Kinetic Study of Catalytic Esterification of Butyric Acid and Ethanol ...

Hindawi Publishing Corporation ISRN Chemical Engineering Volume 2013, Article ID 520293, 6 pages http://dx.doi.org/10.1155/2013/520293

Research Article Kinetic Study of Catalytic Esterification of Butyric Acid and Ethanol over Amberlyst 15 Nisha Singh,1 Raj kumar,2 and Pravin Kumar Sachan3 1

Department of Chemical Engineering, RNCET, Panipat, Haryana 132103, India Department of Chemical Engineering, Bipin Tripathi Kumaon Institute of Technology, Dwarahat, Almora, Uttarakhand 263653, India 3 Department of Biochemical Engineering, Bipin Tripathi Kumaon Institute of Technology, Dwarahat, Almora, Uttarakhand 263653, India 2

Correspondence should be addressed to Raj kumar; [email protected] Received 29 June 2013; Accepted 5 August 2013 Academic Editors: G. Bayramoglu and R. Mariscal Copyright © 2013 Nisha Singh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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.

1. 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

2 0.007 0.006 −rA0 (mol/L·min)

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.

ISRN Chemical Engineering

0.005 0.004 0.003 0.002 0.001 0 0

0.5

2. 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). 2.2. 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.

3. 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.

4. 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.

1

1.5

2

CA0 (mol/L)

Figure 1: Effect of acid concentration on initial reaction rate: (𝐶𝐵0 = 14.84 mol/L).

5. 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 𝐴 + 𝐵 󳨀→ 𝐸 + 𝑊,

(1)

where 𝐴 = butyric acid, 𝐵 = ethanol, 𝐸 = ethyl butyrate, and 𝑊 = water, and −𝑟𝐴 =

𝐾𝑓 (𝑚/𝑉) (𝐶𝐴 𝐶𝐵 − (𝐶𝐸 (𝐶𝑊/𝐾𝑒 ))) 1 + 𝐾𝐵 𝐶𝐵 + 𝐾𝑊𝐶𝑊

,

(2)

where subscript 𝐶𝐴 and 𝐶𝐵 refer to acid and alcohol concentrations, respectively, 𝐾𝑓 is forward reaction rate constant, 𝐾𝑒 is the equilibrium constant of the reaction, 𝐾 is the adsorption equilibrium constants, 𝑚 is the quality of dry resin, and 𝑉 is the volume of the resin. For the initial reaction rate, with no product present, (2) can be reduced to −𝑟𝐴0 = (

𝐾𝑓 𝑚𝐶𝐵0 /𝑉 1 + 𝐾𝐵 𝐶𝐵0

) 𝐶𝐴0 .

(3)

A plot of −𝑟𝐴0 versus 𝐶𝐴0 provides a straight line with slope of (𝐾𝑓 𝑚𝐶𝐵0 /𝑉(1 + 𝐾𝐵 𝐶𝐵0 )). These lines were presented in Figure 1 at temperature 348 K. If (3) is rearranged for variables 𝐶𝐵0 values, the following expression can be obtained: 𝐶𝐴0 𝐶𝐵0 𝑉𝐾𝐵 𝑉 = +( ) 𝐶𝐵0 . −𝑟𝐴0 𝐾𝑓 𝑚 𝐾𝑓 𝑚

(4)

A plot of 𝐶𝐴0 𝐶𝐵0 /(−𝑟𝐴0 ) versus 𝐶𝐵0 produces a straight line with the slope of 𝑉𝐾𝐵 /𝐾𝑓 𝑚 and intersection of 𝑉/𝐾𝑓 𝑚. 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.

3

3230

0.0047

3225

0.0046

3220

0.0045 −rA0 (mol/L·h)

CA0 ∗ CB0 /−rA0 (mol min/L)


3215 3210 3205 3200

0.0044 0.0043 0.0042 0.0041

3195 13

13.5

14 CB0 (mol/L)

14.5

15

In order to evaluate the inhibiting effect of water concentration, (2), with no ester present initially, can be written as −𝑟𝐴0 =

1 + 𝐾𝐵 𝐶𝐵0 + 𝐾𝑊𝐶𝑊0

,

(5)

from which the following equation can be obtained: 𝐶𝐴0 𝐶𝐵0 𝑉 (1 + 𝐾𝐵 𝐶𝐵0 ) 𝑉𝐾 = + ( 𝑊 ) 𝐶𝑊0 . −𝑟𝐴0 𝐾𝑓 𝑚 𝐾𝑓 𝑚

(6)

A plot of 𝐶𝐴0 𝐶𝐵0 /(−𝑟𝐴0 ) versus 𝐶𝑊0 at constant acid and alcohol concentrations gives a straight line with slope of 𝑉𝐾𝑊/𝐾𝑓 𝑚 and intersection of 𝑉(1 + 𝐾𝐵 𝐶𝐵0 )/𝐾𝑓 𝑚. These lines were presented in Figure 3. At temperature 348 K, 𝐾𝑓 𝑚𝐶𝐵0 𝑉 (1 + 𝐾𝐵 𝐶𝐵0 ) 𝑉𝐾𝐵 = 14.40, 𝐾𝑓 𝑚

= 0.0045,

𝑉 = 3012.06, 𝐾𝑓 𝑚

𝑉 (1 + 𝐾𝐵 𝐶𝐵0 ) = 3201.13, 𝐾𝑓 𝑚

(7)

Solving these equations by the “method of averages,” we found the values of 𝐾𝑓 = 4.53 × 10−6 L2 /mol, 𝐾𝐵 = 4.78 × 10−6 L/mol, 𝐾𝑊 = 7.35 × 10−6 L/mol. 5.1. 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 𝐶𝐵0 is taken as constant; hence, (3) becomes a pseudo-first-order rate equation: 𝑤 𝐶 𝐶 . 𝑉 𝐵0 𝐴

0.5

1

1.5

Figure 3: Effect of water concentration on initial reaction rate: catalyst loading = 73.2 kg/m3 ; temperature = 348 K.

Putting 𝐶𝐴 = 𝐶𝐴0 (1 − 𝑋𝐴), 𝑑𝑥𝐴 𝑤 = 𝐾𝑓 𝐶𝐵0 𝐶𝐴0 (1 − 𝑋𝐴) , 𝑑𝑡 𝑉 𝑑𝑥 𝑤 − 𝐴 = 𝐾𝑓 𝐶𝐵0 (1 − 𝑋𝐴 ) , 𝑑𝑡 𝑉 (9) 𝑥𝐴 𝑑𝑋𝐴 𝑤 = ∫ 𝐾𝑓 𝐶𝐵0 𝑑𝑡, ∫ − 𝑉 (1 − 𝑋𝐴 ) 0 𝑤 − ln (1 − 𝑋𝐴) = 𝐾𝑓 𝐶𝐵0 𝑡, 𝑉 where 𝑋𝐴 is the fractional conversion of 𝐴. Thus, a plot of − ln(1 − 𝑋𝐴) against time 𝑡 gives a slope which represents 𝐾. −𝐶𝐴0

6. Results and Discussion

𝑉𝐾𝑊 = 22.18. 𝑘𝑓 𝑚

−𝑟𝐴0 = 𝐾𝑓

0

CW0 (mol/L)

Figure 2: 𝐶𝐴0 𝐶𝐵0 /−𝑟𝐴0 versus 𝐶𝐵0 : temperature = 348 K; catalyst loading = 73.2 kg/m3 .

𝐾𝑓 (𝑚/𝑉) 𝐶𝐴0 𝐶𝐵0

0.004

(8)

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). 6.2. Effect of Catalyst Loading. In Figure 6 experiments work conducted by varying the catalyst loading from 24.4 kg/m3

4

ISRN Chemical Engineering Table 1: Characteristics of the catalysts.

−3

Skeletal density (g cm )

Amberlyst 35 5.32 34 5.20 0.28 329 1.542

Amberlyst 15 4.75 42 4.70 0.36 343 1.416

70 60 Fractional conversion

Property Acidity (eq. H+ kg−1 ) Surface area (m2 g−1 ) Total exchange capacity (eq/kg) Pore volume (cm3 g−1 ) ˚ Mean pore diameter (A)

80

50 40 30

CA0 ∗ CB0 /−rA0 (mol·min/L)

20 3230 3225

10

3220

0 0

3215

200

400 Time (min)

3210

Figure 5: Variation with type of catalyst. Amberlyst 15 (⧫) and Amberlyst 35 (◼), catalyst loading = 48.8 kg/m3 , temperature = 75∘ C, and molar ratio = 1 : 10 (butyric acid : ethanol).

3205 3200 3195 0

0.5

1

1.5

CW0 (mol/L)

3

to 81.4 kg/m . 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 − 𝑋𝐴 ) versus 𝑡 in Figure 7 and get a straight line passing through origin, the slope of which gives the value of 𝐾. The values of 𝐾 are plotted against catalyst loading, 𝑤 (kg/m3 ) in Figure 8, and it is observed that with the increase in 𝑤, there is a linear increase in 𝐾; thus, it validates the model. 6.3. 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/m3 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 𝐾, these data are once again used to plot − ln(1 − 𝑋𝐴 ) versus 𝑡 to get a straight line passing through origin shown in Figure 10 the slope of which gives the value of 𝐾.

Fractional conversion (XA )

Figure 4: (𝐶𝐴0 𝐶𝐵0 /−𝑟𝐴0 ) versus 𝐶𝑊0 : temperature = 348 K; catalyst loading = 73.2 kg/m3 .

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

100

200 300 Time (min)

400

500

Figure 6: Comparison of the effect of catalyst loading (⧫) 24.4 kg/m3 , (◼) 32.5 kg/m3 , (󳵳) 48.8 kg/m3 , (×) 56.9 kg/m3 , (∙) 65.06 kg/m3 , (+) 73.2 kg/m3 , and (−) 81.4 kg/m3 on the conversion of butyric acid with ethanol; molar ratio = 1 : 10 (butyric acid : ethanol), and temperature = 75∘ C.

The values of 𝐾 are plotted against catalyst loading, 𝑤 (kg/m3 ), and it was observed that with the increase in 𝑤, there was a linear increase in 𝐾; thus, it validates the model. 6.4. 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/m3 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.


5

1.8

2

1.6

1.8 1.6

1.2

1.4

1

−(ln (1 − XA ))

− ln (1 − XA )

1.4

0.8 0.6 0.4

1.2 1 0.8

0.2

0.6

0 0

100

200 Time (min)

300

0.4

400

0.2 0

Figure 7: Typical kinetic plot of comparison of different catalyst loadings: (⧫) 24.4 kg/m3 , (◼) 32.5 kg/m3 , (󳵳) 48.8 kg/m3 , (×) 56.9 kg/m3 , (∗) 65.06 kg/m3 , (∙) 73.2 kg/m3 , and (−) 81.4 kg/m3 .

0

200 Time (min)

300

400

Figure 10: Typical kinetic plot of comparison of different molar ratios: (⧫) 1 : 5, (◼), 1 : 10, and (󳵳) 1 : 15, temperature: 348.15 K, and catalyst loading: 73.2 (kg/m3 ).

0.4 0.35

0.006

0.3 Rate constant K (hr−1 )

Rate constant K (min−1 )

100

0.25 0.2 0.15 0.1

0.005 0.004 0.003 0.002 0.001

0.05 0 0

0 0

20

40 60 Catalyst loading (kg/m3 )

80

10 Molar ratio

15

20

Figure 11: Effect of different molar ratios: rate constant (𝐾) versus molar ratio, temperature: 348.15 K, and catalyst loading: 73.2 (kg/m3 ).

Figure 8: Effect on 𝐾 with catalyst loading.

0.9

0.9

0.8

0.8



5

100

0.7 0.6 0.5 0.4 0.3 0.2

0.7 0.6 0.5 0.4 0.3 0.2 0.1

0.1

0

0

0 0

100

200 300 Time (min)

400

500

Figure 9: Comparison of the effect of molar ratio 1 : 1 (⧫), 1 : 5 (◼), 1 : 10 (󳵳), and 1 : 15 (×) on the conversion of butyric acid with ethanol, with catalyst loading of 73.2 kg/m3 and temperature of 75∘ C.

100

200 300 Time (min)

400

500

Figure 12: Comparison of the effect of temperature: 328 K (⧫), 333 K (◼), 338 K (󳵳), 343 K (×), and 348 K (∗), on the conversion of butyric acid with ethanol, with catalyst loading of 73.2 kg/m3 and molar ratio of 1 : 10 (butyric acid : ethanol).

6

ISRN Chemical Engineering 1.8

7. Conclusion

1.6

The kinetics of esterification of butyric acid with ethanol in the temperature ranging between 328.15 K and 348.15 K 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.

− ln (1 − XA )

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

100

200 Time (min)

300

400

Figure 13: Typical kinetic plot of comparison of different temperatures: 328 K (⧫), 333 K (◼), 338 K (󳵳), 343 K (×), and 348 K (∗). 6.2 6.1

References

6 − ln (K)

5.9 5.8 5.7 5.6 5.5 5.4 5.3 2.85

2.9

2.95

3

3.05

3.1

1000 (T)

Figure 14: Arrhenius plot of − ln 𝐾 versus 1/𝑇 (molar ratio = 1 : 10; catalyst loading = 73.2 kg/m3 ).

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: 𝐾 = 𝑘0 exp (−

𝐸0 ), 𝑅𝑇

(10)

where 𝑘0 = frequency factor, 𝐸0 = activation energy, and 𝑅 = gas constant. This data is once again used to plot − ln(1−𝑋𝐴) versus 𝑡 to get a straight line passing through origin, the slope of which gives the value of 𝐾. Equation (1), a plot of − ln 𝐾 versus 1/𝑇, at constant acid and alcohol concentrations, gives a straight line with a slope of (𝐸𝑎 /𝑅) 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.

[1] A. A. Yeramian, J. C. Gottifredi, and R. E. Cunningham, “Vapor-phase reactions catalyzed by ion exchange resins. II. Isopropanol-acetic acid esterification,” Journal of Catalysis, vol. 12, no. 3, pp. 257–262, 1968. [2] A. Izci and F. Bodur, “Liquid-phase esterification of acetic acid with isobutanol catalyzed by ion-exchange resins,” Reactive and Functional Polymers, vol. 67, no. 12, pp. 1458–1464, 2007. [3] M. R. Altiokka and A. C ¸ itak, “Kinetics study of esterification of acetic acid with isobutanol in the presence of amberlite catalyst,” Applied Catalysis A, vol. 239, no. 1-2, pp. 141–148, 2003. [4] K. R. Ayyappan, T. A. Pal, G. Ritu, B. Ajay, and R. K. Wanchoo, “Catalytic hydrolysis of ethyl acetate using cation exchange resin (Amberlyst-15): a kinetic study,” Bulletin of Chemical Reaction Engineering & Catalysis, vol. 4, no. 1, pp. 16–22, 2009. [5] B. Chemseddine and R. Audinos, “Use of ion-exchange membranes in a reactor for esterification of oleic acid and methanol at room temperature,” Journal of Membrane Science, vol. 115, no. 1, pp. 77–84, 1996. [6] B. Saha and M. M. Sharma, “Esterification of formic acid, acrylic acid and methacrylic acid with cyclohexene in batch and distillation column reactors: ion-exchange resins as catalysts,” Reactive and Functional Polymers, vol. 28, no. 3, pp. 263–278, 1996. [7] A. Chakrabarti and M. M. Sharma, “Esterification of acetic acid with styrene: ion exchange resins as catalysts,” Reactive Polymers, vol. 16, no. 1, pp. 51–59, 1991. [8] C. F. Oliveira, L. M. Dezaneti, F. A. C. Garcia et al., “Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia,” Applied Catalysis A, vol. 372, no. 2, pp. 153–161, 2010. [9] S. S. Dash and K. M. Parida, “Esterification of acetic acid with n-butanol over manganese nodule leached residue,” Journal of Molecular Catalysis A, vol. 266, no. 1-2, pp. 88–92, 2007.

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