Recent Developments on the Mechanism and Kinetics of Esterification Reaction Promoted by Various Catalysts

Esters have played a significant role in daily living and chemical industry, such as plasticizers, fragrance, adhesive and lubricants (Joseph et al., 2005; Mbaraka & Shanks, 2006; Krause et al., 2009; Martínez et al., 2011). The vast majority of esters can be prepared using esterification reaction in the chemical engineering industry. Esterification has acquired further improvement from the engineering side; this mainly depends on the research of esterification kinetics. On the other hand, the need to control chemical reactions at the molecular level, which depends critically on the catalytic mechanism, is rapidly increasing (Salciccioli et al., 2011).

The chemical reaction can be expressed as Figure 1.   (Streitwieser et al., 1985;Fei & Zhao, 2009 In addition, Streitweiser considered that step (2) is the rate-determining stage. The total reaction is: The result of the reaction is that the rupture of carboxylic acid acyl-oxygen bond occurs, and the hydroxy of acid is replaced by alkoxy, which is a nucleophilic substitution process of carboxylic acids.
Esterifications of primary alcohols, secondary alcohols with carboxylic acids comply with this mechanism usually. According to this mechanism, the intermediate with a tetrahedral structure is more crowd than reactants, which makes the structure of carboxylic acid and alcohol have a significant effect on the easy of esterification.
The activity of different structures of the alcohol and carboxylic acid in esterification obeys the following order: Alcohol: CH 3 OH > RCH 2 OH > R 2 CHOH; Acid: CH 3 COOH > RCH 2 COOH > R 2 CHCOOH > R 3 CCOOH (Solomons, 1986;March, 1992;TianQuan 1992;Fei & Zhao, 2009).It shows that straight-chain structure is easier than branched-chain structure to esterification, and the more the branched-chain is, the lower the rate will be.
Based on the above mechanism proposed by Streitweiser, Rönnback, et al. developed a esterification kinetic model of carboxylic acid with methanol in the presence of hydrogen iodide though isothermal batch experiments at 30-60℃. The catalyst concentration varied from 0.05 to 10.0 wt%. Because the proton-donation step (1) as well as the subsequent steps (3)-(5) is assumed to be rapid, the simplified mechanism shown in Figure 3: where r is the total rate, r 2 is the rate of step(2), A is CH 3 C(OH) 2 + and E is CH 3 COOCH 3 .
The kinetic and equilibrium parameters included in Equation (5) are estimated from experimental data with regression analysis. Simulation of the model with the estimated parameters revealed that Equation (5) can predict the experimental trends in the acidcatalyzed esterification correctly.

Single-molecule reaction
The tertiary alcohol is prone to generate carbocation under acidic condition. Therefore, the mechanism of tert-esterification is different from that of the primary and secondary alcohol, it follows the single-molecule reaction process(shown in Figure 4):The tertiary alcohol combines with protons to generate protonated alcohol(1); The protonated alcohol is got rid of a molecule water to produce tert-carbocation(2), which is very stable; Then the electrophilic attacking is taken place between the tert-carbocation and oxygen atom of carboxylate, the protonated ester(3) is yielded; The loss of a proton from the protonated ester gives the product(4) (Bart et al., 1994).

Large-steric reaction
Aromatic acid has serious steric hindrance. If there are both ortho-methyls, just as 2.4.6trimethyl benzoic acid, the alcohol molecule is so difficult to access to carboxyl that the esterification cannot occur. However, if 2.4.6-trimethyl benzoic acid is dissolved in the 100% sulfuric-acid solution, acylilum ion will be formed as shown in Figure 5. Then the added alcohol with the acylilum ion produced ester. The reaction will conduct smoothly (Streitwieser et al., 1985).
This kind of esterification could occur for the reason that carbon atom of the acylilum ion is sp 2 hybridized and coplanar with the benzene ring, Then alcohol molecule can be virtually unhindered to attack the acylilum ion from above or below of the molecular plane. Esterifications conducts with this mechanism are merely few.
The alkylation reaction of sulfuric acid with alcohol is irreversible (Aranda et al., 2008).Therefore the reaction rate can be expressed as: where k is rate constant and K is equilibrium constant; C A , C B , C W , C E is the concentration of carboxylic acid, alcohol, water and ester, respectively. In 2009, Fei, et al. studied on the esterification behaviors of neo-polyhydric alcohols with fatty acids catalyzed by sulfuric acid. It was found that the esterification follows the above mechanism.

Lewis acids
Lewis acid is a compound or ionic species that can accept an electron pair from a donor compound to form a Lewis adduct, such as ZnCl 2 , Mg (ClO 4 ) 2 , Sn[N(SO 2 -n-C 8 F 17 ) 2 ] 4 . It is potentially useful on the production of esters as a catalyst, the reason is that it can be easily separated from the reaction media (Sanchez et al., 1992;Cardoso et al., 2009); moreover, comparing with Bronsted acid, template effects are to be expected as Lewis acid sterically bulkier than a proton. In 1964, Anantakrishnan, et al. investigated the esterification reaction of Ac 2 O with 50% MeOH, EtOH, PrOH, and iso-PrOH, in 50% Me 2 CO or Dioxane as solvent, using Lewis acid (ZnCl 2 ) as catalyst.
Along with the scientific and technological progress and social development, more and more researchers have concerned about this issue, including the kinds of catalysts, the mechanism and kinetics of reaction. Thus, the contents containing the reaction mechanism and esterification kinetics have been significantly reviewed according to the type of catalysts as follows.

Metal chloride
Ethyl oleate was synthesized by the esterification of and ethanol catalyzed by SnCl 2 •2H 2 O . Under the circumstance of excess ethanol, the effects of the concentration of the catalyst and oleic acid, and temperature on the reaction rate were investigated. A related esterification mechanism was presented and described as follows: in presence of Sn 2+ ( SnCl 2 •2H 2 O) catalyst, the carbonyl of the fatty acid is polarized to activate of substrate, which makes the nucleophilic attack to the molecules by ethanol become more favorable. Cardoso et al. investigated the effect of different carbonic chain of alcohol (methyl alcohol, ethyl alcohol, n-propyl alcohol, n-butyl alcohol) on the conversion of oleic acid into respective ester. The results showed that the conversion rate was down with the increase of carbon chain of alcohol, which indicated that high bulk hindrance occurs on the hydroxyl of the alcohol, and the efficient attack of them to the polarized carbonyl of oleic acid is reduced. However, it is not clear how the carbonyl is polarized by Sn 2+ .
Kinetic data of esterification of ethanol and oleic acid catalyzed by SnCl 2 (n ethanol : n oleic acid : n SnCl2 = 120 : 1 : 0.01) were measured at the reflux temperature, and kinetic model was obtained as follows, where C is the concentration of oleic acid, t is the reaction time.
The effectiveness of the catalyst SnCl 2 •2H 2 O has been investigated in a broad range of concentrations, and the results are approximately concomitant with a first order dependence in relation to the catalyst concentration. The effect of the temperature on the initial rate of the esterification was determined, showing that an increase in the reaction temperature was caused a corresponding improvement on the reaction rate, especially at a range of 45-75°C. The value of activation energy for the reaction was determined from the data of the initial rate to be 46.69 kJ·mol -1 .
The kinetics of catalytic esterification of castor oil with lauric acid using SnCl 2 •2H 2 O was studied (Kulkarni & Sawant, 2003 The consumption rate of A (-r A ) is expressed by, where C A , C B , C C is the concentration of lauric acid, hydroxyl group and catalyst (mol/mL), respectively.
The catalyst concentration was constant throughout the reaction; therefore, Equation (11) can be rewritten as The integrated form of Equation (12) using fractional conversion is, where X A , X B is fractional conversion of the lauric acid and the -OH group, respectively; C A0 , C B0 is initial concentration of the lauric acid and hydroxyl group (mol/mL), respectively; and k is the second-order rate constant (mL 2 mol -2 min -1 ).
The esterification reaction of castor oil and lauric acid was carried out using SnCl 2 •2H 2 O as catalyst (0.25,0.5,1.0 and 2.0 % w/w catalyst loadings) at 185℃ and the kinetic data were measured. The results showed that there was a good linearity relationship between ln [(1-X B )/(1-X A )] and t. The plot of k′ versus C C was close to a straight line. For the given values of C B0 , C A0 and C C , the kinetic model of esterification of castor oil and lauric acid was obtained at 185°C, It was investigated the esterification of octanoic acid and n-octyl alcohol utilizing metallic chlorides (KCl, CoCl 2 , MgCl 2 , ZnCl 2 , FeCl 3 etc.) in a stirred tank reactor (Santos, 1996). The results showed that the best efficiency of the formatted ester (n-octyl octanoate) was obtained with ferric chloride, indicating that the higher electronegativity of the metallic ion and the existence of free d orbitals in the transition metals are responsible for the higher yield of ester. Thus, the existence of free d orbitals in Fe 3+ (its configuration: 3d 3 ) gives the possibility of the formation of complexes with OH groups of the reactants, which explains the highest activity found for FeCl 3 •6H 2 O in this way. Generally, in the catalytic systems whose bases were constituted by a transition metal, such as Fe, Co, Mn, Zn, the mechanism of esterification can similarly be ascribed to the above mentioned.
The esterification reaction of octanoic acid and n-octyl alcohol was carried out using CoCl 2 •2H 2 O as catalyst (0, 0.0385, 0.077 mol/l) at 70℃ and the kinetic data were measured. The experimental curves suggest that the kinetics of esterification between octanoic acid and n-octyl alcohol can be described by an irreversible second order power model, considering the catalyst concentration as a constant in the kinetic model proposed. The activation energy is seen to have a value of 53 kcal/mol (Urteaga et al., 1994).
The reaction rate went on a power law model of second order, one for the alcohol and one for the acid, It was also observed that the homogeneous reactions were fitted much better than the pseudo-homogeneous reactions to proposed kinetics model. The reason for it is that an important influence of the physical steps that could have place in the global mechanism of this reaction.

Perchlorate
Perchlorates are of great chemical interest and importance. The high electronegativity, together with the relatively low charge density, results in poor complexing ability of the perchlorate ion. Metal perchlorates can therefore act as powerful Lewis acids, with this character mainly being exploited to activate bidentate compounds (Bartoli et al., 2007). Magnesium perchlorate is one of the most active Lewis acids for esterification. By 2003, Gooβen & Döhring synthesized various esters through a decarboxylative esterification of alkyl and aryl carboxylic acids with dialkyl dicarbonates in the presence of 1 mol% of Mg(ClO 4 ) 2 . However, more in-depth investigations on the mechanism of this reaction are underway. nucleophilicity. Herein, the reaction mechanism was presented to explain the observed reaction characteristics. In fact, the more acidic and less nucleophilic phenols react faster than aliphatic alcohols. Thus, the release of the alcoholic proton should be involved in the ratedetermining step. A reasonable mechanistic hypothesis is depicted as follows (Figure 6), Due to the ability of coordinating with 1,3-dicarbonyl compounds, Mg(ClO 4 ) 2 reacts with diethyl dicarbonate to form complex (II), which can undergo the addition of the alcohol to form intermediate (III). An internal proton shift in (III) can produce intermediate (IV), which can irreversibly decompose to the mixed carbonate (VI) and to the carbonic acid monoester (V). Owing to its high instability, (V) immediately produces EtOH and CO 2 . The irreversibility of the last two steps drives the overall process towards (III). This explanation accounts for the formation of the mixed carbonate (VI) as the major product of the reaction (Jousseaume et al., 2003).
Nevertheless, this is a speculative hypothesis; more studies are in progress to find experimental evidence to elucidate the reaction mechanism.

Organometallic compound
Organometallic compounds containing bonds between carbons and metals provide a source of nucleophilic carbon atoms which can react with electrophilic carbon to form a new carbon-carbon bond (Banach et al., 2001;Finelli et al., 2004). Organometallics find practical uses in catalytic processes. For example, Titanium tetrabutoxide, Butylhydroxyoxostannane are well-known catalysts for the esterification between diacid and diol in polyester industry (Grzesik et al., 2000;Prabhakarn et al., 2011). The catalytic mechanism and kinetic model of related catalysts are reviewed in the following discussions.

Titanate
Titanium-based catalysts have been known for many years and actually are widely used for the production of poly (butylene terephthalate) (PBT), poly (trimethylene terephthalate)

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Chemical Kinetics 264 (PTT), and so on. Recently, the developments of titanium-based catalysts make a marvelous progress , such as titanium dioxide based catalyst (C-94) mainly focusing on designing to be stable and having good activity and color.
The mechanism of the mono-esterification between terephthalic acid (TPA) and 1,4-butanediol (BDO) catalyzed by Ti(OBu) 4 was proposed (Tian et al., 2010). As shown in Figure 7, the reaction involves the formation of an adduct between a carbonyl group and Ti atom. Fig. 7. Mechanism of the mono-esterification between BDO and TPA catalyzed by Ti(OBu) 4 Although the reaction system of TPA and BDO is heterogeneous, it can be assumed that the esterification occurs only in the liquid phase. The initial rate method is used to predict the reaction rate. The kinetic model of mono-esterification between TPA and BDO catalyzed by Ti(OBu) 4 in the temperature of 463-483K was investigated (Bhutada & Pangarkar, 1986). The reaction rate r can be described as, where C A , C B is the concentrations of TPA and BDO, respectively, k is the reaction rate constant, n and m is the reaction order with respect to A and B.
The reaction order can be obtained from experiments in which the initial rates are measured at a series of initial reactant concentrations. The results show that the reaction order for TPA and BDO is nearly a constant of 0.7 and 0.9, respectively. Therefore, the rate equation of the esterification is written as follows, where k Ti is the rate constant (L 0.6 ·mol −0.6 ·min −1 ), R 2 is the correlation coefficient.
The reaction of behenic acid with fatty alcohols (decanol, lauryl alcohol, myristyl alcohol and cetyl alcohol) was studied by Tiwari, et al.    (20)) and the catalyzed reactions (Equation (21) (21) Excess of the acids is used to get almost complete conversion of the alcohols, and the unreacted acid was easily removed from the ester as a sodium salt. Assuming that it is first order dependence of the reaction rate on each reactant and the catalyst, the overall rate expression in the integrated form becomes, where X A is fractional conversion of behenic acid; C A0 is initial concentration of behenic acid;  cc u c kk C k, k c is catalyzed reaction rate constant, k uc is uncatalyzed reaction rate constant; C C is catalyst concentration.
The kinetic data show that there is a reasonably good agreement between experimental points and the ones calculated by Equation (22). The values of activation energy obtained for the uncatalyzed (∆E uc ) and catalyzed (∆E c ) reaction of behenic acid with decanol, lauryl alcohol, myristyl alcohol and cetyl alcohol are shown in Table 2.The model Equation (22) is also appropriate for the reaction of erucic acid with cetyl alcohol and oleyl alcohol using TBT as a catalyst

Butylhydroxyoxo-stannane
Stannum-based catalysts such as butylhydroxyoxo-stannane (BuSnOOH) are commonly used for the synthesis of poly (ethylene terephthalate) (PET) , poly (butylene terephthalate) (PBT) and so on. Due to the similarity in coordination number and electronegativity between Ti and Sn, as shown in Figure 8, the mechanism of BuSnOOH catalyst is also proposed by TIAN, et al.
The initial rate method will also be used in this reacting system. The kinetic model of monoesterification reaction between terephthalic acid (TPA) and 1,4-butanediol (BDO) is expressed as follow, where C A and C B is the concentrations of TPA and BDO, respectively; k Sn is the rate constant (L 1.9 ·mol −1.9 ·min −1 ) .

Fluorous metaloxide
In 2004, Otera investigated the transesterification and esterification of various acids and alcohols using 1, 3-disubstituted tetraalkyldistannoxanes as catalyst, and found that the catalytic mechanism different from that proposed for tetraalkyldistannoxanes. In the latter case, the initial step is substitution of the bridging X group to give an alkoxydistannoxane intermediate, (YR 2 SnOSnR 2 OR) 2 , which works as an alkoxyl donor. In contrast, for 1, 3disubstituted tetraalkyldistannoxanes, no substitution takes place at the bridging position b y a n a l k o x y g r o u p . A s d e s c r i b e d a b o v e , t h e b r i d g i n g c h l o r i n e i n 1 , 3 -d i s u b s t i t u t e d tetraalkyldistannoxanes is never substituted by isothiocyanate ion, but experiences strong association.
A related mechanism of the esterification is proposed that both alcohol and acid coordinate on the terminal tin atom (Yoshida et al., 2006), on which the interchange between the hydroxyl groups of carboxylic acids and the alkoxy groups of alcohols takes place. Meanwhile, since water, one of the products, is less fluorophilic, the esterification of carboxylic acids with alcohols should proceed efficiently.
Fluorous distannoxanes exhibit unusually high preference for fluorous solvents over common organic solvents thanks to coverage of the molecular surface with fluoroalkyl groups. While the fluorous biphase technology had been high-lighted mainly in terms of facile separation of products and fluorous catalysts, it has now been revealed that the equilibrium is also controllable under fluorous biphase conditions. Thus, the esterification has been completely driven in the desired direction by use of a 1:1 ratio of starting materials without recourse to any dehydration technique.

Solid acids
The research for solid acids has become active since the early 1970s. In 1971, Isao, et al. investigated the esterification of ethanol with acetic acid on silica-alumina, and a simple kinetic model based on a Langmuir-Hinshelwood mechanism was proposed. Since then, the esterification catalyzed by solid acids is widely studied and largely reported (Jiang et al., 2008;Jothiramalingam & Wang, 2009;Li et al., 2010). Herein, the esterification mechanisms and kinetic models according to the different types of solid acids are reviewed, such as sulfate-supported metal oxides (SO 4 2− /M x O y ), TPA/SnO 2 .

TPA/SnO 2
The esterification of palmitic acid with methanol using 12-Tungstophosphoric acid (TPA)/SnO 2 was investigated (Srilatha et al., 2011). The reaction is shown as follows: As shown in Figure 9, the mechanism of this esterification is proposed,  The catalyst initiates the esterification reaction by donating a proton to palmitic acid molecule. The palmitic acid is then subjected to nucleophilic attack by the hydroxyl group of methanol, and the reaction continues with water elimination.
As the large excess of methanol, it could be safely assumed to be a first-order pseudohomogeneous reaction. The esterification reaction was carried out using 15 wt% TPA/SnO 2 as catalyst w A : w cat =5:1 in the temperature range of 45-65℃and the kinetic data were measured.
The reaction rate can be described as follows: where X A is the conversation of palmitic acid, t is the reaction time, k is the reaction rate constant.

Chemical Kinetics 268
The plots of-ln (1-X A ) versus t at different temperatures showed that there was a good linearity relationship. Linear fitting to lnk -1/T curve was carried out and the reaction rate constant can be described as follows: The activation energy was obtained as 36.33 kJ mol −1 .

SO 4 2− /M x O y
The solid superacid (SO 4 2− /M x O y ) is a new type of catalyst used in esterification (Jiang et al., 2004). M x O y are usually some transition metal oxides such as ZrO 2 and TiO 2 .
In 2010, Rattanaphra, et al. investigated the esterification of myristic acid with methanol catalyzed by sulfated zirconia. The mechanism of esterification is probably following Langmuir-Hinshelwood model (Arata, 2009;Reddy & Patil, 2009). It is possible that methanol and myristic acid are preferentially adsorbed on the Bronsted acid sites of sulfated zirconia during esterification. The hydroxyl group of methanol is protonated by Bronsted acid on the catalyst surface while the protonation of myristic acid on an adjacent site leads to the carbocation. Deprotonation of methanol oxygen produces the nucleophile, which attacks the carbocation to generate a tetrahedral intermediate. As shown in Figure 10 The stirring rate is sufficient to overcome the diffusion limitation of reactive species. Therefore, the performance of pseudo-homogeneous model can be considered as satisfactory to correlate the kinetic data for the esterification. The rate equation can be written as follows: where C A , C B , C E and C W is the concentration of myristic acid, methanol, myristic acid methyl ester and water respectively, k 1 is the forward rate constant and k -1 is the backward rate constant.
The equation can be rearranged to be: where X A is the conversion of myristic acid，C A0 is the initial concentration of myristic acid, K e is the equilibrium constant, M is the concentration ratio of methanol to myristic acid (M=C B0 /C A0 ).

   
The linear coefficient is 0.996, and the activation energy is 22.51 kJ /mol.
In 2004, Jiang, et al. studied the esterification of n-pentanol with benzoic acid using Alpillared clay (PILC) supported SO 4 2− /TiO 2 superacid catalyst (Fang et al., 2010), and find that it is known that the generation of super acid sites in the system of SO 4 2− /M x O y solid superacid is necessarily promoted by the sulfur of the metal oxides, the more acid sites formed, the higher catalytic activity exhibited. Therefore, Al-PILC carrier can effectively enhance the catalytic activity.
In 2010, Zubir, et al. investigated the kinetic behavior of the heterogeneous catalyzed esterification of oleic acid with ethanol using tungstated zirconia as a catalyst in the temperature range of 303.15-323.15K (Otera, 1993 The kinetics of esterification reaction can be expressed using a pseudo-homogeneous second-order equilibrium model in the absence of any intraparticle diffusional limitation as follows: www.intechopen.com

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where C A ,C B , C C , C D is the concentration of oleic acid, ethanol, ethyl oleate and water, respectively, k is the kinetic constants for the forward reaction, K e is the equilibrium constant.
The value of activation energy is 51.9 kJ·mol -1 . The goodness-of-fit of the experimental data to the proposed model is assessed by comparing the experimental reaction rate with the theoretical prediction, and the experimental data are reproduced with errors not greater than 10%.

Ion-exchange resin
Ion-exchange resins, especially the cation-exchange resins such as Dowex, Amberlyst series are manufactured mainly by sulfonation of ethylbenzene first, followed by a cross-link with divinylbenzene (Liu & Tan, 2001;Alexandratos, 2008;Tesser et al., 2010). Because of their selective adsorption of reactants, surface acid site features, and swelling nature, these resins not only catalyze the esterification reaction but also affect the equilibrium conversion. They also show excellent performance such as reusable, mechanical separation, continuous operation as a heterogeneous catalyst in esterification (Yang et al., 2007;JagadeeshBabu et al., 2011;Ju et al., 2011;.
By 1965, Bochner, et al. evaluated the performance of Dowex 50WX-8 as a catalyst for the esterification of salicylic acid with methanol. Through analyzing the experimental results, the Langmuir-isotherm applied to this system to describe the reaction rate. Since then, various mechanisms and kinetics of esterification between acids and alcohols catalyzed by cation-exchange resins were proposed. Here is an overview of the reaction mechanisms and kinetic models according to the type of cation-exchange resins.

Dowex
Dowex, a common cation-exchange resin, is widely used as catalysts in esterification (Vahteristo et al., 2009). In 2005, Kulawska, et al. investigated the esterification of maleic anhydride with octyl, decyl or dodecyl alcohol over Dowex 50Wx8-100, and the kinetic data were measured in the temperature range of 403-433 K (C 0ALC /C 0MA =5:1). Based on the data, a first order reaction was found-first order with respect to acid and zero order with respect to alcohol. The reaction rate can be described as follows, where c M is the concentration of maleic monoester, t is reaction time.
The values of activation energy are 66.0 (±0.65) kJ/mol, 58.6 (±0.4) kJ/mol, and 66.1(±0.4) kJ/mol for dioctyl, didencyl, and didodecyl maleate formation, respectively. However, the effects of the catalyst (the particle size and the concentration) and the mass transfer resistance on the reaction rate were not taken into account.

Recent Developments on the Mechanism and Kinetics of Esterification Reaction Promoted by Various Catalysts 271
The esterification kinetics of acetic acid with iso-butanol catalyzed by Dowex 50 Wx2 was studied (Izci & Bodur, 2007). It is considered to be reversible reactions and can be described by pseudo-homogeneous (PH) model. The general reaction rate expression can be written as follows, where subscripts A, B, E and W is acid, alcohol, ester and water, respectively, k 1 is forward reaction rate constant (L mol −1 min −1 ), K is the equilibrium constant of the reaction.
It is found that this bimolecular type is second order reaction. The values of k 1 for different temperatures are 0.24×10 3 (318K), 1.00×10 3 (333K), 3.07×10 3 (348K). The activation energy was found to be 1.745 kJ·mol −1 in the presence of Dowex 50 Wx2. However, the PH model does not take into account the resin swelling ratio, adsorption of the components and the non-ideal thermodynamic behavior of reactants and products (Ali &Merchant, 2006;Patel & Saha, 2007;Jeřábek et al., 2010;Erdem & Kara, 2011).
The kinetics of Dowex 50 Wx8-catalyzed esterification was studied between acetic acid and benzyl alcohol (Ali & Merchant, 2009). The swelling ratio of Dowex 50 Wx8 in different solvent was measured, and the results show that it decreases in the order of water, benzyl alcohol, acetic acid, benzyl acetate. Water appears to be preferentially adsorbed by the catalyst from a binary solution of acetic acid and water, and hinders the approach of butanol to the protonated acid. Therefore, the water exerts an adverse effect on the esterification rate.
The initial reaction rates of esterification between acetic acid and benzyl alcohol were measured at various conditions, and the Eley-Rideal (ER) model was used to correlate the data and showed a high degree of fit, indicating that the surface reaction between adsorbed alcohol and acid in the bulk is the rate-limiting step during the initial stage of the reaction.
where M cat is the mass of the catalyst, g; a acid , a alc , a ester , a water is the activity of acid, alcohol, ester, water in the liquid phase, respectively; k f is forward reaction rate constant for the esterification, mol g -1 s -1 . K a is activity reaction equilibrium constant. K acid , K alc , K water is adsorption equilibrium constant for the alcohol present in the system, respectively.
The activation energy form the above relationship was found to be 55.4kJ/mol.
However, as the reaction proceeds, the acid adsorption term might have to be introduced and in such an event the reaction kinetics would be represented by a dual site mechanistic, such as Langmuir-Hinshelwood (LH) model.
In 2011, Ju, et al. measured the kinetic data for the esterification of butyric acid with nbutanol over Dowex 50Wx8-400, and correlated with various types of kinetic models. Strong resin water affinity was taken into account, and the non-ideality of the system was considered by applying the no-random two liquid (NRTL) model. The comparisons of the conversion of butyric acid with reaction time between experimental data and ones predicted by these kinetic models reveal that the ER model and LH model are the most reasonable fit for describing the mechanism, with the total average error 12.52%. Surface reaction is the rate determining step, and the affinity between resin and water is found to be not strong. Therefore, the most possible esterification reaction mechanisms can be proposed as following lists: 1. Single site mechanism: the adsorbed butyric acid onto the catalysts reacts with nonadsorbed n-butanol in the bulk. 2. Single site mechanism: the adsorbed n-butanol onto the catalyst reacts with nonadsorbed butyric acid in the bulk. 3. Dual site mechanism: both the reactants adsorbed on the catalyst surface and react there.
In 2007, Ali, et al. studied the esterification of 1-propanol with propionic acid catalyzed by Dowex 50Wx8-400. The experiments were carried out over a temperature range of 303.15-333.15K, and the reaction mechanism for the esterification was proposed (Lilja et al., 2002). The reaction is initiated by the transfer of a proton from the catalyst to the carboxylic acid，and the carbonium ion is formed during the reaction. The ion is accessible for a nucleophilic attack by the hydroxyl group from the alcohol. After that, a molecule of water is lost from the ion. Finally, the catalyst is recovered by the transfer of proton from the ion to the catalyst surface. This mechanism is represented by the following scheme ( Figure 11): Fig. 11. Mechanism for the esterification catalyzed by Dowex 50Wx8-400 The donation of a proton is commonly assumed to be a fast step, and the nucleophilic substitution is usually assumed to be the rate determined step.
Based on the above reaction mechanism, external and internal diffusion can be negligible, and the E-R model (Equation (39)) is applied. The activity coefficients of components can be predicted using UNIFAC model. Meanwhile, by taking into account the strong water affinity for Dowex 50Wx8-400, a correction term (α) was added to the activity term for water in rate expression. The experimental results for the esterification between 1-propanol and propionic acid were modeled according to the above kinetic model. The total average error between the predicted and experimental mole fractions of acid is 1.65%. The activation energy for the forward reaction was estimated to be 67.3 kJ /mol.
The effect of different alcohols on the conversion of propionic acid was investigated using methanol, ethanol, 1-propanol and 1-butanol. The results reveal that the conversion of The acetic acid initiates swelling of the resin, which results in easy accessible acid groups for the reaction and free mobility of all the components. Therefore, a pseudo-homogeneous (PH) kinetic model is applicable. For the reaction, there is, where k 1 , k 2 , k −1 , k −2 is the forward and backward rate constants, respectively, (mol g -1 s -1 ), a i is the activity of each component, and it is calculated using the UNQUAC model, m cat is the mass of catalyst.
The kinetic behavior of esterification of lactic acid with isopropanol over Amberlyst 15 was investigated Kirbaslar et al., 2001), at different temperatures from 323K to 353K. The ER model was used to describe the reaction mechanism that takes place between adsorbed molecules of isopropanol and the molecules of lactic acid in bulk. The adsorption of ester is reported to be negligible. Hence the rate equation can be described as follows, where k 1 is Rate constant for esterification reaction, M is molar ratio of isopropanol to lactic acid, K e is equilibrium rate constant.
The value of activation energy is found to be 22l J/mol, and at each different temperature, the rate constants is 1. The heterogeneous esterification of propionic acid with n-butanol over Amberlyst 35 for the synthesis of n-butyl propionate was investigated (Lee et al., 2002). The esterification is shown as follows,   The kinetics data of the liquid-solid catalytic esterification are correlated with various kinetic models, over wide ranges of temperature and feed composition. The activity coefficients calculated using the NTRL model are utilized to represent the non-ideality behavior of the species in the liquid solutions. Meanwhile, the effects of film diffusion and pore diffusion appear to be negligible at the experimental conditions. The results reveal that the Langmuir-Hinshelwood (LH) model yielded the best representation for the kinetic behavior of the liquid-solid catalytic esterification: The activation energy of reaction is about 6.81kJmol −1 . This model is capable of representing the kinetic behavior of the liquid-solid catalytic esterification at temperatures from 353.15 to 373.15 K over entire range of the experimental feed compositions.
The kinetics of esterification between acrylic acid and propylene glycol in the presence of Amberlyst 15 was investigated (Altıokka & Ödeş, 2009). Taking into account the general esterification reaction as well as polymerization of acrylic acid and products, the overall reaction mechanism is proposed to be: The reaction rate constants k 1 , k 2 , k 3 and k 4 , in Equation (54) are determined, and shown in Table 4, where k 1 , k 2 is forward and backward reaction rate constants in Equation (51), and k 3 , k 4 is forward reaction rate constants in Equation (52) and Equation (53), respectively,(L min −1 mol −1 ), T is absolute temperature in K.
Together with the experimental data, the concentration-time curves based on the model were obtained under given reaction conditions; there is a reasonably good agreement between calculated curves and experimental points.

Conclusions
This chapter discussed esterification mechanisms, and evaluated the kinetics objectively and quantitatively, which provided a most effective way to select catalyst and design reactor for different esterification systems. It is discovered that some new catalysts (such as lipases, room temperature ionic liquids) have being used in esterification; nevertheless, there are few research on the case. Herein, it is worthy to be investigated deeply.