Kinetics Study of Esterification Reaction of 2-Methyl-4 ... · 2-Methyl-4-Chlorophenoxyacetic Acid...

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 9 2011 Article A112

Kinetics Study of Esterification Reaction of2-Methyl-4-Chlorophenoxyacetic Acid

(MCPA Acid)

Pei San Kong∗ Mohamed Kheireddine Aroua†

Aziz Abdul Raman‡

∗University of Malaya, [email protected]†University of Malaya, mk [email protected]‡University of Malaya, [email protected]

ISSN 1542-6580Copyright c©2011 De Gruyter. All rights reserved.

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Kinetics Study of Esterification Reaction of2-Methyl-4-Chlorophenoxyacetic Acid (MCPA

Acid)∗

Pei San Kong, Mohamed Kheireddine Aroua, and Aziz Abdul Raman Ir.

Abstract

MCPA ester is a postemergence and selective herbicide widely used in control-ling weed growth. This is due to the low solubility of MCPA ester in water whichdiffers from majority of the other herbicides. With low solubility, the chance ofwater pollution is lesser comparatively. MCPA ester can degrade in soil by bio-logical and biotic mechanism which reduces the soil pollution. Despite the wideapplication of this ester, the kinetic data on synthesis of MCPA ester is still con-sidered as a proprietary data and it is not available in the open literature. Thereforekinetic studies are conducted in this work. MCPA ester was synthesized by react-ing MCPA acid and 2-ethylhexanol in the presence of sulfuric acid as a catalyst.The parameters studied were reaction temperature, catalyst concentration and al-cohol to acid molar ratio. The reaction was conducted in a jacketed batch reactorand samples were taken at an appropriate time intervals. The concentration MCPAester was determined by gas chromatography mass spectrometry (GC-MSD) anal-ysis. The experimental data were fitted with proposed homogeneous integratedsecond order kinetic model and the fitting accuracy at 373K, 383K and 393Kwere 0.99, 0.95 and 0.99 respectively. The activation energy and frequency factorwere estimated to be 71.559 kJmol−1 and 1.221 x 107 Lmol−1 min−1 respectively.Kinetic constant values were 0.844 x 103 to 6.331 x 103 Lmol−1 min−1 within therange of the temperature and concentration studied. As predicted, the activationenergy decreases with increases in catalyst concentration and the values at 0.01M,0.1M and 0.5M catalyst concentration were 73.6, 71.7 and 69.4 kJmol−1 respec-tively.

KEYWORDS: kinetic, esterification, MCPA acid, 2-ethylhexanol, sulfuric acid

∗The authors would like to thank the University of Malaya for providing IPPP Grant No.:PS139/2009C and Fellowship Scheme. Special thanks to the staffs and postgraduate studentsof Chemical Engineering Department, University of Malaya.



Introduction �Esterification reaction of MCPA acid and 2-ethylhexanol (2-EH) in the presence of strong mineral acid can produce MCPA ester. MCPA acid is a type of carboxylic acid. According to Ronnback et al. (1997), esterification of carboxylic acids with alcohols in liquid-phase reaction could produce ester. The carboxylic acid esters have enormous practical importance. For example, industries produce million tons of polyesters via esterification reaction for application in pharmaceuticals, cosmetic, herbicides, pesticides and fragrances.

Previous studies done by Wasewar et al. (2009) reported that thermodynamic equilibrium limits the esterification reaction. It is reversible and occurs at a slower rate. It is also faces challenges with product purification. Ronnback et al. (1997) stated the equilibrium constants of esterification reactions range from 1 to 10, which includes amount of reactants exist in the equilibrium mixture. Esterification reaction could achieve higher conversion by removing the produced water continuously. One of the ways is to add appropriate solvent. For example, DuPont and Lefebvre (1996) and Yadav and Devi (2004) stated the organic solvent addition during the esterification process can result formation of azeotrope with water. The organic phase can be separated and the solvent can be recycled and reused later. In this case, the role of solvent is purely to remove the water formed during esterification. However, ternary azeotrope esterification method will reduce the catalyst activity. Malone and Doherty (2000) also reported that formation of ternary azeotrope can cause difficulty in operating esterification process economically and especially during purification process.

Esterification process can take place by using excess amount of one of the reactant. This process becomes slightly inefficient compared to reactive distillation technique as it needs larger reactor volume. However, using excess amount one of reactants can eliminate usage of solvent (Wasewar et al., 2009). Balland et al. (2002) studied solvent free reaction where the reaction was done without solvent. The equipment size for solvent free reaction are smaller because of the absence of unit operations related to the solvent such as storage, loading and recycling tanks. Besides, solvent free reaction has higher reaction rate and it is possible to increase the productivity. The carboxylic acids esterification with a high molecular weight of alcohol can take place in a concentrated free solvent because of the formation of hydrophilic aggregates in the oleophobic medium (Lacaze-Dufaure and Mouloungui, 2000). Esterification reaction can also take place by removing one of the products continuously by reactive distillation. The reactive distillation is a combination of reactor and distillation column in a single operating vessel. Among the advantages of reactive distillation column are reduced equipment and recycling costs, higher

1Kong et al.: Kinetics Study of Esterification Reaction

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selectivity, reduced energy usage and elimination of solvents usage (Malone and Doherty, 2000). Therefore, the reaction kinetics is important to model reactor or any reactive distillation column. It should be known for esterification of MCPA acid with 2-ethylhexanol.

According to Grabinska-Sota et al. (2003), PMRA Health Canada (2007) and U.S. EPA (2004), the parent compound of MCPA ester is MCPA. MCPA is an herbicide in the phenoxyacetic acid family. It is postemergence for selective control of broadleaf weeds but not including grasses.� In a report produced by PMRA Health Canada (2007), MCPA can produce an overload of auxin (also known as indole-3-acetic), a plant hormone which causes target plants to die. U.S. EPA (2004) also reported the plant can absorb MCPA through the leaves and roots and it translocates throughout the plant by using xylem and phloem. Therefore, MCPA can break through the soil surface and even a small surface area of the plant or seedling exposes to MCPA ester can cause the whole plant to die. It is efficient to prevent the crop competing with other plants for resources and water due to damage of competing plants. MCPA ester can improve the ability of MCPA to enter the targeted plant. The ester binds within target site of the plant and causes the plant to die.

There are four herbicides related to MCPA, which are MCPA acid, MCPA sodium salt, MCPA dimethylamine salt (DMAS) and MCPA ester. Among the four forms of MCPA, MCPA ester attracts most attention because of its application and environmental friendliness. MCPA ester is low volatile esters because it has high boiling point. Unlike low volatile MCPA esters, amine salt is easy to volatilize and the subsequent drift may avoid damage to the target plants (U.S. EPA, 2004). While sodium salt is a water soluble herbicide, sodium salt can revert to acid form under acidic conditions. Therefore, MCPA ester is more environmental friendly compared to sodium and DMAS form (PMRA Health Canada, 2007). The low water solubility of ester can prevent the ecological effects by hydrolysis route. The study adopted by MCPA Task Force Three (Pigott and Van. Ravenzwaay, 2000) based on solubility limit indicated there is no effect on different marker for species tested. The Brixham Environmental Laboratory in the UK reported the pH of 5, 6 and 7 buffer solubility in aqueous media was below100 μg/L. PMRA Health Canada (2007) also reported the ester form has low solubility at about 120�g/L.

In comparing the user friendliness, the salt form herbicide is not user friendly as it needs applying aqueous solution to water soluble substrate. Because of hygroscopic property of salt form herbicide can cause clumping and stickiness, therefore, addition of drying agent is required to avoid stickiness. However, MCPA ester does not need the mentioned extra steps during application (Harwell and Lowell, 2003).

2 International Journal of Chemical Reactor Engineering Vol. 9 [2011], Article A112



MCPA Task Force Three pointed out that absorption of soil is low because the esters hold the soil strongly. So, it is further limiting the potential for MCPA ester dispersion in the environment. Grabinska-Sota et al. (2003) reported that MCPA can degrade in soil by biotic and biological mechanism. The experiment done by Smith and Hayden (1980) discovered the recovery of applied MCPA ester from the moisture soil is 32 % lower than high recovery of applied MCPA ester in air-dried soil. It is also proven that the applied MCPA ester was lost in the moisture soil due to biological mechanisms. MCPA Task Force Three also summarized that bioaccumulation in aquatic mammals such as fish is unlikely. The tested aquatic species can quickly hydrolyze esters in the bloodstream and metabolic activities. Experiments also confirmed the absence of ester in the blood even after a substantial dose. Therefore, the rapid hydrolysis in non-sterile media shows that persistence of ester in the environment will be very low. All these factors contribute positively towards MCPA esters as a reliable herbicide. Reaction Mechanism Ali et al. (2007) termed esterification reaction as an intermolecular dehydration reaction, which is important and a common reaction in chemical industry. Water formation of esterification reaction attributes to the reverse reaction. Liu et al. (2006b) reported the catalytic activity of sulfuric acid was strongly inhibited by water in esterification reaction. The water formed in the esterification inspires the reverse reaction, which is hydrolysis of MCPA ester to MCPA acid. Figure 1 shows the reversible esterification reaction step:

Figure 1. Esterification step of production of MCPA Ester

It is difficult for esterification reaction to occur without catalyst because it takes several days to attain the equilibrium. Without catalyst, the reaction is initiated by weak carboxylic acids only. The effective homogenous catalysts used for esterification of carboxylic acids are strong liquid mineral acids such as sulfuric acid, hydrogen chloride and hydrogen iodide. The homogenous sulfuric acid catalysts have been well studied and documented in the literatures (Ronnback

MCPA acid 2-ethylhexanol MCPA ester Water




et al., 1997 and Liu et al., 2006a). The availability of free protons in liquid–liquid catalytic reaction mixture can result faster reaction rate (Ali et al., 2007). According to Liu et al. (2006a and 2006b), the slowest step of the esterification reaction is nucleophilic attack of alcohol on protonated carbonyl group of the carboxylic acid, step (4) in the Figure 2. The role of catalyst is to activate and promote the protonated carbonyl oxygen on the carboxylic group. Transferring H+ ion from sulfuric acid to MCPA acid (carbonyl group) initiates the mechanisms of MCPA acid esterification reaction. Then, the positive H+ ion attaches to the oxygen which is double bonded to the atom carbon. Transfer of the proton (from double bond) to the oxygen atom gives carbon atom a positive charge. This resulted positive charge on carbon atom. Subsequently, the hydroxyl group of alcohol attacks positive charge on carbonyl group and resulted the ester formation. The water molecule will lose from the ion at the same time (Ali et al., 2007 and Liu et al., 2006b). The Figure 2 shows the simple mechanism steps involved in esterification of carbonyl acid:

Figure 2. Mechanistic route of acid catalyzed esterification (Y. Liu et al., 2006b)

However, literature reviews found that there is no published data on kinetics and operating parameter of MCPA acid esterification with 2-ethylhexanol. The kinetics parameter should be determined because the designs of reactors are based on rate equations (Ronnback at al., 1997). Therefore in this study, kinetic data for esterification of MCPA ester will be determined by using solvent free esterification reaction catalyzed by homogenous sulfuric acid. The kinetic model will be proposed based on homogenous catalyzed second order reaction model. The justification for using second order kinetic model will be discussed in later session. The proposed kinetic model will be fitted with the experimental data. The model that correlates well with the experimental data will be used to generate the kinetic data. Besides, the effect of reaction temperature, catalyst concentration and molar ratio are also studied.




Experimental

Materials

The main reactants used in the kinetic study are technical grade of MCPA acid, (> 95.0% purity, Sigma Adrich), 2-ethylhexanol (> 99.0% purity, Fluka) and sulfuric acid (98.0% purity, R&M laboratory grade). Analytical MCPA Ethylhexyl ester (>95.0%, Fluka) and 2-ethylhexanol (99.0%, Sigma Adrich) standard were used for data analysis. Pure helium (99.0%, Mox) was used as a carrier gas in the gas chromatography analysis.

Equipments and procedure

Figure 3 shows the schematic diagram of the experiment setup. The experiments were conducted in a 150 ml jacketed batch reactor equipped with vessel head and plastic stoppers. The jacketed reactor was connected to a circulating oil bath to ensure the temperature is constant throughout the experiment. Sulfuric acid was diluted in 0.01M, 0.1M and 0.5M 2-ethylhexanol. The mixture was well mixed and pre-heated by circulating jacketed oil to the desired reaction temperature. The water formed during the reaction was removed by using high reaction temperature above the boiling point of water 100 oC. The water removal was also facilitated by collection of water in a small collector. MCPA acid was then loaded into reactor to start the reaction after desired temperature was reached and this point was taken as zero time for the run. Samples were withdrawn by a glass syringe at appropriate time intervals as shown. The samples were immediately diluted into cold solvent to stop further reaction from taking place and analyzed by using gas chromatography.

Figure 3. Schematic diagram of experiment setup




Analysis

Samples were analyzed by using Agilent 7890A Gas Chromatography with GC-MSD detector. DB-XLB column (30m x 0.25mm x 0.25μm) was selected in this kinetic analysis study. Helium was used as carrier gas at constant speed of 0.8 ml/min. Split modes at 1: 100 and injection temperature of 250 °C was used. The temperature program was set at 50 °C for 1 min, 50°C to 100°C at 25°/min, 100°C to 320°C at 10°/min and 320°C for 1 min.

Results and Discussions

Kinetic model

Kinetics modeling of homogeneous catalyzed esterification reaction should include the mechanistic steps involved. Classical definition of kinetics reaction considers carbonyl acid as limiting reagent in the process in determining order of esterification reaction (Carmo et al., 2009). The catalyst, reactant and product concentrations should be included to develop a rate equation (Lilja et al., 2005).

There are many kinetic models for esterification as reported by many researchers. The models include simple power law, least square method, initial reaction method, non-linear equation fitting method and integration equation method. It was found that homogenous esterification reaction is a second order reaction since it was found to be a first order reaction for both reactants by previous researchers in different kinetic models (Aranda et al., 2008; Berrios et al., 2007; de Jong et al., 2009; DuPont & Lefebvre, 1996; Liu et al., 2006a; Nowak, 1999; Skrzypek et al., 1998; Yadav & Devi, 2004).

Preliminary, simple power law method was used to generate activation energy however it gives lower R-squared value than integration linearized second order mathematical equation, 0.99. Therefore, integration linearized method is using to generate kinetic parameters in this study as it gives better linear fitting.

All the methods used to develop kinetic model are based on rate equation and it can be derived to different forms of mathematical equation. The homogenous reaction can be described as below:

A+B+C E+W+C

)( 11 WEBAAE CCkCCkCcrr �� (1)

CC, CA, CB, CE and CW are catalyst, MCPA acid, 2-ethylhexanol, MCPA ester concentration and water concentration respectively. The symbol k1 and k-1




are rate constant for forward reaction and reverse rate constant respectively. In this study, the kinetic model was derived based on the following assumptions: 1. The reverse kinetic constant does not consider in overall rate of reaction. 2. The mass transfer between acid and alcohol is negligible as the reaction is

conducted at 500 rpm stirring speed which eliminated the mass transfer effect (Teo and Saha, 2004).

3. The volume of mixture is constant throughout experiment by assuming negligible water vapor flowing out.

Table 1 shows composition of reactants and products respectively in the initial feed reactants and final reaction mixture. There is no ester and water present at the beginning of the reaction. The symbol ‘a’ and ‘b’ is implicitly recognized as stoichiometric coefficients for MCPA acid and 2-EH.

Table 1. Composition of the initial and final mixture in constant volume reaction

Time (min) Concentration (mol/L) MCPA acid 2-ethylhexanol MCPA ester wate

r Initial molar of feed

t = 0 CAO CBO 0 0

Final mixture t = t1 CAO(1-X) )( X

ab

CC

CAO

BOAO �

CAO X CAO X

Equation (2) shows general mole balance for batch system and the

conversion of the MCPA acid is shown in Equation (3). By assuming constant stoichiometry volume, V = Vo during the reaction and elimination of reverse rate constant of k-1, the kinetic model in term of conversion reaction rate is shown in Equation (4).

The excess alcohol and varying initial alcohol to acid molar ratio was used in generation of kinetic data done by (Lilja et al., 2005; M.C. de Jong et al., 2009 and Teo and Saha, 2004). The authors stated that excess alcohol can increase final conversion of product. Equation (4) can be linearized by integrating it mathematically, as shown in Equation (5). This kinetic model, Equation (6) is referred to Liu et al. (2006b), linear integration method.

AO

A

NVr

dtdX

�� (2)




%100��Ao

E

CCX (3)

))(1( XCCXCcCk

dtdX

AO

BOAOc �� (4)

tCCkX

XCCInCC

CAOcc

AOBO

AOBO

AO ��

��

])1(

))(()[( (5)

tCkX

XCCInCC

CAO

AOBO

AOBO

AO1]

)1())(()[( �

��

� (6)

Since catalyst concentration was constant throughout the reaction, so it was removed from Equation (5). Where, k1 = kcCc is formed. The kinetic constant, k1 can be determined by plotting

])1(

))(()[(X

XCCInCC

C AOBO

AOBO

AO

��

�versus t.

Rate constant is strongly temperature dependent and it can be well expressed by Arrhenius equation, as shown in Equation (7). The value of activation energy, EA and frequency factor can be determined by plotting In k versus 1/T as shown in Equation (8). Where frequency factor can be obtained from intercept and activation energy obtained from slope of the plot.

��

� ��

RTEkk A

O exp (7)

RTEkInkIn A�� ][][ 0

(8)

Kinetic Parameters

Esterification of MCPA acid at 373K, 383K and 393K reaction temperatures was used to plot the integrated second order mathematical rate equation. Figure 4 shows the second order kinetics plot for Equation (6). The linearity of the data




shows the validity and accuracy of the second order kinetic model. The R-Squared values from the plot at 373K and 393K are more than 0.99. The R-Squared value for 383 K is about 0.95 which is acceptable too.

The plot also shows that it is first-order reaction dependent on acid and alcohol concentrations respectively. The MCPA acid esterification was confirmed is second order for overall reaction rate. From the plot, it can be seen the slope increases as reaction temperature increases, which means that rate constant value, k1 increases too. The rate constant obtained are 0.915 x103 Lmol-1min-1, 1.993 x103 L mol-1min-1 and 3.049 x103 Lmol-1min-1 at 373K, 383K and 393K reaction temperature for 0.01M catalyst concentration. Table 2 summarizes the rate constants for 0.01M, 0.1M and 0.5M.

R²�=�0.9938

R²�=�0.9542

R²�=�0.998

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 20 40 60 80 100 120

(CAO/C

BO�C

AO)In((CBO/C

AO)�X/(1�X))

Time�(min)

T�=�373�K

T�=�383�K

T�=�393�K

Figure 4. Second order plot for esterification of MCPA acid in different reaction temperature. Constant parameter at 0.01 M catalyst concentration and 1.5 initial alcohol to acid molar ratio

� Arrhenius plot of MCPA esterification was used to determine activation energy and frequency factor. Figure 5 shows the linearized Arrhenius Equation, (Equation (8)) at catalyst concentration of 0.01 M and 1.5 alcohols to acid molar ratio. Frequency factor can be determined from the intercept and the activation energy from the slope of the plot. The activation energy and frequency factor for esterification reaction at different catalyst concentration, 0.01M, 0.1 M and 0.5 M is summarized in Table 2. Standard deviation, standard mean error (S.M.E) and 95% confidence limit were used to verify the accuracy of experimental data.




1)(

,tan2

�

��

�

nxx

ISdeviationdardS (9)

nSSerrormeandardS

x��,tan (10)

95 % confidence limit is interval estimate for the mean. Confidence interval creates a lower and upper limit for the mean. For example, 95 % of the experimental data are within the maximum and minimum boundary as in normal distribution of statistic study (Snedecor and Cochran, 1989). 95% confident level means the result of an action will probably meet expectations 95% of the time. Table 2 shows the kinetic constant, activation energy and frequency factor values were within the 95 % confidence limit. Therefore, it can be said this model is sufficient to generate accurate kinetic data from experiment esterification reaction data.

Table 2. Kinetic parameters for MCPA acid esterification

Cc (mol L-1) T (K) K1 (L mol-1min-1)

Ea ±3.009 (kJ/mol)

ko ±1.166 x107 (L mol-1min-1)

0.01 373 0.915 x 103

73.597 1.935 x107 383 1.993 x 103 393 3.049 x 103

0.1 373 0.844 x 103

71.731 1.039 x107 383 2.181 x 103 393 2.720 x 103

0.5 373 2.017 x 103

69.354 9.057x106 383 2.439 x 103 393 6.331 x 103

Average value 71.559 1.221 x107 *± 95% confident bound




R²�=�0.9767

�7.5

�7

�6.5

�6

�5.5

�5

�4.5

�4

2.52E�03 2.54E�03 2.56E�03 2.58E�03 2.60E�03 2.62E�03 2.64E�03 2.66E�03 2.68E�03 2.70E�03In�k�(L�mol��1

�min�1)

1/�T�(�K�1)

�Figure 5. Arrhenius Plot of MCPA acid esterification at catalyst concentration 0.01 M

The average activation energy and frequency factor for MCPA acid

esterification is estimated at 71.559kJ/mol and 1.221 x 107 Lmol-1min-1 respectively. This value is reasonable when compared to the work done by J. Skrzypek et al., (1998) although different reactant was used. The activation energy of esterification phthalic anhydride with 2-ethylhexanol catalyzed by sulfuric acid is 47.31 kJ/mol and frequency factor is 1.66 x106 L mol-1 min-1. Besides, esterification of acrylic acid with 2-ethylhexanol catalyzed by sulfuric acid is reported as 56 kJ/mol (Nowak, 1999). It was found that by increasing catalyst concentration, the activation energy decreases as summarized in Table 2. Previous work done by Aranda et al. (2008), reported that activation energy decreases as catalyst concentration increases. The homogenous catalyzed esterification of free fatty acid and methanol by M. Berrios et al. (2007) shown that the activation energy was 50.745kJ/mol at 5 % sulfuric acid. The lower value 44.559 kJ/mol of activation energy obtained at 10% sulfuric acid concentration. These works indicate that lower activation energy obtained when higher sulfuric acid amount used as catalyst. The finding of this work can prove the kinetic model correlates well with experimental data. Effect of reaction temperature Temperatures of 373K, 383K and 393K were used for studying the effect of reaction temperature at constant catalyst concentration. Figure 6, 7 and 8 shows the plot of MCPA acid conversion versus time for three different catalyst concentrations. From the plots, it can be observed that the conversion of acid




increases with increases in temperature. Increasing of 10K from 373K reaction temperature can increase about 8.4% acid conversion. About 13.3% of higher acid conversion can be obtained when reaction temperature increased about 20K (373K to 393K). Ali et al. (2007) reported that there are more collisions and higher successful collisions occur among molecule-molecule mixture at higher temperature. The successful collisions have enough energy to break the bonds and form the product. Therefore, higher conversion of ester can be achieved at higher reaction temperature.

The increase in conversion with temperature was also obtained by the works of Ali et al. (2007) and Lilja et al.� (2005). It was indicated that reaction rate increases with increases of temperature at esterification reaction. Wasewar et al. (2009) also reported the reaction rate constant for esterification is a function of temperature. The forward rate constant is normally faster than reverse rate constant with increase in temperature. However, the changes were not significant at further higher temperature. The average increment acid conversion percentage in 383K-393K (4.89%) is lower that 373K-383K (8.39%). This shows the changing of acid conversion in higher reaction temperature is slightly significant only. This is supported by Wasewar et al. (2009)’s work. Ghaziaskar et al. (2006) stated that high temperature may promote side reactions, such as dehydration, if their activation energy is higher than esterification. Therefore, the formed ester yield may reduce. Alternatively, high temperatures would decrease density and solubility of reactants, which also can reduce the rate of the esterification reaction.

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0 50 100 150 200 250 300 350

Conversion�of�acid

Time�(min)

T�=�373�K

T�=�383�K

T�=�393�K

Figure 6. Effect of reaction temperature on the conversion of MCPA acid in esterification reaction at 0.01 M catalyst concentration and 1.5 initial alcohol to acid molar ratio




0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300 350

Conversion�of�acid,�X

Time�(min)

T�=�373�K

T�=�383�K

T�=�393�K

Figure 7. Effect of reaction temperature on the conversion of MCPA acid in esterification reaction

at 0.1 M catalyst concentration and 1.5 initial alcohol to acid molar ratio

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300 350


Time�(min)

T�=�373�K

T�=�383�K

T�=�393�K

Figure 8. Effect of reaction temperature on the conversion of MCPA acid in esterification reaction

at 0.5 M catalyst concentration and 1.5 initial alcohol to acid molar ratio




�

Effect of catalyst concentration

The effect of catalyst concentration on esterification rate was done by varying catalyst concentration to determine catalyst concentration in production of MCPA esters. The amount of sulfuric acid used was 0.0096 M in phthalic anhydride and 2-ethylhexanol esterification (Skrzypek et al., 1998); 1×10�3M H2SO4 in esterification of acetic acid and methanol (Liu et al., 2006b); 0.1 wt% of H2SO4 in benzoic acid and n-octyl alcohol/isooctyl (2- ethylhexyl) alcohol esterification (Skrzypek et al., 2003). The mentioned amount of catalyst concentration in different reactant can be used as reference or at this study. Based on the literatures, the catalyst concentration used was in between 0.01M-0.5M. Figure 9, 10 and 11 are the plots of MCPA acid conversion versus reaction time at different reaction temperatures. From the plots, it can be seen that the acid conversion increases as the amount catalyst concentration increases. The average conversion of acid increases about 7.7% when the 10fold of 0.01M catalyst concentration used. When increase from 0.01M to 0.5M H2SO4 gave about 15.0 % of higher acid conversion in this work.

It can be observed the acid conversion increases until about 120 minutes of reaction time and beyond this point the acid conversion begin to fluctuate. It means that reverse reaction sets in after about 120 minutes of reaction time. Therefore, kinetic model can be safely established within 120 minutes reaction time and during this time it can be assumed that no reverse reaction occurs. The assumptions in this work are similar to work done by Yadav and Devi (2004). The authors reported the initial rate of reaction increases with the increases of catalyst concentration. The acid conversion increases with increased in catalyst concentration. Saha and Sharma (1996) also stated that by increasing the catalyst concentration, higher availability of catalyst ion in the reaction medium which can increase reaction rate. The hydronium ions from sulfuric acid lead forming of ester between carbonyl and hydroxyl group.




0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300 350


Time�(min)

0.01�M

0.1�M

0.5�M

Figure 9. Effect of catalyst concentration on the conversion of MCPA acid in esterification

reaction at 1.5 initial alcohol to acid molar ratio and 373K reaction temperature

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300 350

Conv

ersi

on�o

f�aci

d,�X

Time�(min)

0.01�M

0.1�M

0.5�M

Figure 10. Effect of catalyst concentration on the conversion of MCPA acid in esterification

reaction at 1.5 initial alcohol to acid molar ratio and 383K reaction temperature




0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300 350

Conv

ersi

on�o

f�ac

id,�X

Time�(min)

0.01�M

0.1�M

0.5�M

Figure 11. Effect of catalyst concentration on the conversion of MCPA acid in esterification reaction at 1.5 initial alcohol to acid molar ratio and 393K reaction temperature

Figure 12 shows the plot of rate constant versus catalyst concentration. It shows that the rate constant increases linearly when increases in catalyst concentration at constant reaction temperature of 383K. Similar linear dependent catalyst concentration plots at reaction temperature 373K and 393K were also obtained. It proves that the reaction rate constant is linearly dependent on the catalyst concentration. In other words, the model corresponded well with the experimental data.

The assumption of rate constant is linearly dependent on catalyst concentration is supported by DuPont and Lefebvre (1996); M.C. de Jong et al. (2009) and K. Wasewar et al. (2009) observations. Ronnback et al. (1997) stated that reaction rate is the function of catalyst amount used. Similarly M.C. de Jong et al. (2009) reported that the availability H+ ions in the reaction mixture increase when the quantity of catalyst increase. The catalyst concentration influences the reaction rate or rate constant. Altiokka and Odes (2009) and Ali et al. (2007) also explained the reaction rate is linear dependent on catalyst loading because of the availability active free proton which is proportional to the amount of catalyst. The kc value can be obtained from the linear equation of the plot rate constant versus catalyst concentration based on the equation, k1 = kcCc. The value




of kc obtained was 0.0025 L mol-2 min-2 at 373K reaction temperature and 1.5 alcohol to acid molar ratio.

R²�=�0.9494

0

0.0005

0.001

0.0015

0.002

0.0025

0 0.1 0.2 0.3 0.4 0.5 0.6rate�co

nstant�(�L�m

ol�1min

�1)

Catalyst�concentration� (M)

�Figure 12. Effect of catalyst concentration on the esterification of MCPA acid at 373 K reaction

temperature and 1.5 alcohol to acid feed molar ratio

Effect of initial reactant molar ratio of alcohol to acid

The effect of initial reactant molar ratio was studied by varying molar ratio of the reactants. Figure 13 shows the effect of initial alcohol to acid molar ratio on acid conversion. The acid conversion increases when the alcohol feed increases. The 0.9 alcohol to acid molar ratio (excess of MCPA acid esterification) was conducted to compare the effect of initial molar ratio in conversion of acid. It resulted in low purity ester formation when excess of MCPA acid was used in reaction. Excess of MCPA acid reactant limits the acid conversion because MCPA acid is the limiting reactant in MCPA ester esterification. The average acid conversion increases about 14.6% and 17.6% when molar ratio of alcohol to acid increase from 0.9 to 1.5 and from 0.9 to 2 respectively. The experiment conducted with excess of alcohol gave better conversion. Excess alcohol used in reaction also supported by Lilja et al. (2005) in esterification on propanoic acid and ethanol. Final conversion of acid can be increased by increasing amount of alcohol concentration in esterification reaction. Besides, it shows that at high initial amount of acid, esterification rate are lower. Ali et al. (2007) found that by increasing initial amount of alcohol can increase the conversion of propionic acid in esterification reaction with 1-propanol. Teo and Saha, (2004) reported the equilibrium conversion increase by increasing initial isoamyl alcohol-to-acetic acid. M.C. de Jong et al. (2009) reported the




results of reaction rate and equilibrium conversion increase with a decreasing myristic acid to alcohol feed ratio.

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 50 100 150 200 250 300 350


Time�(min)

alc:acid�=�0.9

alc:acid�=�1.5

alc:acid�=�2

Figure 13. Effect of initial alcohol to acid molar ratio on the conversion of MCPA acid in esterification reaction at 0.1 M catalyst concentration and 373K reaction temperature

Conclusions

Sulfuric acid can be used in esterification of MCPA acid. The kinetic model was developed by using integrated second order mathematical equation. The proposed kinetic model was correlated well with experimental data by giving good fitting regression which proved that MCPA ester esterification is second order reaction.

The average activation energy and frequency factor were estimated at 71.559 kJ mol-1 and 1.221 x 107 L mol-1 min-1 respectively. Besides, the kinetic constant data in various reaction temperature and catalyst concentration were generated. The activation energy was 73.597 kJmol-1, 71.731 kJmol-1 and 69.354 kJmol-1 at 0.01M, 0.1M and 0.5M catalyst concentration respectively. It means that the activation energy decreases when the applied catalyst concentration increases. The rate constant was found linear dependent on catalyst concentration which shows the model corresponded well with the experimental data.

Increasing of 10K from 373K reaction temperature can increase about 8.4% acid conversion. About 13.3% of higher acid conversion can be obtained when reaction temperature increased about 20K (373K to 393K). Significantly, it




shows the conversion of acid increases when reaction temperature increases. The conversion of acid increases about 7.7% when the catalyst concentration was increased from 0.01M to 0.1M. When 0.5M catalyst concentration was used about 15.0 % higher acid conversion was obtained. The average acid conversion increases about 14.6% and 17.6% when alcohol to acid molar ratio increases from 0.9 to 1.5 and from 0.9 to 2 respectively. These results show that acid conversion increases when increasing reaction temperature, catalyst concentration and alcohol to acid molar ratio. Symbols CA Concentration of MCPA acid, mol L-1 CAO Concentration of initial MCPA acid, mol L-1 CBO Concentration of initial 2-ethylhexanol, mol L-1 CE Concentration of MCPA ester, mol L-1 Cw Concentration of water, mol L-1 Ea Activation energy, kJ mol-1 K0 Pre-exponential factor, L mol-1 min-1 K1 Forward reaction equilibrium rate constant, L mol-1 min-1 K-1 Reverse reaction equilibrium rate constant, L mol-1 min-1 kc Catalyst dependency rate constant, L mol-2 min-2 R Real gas constant, J K-1mol-1 rA Reaction rate, mol L-1 min-1 T Reaction temperature, K t Reaction time, min V Volume, L X Conversion Abbreviations MCPA ester 2-methyl-4-chlorophenoxyacetic 2-ethylhexyl ester MCPA acid 2-methyl-4-chlorophenoxyacetic acid 2-EH 2-ethylhexanol H2SO4 Sulfuric acid H2O Water




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