PAA increased from 9 to 60% with increase in temperature from 403 to 463 K. Selectivity was 100% at all temperatures, as the ester was the only product formed. Increase in conversion with increase in temperature is in conformity with the endothermic nature of esterification where the forward reaction is favored by increase in temperature.20 3.1.3. Effect of Mole Ratio of Reactants. In order to study the effect of mole ratio of reactants, a set of reactions was carried out by varying molar ratio of reactants and keeping all other conditions constant. The results of the study are presented in Figure 3. Conversion of PAA was increased from 35 to 60% as the PAA:PC mole ratio was changed from 1:1 to 1:2, and it remained constant for ratios greater than or equal to 1:3. This is in agreement with the very well established fact that the equilibrium can be shifted toward product side when one of the reactant is used in high 3830
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Scheme 1. Schematic Representation of Catalytic Esterification of Phenyl Acetic Acid with p-Cresol under Microwave Irradiation
Figure 4. Effect of reaction time on conversion of PAA under conventional and microwave heating. Reaction conditions: temperature 463 K, catalyst amount 100 mg, mole ratio (PC:PAA) 2:1, maximum power 1000 W.
concentration.21 However, the enhancement in conversion of PAA was almost negligible as the yield of ester was increased from 35 to 38% when the molar ratio of PAA:PC was varied from 1:1 to 3:1. Thus, the results indicate that the yield of ester was increased more pronouncedly with the concentration of PC than with the concentration of PAA. This could be due to the mechanism of reaction, which involves protonation of PAA by Brønsted acid site of the catalyst followed by the addition of PC and elimination of water. Thus the ready availability of PC for nucleophilic reaction with protonated acid increases the yield of ester and also when present in excess PAA will exist mainly as dimer, which may have a bearing on the reactivity. 3.1.4. Effect of Reaction Time. Effect of reaction time on the conversion of PAA was studied under optimum reaction conditions (temperature 463 K, mole ratio of PC:PAA 2:1, and catalyst amount of 100 mg). Under microwave heating time of irradiation was varied from 5 to 60 min. To compare rate of reaction with conventional heating, time period was varied from 30 to 360 min. Results are shown in Figure 4. Initially, the rate of reaction was faster and as the reaction approached equilibrium, conversion increased very slowly before the steady state was attained. Similar observations have been reported by others.22 Equilibrium conversion was obtained under microwave heating in 50 min while it took 360 min with conventional thermal heating. Further, the equilibrium conversion with microwave heating was found to be slightly higher than the conversion obtained with conventional heating. This can be attributed to the different modes of heat transfer from source to reaction mixture. Under conventional heating, energy is transferred slowly in a series fashion from mantle to oil bath, from oil bath to reaction vessel, and from reaction vessel to the reaction mixture, whereas the microwave irradiation produces efficient internal heating by direct coupling of microwave energy with the molecules present in the reaction mixture. Further since water is a very good absorber of microwave radiation than other reactants and products of the reaction mixture. The higher conversion with microwave heating is probably due to better removal of water from the reaction mixture by microwave heating. However some part of the water always comes back to the reaction mixture, as the reactions were conducted mounting a condenser over a reaction vessel.
Figure 5. The conversion of PAA under microwave irradiation: a) with initially added ester (5 mmol), b) with initially added water (10 mmol), c) with initially added water (5 mmol), d) with condenser, and e) without condenser. Reaction conditions: temperature 463 K, catalyst amount 100 mg, mole ratio (PC:PAA) 2:1, maximum power 1000 W.
3.1.5. Effect of Water Removal. It has been well established that the equilibrium in esterification can be shifted toward the ester side by removing one of the products, either ester or water continuously from the reaction mixture.23 In the esterification of PAA with PC (Scheme 1), it is highly difficult to remove ester as soon as it is formed, because of its high boiling point. However as the reactions were carried out at 463 K, water formed in the reaction can be removed easily. Hence to study the effect of water removal on equilibrium conversion, a set of reactions was conducted without using a condenser and allowing the water formed during the reaction to evaporate. Equilibrium conversion which was 75% when water formed in the reaction was not removed increased to 90% with the removal of water. Further to check the effect of initially added water and ester on the rate of conversion, evolution of conversion with time was studied with 5 and 10 mmol of initially added water and 5 mmol of ester. A large decrease in the conversion was observed in all cases. The corresponding results of these experiments are shown in Figure 5. 3.2. Kinetic Studies. 3.2.1. Equilibrium Constant. The equilibrium constant for the esterification of phenylacetic acid with p-cresol over H-β zeolite was determined at different temperatures 3831
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Figure 6. The equilibrium conversion of PAA under microwave irradiation at different temperatures. Reaction conditions: catalyst amount 100 mg, mole ratio (PC:PAA) 2:1, maximum power 1000 W. Figure 8. Plot of conversion of PAA with reaction time at different temperatures. Reaction conditions: catalyst amount 20 mg, mole ratio (PC:PAA) 2:1, maximum power 1000 W.
Table 1. Water Content of Reaction Mixture As Estimated by Karl Fischer Titrationa water content of reaction mixture at different reaction temperatures (mmol)
ln K ¼
-ΔH° ΔS° þ RT R
433 K
448 K
463 K
5
0.05
0.10
0.19
0.32
10 15
0.08 0.11
0.19 0.29
0.31 0.46
0.56 0.72
20
0.14
0.33
0.58
0.82
30
0.20
0.46
0.72
0.91
40
0.27
0.54
0.79
0.98
Reaction conditions: catalyst amount 20 mg, mole ratio (PC:PAA) 2:1, maximum power 1000 W.
using the eq 1. Equilibrium conversion of PAA at different temperatures is shown in Figure 6. Equilibrium conversion was achieved within 60 to 90 min depending on the reaction temperature. The value of the equilibrium constant enhanced with an increase in temperature indicating the endothermic nature of the reaction. Reaction enthalpy (ΔHo) and reaction entropy (ΔSo) were obtained from eq 2 by plotting ln K against T-1 (Figure 7), and the corresponding values were found to be 99.8 kJ mol-1 and 220.3 J K-1 mol-1, respectively. The positive reaction enthalpy suggests that the esterification reaction is endothermic CE CW Xe 2 ¼ CA C B fð1 - Xe ÞðM - Xe Þg
418 K
a
Figure 7. Arrhenius plot for equilibrium constant K.
K ¼
reaction time (min)
ð1Þ
Karl Fischer titration method is given in Table 1. Experimental data from Figure 8 and Table 1 have been used for the estimation of kinetic parameters and activation energy. Hence the proposed kinetic model is applicable for conversion of PAA under the above-mentioned experimental conditions. The conventional way to describe the kinetics of heterogeneously catalyzed systems is by using Langmuir-Hinshelwood-Hougen-Watson (LHHW) model.24 Kinetics of esterification of carboxylic acid with alcohol by heterogeneous catalysts has been described in the literature using this model where the model assumes that the reactants are chemisorbed over catalytic surface and the surface reaction is the rate determining step.25-28 For the following simple bimolecular esterification reaction AcidðAÞ þ AlcoholðBÞ S EsterðEÞ þ WaterðWÞ
ð2Þ
3.2.2. Kinetic Modeling. The kinetic behavior for the esterification of PAA with PC over H-β zeolite has been studied experimentally over reaction temperatures from 418 to 463 K using 20 mg of catalyst with a molar ratio of the feed equal to 2.0. The conversion of PAA at various temperatures is shown in Figure 8. Water content of the reaction mixture as estimated by
with surface reaction as rate limiting step, the rate equation obtained using LHHW model is given by eq 3 2 3 K E K W CE C W K K C C A B A B 6 7 6 7 KS 7 ð3Þ ð-rs Þ= ¼ ks 6 2 4f1 þ KA CA þ KB CB þ KE CE þ KW CW g 5 3832
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Table 2. Surface Reaction Rate Constant (ks) and Surface Equilibrium Constant (KS) at Different Temperatures surface reaction
surface
temperature
rate constant ks
equilibrium
(K)
(g cat dm3 mol-1 S-1)
constant KS
418
0.1214
1.3546
433
0.3909
2.3512
448
0.9443
3.4313
463
2.6209
4.4156
Table 3. Adsorption Equilibrium Constants at Different Reaction Temperatures adsorption equilibrium constants (dm3 mol-1) temperature (K)
KA
KB
KE
KW
418
1.9889
1.2217
1.8545
0.5475
433
1.6452
0.7254
1.2244
0.3346
448
1.2984
0.5325
0.6655
0.1425
463
1.0115
0.3635
0.4226
0.0913
Figure 9. Arrhenius plot for the surface reaction rate constant ks.
Concentrations of the reactants and the products can be expressed in terms of initial concentration of PAA (CAO), conversion of PAA (XA), and PC/PAA molar ratio (M) defined according to the equations CA ¼ CAO ð1 - XA Þ
ð4Þ
CB ¼ CAO ðM - XA Þ
ð5Þ
CE ¼ CAO XA
ð6Þ
Substituting eqs 4-6 in eq 3, the final simplified equation obtained is represented by eq 7 =
ðrexp Þ ¼ ð-rs Þ
=
2
3 KE ðCAO XA Þ KW CW KA KB CAO 2 ð1 - XA ÞðM - XA Þ 6 7 KS 7 ¼ ks 6 4f1 þ K C ð1 - X Þ þ K C ðM - X Þ þ K ðC X Þ þ K C g2 5 A AO A B AO A E AO A W W
Figure 10. Effect of temperature on adsorption equilibrium constants.
function of temperature as follows
-13033 ks ¼ 5:310 exp T
ð9Þ
2915 T
ð10Þ
8
ð7Þ
KA ¼ 1:910-3 exp
3
where K values are adsorption equilibrium constants (dm mol-1), ks is surface reaction rate constant (mol dm-3 g cat-1 s-1), KS is surface equilibrium constant for esterification reaction, C is concentration (mol), and subscripts A, B, E, and W refer to PAA, PC, PCPA, and water, respectively. The experimental rates were calculated using eq 8 AmountofPCPAformedðmoldm-3 Þ ðrexp Þ= ¼ ðMassofcatalyst, gÞðsÞ
KE ¼ 3:410-7 exp
ð8Þ
Surface rate constant ks, adsorption equilibrium constants for PAA (KA), PC (KB), and PCPA (KE), and water (KW) were determined at different temperatures using a Nonlinear Least Square Method. The results thus obtained are summarized in Table 2 and Table 3. The dependency of rate and adsorption equilibrium constants on temperature is shown in Figure 9 and Figure 10, respectively. These rate parameters are expressed as a
5100 KB ¼ 5:910 exp T -6
ð11Þ
6502 T
ð12Þ
8034 T
ð13Þ
KW ¼ 2:610-9 exp
where the energy of activation values are given in J mol-1. The values of rate constants at different temperatures were calculated, and an Arrhenius plot of ln ks vs 1/T (Figure 9) was used to estimate the frequency factor and energy of activation. The values of the frequency factor ko and the energy of activation for the 3833
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Figure 11. Parity plot between experimental rate and simulated rate by LHHW model.
esterification of PAA with PC over H-β zeolite were obtained as 5.3 108 cm3 mol-1 s-1 and 25.8 kcal mol-1, respectively. A parity plot between simulated and experimental values of conversion is shown in Figure 11. The regression coefficient between the simulated results and the experimental values is found to be 0.987. The very high R2 value indicates a very good agreement between experimental and simulated values of conversion. This further justifies the selection of the rate model for the reaction.
4. CONCLUSIONS p-Cresyl phenylacetate can be synthesized by a clean and green process with enhanced reaction rate under microwave heating using H-β zeolite as heterogeneous catalyst under solvent free condition. Microwave heating was found to be superior to the conventional thermal heating in terms of yields in short reaction periods. Experimental analysis showed that 2:1 mol ratio of reactants (p-cresol:phenylacetic acid), 100 mg of catalyst amount, 60 min of reaction time, 463 K of reaction temperature, microwave irradiation with removal of water are optimum conditions for maximum ester yield. The LHHW formalism was used to model the kinetics of the reaction, and the parameters were obtained by using Nonlinear Least Squares method. Experimental values of rate constant were found to be in very good agreement with the simulated values. ’ AUTHOR INFORMATION Corresponding Author
*Tel./Fax: þ91-80-26615865/26426796. E-mail: BHATYS@ yahoo.com.
’ ACKNOWLEDGMENT This work was funded by the Department of Science and Technology, New Delhi. The authors thank the Principal and the Governing Council of Bangalore Institute of Technology for the facilities provided. Thanks are due to S€ud Chemie India Pvt Ltd. for providing the zeolite sample. ’ REFERENCES (1) Junzo, O.; Joji, N. Esterification: Methods, Reactions and Applications; WILEY-VCH GmbH & Co.: Weinheim, 2009; p293.
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