Ion-exchange Resin Catalyzed Esterification of Lactic Acid with Isopropanol : a Kinetic Study

The kinetic behavior of esterification of lactic acid with isopropanol over an acidic cation exchange resin, Amberlyst 15, was studied under isothermal condition. Isopropyl lactate synthesized in this reaction is an important pharmaceutical intermediate. The experiments were carried out in a stirred batch reactor in the temperature range of 323.15 to 353.15 K. The effect of various parameters such as temperature, molar ratio and catalyst loading was studied. Variation in parameters on rate of reaction demonstrated that the reaction was intrinsically controlled. Kinetic modeling was performed using Eley-Rideal model which acceptably fits the experimental data. The activation energy was found to be 22.007 kJ/mol and frequency factor was 0.036809 l2 g-1 mol-1 min-1 for forward reaction. The value of entropy for the forward reaction was found to be 182.317 J K-1 mol-1 . © 2011 BCREC UNDIP. All rights reserved

investigated.Typical homogenous catalysts like H2SO4, HCl and ClSO3OH are used but due to their miscibility with the reaction medium, separation becomes a problem, hence ion exchange resins are preferred over homogeneous acids due to ease of product separation and catalyst recovery [6].The drive to develop green processes has led to the development of solid acid catalysts to a significant increase in research activities both in academic and industrial sections.These materials can replace the corrosive liquid acids currently used in many industries [7].Therefore heterogeneous catalyst become an attractive alternative catalyst which is non-polluting, non corrosive and has long activity life.Scientific literature contains some reports on the use of heterogeneous acid catalysts bcrec_791_2011 Copyright © 2011, BCREC, ISSN 1978-2993 for esterification.The most popular solid acid catalysts used to produce esters have been ionexchange organic resins, such as amberlyst 15, Amberlyst 35, Amberlyst 36, Dowex 50 W etc [8][9][10][11][12][13][14][15].In the recent years a lot of studies were subjected to ion-exchange resins, which are distinguished among solid-state catalysts by their reactivity and selectivity.They are also relatively cheap and could be reused many times [9][10][11][12][13][14][15][16] Lactic acid and their esters are used in food industry for preservation and flavoring purposes, as well as in the pharmaceutical and cosmetic industries [9].Sanz et al [17,18] studied the ion exchange resin catalyzed esterification of lactic acid with methanol and ethanol and reported that Amberlyst 15 is most suitable catalyst for the reaction.Zang et al [19] investigated the esterification of lactic acid with ethanol in the presence of five different cation exchange resins.Their investigations indicated that pseudo homogeneous second order model was the most appropriate model to describe the kinetic behavior.Studies on esterification reaction of lactic acid with isopropanol has been done by Yadav et al [3] over Amberlyst 36 at higher molar ratios of reactants (i.e.lactic acid to iso-propanol upto 1:44).
They also studied the application of different heterogeneous catalyst such as Amberlyst 15, Indion 130, Amberlite-120, Dowex 50 W, and Filtrol-44.Amongst these Amberlyst 15, Amberlyst 36 and Indion-130 were observed to be most effective.By using the large excess of one of the reactants the reaction yield of other compound can be enhanced but because of the presence of the large excess of one of the reactant, the recovery of the product and the reactant becomes more expensive [18].
The present study aims at to provide insight into the kinetics of the esterification of lactic acid with isopropanol over amberlyst 15 at low molar ratios and compare the results with data available for higher molar ratios [3] and to investigate the effect of reaction temperature, catalyst loading and molar feed ratio.

Apparatus and Procedure
Reaction kinetics was studied in a three-necked glass reactor of 500 ml capacity operating in a batch mode.A reflux condenser was used to avoid the loss of volatile compounds.The catalyst was suspended in the reaction mixture with the help of stirrer.A simple pretreatment was given to the catalyst to convert it to acid form.The catalyst was initially washed with distilled water four to five times and was dried at ambient conditions for 4-5 hrs.Further the catalyst was washed by dipping in 0.1 N hydrochloric acid solution for half an hour.This solution was then filtered to separate the catalyst and dried at atmospheric conditions for about 48 hrs.In all the experiments, a known amount of lactic acid and the catalyst were charged into the reactor and heated to the desired temperature.Temperature inside the reactor was controlled within the accuracy of ±0.5 K.All of the reactants charged in the reactor were volumetrically measured.The progress of the reaction was followed by withdrawing samples at regular intervals, small enough to consider them negligible compared to the volume of the reaction mixture.

Analysis
To measure the concentration of unreacted acid in the reaction mixture, each sample was analyzed volumetrically by 1 N NaOH as titrant and phenolphthalein as an indicator.

Results and Discussion
The esterification reaction of lactic acid with isopropanol was studied in a batch reactor in the presence of acidic ion exchange resin catalyst; Amberlyst 15.The kinetics of esterification of lactic

Kinetics
Esterification of lactic acid with iso-propanol can be represented as: Reaction mechanism for this reaction can be well studied by Eley-Rideal model [20] that is the reaction takes place between adsorbed molecules of alcohol and the molecules of acid in bulk solutions.The adsorption of ester is reported to be negligible [20][21][22].Hence the rate equation is: (2) As stated in the literature the value of KB, Kw were found to be negligible for esterification of lactic acid with lower alcohols [3,23].As in absence of any product initially N=0, Equation 2 reduces to (3) Putting k1 CA0 (w/V) = k in equation 3 and then integrating the equation: Which can be written as: Y=kt+C (4) where The value of C at t=0 , XA=0

Effect of catalyst loading on reaction
The catalyst loading was varied from 4.73 kg/m 3 to 14.21 kg/m 3 (of the reaction mixture), which corresponds to 1 to 3 % (Weight/volume) of reaction mixture at a temperature of 353.15K and molar ratio, M of 1.5.Fig. 1 illustrates the variation in fractional conversion (XA) of lactic acid for different catalyst loading at specified temperature.Fractional conversion was found to be increasing with the increase in catalyst loading.Equilibrium conversion increases with increase in catalyst loading which could be due to more number of available acid sites for the reaction.For reactions catalyzed by the amberlyst series resins, the intraparticle diffusional resistances are usually negligible [20].
Thus typical second order rate equation was used as given in Equation ( 4) to find the values of rate constant k.Fig 2 .shows the plot of Y vs. time at different catalyst loading is straight line and the slope of these lines gives the rate constant.The rate constants thus calculated at different catalyst loading were plotted with catalyst loading (w) as shown in Fig. 3.It is seen that the rate is linearly proportional to w (weight of the catalyst) which is in accordance with the observation reported in [3, 20, and 22].

Effect of Temperature
To study the effect of the temperature is important since it is useful in calculating the activation energy of the reaction.The effect of temperature on conversion under otherwise similar conditions was studied in the range of 323.15 to 353.15 K (Fig. 4).Lui and Tan [21] have reported that ester conversion is dependent on the temperature and conversion increases with the increase in temperature.The plots Y vs. time gives a straight line as shown in Fig 5 and the rate constants for forward reaction at different temperatures were calculated using whole data and the values are tabulated in table 2. Temperature dependency of the specific reaction rate constant was expressed in terms of Arrhenius equation, which follows, (5) Where, ko = Frequency factor; E1 = Activation energy, and R = Gas constant (8.314JK 1 mol 1 ) For the linear plot of ln k1 versus to 1000/T Fig. 6, activation energy (E1) and frequency factor (ko) for the forward reaction was determined.E1 was found to be 22.0079 kJ/mol and k0 was found to be 3.681*10 -2 l 2 g -1 mol -1 min -1 .The increase in the value of the equilibrium constant with an increase in temperature indicates that the reaction is endothermic in nature.Reaction enthalpy ( H ) and reaction entropy (S ) for forward reaction were obtained from Equation ( 6) by plotting ln K against 1000/T [7].(6) Where K = equilibrium constant, R = universal gas constant.The value of enthalpy suggests that the esterification reaction is endothermic in nature.The linear plot yields S o = 182.317J K -1 mol -1 and H o = 70.022kJ mol -1

Effect of molar ratio
initial molar ratio of lactic acid to isopropanol was varied from 1 to 3 for the esterification reaction at a catalyst loading of 11.84 kg/m 3 at 353 K.As can be seen from

Conclusions
Kinetic behavior has been investigated experimentally for the synthesis of isopropyl lactate using Amberlyst 15 as a catalyst.The experimental data were obtained in a batch reactor within the temperature range (323.15K to 353.15 K).Reaction entropy and enthalpy for the esterification of lactic acid with isopropanol over Amberlyst 15 at different temperatures were found to be S o = 182.317JK -1 mol -1 and H o = 70.022kJmol -1 .Activation energy and frequency factor for the forward were found to be 22.0079 kJ/mol and 0.036809 l 2 g -1 mol -1 min -1 respectively.The reaction is endothermic in nature.The reaction kinetics was well represented by Eley-Rideal model.The reaction rate increased with the increasing temperature of the reaction, weight of the catalyst, and molar ratio.The experimental work showed that esterification of lactic acid with isopropanol can successfully be carried out at low molar ratio also.

Table 1 .
Experimental conditions and range of fractional conversion obtained acid was studied at different temperatures from 323-353 K, different molar ratios (M i.e. molar ratio of isopropanol to lactic acid) and varying the catalyst loading as given in table 1.

Table 2 .
Rate constant values at different temperature