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Ind. Eng. Chem. Res. 2007, 46, 1058-1062
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of Hydrolysis of Benzaldehyde Dimethyl Acetal over Amberlite IR-120 Mehmet R. Altıokka* Department of Chemical Engineering, Anadolu UniVersity, 26470, Eskis¸ ehir, Turkey
Halit L. Hos¸ gu1 n Department of Chemical Engineering, Eskis¸ ehir Osmangazi UniVersity, 26480, Eskis¸ ehir, Turkey
The kinetics of the hydrolysis of benzaldehyde dimethyl acetal has been studied using a circulated batch reactor in dioxane. Amberlite IR-120, in its acidic form, was used as a heterogeneous catalyst. Kinetic expression for the formation of acetal was also determined since the reaction is reversible. In the temperature range 298-328 K, the equilibrium constant for hydrolysis of benzaldehyde dimethyl acetal was found to be Ke ) exp(8.67 - 1880/T) mol‚L-1 where T is the absolute temperature in Kelvin. In the presence of catalyst, the reaction has been found to occur between an adsorbed water molecule and a molecule of acetal in the bulk phase (Eley-Rideal model). It was also observed that benzaldehyde adsorbed by the catalyst has an inhibiting effect on the reaction rate. From this model it was concluded that the reaction is a “surface reaction control” and its rate will be given by the expression -rW ) [k(m/V)(CACW - ((CBACM2)/Ke))]/[1 + KBACBA + KWCW] where concentrations are in the unit of mol‚L-1. It was shown that temperature dependency of the hydrolysis rate constant can be given by k ) exp(9.4 - 4915/T) L2‚(g-dry resin)-1‚mol-1‚min-1. The adsorption equilibrium constants related to benzaldehyde and water were also calculated to be KBA ) exp(7292/T - 24.9) L‚mol-1, KW ) exp(1296/T - 4.4) L‚mol-1, respectively. Introduction Aldehydes used in the fragrance industry are unstable under the alkali conditions and are also prone to chemical attack by oxygen, resulting in the change in odor and color of the finished product. Therefore, these aldehydes need to be stabilized by acetals since they give no reaction in alkali media.1 It is also reported that acetals can be used as an additive that increase the cetane number of the fuel and help in the combustion of the resulting mixture.2,3 Additionally, acetals are used as intermediates for the synthesis of various industrial chemicals and organic solvents.4,5 Specifically, benzaldehyde dimethyl acetal, studied in this work, has sweet-green and warm odor remotely reminiscent of nuts and bitter almonds. It is also in flavor compositions for the imitation cherry, fruit, almond, nut, etc.6 Acetals are synthesized by addition of 2 mol of alcohol to an aldehyde carbonyl group. The reaction is reversible and commonly catalyzed by acid.7 Both homogeneous and heterogeneous catalysts have been used for this purpose. While p-toluenesulfonic acid, hydrochloric acid, and sulfuric acid can be given as the example of homogeneous catalysts, a cationexchange resin in the acidic form can serve as a heterogeneous catalyst. The use of heterogeneous catalyst has the inherent advantages over catalysis effected by dissolved electrolytes since they eliminate the corrosive environment and can be easily removed from the reaction mixture by decantation and filtration. It is also claimed that heterogeneous catalyst suppresses the side reaction, leading to the higher purity of product.8 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +90-222-321 35 50/6501. Fax: +90-222-323 95.
Many researchers have studied the synthesis of acetals in the presence of heterogeneous catalysis. The reaction between ethanol and acetaldehyde was realized using the acid resin Amberlyst 18 as a catalyst in a batch reactor. It is found that the experimental data verify the model based on a LangmuirHinshelwood rate expression.9 The same reaction has also been studied by using various types of heterogeneous catalysts. It is reported that Amberlyst 15 is the most active catalyst among the other catalysts in acidic form.10 A heterogeneous kinetic model related to the acetal formation from n-octanal and methanol has been developed to observe that the Eley-Rideal type of mechanism prevails with chemisorption of the aldehyde on the active sites.11 In other works, the heterogeneous catalytic acetalization reaction has been studied in batch mode, in semibatch reactive distillation mode, and in a continuous reactive distillation column. The data obtained from the batch reactor were explained with a pseudo-homogeneous model. More than 98% conversion of aldehyde was obtained in semibatch reactive distillation mode.12,13 In this work, the development of a kinetic model for the hydrolysis of benzaldehyde dimethyl acetal was the goal. The hydrolysis reaction was realized in a batch reactor using Amberlite IR-120 as a heterogeneous catalyst. The kinetic model obtained from hydrolysis reaction can also be used for the formation of acetal since the reaction is reversible. Experimental Section Chemicals. Amberlite IR-120, an acidic cation-exchange resin, was obtained from Rohm and Haas Co. The water content of the resin was determined to be 49% and this value was taken into account in the mass balance. The resin was in the form of
10.1021/ie060716o CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
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beads of 26-45 mesh in size and it keeps its size during the experiments. Benzaldehyde dimethyl acetal of 97% purity (Merck), methanol of 99% purity (Lab-Scan), benzaldehyde of >99% purity (Merck), and 1,4-dioxane of 99.8% purity (Carlo Erba) were used in all experiments. The water used in the hydrolysis reaction was pure distilled water. Apparatus. The reactor consisted of a stainless steel vessel of 400 mL capacity fitted with a sampling device and a conical bottom. The conical end of the reactor has an outlet fitted with stainless steel lattice to prevent the transportation of catalyst beads. The reaction mixture was circulated, by means of a pump, between the bottom and top of the reactor. The temperature was controlled within (0.1 K by circulating water from a thermostat into the water jacket of the reactor. Procedure. In a typical run, dioxane and one reactant were placed in the reactor. A known weight amount of the catalyst was added and circulation of the reactor contents commenced. After a steady value of the desired temperature was attained, the second reactant, at the same temperature, was added and this was taken as zero time for a run. Two milliliters of a liquid sample was withdrawn from the reactor at regular intervals for analysis. Analysis. The samples were analyzed on a gas chromatograph (HP 5890 Series II) equipped with flame ionization detector (FID) and the compounds were separated by a capillary column (DB-WAX). The column temperature was programmed with a 3 min initial hold at 40 °C, followed by a 5 °C‚min-1 ramp-up to 100 °C, held for 3 min, and then a 40 °C‚min-1 ramp-up to 220 °C, held for 4 min. The water content of the reaction mixture was measured by Karl Fischer titration (Metrohm KF-784). Interpretation of Experimental Results The effects of the parameters, such as reactant molar ratio, temperature, amount of catalyst, and product concentration, on the reaction rate were studied. The influence of external or pore diffusion can be neglected as reported in previous studies.14,15 It was also stated that the deactivation was insignificant in the period of the reaction time.8 Determination of the Equilibrium Constant. To determine the equilibrium constant, the mixture of benzaldehyde dimethyl acetal and water, each 1 M, was used. The amount of catalyst was 2 g on dry basis per liter of reaction mixture. The temperature of the reaction was fixed at 298 K. The sample withdrawn from the reactor at regular intervals was analyzed. When the composition of the last three successive samples was found to be unchanged, it was concluded that the mixture was at equilibrium. Then, the same procedure was performed at the temperature 313 and 328 K. With use of the equilibrium expression of
Figure 1. Temperature dependency of equilibrium constant.
constant Ka. This is also an expected result since the activities of species can be taken to be the concentrations in dilute solutions as used in this work. Figure 1 also reveals that the temperature dependency of the equilibrium constant can be given by
Ke) exp(8.67 - 1880/T) mol‚L-1
(2)
where T is the absolute temperature in Kelvin. Effect of Catalyst Loading. The catalyst loading was varied over a range of 0-3 g‚L-1 of reaction mixture, on a dry basis, keeping the other parameters constant. The result is shown in Figure 2. As seen from this figure, the initial reaction rate is linearly increasing with catalyst loading, since the active surface area is proportional to the amount of catalyst.
Figure 2. Effect of catalyst loading on the initial reaction rate.
Ka [M]2[BA] Ke ) ) Kγ [A][W]
(1)
the equilibrium constant was calculated at different temperatures. The temperature dependency of the equilibrium constant was determined from Ln Ke versus 1/T plot given in Figure 1. From the slope of the line given in Figure 1, the heat of reaction was found to be 15.66 kJ‚mol-1, which is in good agreement with the literature data of 15.72 kJ‚mol-1 obtained from the heat of formation of the species in the reaction. In this calculation, the heat of formation of benzaldehyde dimethyl acetal was calculated by the Joback Group Contribution Method and that of the others was already tabulated in the literature.16 This comparison indicated that Ke is equal to thermodynamical equilibrium
Effects of Acetal, Alcohol, Water, and Aldehyde Concentrations. The initial reaction rate was determined experimentally by altering the concentration of the component under investigation while keeping that of the other components constant. The results are shown in Figure 3-6. From these figures it is evident that while water and benzaldehyde were adsorbed on the surface, acetal and methyl alcohol were not. It was also reported that the adsorption of solvent (dioxane) on the surface of Amberlite resin was negligible.14,15,17 Reaction Mechanism and Kinetics. Experiments showed that hydrolysis of acetal was much faster than synthesis of acetal. Therefore, kinetic parameters for the acetal synthesis were obtained from the hydrolysis of acetal in a batch reactor. This is always possible since the reaction is reversible.
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Figure 3. Effect of acetal concentration on initial reaction rate. Figure 6. Effect of benzaldehyde concentration on initial reaction rate.
Figure 4. Effect of water concentration on initial reaction rate. Figure 7. 1/-rW0 vs 1/Cw0 for eq 6 at different temperatures.
Figure 5. Effect of methyl alcohol concentration on initial reaction rate. Figure 8. 1/-rW0 vs CBA0 for eq 8.
The conclusion obtained from Figures 3-6 implies that the reaction mechanism can be represented by the Eley-Rideal model, which is the reaction taking place between adsorbed molecules of water and the molecules of acetal in the bulk solution. On the other hand, Figure 6 reveals that benzaldehyde molecules adsorbed by the catalyst have an inhibiting effect on the reaction rate. Depending on this approach, the reaction mechanism can be given as follows:
Figures 3-6 also show that the reaction rate is highly temperature sensitive. Therefore, it is reasonable to accept that the overall reaction is controlled by the surface reaction step. Under these assumptions the general reaction rate expression can be given by
W + S S W.S adsorption of water
(3a)
W.S + A S BA.S + 2M surface reaction
(3b)
BA.S S BA + S desorption of benzaldehyde
(3c)
A + W S BA + 2M overall reaction
(3d)
(
)
CBACM2 m CACW V Ke -rW ) 1 + KBACBA + KWCW k
(4)
where subscript A, W, BA, and M refer to acetal, water, benzaldehyde, and methanol, respectively, k is the reaction rate constant for hydrolysis, Ke is the equilibrium constant of the
Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1061 Table 1. Values of the Slope and Intersection of the Lines Given in Figures 3, 7, and 8 eq 5, Figure 3
eq 6, Figure 7
eq 8, Figure 8
T (K)
k(m/V)CW0/(1 + KWCW0)
VKW/(kmCA0)
V/(kmCA0)
V(1 + KWCW0)/(kmCW0CA0)
VKBA/(kmCW0CA0)
298 313 328
0.0008 0.0022 0.0041
648.54 232.93 116.01
593.45 139.84 77.93
1207.10 414.07 222.51
416.46 193.03 80.25
Table 2. Calculated Values of k, KBA, and KW T (K)
k, ×104 [L2‚(g-dry resin)-1‚mol-1‚min-1]
KBA (L‚mol-1)
KW (L‚mol-1)
298 313 328
7.9897 19.5876 35.9794
0.5674 0.2649 0.0595
0.9397 0.7343 0.6337
overall reaction, KBA and KW are the adsorption equilibrium constants, m is the quantity of dry catalyst, and V is the volume of the reaction mixture. For the initial reaction rate, with no product present, eq 4 can be reduced to
kmCW0/V -rW0 ) C 1 + KWCW0 A0
(5)
A plot of -rW0 versus CA0 provides a straight line with a slope of kmCW0/V(1 + KWCW0). These lines are presented in Figure 3 at different temperatures. If eq 5 is rearranged for variable CW0 values, the following expression can be obtained:
VKW 1 1 V ) + -rW0 kmCA0 kmCA0 CW0
Integrated Rate Expressions. If the reaction rate given in eq 4 is expressed in terms of conversion of water, the following equation can be obtained:
-rW ) CW0
dXW ) dt
[
]
4(C3W0X3W) km 2 CW0(1 - XW)(M - XW) V Ke [1 + KBACW0XW + KWCW0(1 - XW)]
(12)
A numerical integration by the Runga-Kutta method gave several points (xi,ti) which allowed us to plot the calculated curve shown in Figure 9at 313 K, under the condition of M ) CA0/ CW0 ) 1, m/V ) 2 g of dry catalyst‚L-1, and CW0 ) 1. A good agreement between calculated curve and experimental points is observed.
(6)
A plot of 1/-rW0 versus 1/CW0 provides a straight line with slope of V/kmCA0 and intersection of VKW/kmCA0. These lines were presented in Figure 7 at different temperatures. To determine the inhibiting effect of benzaldehyde concentration eq 5, with no methanol present initially, can be written as
m k (CACW) V -rW ) 1 + KBACBA + KWCW
(7)
Figure 9. Experimental points and calculated curve from eq 12 at 313 K.
Conclusions
from which the following equation can be obtained
A plot of 1/-rw0 versus CBA0 provides a straight line with slope of VKBA/kmCW0CA0 and intersection of V(1 + KWCW0)/ kmCW0CA0. These lines were presented in Figure 8 at different temperatures. The values of the slope and intersection of the lines, shown in Figures 3, 7, and 8, have been tabulated in Table 1. Solving these equations by the method of averages18 k, KBA, and KW were found at different temperatures. The resulting values are given in Table 2. Effect of Temperature. Applying the Arrhenius equation to the values given in Table 2, the temperature dependency of the constants was found to be
Kinetic data for the formation of benzaldehyde dimethyl acetal has been obtained by studying the hydrolysis of benzaldehyde dimethyl acetal since the reaction is reversible. The reaction has been investigated in a circulated batch reactor in dioxan in the presence of Amberlite IR-120 as a heterogeneous catalyst. It has been indicated that the equilibrium constant for the hydrolysis of acetal, in the temperature range 298-328 K, was found to be Ke) exp(8.67 - 1880/T) mol‚L-1 where T is absolute temperature in Kelvin. It was also shown that the reaction occurs between an adsorbed water molecule and a molecule of acetal in the bulk phase (Eley-Rideal model). Additionally, the inhibiting effect of benzaldehyde on the reaction rate was observed since it is adsorbed by the catalyst. From these analyses, it was concluded that the reaction is controlled by surface reaction and its rate and its related constants will be given by eq 4 and eqs 9-11, respectively.
k (L2‚(g-dry resin)-1‚mol-1‚h-1) ) exp(9.4 - 4915/T)
Nomenclature
V(1 + KWCW0) VKBA 1 ) + C -rW0 kmCW0CA0 kmCW0CA0 BA0
(8)
(9)
KBA(L‚mol-1) ) exp(7292/T - 24.9)
(10)
KW(L‚mol-1) ) exp(1296/T - 4.4)
(11)
A, W, BA, and M ) acetal, water, benzaldehyde, and methanol, respectively E ) activation energy, kJ‚mol-1 k ) rate constant, mol‚h-1‚g-1
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Ke ) equilibrium constant Ki ) adsorption equilibrium constant for component i m ) catalyst mass, g M ) CA0/CW0 R ) 8.314, J‚mol-1‚K-1 S ) vacant adsorption site T ) temperature, K xi ) mole fraction of component i V ) volume of the reaction mixture Subscripts A, W, BA, and M ) acetal, water, benzaldehyde, and methanol, respectively e ) at equilibrium state 0 ) at initial condition a ) activity γ ) activity coefficient Literature Cited (1) Solomons, G. T. W. Organic Chemistry; John Wiley and Sons: New York, 1996. (2) Bonnhoff, K.; Obenaus, F. Dieselkraftstoff. DE2911411, 1980. (3) Oppenlaender, K.; Merger, F.; Strickler, R.; Hovemann, F.; Schmidt, H.; Starke, K.; Stork, K.; Vodrazka, W. Verwendung von Polya¨thern und Acetalen auf der Basis von Methanol und/oder A ¨ thanol als Kraftstoffe fu¨r Dieselmotoren sowie diese Komponenten enthaltende Kraftstoffe fu¨r Dieselmotoren. EP0014922, 1980. (4) Kaufhold, M.; El-Chahawi, M. Process for preparing acetaldehyde diethyl acetal. U.S. Patent 5527969, 1996. (5) Aizawa, T.; Nakamura, H.; Wakabayashi, K.; Kudo, T.; Hasegawa, H. Process for producing acetaldehyde dimethylacetal. U.S. Patent 5362918, 1994. (6) Arctander, S. Perfume and FlaVor Chemicals (Aroma Chemicals); Published by the Author: Montclair, NJ, 1969.
(7) Pine, S. H. Organic Chemistry; McGraw-Hill: New York, 1987. (8) Altıokka, M. R.; Citak, A. Kinetics Study of Esterification of Acetic acid with Isobutanol in the Presence of Amberlite Catalyst. Appl. Catal., A 2003, 239, 141. (9) Silva, V. M. T. M.; Rodrigues, A. E. Synthesis of Diethylacetal: Thermodynamic and Kinetic Studies. Chem. Eng. Sci. 2001, 56, 1255. (10) Capeletti, M. R.; Balzano, L.; Puente, G.; Laborde, M.; Sedran, U. Synthesis of Acetal (1,1-diethoxyethane) From Ethanol and Acetaldehyde over Acidic Catalysts. Appl. Catal., A 2000, 198, L1. (11) Yadav, G. D.; Pujari, A. A. Kinetics of Acetalization of Perfumery Aldehydes with Alkanols over Solid Acid Catalysts. Can. J. Chem. Eng. 1999, 77, 489. (12) Chopeda, S. P.; Sharma, M. M. Reaction of Ethanol and Formaldehyde: Use of Versatile Cation-Exchange Resins as Catalyst in Batch and Reactive Distillation Columns. React. Funct. Polym. 1997, 32, 53. (13) Chopeda, S. P.; Sharma, M. M. Acetalization of Ethylene Glycol with Formaldehyde Using Cation-Exchange Resins as Catalysts: Batch versus Reactive Distillation. React. Funct. Polym. 1997, 34, 37. (14) Darge, O.; Thyrion, F. C. Kinetics of the Liquid Phase Esterification of Acrylic Acid with Butanol Catalysed by Cation Exchange Resin. J. Chem. Technol. Biotechnol. 1993, 58, 351. (15) Dassy, S.; Wiame, H.; Thyrion, F. C. Kinetics of the Liquid-Phase Synthesis and Hydrolysis of Butyl Lactate Catalysed by Cation Exchange Resin. J. Chem. Technol. Biotechnol. 1994, 59, 149. (16) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids; McGraw-Hill: New York, 1999. (17) Choi, J. I.; Hong, W. H.; Chang, H. N. Reaction kinetics of lactic acid with methanol catalyzed by acid resins. Int. J. Chem. Kinet. 1996, 28, 37. (18) Jenson, V. G.; Jeffreys G. V. Mathematical Methods in Chemical Engineering; Academic Press: New York, 1977.
ReceiVed for reView June 6, 2006 ReVised manuscript receiVed December 3, 2006 Accepted December 4, 2006 IE060716O
