Computational and Theoretical Chemistry 974 (2011) 100–108

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Computational and Theoretical Chemistry

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Formation of heavy adducts in esterification of acrylic acid: A DFT study

Sławomir Ostrowski a, Małgorzata E. Jamróz a,⇑, Jan Cz. Dobrowolski a,b,⇑a Industrial Chemistry Research Institute, 8 Rydygiera Street, 01-793 Warsaw, Polandb National Medicines Institute, 30/34 Chełmska Street, 00-725 Warsaw, Poland

a r t i c l e i n f o

Article history:Received 6 April 2011Received in revised form 15 July 2011Accepted 15 July 2011Available online 23 July 2011

Keywords:Acrylic acidAcrylatesActivation energyDFTEsterificationSide reactions

2210-271X/$ - see front matter � 2011 Elsevier B.V.doi:10.1016/j.comptc.2011.07.016

⇑ Corresponding authors. Address: Industrial CheRydygiera Street, 01-793 Warsaw, Poland. Tel.: +48 222 568 2421 (J.Cz. Dobrowolski).

E-mail addresses: Malgorzata.Jamroz@ichp.pl (M(J.Cz. Dobrowolski).

a b s t r a c t

Esterification of acrylic acid with alcohols and the side reactions were studied at the B3LYP/6-31G�� level.The main model reaction is predicted to be endoergic which is in line with experimental findings. Itappeared that the activation barriers for the esterification reactions in vacuum are equal to ca.68 ± 3 kcal/mol, they decrease in presence of the H+ ion to ca. 47.5 ± 3.5 kcal/mol, and are relatively insen-sitive to change of environment simulated by PCM method (a matter of 0.5 kcal/mol). Out of four side reac-tions studied, the lowest activation barrier (36.5 ± 1.5 kcal/mol) is for addition of the acrylic acid moleculeto double bond in acrylates. Next, relatively easily occurring side reactions are the additions of water andalcohols to acrylates (barriers of ca. 48 ± 1 kcal/mol in presence of the H+ ion). Activation barrier for dimer-isation of acrylic acid, i.e., addition of one molecule to the double bond of the other, in catalytic reaction isequal to 56.5 kcal/mol. Finally, the addition of alcohol to the acid dimer (leading to the same product asaddition of acid to an acrylate) needs to overcome the 61 kcal/mol barrier. Based on the above results wediscuss qualitatively our experimental findings of technology using heterogeneous acid catalysts.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Acrylates are intermediate products of high commercial valueused chiefly in the manufacturing of polymers and copolymers.They are particularly important in formulation of water-based ac-rylic emulsions for production of coatings, polishes, carpet backingcompounds, adhesives, and sealants. For acrylic based polymers,they are primarily applied in construction, paper and textilesindustry as well as for curing polymeric materials, used for in-stance in dentistry [1–5].

In 2002–2007 the world demand for acrylic acid increased ca.3.4% per year. Although, a stagnation at the acrylic acid marketwas observed for the next few years, now a 1.6% increase is antic-ipated. It is important that more than half of the world productionof acrylic acid is used to synthesize various acrylic acid esters,chiefly n-butyl, and 2-ethylhexyl acrylates.

Acrylates are obtained by direct acid catalyst esterification of ac-rylic acid with alcohols at elevated temperature [5–11]. This is anequilibrium process. A disadvantage of such an esterification is thatthe subsequent reactions occur in these very conditions. The uncon-verted starting alcohol or acrylic acid reacts with double bond of theester formed. As a result, the alkoxyesters and acryloiloxyesters (theoxyesters) are formed. Moreover, acrylic acid is also involved in

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mistry Research Institute, 82 568 2021 (M.E. Jamróz), 48

.E. Jamróz), janek@il.waw.pl

dimerization to 3-acryloiloxypropionic acid. Furthermore, waterformed in esterification is added to acrylic moiety and hydroxyest-ers or hydroxyacid are formed, too. The generation of these by-prod-ucts can be seriously disadvantageous for the production costs.

Esterification reaction is apparently well recognized as is ex-plained in each fundamental organic chemistry textbook (e.g.[12]). However, although ab initio studies on esterification wereundertaken just in late 1970s and early 1980s [13,14], now, thisrelatively simple reaction is very seldom taken into considerationby using modern computational techniques. Indeed, a very recentDFT study on mechanism of direct esterification of the p-nitroben-zoic acid with n-butanol has shown the diethyl chlorophosphate todecrease by as much as ca. 20 kcal/mol [15]. Moreover, the reactioncan proceed via different channels: one of them, through a four-membered ring, is being a one-step reaction, whereas the otherproceeds through two steps. In another very recent study the mainfocus of the computational part of investigation of esterificationkinetics was to tell more about adsorption of acetic acid on a cat-alyst [16]. The energetics and polyesterification mechanisms ofsuccinic acid with ethylene glycol were recently investigated atthe B3LYP/6-31G�� level followed by IRC analysis [17]. It was foundthat the polyesterification ran in self-catalyzed concerted and non-self-catalyzed stepwise mechanisms. The self catalysts includeacid, alcohol and water. In conclusion, it was found that the opti-mum channel of the esterification is a stepwise mechanism andthat the self acid catalyst and dimerization are the rate-limitingsteps. Also, for large set of esters the enthalpies of formation wereelucidated by theoretical methods [18,19].

S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108 101

The experimental determination of the equilibrium constant,kinetic and thermodynamic parameters for the ethyl acrylate for-mation were accomplished in Refs. [20–24]. It appeared that thereaction is endothermic and equals ca. 6.5 kcal/mol [20,21]. Theestimated activation barriers practically did not vary with the cat-alyst used and was equal to ca. 15.5 kcal/mol [20–22]. On the otherhand, for butyl acrylate, the reaction enthalpy was found to beequal from ca. 7.5 kcal/mol [25] to ca. 16.5 kcal/mol [26], and thebarrier varied from 14 to 20 kcal/mol depending on the catalystused [26–29]. The physico-chemical aspects of the acrylic acidesterification with 2-ethyl-1-hexyl alcohol were studied onlyrarely [30,31]. The estimated enthalpy of the reaction was deter-mined to be 16.5–18.0 kcal/mol [30] and the reaction barrier toca. 17–18.5 kcal/mol [30] and 13.5 kcal/mol [31].

Recently, we have been developing technology of acrylic acidand acrylates production based on waste glycerol derived fromFAME manufacturing. Many of acrylate producers improve processof esterification by recovering and recycling reactants from theirhigher boiling adducts formed during the processing [7]. Neverthe-less, there is still little evidence to suggest which heavy adductsformation is favored in acrylate manufacturing. Therefore, it isimportant to recognize the physicochemical properties and energybarriers of the secondary reactions leading to the main by-products(Scheme 1). This was the main motivation for the computationalstudy.

2. Calculations

All DFT calculations were performed by using the B3LYP func-tional [32,33], for which the reliability in calculations of the groundstate geometries has been widely assessed [34]. The standard 6-31G�� basis set was applied throughout the study. Stationary struc-tures have been recognized as true minima after checking for thelack of imaginary harmonic frequencies. The relative abundancesof the most stable conformations were estimated using the Gibbsfree energies at 298.15 K, DG, and referred to the most stable

C H2

OH

O+ C H2

OH

O

C H2

O

OR + C H2

OH

O

C H2

O

OR + R OH

OH

O

O

OC H2 + R OH

C H2

O

OR H OH+

H+

H+

H+

Scheme 1. Main secondary reactions related

conformer. The influence of the solvent was also studied andthe solute–solvent interactions were calculated by using theTomasi’s polarized continuum model with the integral equationformalism (IEF–PCM) [35,36]. In this procedure, the solvent ismimicked by a dielectric continuum with dielectric constant esurrounding a cavity with shape and dimension adjusted to thereal geometric structure of the solute molecule. The latter polar-izes the solvent which, as a response, induces an electric field(the reaction field) which interacts with the solute. In the IEF–PCM, the electrostatic part of such an interaction is representedin terms of an apparent charge density spread on the cavity sur-face. Three solvents were considered: chloroform, ethanol, andwater. Because of the way, the PCM methods simulate the sol-vent surroundings, the solvents were chosen to reveal the effectof increasing permittivity of the solvent rather than the solventpossibility to be the H+ ions carrier. The dielectric constantsfor the solvent used were 4.71, 24.85, and 78.36 for chloroform,ethanol, and water, respectively. Full geometry optimizationswere performed both in the gas phase and in solvents. All sta-tionary points (minima and TSs) were confirmed by calculatingthe harmonic vibrational frequencies, using analytical secondderivatives. All calculations were performed by using Gaussian09 packages of programs [37].

3. Results and discussion

Our experimental study on esterification is focused on develop-ing acrylates technology, in particular on improvement of manu-facturing of ethyl, butyl, and 2-ethylhexyl acrylates [38]. Anumber of technologies currently used has still a severe shortcom-ing – they are performed with a homogeneous acidic catalysthighly corrosive and generating heavy waste, difficult to separatefrom the product. A solution to these problems can be the use ofan efficient heterogeneous catalysts. Therefore, we apply solid acidcatalysts in the form of sulfonated polystyrene/divinylbenzene re-sin, which are well recognized to be cheap, active and durable

OH

O

O

OC H2

3-acryloiloxy propionic acid (dimer)

O

O

O

OC H2 R

3-acryloiloxy propionic ester

OO

OR

R

3-Alkoxy propionic ester

OO

OR

H

3-Hydroxy propionic ester

H+

H+

to esterification process of acrylic acid.

Fig. 1. The B3LYP/6-31(d,p) most stable conformers of acrylic acid (AA), butyl alcohol (BuOH), and butyl ester (BA) and their the most stable protonated conformers.

Table 1The B3LYP/6-31G(d,p) calculated Gibbs free energies andrelative Gibbs free energies of reactants, transition statesand products of acrylic acid esterification with methyl, ethyl,and butyl alcohols calculated with and without presence of H+

catalyst (for abbreviations see Fig. 2).

System DG (kcal/mol)

Without H+ catalystAA1 + MeOH 0.0TS–MA(H2O) 71.2MA(H2O) 5.4

AA1 + EtOH 0.0TS–EA(H2O) 64.9EA(H2O) 5.8

AA1 + BuOH 0.0TS–BA(H2O) 64.6BA(H2O) 5.9

With H+ catalystAA1(H+) + MeOH 0.0TS–MA(H2O) (H+) 57.8MA(H2O)(H+) 9.2

AA1(H+) + EtOH 0.0TS–EA(H2O)(H+) 46.8EA(H2O)(H+) 9.0

AA1(H+) + BuOH 0.0TS–BA(H2O)(H+) 44.1BA(H2O)(H+) 9.1

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industrial catalysts. In particular, we tested activity and selectivityof the esterification over Amberlyst 39, 46, 70, and 131. The small-est amounts of byproducts and quite a high yield were obtained inpresence of Amberlyst 70.

The following reactions were performed experimentally: acrylicacid with (a) ethyl alcohol, (b) n-butyl alcohol, and (c) 2-ethyl-1-hexyl alcohol. The reaction conditions depended on the alcoholused for the esterification. For the ethyl acrylate formation, thetemperature was 355 K and atmospheric pressure. The butyl and2-ethyl-1-hexyl acrylates were obtained at the temperature rangeof 393–403 K under 4–5 bars. For all the acrylates, the main by-products are formed in the subsequent reactions of the appropriateester with alcohol, acrylic acid, and water (being a product of esterformation). The exemplary content of products is presented inTable 1SI of supplementary materials.

The limitation of byproducts formation is one of the most chal-lenging tasks of our research. To better understand the investi-gated processes, we performed the following computationalstudies. First, the main esterification reactions of the acid and alco-hols in vaccuo were considered. Second, the reactions were calcu-lated in presence of H+ ion to simulate the presence of acidiccatalyst. Finally, the influence of solvent surrounding was takeninto account by applying the IEF–PCM model of the water, metha-nol, and chloroform solvents. Then, the acid dimerization reaction,that is the acidic OH addition to the double bond of another acidmolecule, was studied with and without presence of a proton. Inthe subsequent steps the addition of water, alcohol and acrylic acidto double bond of esters previously formed was studied in pres-ence and absence of catalytic proton.

It is important, that in the experimental kinetic studies on influ-ence of catalyst concentration on reaction productivity it appearedthat the process can be described as quasi-homogeneous. This jus-tifies the theoretical model(s) applied in this study in which theitemized details of the catalytic surface have not been considered.Also, it is known that the presence of a proton decreases the barrierheight in the gas phase and therefore the influence of H+ ion on thereaction course can be accepted as a first step in modeling an acidcatalyzed reaction [38–41].

3.1. Esterification

The modeling of the acrylic acid esterification reactions bymethyl, ethyl, and butyl alcohol and the selected side reactionsin absence and in presence of acidic catalyst simulated by H+ ionwas performed by using B3LYP/6-31(d,p) calculations. In the intro-ductory step, the conformational spaces of each molecule partici-pating in the reaction (acrylic acid, methyl, ethyl, and butyl

alcohols, as well as the appropriate esters) were determined(Fig. 1). The same was done for each of the protonated forms(Fig. 1). First, the reactions without catalyst were considered. Theacrylic acid esterification reactions studied at the B3LYP/6-31G(d,p) were endoergic (Table 1). This is in agreement with theexperimental knowledge on the esterification processes. The acti-vation barriers for these reactions (Fig. 2) were found to be the fol-lowing: 65, 65, and 71 kcal/mol for synthesis of butyl, ethyl andmethyl, acrylates, respectively (Table 1). As expected, the activa-tion barriers decrease significantly when H+ ion is accompanyingthe reaction. Indeed, the barriers in the catalytic process are thefollowing: 44, 47, and 58 kcal/mol for synthesis of butyl, ethyland methyl, respectively (Table 1). We found that the catalyticreactions were even more endoergic than the non-catalytic pro-cesses (Table 1). Finally, we checked whether the solvent sur-rounding has an influence on the barrier height or not. Itappeared, that this influence (simulated by using the IEF–PCM sol-vent model) is practically meaningless (a matter of 0.5 kcal/mol,Table 2). This means, that the center of reaction remains practicallyimpenetrable by the solvent molecules in the reaction course and

Fig. 2. The B3LYP/6-31G(d,p) reactants (reactant complexes), transition states, and products of esterification of acrylic acid with methyl, ethyl, and butyl alcohols in absenceand presence of an acidic catalyst (H+) (AA – acrylic acid; MeOH – methyl alcohol; EtOH – ethyl alcohol; BuOH – butyl alcohol; MA – methyl acrylate; EA – ethyl acrylate; BA –butyl acrylate; TS – transition state).

S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108 103

that the charge distribution around this center for reactants, tran-sition states, and products remains quite stable within the frame ofthe IEF–PCM model applied.

Last but not least, let us comment on the applied methodology.The primary source of protons are the SO3H groups of a catalyst.Although in the very early step of reaction water is not yet presentin the system in an important amount, the H3O+ cation is a moreprobable H+ carrier than the H+ ion itself. Therefore, we tried tomodel the reactions with presence of the hydronium cation. Theproton jumps from the SO3H groups to the closest basicity center.It is likely, that in presence of alcohol it will be the alcohol hydro-xyl group. When the water molecules concentration is increasedduring the reaction course, the H3O+ ion formation start to be pop-ulated as well. However, the calculated systems are very flexibleand we could not find the TSs when the hydronium was used

instead of H+ cation. This probably could be found assuming someadditional constrains, yet, the error which could result from suchan assumption, would be difficult to estimate. In this context, useof chloroform looks bizarre because it can hardly be a proton car-rier. This very solvent was used only for the purpose to estimate arole of changeable permittivity of the reaction environment.

3.2. Side reactions

The acrylic acid esterification is accompanied by a series of sidereactions. This is a result of the presence of double bond in both thereactant and the acrylates. The acidic media activate the OH groupsin the acid and alcohols, but, simultaneously they activate the dou-ble bonds in the reactant and in the products. This is the main dif-ficulty in industrial operation with esterification. In our

Table 2The B3LYP/6-31G(d,p) calculated relative Gibbs free energies (kcal/mol) of reactants,transition states and products of acrylic acid esterification with the methyl, ethyl, andbutyl alcohols in vacuum and in solvents simulated by the IEF–PCM method (forabbreviations see Fig. 2).

System Gas phase Water Ethanol ChloroformDG298 DG298 DG298 DG298

AA1(H+) + MeOH 0.0 0.0 0.0 0.0TS–MA(H2O)(H+) 57.8 57.6 57.6 57.5MA(H2O)(H+) 9.2 9.5 9.5 9.4

AA1(H+) + EtOH 0.0 0.0 0.0 0.0TS–EA(H2O)(H+) 46.8 46.7 46.7 46.7EA(H2O)(H+) 9.0 9.3 9.3 9.2

AA1(H+) + BuOH 0.0 0.0 0.0 0.0TS–BA(H2O)(H+) 44.1 44.7 44.6 44.6BA(H2O)(H+) 9.1 9.4 9.3 9.3

Table 3The B3LYP/6-31G(d,p) calculated relative Gibbs free energies(kcal/mol) of reactants, transition states and products ofacrylic acid dimerization reaction calculated with andwithout presence of H+ catalyst (for abbreviations see Fig. 3).

System DG298

Without H+ catalystAA1 + AA1 0.0TS–AoxPA 68.5AoxPA 4.1

With H+ catalystAA1(H+) + AA1 0.0TS–AoxPA(H+) 56.3AoxPA(H+) �5.7

104 S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108

experimental studies, we detected addition products of water, alco-hols and acrylic acid to double bonds in acrylates. These reactionslead to quite a number of byproducts. First, we examined the so-called dimerization of acrylic acid, i.e., the carboxylic OH group addi-tion to double bond of the other acrylic acid molecule leading to for-mation of 3-acryloiloxypropionic acid (Fig. 3, Table 3). Observe, thatthe non-catalytic process is endoergic whereas the catalytic one isexoergic (Table 3). The barrier of the non-catalytic reaction is68.5 kcal/mol and is found to decrease by ca. 12 kcal/mol in presenceof a catalyst (Table 3). Next, we studied addition of water and alco-hols to double bonds of acrylates (Fig. 4). These reactions are exoer-gic: a matter of 3–5 kcal/mol (Tables 4 and 5). The barriers in thereactions with the water molecule and methanol do not depend onthe ester type and are equal to ca. 48 (±1) kcal/mol (Tables 4 and5). In the case of the addition of the acrylic acid molecule to acrylatesthe reaction is slightly exoergic (ca. 1.5 kcal/mol) and the barriers areequal to ca. 36.5 (±1.5) kcal/mol (Table 6).

3.3. Energetics

Now, let us summarize the energetics of the studied main andside reactions. First, the acrylic acid esterification is endoergic inline with the experimental findings. The presence of the H+ ionscut down the barrier heights. However, the endoergicity of the cat-alytic processes is larger than that of the non-catalytic reactions.Thus, use of a catalyst facilitates the reaction, but more energymust be supplied to run the process.

According to our calculations, the additions of the acrylic acidmolecule to acrylates are the easiest processes: they exhibit thesmallest activation barriers. Surprisingly, apparently similar

Fig. 3. The B3LYP/6-31(d,p) optimized reactants, transition states, and products of acrylic– acryloiloxypropionic acid; TS – transition state).

addition reaction of two acid molecules exhibits the barrier higherthan the above reaction by ca. 20 kcal/mol (Tables 3 and 6). Thereis however, a small but significant difference: two protons are in-volved in the addition of acid to an acrylate catalyzed by H+ ion,whereas three protons are involved into the analogous acid dimer-ization. In the former case, one of the protons can be localized atthe double bond of acrylate (Fig. 4), whereas in the latter case,the presence of three protons forces a specific conformation ofthe system blocking the double bond accessibility (Fig. 5). Note,that the addition of alcohol to the acid dimer (leading to the sameproduct as addition of acid to an acrylate) needs the 61 kcal/molbarrier to be overcome (Table 7). This means that this is the mostdifficult reaction in the whole reaction system. Finally, the forma-tion of alkoxy and hydroxy propionic acid esters requires ca.48 kcal/mol barrier to be overcome regardless the ester and alcoholtypes (Tables 4 and 5). Thus, the most important side reactions areall additions to double bond moiety of acrylic acid esters.

Last but not least, the proton affinities (PA) are listed in Table 8and Table 2SI of supporting information. The PA values are calcu-lated for the most stable forms of the neutral and protonated com-pounds. There is a regular tendency to increase the proton affinityfrom methyl, through ethyl, to butyl derivative regardless the typeof the studied compounds. The proton affinity tends to increasewith the number of oxygen atoms in the molecular structure. Forthe studied reactions, the number of O-atom in the products isgreater than in the reactants therefore, the protonation shifts thereaction towards the products.

3.4. Computational vs. experimental findings

Presentation of the computational results always bears a ques-tion about quantitative agreement of the energetical values found

acid (AA1) dimerization in absence and presence of H+ ion (AA – acrylic acid; AoxPA

Fig. 4. The B3LYP/6-31(d,p) reactants (reactant complexes), transition states, and products of water addition (a) and MeOH addition (b), and acrylic acid addition (c) reactionsto the double bonds of methyl, ethyl and butyl acrylates in presence of an acidic catalyst (H+) (AA – acrylic acid; MA – methyl acrylate; EA – ethyl acrylate; BA – butyl acrylate;TS – transition state; MP – methyl propanoate; EP – ethyl propanoate; BP – butyl propanoate; OHMP – hydroxy methyl propanoate; OHEP – hydroxy ethyl propanoate; OHBP– hydroxy butyl propanoate; MMP – metoxy methyl propanoate; MEP – metoxy ethyl propanoate; MBP – metoxy butyl propanoate; AoxMP – acryloiloxy methyl propanoate;AoxEP – acryloiloxy ethyl propanoate; AoxBP(H+) – acryloiloxy butyl propanoate).

S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108 105

with the experimental ones. This, however, is a very complex prob-lem. First, computations require simplifications and use of meth-ods guaranteeing finite time of calculations. In the case of manyside reactions of non-rigid molecules, this is a crucial factor. In-deed, this has been the purpose of use of not very sophisticatedB3LYP/6-31G�� level in this study. Moreover, it is known, that theB3LYP functional does not yield a correct barrier height [39,40],which is a consequence of parametrizing it to correctly reproducethermochemical not kinetic data [42,43]. Last but not least, thelargest and unpredictable errors arise from not fully adequatemechanism of reaction assumed in calculations. First of all, this in-cludes differences in surrounding of reacting system. The PCM-likemodels of the solvent may be of some help, yet, much better solu-tion to this problem is to combine PCM with a supermolecule ap-proach for the first solvation sphere. For non-rigid systems this isagain a very difficult requirement to be satisfied. As a result, the

computationally studied reaction is considered in an abstract vac-uum whereas the real reaction may be, for instance, sensitive totraces of water facilitating proton jump from one to the other elec-tron donor center. Similar can be said about a catalyst with definedstructure at the atomic scale, where some positive and negativecenters facilitate the reaction.

For the reaction studied in this paper there are limited thermo-dynamic and kinetic data [20–31]. They say, that by using variousacid catalysts and alcohols, the barrier for esterification falls intothe range from ca. 15 to 20 kcal/mol, whereas for the reaction mod-el assumed in this paper the barriers are ca. three times larger. Theexperimental thermochemical data for the side reactions areunavailable. The discrepancy between the calculated and theexperimental data is difficult to be unequivocally interpreted. Itmay indicate that the assumed mechanism is oversimplified. Itmay indicate an error in the experimental data larger than given

Fig. 4 (continued)

Table 4The B3LYP/6-31G(d,p) calculated relative Gibbs free energies(kcal/mol) of reactants, transition states and products of thewater molecule addition to the double bond of the methyl,ethyl, and butyl acrylates in presence of the H+ ion (forabbreviations see Fig. 4).

System DG298

MA(H+) + H2O 0.0TS–OHMP(H+) 47.5OHMP(H+) �3.1

EA(H+) + H2O 0.0TS–OHEP(H+) 48.2OHEP(H+) �3.6

BA(H+) + H2O 0.0TS–OHBP(H+) 48.5OHBP(H+) �3.7

Table 5The B3LYP/6-31G(d,p) calculated relative Gibbs free ener-gies (kcal/mol) of reactants, transition states and productsof the methanol molecule addition to the double bond ofthe methyl, ethyl, and butyl acrylates in presence of the H+

ion (for abbreviations see Fig. 4).

System DG298

BA(H+) + MeOH 0.0TS–MBP(H+) 47.8MBP(H+) �5.4

MA(H+) + MeOH 0.0TS–MMP(H+) 47.1MMP(H+) �4.7

EA(H+) + MeOH 0.0TS–MEP(H+) 47.6MEP(H+) �5.2

Table 6The B3LYP/6-31G(d,p) calculated relative Gibbs free ener-gies (kcal/mol) of reactants, transition states and productsof the acrylic acid molecule addition to the double bond ofthe methyl, ethyl, and butyl acrylates in presence of the H+

ion (for abbreviations see Fig. 4).

System DG298

MA(H+) + AA1 0.0TS–AoxMP(H+) 36.0AoxMP(H+) �1.4

EA(H+) + AA1 0.0TS–AoxEP(H+) 37.0AoxEP(H+) �1.7

BA(H+) + AA1 0.0TS–AoxBP(H+) 37.5AoxBP(H+) �1.8

106 S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108

in the papers. It may indicate incompatibility(ties) of the reactionconditions between the two studies. However, at the present stageof study, one may believe, that a constant systematic error is asso-ciated with all the calculated data and that qualitative conclusionscan be drawn and can be use to interpret the experimentalfindings.

Therefore, another important question arises: what is the signif-icance of our computational findings for interpretation of experi-mental systems. First, note that for practical reasons, aheterogenous catalysis is applied in industrial processes. However,a heterogenous catalysis is hardly reproducible by using a sole pro-ton to simulate the catalytic act. So, the calculations are likely tosimulate homogenous rather than heterogenous system. The sur-rounding is argued to have a minor influence on the reactioncourses, the assumption which is rather strong, yet, it is quite dif-ficult to go beyond this condition.

Let us now compare qualitatively the results of acrylic acidesterification in presence of heterogeneous acid catalysts with theresults of our modeling. First, in the experiments the main byprod-ucts are alkoxy and hydroxy propionic acid esters. Despite the factthat the addition of acid to esters is the easiest, in the experimental

conditions, concentration of acid is much lower than that of thealcohol. So, in the experimental systems the alkoxy propionatesare much more abundant than the acryloiloxypropionates. Second,the presence of hydroxy propionic acid esters may seem strange

Fig. 5. The B3LYP/6-31(d,p) reactants (reactant complexes), transition states, and products of methanol esterification of acrylic acid dimer in absence and in presence of anacidic catalyst (H+) (MeOH – methanol; AoxPA – acryloiloxypropionic acid; TS – AoxMP – transition state for acryloiloxy methyl propanoate; AoxMP – acryloiloxy methylpropanoate).

Table 7The B3LYP/6-31G(d,p) calculated Gibbs free energies(kcal/mol) of reactants, transition states and productsof the methyl alcohol esterification of the dimer ofacrylic acid in absence and in presence of the H+ ion(for abbreviations see Fig. 5).

System DG298

Without H+ catalystAoxPA + MeOH 0.0TS–AoxMP 81.3AoxMP + H2O 15.4

With H+ catalystAoxPA(H+) + MeOH 0.0TS–AoxMP(H+) 60.9AoxMP(H+) + H2O 6.3

Table 8The B3LYP/6-31G�� calculated proton affinities (kcal/mol) of the selected systemsbased on Gibbs free enthalpies calculated for 298 K and 1 atm.

System Proton affinity (kcal/mol)

Methanola �184.6Ethanola �190.5Butanola �193.0Acrylic acidb �195.8Acrylic acidc �159.6Methyl acrylateb �200.3Ethyl acrylateb �203.5Butyl acrylateb �205.0Acryloiloxy propionic acidb �217.6Hydroxy methyl propanoateb �211.9Hydroxy ethyl propanoateb �214.2Hydroxy butyl propanoateb �215.1Acryloiloxy methyl propanoateb �212.4Acryloiloxy ethyl propanoateb �214.6Acryloiloxy butyl propanoateb �215.6Metoxy methyl propanoateb �216.8Metoxy ethyl propanoateb �219.2Metoxy butyl propanoateb �220.2

a Protonation at the hydroxyl O-atom.b Protonation at the O atom of the C@O group.c Protonation at the C@C double bond.

S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108 107

taking into account the fact that concentration of water in the prod-uct is even smaller than that of acid (a matter of ca. 5%). However, itis quite easy to understand that fact. Water, even if it is in minorquantities, is attached to acidic groups (usually SO3H) of heteroge-

neous catalysts. Thus it is always accompanying to proton activat-ing the double bonds. Therefore, locally, in close vicinity of thecatalytic centers concentration of water is high. Finally, the acrylicacid dimers are detectable, but, in minor concentrations, due to thefact that the acid is reacted with high excess of alcohols. Interest-ingly, the book-mechanism of acrylic acid dimerization is the Mi-chael addition in presence of anions, while in the experimentalsystem the cations predominate in the system. Thus, either a com-petitive mechanism may occur or small amounts of water induceacid dissociation and possibility for nucleophilic attack to doublebond of the other acrylic acid molecule.

4. Conclusions

The esterification of acrylic acid molecule with alcohols isindustrially a very important reaction leading to acrylates whichare monomers for obtaining water-based polyacrylic emulsionsfor production of coatings, polishes, carpet backing compounds,adhesives, sealants and polymers used in construction, paper andtextiles industry, as well as for curing polymeric materials, usedin dentistry. Unexpectedly, neither acrylate formation nor the sidereactions were studied by computational chemistry methods, sofar.

In this study, we present results of the B3LYP/6-31G�� calcula-tions on energetic and barriers of esterification of the acrylic acidmolecule with methyl, ethyl, and butyl alcohols. Because of doublebond moiety present in acrylate molecule, it is useful for furtherpolymerization, however, it is also the target of subsequent alcoholattack to perform alkoxy propionic acid esters, as well as someother side products connected with undesired additions to acry-late’s double bond. Deeper inspection into formation of the sideproducts in light of computational methods, was the aim of thisstudy.

We limited the investigations to by-products observed by us inthe experimental studies in significant extents, i.e., acrylic acid di-mer, alkoxy propionic acid esters, hydroxy propionic acid ester,and the product of addition of acrylic acid to acrylate in presenceand in absence of the H+ cation. As a result, we established, thatthe latter reaction exhibits the lowest activation barrier(36.5 ± 1.5 kcal/mol). Next, additions of water and alcohols to acry-lates needs ca. 48 ± 1 kcal/mol to overcome the reaction barrier.The most difficult reaction to run is the alcohol addition to the aciddimer with ca. 60 kcal/mol activation barrier.

108 S. Ostrowski et al. / Computational and Theoretical Chemistry 974 (2011) 100–108

Although, the studied processes need much more attention andhigher computational level to confirm quantitative analysis of thereaction energetics, qualitatively they help us in understanding theresults of our experiments on heterogeneous catalytic esterifica-tion of acrylic acid.

Acknowledgments

This work was supported by Ministry of Science and HigherEducation in Poland Grant No. POIG 01.03.01-00-010/08. The com-putational Grant G19-4 from the Interdisciplinary Center of Math-ematical and Computer Modeling (ICM) at the University ofWarsaw is gratefully acknowledged.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.comptc.2011.07.016.

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