kinetics of esterification of aromatic carboxylic

161–167

Kinetics of esterification of aromatic carboxylic acids over zeolites H␤and HZSM5using dimethyl carbonate

Sharath R.Kirumakki a,N.Nagaraju a,Komandur V.R.Chary b,

Sankarasubbier Narayanan b,∗

a Department of Chemistry,St.Joseph’s College P.G.Center,46Langford Road,Bangalore560027,India

b Inorgani

c an

d Physical Chemistry Division,Indian Institut

e o

f Chemical Technology,Hyderabad500007,India

Received16December2002;received in revised form14February2003;accepted14February2003

Abstract

Esterification of benzoic acid and substituted benzoic acids has been carried out efficiently over zeolite H␤and HZSM5in an autoclave using dimethyl carbonate(DMC)as the methylating agent.The wide applicability of this esterification method to esterify a wide range of aromatic carboxylic acids has been demonstrated.This method is a viable and safe alternative to other esterification processes,involving harmful alkylating agents and chemicals.Strong acid sites are responsible for the reaction.The reaction proceeds inside as well as outside the pores of the zeolites.The pore architecture of the zeolites and mass transfer limitations come into play only when the molecular diameter of the reactant molecules is greater than the pore size of the zeolites.Kinetic study shows that the reaction isfirst-order with respect to acid concentration.

©2003Elsevier Science B.V.All rights reserved.

Keywords:Esterification;Aromatic carboxylic acids;Dimethyl carbonate;Zeolites;Kinetics

1.Introduction

Esterification reaction is one of the fundamental processes in organic synthesis and has been exten-sively used[1].A number of techniques including microwave applications have been developed for es-terification process and new ones continue to appear [2–4].Most of the reported procedures for the synthe-sis of methyl esters require the use of sulfuric acid, hydrochloric acid and toxic chemicals like dimethyl sulfate or methyl iodide or unsafe reagents such as diazomethane,which are environmentally hazardous and unacceptable.Considering the impact of these ∗Corresponding author.Tel.:+91-40-27193200;

fax:+91-40-27160921.

E-mail address:snarayanan@iict.ap.nic.in(S.Narayanan).chemicals on the environment there is an urgent need to develop a more eco-friendly method for the produc-tion of the methyl esters.The use of zeolites as solid acid catalysts in organic transformations in place of conventional catalysts is on the rise[5–8].The main advantages of using zeolites are that they are environ-mentally benign,can be reused and are thermally sta-ble.Protonated forms of zeolites have been found to be efficient catalysts in esterification reactions[9–11]. Generally,alcohols are used as esterifying agents and water is a by-product of the reaction.In order to achieve higher yields of the ester removal of water during the reaction becomes inevitable.Moreover, when zeolites are used the water that is formed during the reaction could lead to the deactivation of the cat-alyst.In order to overcome this problem,using an al-ternate methylating agent in place of methanol would

0926-860X/$–see front matter©2003Elsevier Science B.V.All rights reserved. doi:10.1016/S0926-860X(03)00152-2

162S.R.Kirumakki et al./Applied Catalysis A:General 248(2003)161–167

be interesting.Dimethyl carbonate (DMC)is one such alkylating agent that has found applications in vari-ous organic transformations.It is a non-toxic building block,which can be used as a green substitute for toxic and corrosive reagents.Shieh et al.[12,13]have em-ployed dimethyl carbonate to esterify carboxylic acids using 1,8-diazo-bicyclo[5,4,0]undec-7-ene (DBU)as the catalyst and also by using microwave irradiation technique.Dimethyl carbonate,when used as an es-terifying agent,CO 2and methanol are formed as the by-products and they do not interfere in the reaction.Our objective has been to synthesize organic chem-icals through catalytic routes and we have used ox-ides,clays and zeolites for Friedel Crafts alkylation reactions [14,15].The use of zeolites as solid acid catalysts has also been extended for esterification of carboxylic acids [16–18].DMC has been found to be an efficient methylating agent in the esterification of salicylic acid [18].We found that in this reaction the acid functionalities of zeolites play a pivotal role in the yield of the ester.In the present investigation,we have chosen benzoic acid and various substituted ben-zoic acids for esterification with DMC over zeolites H ␤,HY and HZSM5to study the influence of zeolite pore architecture and the reactant size on the reaction.Mass transfer limitations reduce the effectiveness of the catalyst and in most liquid phase reactions resis-tance to mass transfer is large enough to compete with surface reaction.Madon and Boudart [19]have devel-oped a criterion to assess whether the catalytic activity is independent of mass transfer limitations by using a number of Pt/SiO 2samples with different metal load-ing having similar metal dispersion in the liquid phase hydrogenation of cyclohexene.Sato et al.[20]studied the mass transfer limitations in mesopores of Ni-MgO

Table 1

Physico-chemical properties of the zeolites used Zeolite (Si/Al)

BET surface area (m 2/g)Acidity a (mmol/g)NH 3Pore dimension (Å)

Yield of methyl benzoate b (%)Weak (A)Medium (B)Strong (C)H ␤(8)434 1.03 2.3 4.50 5.5×5.5987.6×6.4HY (30)730 1.51 1.700.997.4×7.414HZSM5(25)

400

1.

1.16

3.05

5.3×5.690

5.1×5.8

a Estimated by STPD of NH 3[21].

b

Catalyst weight,1.5g;benzoic acid:DMC,1:4(mol/mol);reaction temperature,423K;reaction time,4h.

catalysts under liquid phase hydrogenation conditions.It has been consequently elucidated that the mass transfer of reactants is restricted in pores smaller than the size of the reactant molecule.The present study brings out the mass transfer limitations in the zeolite pores,when the reactant size is more than the pore diameter and by using reactant molecules of different dimensions.It also demonstrates the wide applicabil-ity of dimethyl carbonate for esterification of substi-tuted aromatic carboxylic acids over H ␤and HZSM5.2.Experimental

Zeolites ␤,Y and ZSM5were screened for the esterification of various benzoic acids by dimethyl carbonate.Protonated forms of the zeolites Y and ZSM5were obtained from Conteka,The Netherlands.The sodium form of zeolite ␤was obtained from United Catalysts India Limited and was converted to its protonated form following the conventional ion ex-change procedure using aqueous 0.5M NH 4NO 3so-lution followed by calcination at 823K.The relevant physico-chemical properties of the zeolites used are given in Table 1.Acidity of the zeolites was mea-sured by stepwise temperature-programmed desorp-tion of ammonia on a Micromeritics pulse chemisorb 2700by bracketing the temperature of desorption.The details of acidity measurements are described else-where [21].The esterification reactions were carried out in a stainless steel autoclave reactor (internal vol-ume:300cm 3)under autogenous pressure at the stip-ulated reaction temperature.The reactants were taken directly in the autoclave along with the catalyst usu-ally 1.5g,kept in a hot air oven and the reaction was

carried out for a definite period of time.The benzoic acid to DMC molar ratio was1:4for the preliminary experiments and the total volume of the reactants was 20cm3.For example,in the case of benzoic acid,6.5g, it was taken along with1.5g of the catalyst.For the substituted benzoic acids,the weight of the acid was adjusted to get1:4molar ratio of acid:DMC.After the stipulated reaction time,the reactor was cooled to room temperature and a suitable solvent(usually acetone)was added to dissolve the acid in the re-action mixture and also to wash the products,then filtered to separate them from the catalyst.The re-action products were analyzed by GC–MS(column: HP-1MS15m×0.25mm×0.25␮m).Blank reactions were also carried out in the absence of catalyst.Ki-netic run were carried out at408K for salicylic acid, 3-methoxy-4-hydroxy benzoic acid and for3-phenoxy benzoic acid the reactions were carried out at423K as yield was very low at lower temperatures.The quan-tity of the catalyst and the reaction time was varied to generate kinetic data.

3.Results and discussion

Zeolites H␤,HY and HZSM5were screened for the esterification of benzoic acid.Zeolites H␤and HZSM5gave methyl ester yield>90%where as HY gave only14%(Table1).We have mentioned that strong acid sites are responsible for salicylic acid esterification reaction[18].Among the zeolites H␤, HY and HZSM5studied,H␤and HZSM5have more strong acid sites and HY has much less of the same (Table1).This explains why HY is not as active as the other two for the esterification reaction.H␤and HZSM5show activities which commensurate with their strong acidity.Because of comparable activi-ties,zeolites H␤and HZSM5were chosen for further studies.The preliminary results of the esterification reaction of different benzoic acids with DMC over H␤and HZSM5are given in Table2.Esterification of benzoic acids using dimethyl carbonate proceeded smoothly over these zeolites.A clean regioselectivity was observed,the selectivity towards the ester was 100%in all cases.This method of esterification is suitable to efficiently esterify even relatively inactive substituted benzoic acid like3,5-dinitro benzoic acid (entry8).Table2

Esterification of benzoic acids with dimethyl carbonate over zeo-lites

Entry no.Compound Molecule

diameter

(Å)

Yield of the ester(%)

H␤HZSM5

1 4.390

2 4.69792

3 4.59791

4 4.39591

5 4.39590

6 5.77553

7 5.78051

87.17740

9∼7.53525

Zeolite weight,1.5g;acid:DMC,1:4(mol/mol);reaction temper-ature,423K;reaction time,4h.

In benzoic acids,the aromatic ring does not partic-ipate in the resonance stabilization of the carboxylate ion.Substituents on the ring influence the acidity pri-marily by the inductive effect.The yield of ester de-pends on the substituents on the ring as well as on the1S.R.Kirumakki et al./Applied Catalysis A:General248(2003)161–167

type of zeolite used.From our previous study[18],we found that the zeolitic acidity is a controlling factor in the esterification of salicylic acid with DMC at408K. However,at higher temperatures,say423K,the yield of the ester was independent of the acidity of the ze-olites.Preliminary studies were carried out at423K to bring forth the effect of zeolite pore architecture in the esterification reaction.For mono-substituted benzoic acids with molecular diameter less than5Å, the yield of ester is above90%when H␤and HZSM5 are used(entries1–5).However,in case of di-or tri-substituted benzoic acids(entries6–8)the pore size of the zeolites seem to play an important role in the conversion of the carboxylic acid.The yield of esters is between75and80%on H␤whereas it is below55%on HZSM5.The pores of H␤are large enough to accommodate these reactant molecules whose diameter is more than5.7Åwhereas the pores of HZSM5could not accommodate them(see Table1 for pore dimensions).The fact that the esterification reaction does occur on HZSM5,though to a relatively less extent suggest the involvement of the active sites, which may be outside the pores of zeolite.Even though the pores are large enough in H␤,one does not preclude the possibility of the reaction taking place outside the pore as well.When a bulky substituent is present in the ring like in the case of m-phenoxy benzoic acid(entry9,diameter>7.5Å),the yield of the ester decreases drastically to around35%on H␤and25%on HZSM5.The reactant molecule is large and the pores of H␤cannot be accessed eas-ily thus leading to low conversion.This study sug-gests that the active centers for these reactions are available inside as well as outside the pores of the zeolites.

The reaction of benzoic acid with DMC carried out in the absence of zeolite catalyst did not yield any methyl ester.This suggests that DMC interacts with the active sites on the zeolites to form an ac-tivated complex,which would further react with the carboxylic acid to yield the corresponding ester.From our earlier investigation,we found out that the active centers for the esterification reaction with DMC are the strong acid sites present in zeolites[18]. Investigations on the reproducibility and reusability of the catalyst were conducted on the esterification of benzoic acid with DMC.The catalysts were reused once afterfiltration,washing and calcination at823K.The activity decreased by only∼3%with respect to methyl benzoate formation.

4.Kinetic study

The mass transfer limitations in the micropores of the zeolites H␤and HZSM5are studied as fol-lows.Reactant molecules varying in molecular di-mension,viz.2-hydroxy benzoic acid(entry2), 3-methoxy-4-hydroxy benzoic acid(entry7)and 3-phenoxy benzoic acid(entry9)were selected.The reaction rate was calculated from the conversion of the acid to its corresponding ester.A plot of−ln(1−conv) versus time was found to be linear in all the reactions, indicating afirst-order dependence of the rate on

the Fig.1.(a)First-order dependence plot for the esterification reaction with dimethyl carbonate over zeolite H␤:3-phenoxy benzoic acid (᭹)at423K,3-methoxy-4-hydroxy benzoic acid(᭿)and salicylic acid(᭡)at408K.(b)First-order dependence plot for the ester-ification reaction with dimethyl carbonate over zeolite HZSM5: 3-phenoxy benzoic acid(᭹)at423K,3-methoxy-4-hydroxy ben-zoic acid(᭿)and salicylic acid(᭡)at408K.

S.R.Kirumakki et al./Applied Catalysis A:General 248(2003)161–167165

concentration of carboxylic acid (Fig.1a and b ).The rate constants were calculated from the slopes of the plots.To understand the diffusion limitations,the es-terification reaction was carried out by varying the weight of the zeolite powder.The rate of reaction (k )was calculated in each case (Table 3).The plot of k versus the weight of zeolite for the three

different Fig.2.(a)Plot of rate constant (k )vs.weight of the zeolites H ␤(᭡)and HZSM5(᭹)for esterification reaction of salicylic acid with DMC;SA:DMC,1:4(mol/mol);reaction temperature,408K.(b)Plot of rate constant (k )vs.weight of the zeolites H ␤(᭡)and HZSM5(᭹)for esterification reaction of 3-phenoxy benzoic acid with DMC;acid:DMC,1:4(mol/mol);reaction temperature,423K.(c)Plot of rate constant (k )vs.weight of the zeolites (i)H ␤(᭡)and (ii)HZSM5(᭹)for esterification reaction of DMC:3-methoxy-4-hydroxy benzoic acid;acid:DMC =1:4(mol/mol);reaction temperature =408K.

molecules is given in Fig.2a–c .As can be seen,k for the esterification reaction of salicylic acid on both H ␤and HZSM5increases with zeolite weight.For the es-terification of 3-methoxy-4-hydroxy benzoic acid the rate constant increases linearly only on H ␤;however on HZSM5the rate constant was not a linear function of the zeolite weight.For the esterification of bulky

166S.R.Kirumakki et al./Applied Catalysis A:General 248(2003)

161–167

Fig.2.(Continued )

3-phenoxy benzoic acid (pore dimension:≈7.5Å),the rate constant did not show any linear relationship with catalyst weight on both the zeolites H ␤and HZSM5indicating mass transfer limitations for these molecules.We have earlier given the mechanism for the esterification of salicylic acid with DMC over zeolites [18].Considering that the mechanism of es-terification reaction of carboxylic acid is the same on both zeolites,it is reasonable to assume that the pore architecture plays a role in conversion especially in the case of bulky acid molecules such as 3-phenoxy ben-zoic acid.The fact that a small molecule like salicylic acid,which can enter the pores of the zeolites easily,gives good conversion supports the argument that the active sites are present mostly inside the zeolite pore.In the case of large molecules such as 3-phenoxy ben-zoic acid (entry 9)one still observes some reaction over the zeolites in spite of the fact that this molecule cannot enter the zeolite pore.Obviously,one tends to believe that some active sites must be available on the outside of the pore as well,may be around the pore mouth of the zeolite.Of course,the conversion is very low (25and 35%for HZM5and H ␤,respectively)suggesting that only a fewer no of active sites are outside the pore than you would find them inside the pore.Therefore,one concludes that the active sites are available more inside the zeolite pore than on the outside.

S.R.Kirumakki et al./Applied Catalysis A:General248(2003)161–167167 Table3

Rate constants for the esterification reaction of benzoic acids with

DMC

Weight of the catalyst(g)Rate constant,k(×103min−1)

H␤HZSM5

a b c a b c

0.5 1.510.60.90.50.4

1.0 3.7 1.7 1.2 1.80.80.7

1.5 5.4

2.6 1.4 2.3 1.0 1.0

2.07.9

3.4 1.5 2.9 1.0 1.0

4.012.5

5.6 1.6 3.5 1.1 1.1

a,salicylic acid;b,3-methoxy-4-hydroxy benzoic acid;c, 3-phenoxy benzoic acid;acid:DMC,1:4(mol/mol);reaction tem-perature,408K for a,b and423K for c.

5.Conclusion

An efficient esterification of benzoic acid and sub-stituted benzoic acids is reported over zeolites H␤and HZSM5for thefirst time.DMC is a safe and versatile methylating agent for the esterification of aromatic carboxylic acids.This esterification reaction is facile and can be used to esterify a wide range of carboxylic acids over zeolite catalysts.Acidity of zeolites plays an important role in the conversion levels as evidenced from the differences over HY, H␤and HZSM5zeolites.The study suggests that the active sites are inside as well as outside of the zeolitic pores.The pore architecture of the zeolite comes into play when the molecular diameter of the reactant molecules is greater than5.7Å.For a zeolite to be an efficient catalyst for esterification of aromatic carboxylic acids the right pore size and strong acidity are the determining factors.Kinetic studies reveal that the esterification reaction is afirst-order with respect to the carboxylic acid concentration. Acknowledgements

SN thanks CSIR,New Delhi for the award of an Emeritus Scientist Position at IICT,Hyderabad.SRK thanks the Department of Science and Technology (DST),New Delhi for the JRF fellowship and the Director,Indian Institute of Chemical Technology (IICT)for granting permission to utilize the research facilities.

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