Kinetic Study of Catalytic Esterification of Butyric Acid and Ethanol over Amberlyst 15

The esterification reaction of butyric acid with ethanol has been studied in the presence of ion exchange resin (Amberlyst 15). Ethyl butyrate was obtained as the only product which is used in flavours and fragrances. Industrially speaking, it is also one of the cheapest chemicals, which only adds to its popularity. The influences of certain parameters such as temperature, catalyst loading, initial concentration of acid and alcohols, initial concentration of water, and molar ratio were studied. Conversions were found to increase with an increase in both molar ratio and temperature. The experiments were carried out in a batch reactor in the temperature range of 328.15–348.15 K. Variation of parameters on rate of reaction demonstrated that the reaction was intrinsically controlled. Experiment kinetic data were correlated by using pseudo-homogeneous model. The activation energy for the esterification of butyric acid with ethanol is found to be 30 k J/mol.


Introduction
Organic esters are important fine chemicals used widely in the manufacturing of flavors, pharmaceuticals, plasticizers, and polymerization monomers.They are also used as emulsifiers in the food and cosmetic industries.Several synthetic routes are available for obtaining organic esters, most of which have been briefly reviewed by Yeramian et al. [1].Several synthetic routes have been used to make organic ester, but the most-used methodology for ester synthesis is direct esterification of carboxylic acids with alcohols in the presence of acid/base catalysts.Esterification is used.The most acceptable method is to react the acid with an alcohol catalyzed by mineral acids, and the reaction is reversible [2].Esterification of carboxylic acid with alcohol in the presence of acid catalyst has been the subject of investigation of many research workers [3].The traditional industrial process of synthesizing esters using homogeneous acid catalyst is conveniently replaced by solid acid catalyst, ion exchange resins, clay, and so forth [4].The ion exchange resin is a promising material for replacement of homogeneous acid catalysts.The use of ion exchange resins as solid catalyst has many advantages over homogeneous acid catalysts.They eliminate the corrosive environment; they can separate from liquid reaction mixture by filtration and have high selectivity [5].They can also be used repeatedly over a prolonged period without any difficulty in handling and storing them.And the purity of the products is higher since the side reactions can be completely eliminated [6].Fatty acid esters are used as raw materials for emulsifiers, oiling agents for foods, spin finish and textiles, lubricants for plastics, paint and ink additives and for mechanical processing, personal care emollients, and surfactants and base materials for perfumes.They are used as solvents, cosolvents, and oil carriers in the agricultural industries [7].The esters of biobased organic acids fall into the category of benign or green solvents and are promising replacements for halogenated petroleum-based solvent in a wide range of applications [8].
The ester of butyric acid and ethanol, namely, ethyl butyrate, finds wide applications.It is commonly used as ISRN Chemical Engineering artificial flavoring such as pineapple flavoring in alcoholic beverage, as a solvent in perfumery products and as a plasticizer for cellulose.Ethyl butyrate is one of the most common chemicals used in flavors and fragrances.It can be used in a variety of flavors: orange (most common), cherry, pineapple, mango, guava, bubblegum, peach, apricot, fig, and plum.Industrially speaking, it is also one of the cheapest chemicals, which only adds to its popularity.Although, to the best of our knowledge, no researcher has reported the kinetics of esterification of butyric with ethanol in the presence of Amberlyst 15, there are a number of studies related to the other esterification reactions catalyzed by solid resins.

Experimental
2.1.Chemicals.Ethanol of >98.5% purity (Merck) and butyric acid of 99.8% purity (Merck) are used as the reactants.The reaction occurred in the solution of dioxane of 99.0% purity (Merck).All the aqueous solutions used in the analysis are prepared with distilled water, sodium hydroxide (SD Fine Chemicals Ltd., Boisar, India), and phenolphthalein indicator, (Qualigens Fine Chemicals, Mumbai, India).

Catalyst. Amberlyst 15 (Rohm and Hass
) is used as catalyst.It is synthesized from styrene-divinylbenzene by suspension polymerization and finally sulfonated by sulfuric acid.The esterification reaction of butyric acid with ethanol was studied using different catalysts.The catalytic activity of Amberlyst 15 is higher than the Amberlyst 35.It is due to Amberlyst 15 has higher surface area and pore volume than Amberlyst 35.So, Amberlyst 15 is found to be the best catalyst to generate the data.

Apparatus and Procedure
Esterification reaction of butyric acid with ethanol with the initial concentrations of 1.44 mol/L and 14.84 mol/L, respectively, was carried out in three-necked flask of 500 mL capacity fitted with a reflux condenser, a thermometer, and a magnetic stirrer.The initial volume of reaction mixture was approximately 150 mL.A reflux condenser is used to avoid the loss of volatile compounds.The ion exchange resin suspends in the reaction mixture with the help of stirrer.Temperature inside the reactor is controlled within the accuracy of ±0.5 K.In all the experiments, a known amount of butyric acid and the catalyst charged into the reactor and heated to the desired temperature.All of the reactants charged in the reactor are volumetrically measured.This time is considered the starting point of the reaction.Samples were withdrawn at a regular time intervals for analysis.The progress of the reaction was followed by withdrawing small enough samples to consider them negligible compared to the volume of the reaction mixture at regular intervals.

Analysis
All samples were measured in measuring cylinder of 1 mL, taken periodically, and titrated against 1 N standard NaOH solution using phenolphthalein as an indicator.

Kinetic Modeling
A pseudo-homogeneous model cab was effectively applied to correlate the kinetic data of the liquid solid catalytic reaction.
The esterification reactions are known to be reversible reaction of second order.The general rate expression can be given by where  = butyric acid,  = ethanol,  = ethyl butyrate, and  = water, and where subscript   and   refer to acid and alcohol concentrations, respectively,   is forward reaction rate constant,   is the equilibrium constant of the reaction,  is the adsorption equilibrium constants,  is the quality of dry resin, and  is the volume of the resin.
For the initial reaction rate, with no product present, (2) can be reduced to A plot of − 0 versus  0 provides a straight line with slope of (   0 /(1 +    0 )).These lines were presented in Figure 1 at temperature 348 K.If (3) is rearranged for variables  0 values, the following expression can be obtained: A plot of  0  0 /(− 0 ) versus  0 produces a straight line with the slope of   /   and intersection of /  .These lines are presented in Figure 1 at 348 K. From the slope and lines shown in Figure 2, the following equation can be held.In order to evaluate the inhibiting effect of water concentration, (2), with no ester present initially, can be written as from which the following equation can be obtained: Solving these equations by the "method of averages, " we found the values of

Integrated Rate Expression.
If the reaction rate given in (3) is expressed in terms of conversion of butyric acid, the following equation can be obtained.Adsorption effect of alcohol and water is neglected as discussed earlier and as alcohol is taken in far excess, so  0 is taken as constant; hence, (3) becomes a pseudo-first-order rate equation: Putting where   is the fractional conversion of .Thus, a plot of − ln(1 −   ) against time  gives a slope which represents .

Results and Discussion
The esterification reaction of butyric acid with ethanol was studied in a batch reactor in the presence of acidic ion exchange resin catalyst, Amberlyst 15.The kinetics of esterification of butyric acid is studied at different temperatures from 328.15 to 348.15 K with different molar ratios and varying catalyst loading (Figure 11).A pseudo-homogeneous model has been used to describe the esterification reaction catalyzed by Amberlyst 15.The effects of temperature, catalyst loading, and molar ratio are studied.
6.1.Catalyst Performance.In Figure 5, a comparison of the behavior of the Amberlyst 15 and Amberlyst 35 catalysts is shown.Catalysts are used to access their efficacy in the esterification of butyric acid with ethanol.Amberlyst 15 is shown to be an effective catalyst as compared to Amberlyst 35.The catalytic activity of Amberlyst 15 has higher activity than Amberlyst 35.It is due to Amberlyst 15 has higher total exchange capacity, surface area, and pore volume than Amberlyst 35.So, Amberlyst 15 is found to be the best catalyst to generate the data (Table 1).   to 81.4 kg/m 3 .Keeping butyric acid and ethanol molar ratio 1 : 10 and and the reaction temperature 75 ∘ C. It is found that with increase in catalyst loading, the conversion of butyric acid increased with the proportional increase in the active sites.At higher catalyst loading, the rate of mass transfer is high, and therefore, there is no significant increase in the rate [9].This data is once again used to plot − ln(1 −   ) versus  in Figure 7 and get a straight line passing through origin, the slope of which gives the value of .

Effect of Catalyst
The values of  are plotted against catalyst loading,  (kg/m 3 ) in Figure 8, and it is observed that with the increase in , there is a linear increase in ; thus, it validates the model.

Effect of Molar Ratio. The concentration of ethanol had
an influence on the reaction rate and on the conversion.The results are given in Figure 9.It can be seen that the conversion of acid increases with increasing the initial molar ratio of ethanol to butyric acid.For the esterification with ethanol, the equilibrium conversion of butyric acid increases from 31% to 81% on varying ethanol to butyric acid ratio from 1 : 1 to 1 : 15 for catalyst loading 73.2 kg/m 3 at temperature 348 K (Figure 4).The result observed for the effect of initial molar ratio of ethanol to butyric acid indicated that the equilibrium conversion of butyric acid can be effectively enhanced by using a large excess of ethanol.
For calculating , these data are once again used to plot − ln(1 −   ) versus  to get a straight line passing through origin shown in Figure 10 the slope of which gives the value of .The values of  are plotted against catalyst loading,  (kg/m 3 ), and it was observed that with the increase in , there was a linear increase in ; thus, it validates the model.

Effect of Temperature.
The study of the effect of temperature is very important because this information is useful in calculating the activation energy.In order to investigate the effect of temperature, esterification reactions are carried out in the temperature region from 328 to 348 K while keeping the acid : alcohol ratio at 1 : 10 and the catalyst loading of 73.2 kg/m 3 dry resin (Figure 13).With an increase in reaction temperature, the initial reaction rate or the conversion of the butyric acid is found to increase substantially as shown in Figure 12.It shows that the higher temperature yields the greater conversion of the acid at fixed contact time.Thus, 75 ∘ C is chosen as the maximum working temperature for the butyric acid esterification.Arrhenius law expresses the temperature dependency of the rate constant: where  0 = frequency factor,  0 = activation energy, and  = gas constant.This data is once again used to plot − ln(1−  ) versus  to get a straight line passing through origin, the slope of which gives the value of .
Equation (1), a plot of − ln  versus 1/, at constant acid and alcohol concentrations, gives a straight line with a slope of (  /) as shown in Figure 14.
The study on the effect of temperature is very important for heterogeneously catalyzed reaction as this information is useful in calculating the activation energy for this reaction.An Arrhenius plot for the initial rate of reaction is given in Figure 14.The activation energy for the esterification reaction based on the initial rate is found to be 30 KJ/mol.

Conclusion
The kinetics of esterification of butyric acid with ethanol in the temperature ranging between 328.15 K and 348.15K and at molar ratio 1 : 5 to 1 : 15 is investigated experimentally in a stirred batch reactor using Amberlyst 15.The optimum conditions for synthesizing ethyl butyrate have been delineated.It has also been observed that the initial reaction rate increases linearly with acid and alcohol concentrations.And rate of reaction is found to increase with the increase in catalyst loading, temperature, and molar ratio.Eley-Rideal model is applied to study the kinetics of the reaction.As a result, it is concluded that the acidic resin, Amberlyst 15, is a suitable catalyst for this reaction, since in the presence of it, the reaction has been found to take place between an adsorbed alcohol molecule and a molecule of acid in the bulk phase (Eley-Rideal model).It is also observed that water has inhibiting effect of reaction.Temperature dependency of the kinetic constants has been found to apply Arrhenius equation.The activation energy is found to be 30 KJ/mol.
Loading.In Figure 6 experiments work conducted by varying the catalyst loading from 24.4 kg/m 3

Table 1 :
Characteristics of the catalysts.