Fuel 102 (2012) 72–77

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Esterification of lauric acid with butanol-1 over H3PW12O40 supported on MCM-41

Varsha Brahmkhatri, Anjali Patel ⇑Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India

h i g h l i g h t s

" 12-tungstophosphoric acid anchoredto MCM-41 was synthesized andcharacterized.

" Catalytic activity for esterification oflauric acid with n-butanol.

" Catalyst was recovered and reusedup to four cycles.

" Esterification of lauric acid followssecond order rate law.

" Activation energy was found to be78 kJ mol�1.

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.05.053

⇑ Corresponding author. Tel.: +91 265 2795552.E-mail address: aupatel_chem@yahoo.com (A. Pate

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 May 2010Received in revised form 23 May 2012Accepted 29 May 2012Available online 19 June 2012

Keywords:HeteropolyacidHeterogeneous catalysisEsterificationLauric acidBuOH

a b s t r a c t

Heterogeneous acid catalyst comprising 12-tungstophosphoric acid and MCM-41 was synthesized andcharacterized by surface area measurement (BET method), X-ray diffraction (XRD) and solid state 31PMAS–NMR. The catalytic activity was evaluated for esterification of fatty acid, lauric acid with 1-butanol.Influence of various reaction parameters such as catalyst concentration, acid/alcohol molar ratio andreaction time were studied to optimize the conditions for maximum yields. Also the catalyst was recov-ered and reused up to four cycles. A study on the kinetic behaviour was carried out by classical methodand it was found that esterification of lauric acid follows second order rate law. The influence of temper-ature on rate constant was also studied and the activation energy was found to be 78 kJ mol�1.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The esterification of fatty acid with short chain alcohols is veryimportant due to their industrial applications. The fatty acid estersconstitute an important class of useful chemical intermediates inthe synthesis of several products such as amides, sulfonates andfatty alcohols. These compounds can be used as solvents, spreadingor softening agents in polymers. They are also used in the textile,cosmetic and rubber industries [1]. For example, esters produced

ll rights reserved.

l).

from long-chain fatty acids (12–20 carbon atoms) as well asshort-chain fatty acids (2–8 carbon atoms) are used increasinglyin the food, detergent, and plasticizer, and lubricant, cosmeticand pharmaceutical industries [2].

Traditionally the fatty acid esters are produced in batch reactorsusing strong acids like sulphuric acid [3,4]. These processes sufferfrom high cost separation procedures, large energy consumptionand the production of polluting by-products. As these reactionsare equilibrium limitated, high conversions can be only obtainedby using a large excess of alcohol which is again not desired dueto the economic reasons. A number of reports are available onthe esterification reactions of fatty acids using various heteroge-neous catalysts [5–15].

Table 1Main characterization of catalyst.

Catalyst Surface area(m2/g)

d (Å) Porediameter

Mesopore volume(cm3/g)

MCM-41 659 47.90 0.7930% PW/MCM-41 360 30.13 0.50

V. Brahmkhatri, A. Patel / Fuel 102 (2012) 72–77 73

Catalysis by supported heteropolyacids offers relevant possibil-ities for sustainable production. They have been greatly expandedduring the past few years from the viewpoint of their variety ofstructures and compositions. They provide the opportunities fortuning their chemical properties, such as acidities, and reactivitiesby choice of appropriate support. Considering acidic properties,they have found enormous applications in various industriallyimportant classes of reactions such as alkylations, acylations, andesterifications. A literature survey shows that, heteropolyacidssupported onto various supports have been extensively studiedin esterification reactions [16–29]. Our group is also working inthe field of Supported heteropolyacids and we have establishedthe use of supported heteropolyacids as solid acid catalysts in var-ious organic transformations [30–32], especially for esterificationof primary and secondary alcohols. The obtained promising resultsencouraged us to extend the work for esterification of fatty acids.

In the present paper, esterification of lauric acid with butanolwas studied over 30% 12-tungstophosphoric acid anchored toMCM-41 by varying different reaction parameters such as moleratio, amount of the catalyst and reaction time to optimize the con-ditions for maximum yields. The kinetics of the reaction was alsostudied over this catalyst. The rate of reaction and order of reactionwas determined. The effect of temperature on rate constant wasstudied, and the activation energy was calculated by fitting theresults into Arrhenius equation.

2. Experimental

2.1. Materials

All chemicals used were of A.R. grade, H3PW12O40, nH2O (Lobachemie, Mumbai). CTAB, tetraethyl orthosilicate TEOS, lauric acidand n-butanol were used as received from Merck.

2.2. Synthesis of support (MCM-41)

MCM-41 was synthesized using reported procedure [33] withslight modification. Surfactant (CTAB) was added to the very dilutesolution of NaOH with stirring at 60 �C. When the solution becamehomogeneous, TEOS was added drop wise and the obtained gel wasaged for 2 h. The resulting product was filtered, washed with dis-tilled water, dried at room temperature. The obtained materialwas calcined at 555 �C in air for 5 h and designated as MCM-41.

2.3. Synthesis of catalyst (PW anchored to MCM-41)

A series of catalyst containing 10–30% of PW anchored to MCM-41 were synthesized by impregnation. 1 g of MCM-41 was impreg-nated with an aqueous solution of PW (0.1/10–0.3/30 g/ml ofdouble distilled water) and dried at 100 �C for 10 h. The obtainedmaterials were designated as 10% PW/MCM-41, 20% PW/MCM-41,30% PW/MCM-41 and 40% PW/MCM-41 respectively.

2.4. Characterization

The main characterization of the support MCM-41 and catalyst30% PW/MCM-41 such as surface area measurement, average porediameter and pore volume are presented in Table 1.

For 30% PW/MCM-41, theoretically calculated (%): W, 19; P,0.32. Results obtained by EDS (%): W, 18; P, 0.30; Si, 27.8; O,53.9. The results obtained from EDS were in good agreement withthe theoretical values.

Adsorption–desorption isotherms of samples were recorded ona Micromatries ASAp 2010 surface area analyzer at �196 �C. Fromadsorption–desorption isotherms surface area was calculated

using BET method. The XRD pattern was obtained by using PHILIPSPW-1830. The conditions used were: Cu Ka radiation (1.5417 Å),scanning angle from 0� to 60�. The 31P NMR spectra were recordedat 121.49 MHz using a 7 mm rotor probe with 85% phosphoric acidas an external standard. The spinning rate was 5–7 kHz. Catalystsamples after treatment were kept in a desiccator over P2O5 untilthe NMR measurement.

2.5. Catalytic reaction

The esterification of lauric acid with alcohols were carried out ina 50 ml batch reactor provided with a double walled air condenser,Dean–Stark apparatus, magnetic stirrer and a guard tube. Dean–Stark apparatus was attached to a round bottom flask to separatethe water formed during the reaction. The reaction mixture wasrefluxed at 90 �C for 4 h. The obtained products were analyzed ona Gas Chromatograph (Nucon-5700) using BP1 capillary column.Products were identified by comparison with the authentic sam-ples and finally by Gas Chromatography–Mass Spectroscopy (GC–MS).

3. Results and discussion

The decrease in surface area, pore size and pore volume with in-crease in PW loadings reveals that PW was well dispersed insidethe hexagonal pores of MCM-41 and any pore blockage did nottake place.

The XRD patterns of pure MCM-41 30% PW/MCM-41 are shownin Fig. 1. The XRD pattern of the calcined MCM-41 showed a sharppeak around 2h = 2� and few weak peaks in 2h = 3–5�, which indi-cated well-ordered hexagonal structure of MCM-41. No separateHPA crystal phase of PW was observed in the 30% PW/MCM-41.Further the absence of characteristic peaks of PW indicates thatPW is finely dispersed inside the hexagonal channels of MCM-41.Hence, there must be some chemical interaction between the hostMCM-41 and the guest PW.

31P NMR is the most important method to study chemical envi-ronment around the phosphorous in heteropoly compounds. The31P NMR spectra of PW and 30% PW/MCM-41 are shown inFig. 2. The value of chemical shifts for PW is �15.626 ppm and isin good agreement with the reported value [34].

The value of chemical shift for 30% PW/MCM-41 is�15.523 ppm. No appreciable change in chemical shift value re-veals that the surrounding environment for the supported PW doesnot change even after supporting to MCM-41. In other words theKeggin ion structure is retained after anchoring to MCM-41.

3.1. Esterification of lauric acid with n-butanol

The esterification of lauric acid is an equilibrium-limited reac-tion. In order to overcome the equilibrium limitation, it is necessaryeither to carry out esterification by taking one of the reactant in ex-cess. The yields can be increased by increasing the concentration ofeither alcohol or acid. In a practical means, to obtain maximum yieldfor economic reasons, the reactant that is usually less expensive istaken in excess. In present study corresponding alcohol is taken in

Fig. 1. XRD patterns of MCM 41 and 30% PW/MCM-41.

74 V. Brahmkhatri, A. Patel / Fuel 102 (2012) 72–77

excess. The esterification of lauric acid with alcohol is shown inScheme 1.

3.2. Effect of amount of PW supported onto MCM-41

The esterification reaction was carried out with 10% PW/MCM-41, 20% PW/MCM-41, 30% PW/MCM-41 and 40% PW/MCM-41. Theobtained results are shown in Fig. 3. It is observed from the fig-thatwith increase in the % loading of PW onto MCM-41, yield also in-creases. For 10% and 20% loading, yields for butyl laurate are con-siderably low. For 30% and 40% loading, the difference in yield isnot that much appreciable. Hence the catalyst containing 30% load-ing of PW, i.e. 30% PW/MCM-41 was used for the detail study.

Fig. 2. 31P NMR (MAS) spectra o

R-OH+

Lauric acid alcohol

30%

Scheme 1. Esterification of l

3.3. Effect of mole ratio

The reaction was carried out by varying mole ratio of lauric acidto n-butanol, with 0.2 g of the catalyst for 4 h at 90 �C. It is ob-served that with increase in concentration of alcohol the yield ofthe product; butyl laurate increases (Fig. 4). With 1:2 mol ratio oflauric acid to n-butanol maximum yields up to 95 was achieved.With further increase in n-butanol concentration not much differ-ence was observed. Hence, further reactions were carried out in1:2 mol ratio of acid to alcohol.

3.4. Effect of amount of catalyst

To study the effect of the amount of the catalyst, the reactionwas carried out with different amount of the catalyst keeping themole ratio of acid to alcohol 1:2 for 4 h at 90 �C. The yield of butyllaurate is reported in Fig. 5. It is seen from the figure that initiallythe activity increases with an increase in the amount of the cata-lyst. On further increase with the amount from 0.25 to 0.25 g ofthe catalyst, maximum yields of ester was obtained. The effect ofthe amount of the catalyst was studied keeping all other parame-ters (temperature, reaction time, mole ratio of alcohol to acid) con-stant. Under these conditions, no increase in the yield withincrease in amount of catalyst indicates the attainment of the max-imum equilibrium. The obtained results are as expected. Hence,0.2 g is the optimized amount of the catalyst.

3.5. Effect of reaction time

With increase in reaction time the yield of butyl laurate in-creases (Fig. 6). Upto 3 h maximum yield of butyl laurate wasachieved. Further it does not increase with time.

f PW (a), 30%/MCM-41 (b).

PW/MCM-41

Alkyl laurate

auric acid with alcohol.

10 20 30 400

20

40

60

80

100

% Y

ield

% loading of PW

Fig. 3. Dependence of butyl laurate yield on the PW content supported on MCM-41.

1:1 1:1.5 1:2 1:30

20

40

60

80

100

% y

ield

Mole ratio (acid/alcohol)

Fig. 4. Effect of molar ratio of acid/alcohol on the butyl laurate yield (reactionconditions: 0.2 g catalyst, 90 �C, 3 h).

75 100 125 150 175 200 2250

20

40

60

80

100

% Y

ield

amount of catalyst (mg)

Fig. 5. Effect of catalyst amount on the butyl laurate yield (reaction conditions:acid/alcohol 1:2, 90 �C, 3 h).

V. Brahmkhatri, A. Patel / Fuel 102 (2012) 72–77 75

3.6. Effect of reaction temperature

With increase in reaction temperature the yield of butyl laurateincreases (Fig. 7). At 90 �C maximum yield was obtained.

The optimum conditions for esterification of lauric acid using30% PW/MCM-41 are: mole ratio of acid to alcohol 1:2; amountof catalyst 0.2 g; reaction temperature 90 �C and reaction time 3 h.

3.7. Recycling of the catalyst

The catalyst was recycled in order to test its activity as well asstability. The catalyst was separated from the reaction mixtureonly by simple filtration, washed with conductivity water till thefiltrate is free from the acid dried at 100 �C and the recovered cat-alyst was charged for the further run. There is no appreciablechange in the yield of butyl acetate using regenerated catalyst upto four cycles (Fig. 8).

3.8. Leaching and heterogeneity tests

Any leaching of the active species from the support makes thecatalyst unattractive, and hence it is necessary to study the stabil-ity as well as leaching of PW from the support. Rigorous proof ofheterogeneity can be obtained only by filtering the catalysts atthe reaction temperature before completion of the reaction andtesting the filtrate for activity [35]. The leaching of W from 30%PW/MCM-41 was confirmed by carrying out an analysis of the usedcatalyst (EDX) as well as the product mixtures (AAS). Analysis ofused catalyst did not show any appreciable loss in W content ascompared to the fresh catalyst. Analysis of the product mixturesshows that if any W was present it was below the detection limit,which corresponds to less than 1 ppm. These observations stronglysuggest no leaching of any active species form the support. For therigorous proof of heterogeneity, a test was carried out by filteringthe catalysts from the reaction mixture at 90 �C after 2 h, and thefiltrate was allowed to react up to 4 h. The reaction mixture of4 h and the filtrates were analyzed by gas chromatogram. Therewas no change in the yield, indicating the present catalyst falls intocategory C [35]. On the basis of these results, it can be concludedthat there is no leaching of PW species from the support MCM-41, and the present catalyst is truly heterogeneous in nature.

The EDS analysis of the reused catalyst R1-30% PW/MCM-41was also carried out. The elemental analysis values are close to

1 2 3 40

20

40

60

80

100

% Y

ield

Reaction time (h)

Fig. 6. The yield of butyl laurate in the course of reaction (reaction conditions: acid/alcohol 1:2, 0.2 g catalyst, 90 �C, 3 h).

70 80 90 1000

20

40

60

80

100

Reaction temperature (oC)

% Y

ield

Fig. 7. Effect of reaction temperature on the butyl laurate yield (reaction condi-tions: acid/alcohol 1:2, 3 h).

fresh catalyst 1st 2nd 3rd 4th0

20

40

60

80

100

% Y

ield

number of recylces

Fig. 8. PW/MCM-41 recycling in esterification of lauric acid with n-butanol(reaction conditions: acid/alcohol 1:2, 0.2 g catalyst, 90 �C, 3 h).

0

0.05

0.1

0.15

0.2

0.25

0 30 60 90 120 150 180

Time t (min)

(1/(b

-a))l

n((a

(b-x

))/(b

(a-x

))

Fig. 9. Second order plot for esterification reaction over 30% PW/MCM-41.

0

0.5

1

1.5

2

2.5

0.006 0.008 0.01 0.012 0.014 0.016 0.018

Catalyst concentration (mmol)

Rat

e co

nsta

nt x

10-

3

Fig. 10. Plot of rate constant for catalyst concentration.

-0.5

0

0.5

1

1.5

2

0.00265 0.0027 0.00275 0.0028 0.00285 0.0029 0.00295

1/T

lnk

Fig. 11. Arrhenius plot for esterification of lauric acid with n-butanol over 30% PW/MCM-41.

76 V. Brahmkhatri, A. Patel / Fuel 102 (2012) 72–77

the fresh catalyst. This also confirms the heterogeneous mode ofaction of the catalyst.

4. Kinetics

A detail study on kinetic behaviour for esterification of lauricacid with 1-butanol was carried out over 30% PW/MCM-41. In allthe experiments, reaction mixtures were analyzed at fixed inter-vals of time using gas chromatography.

4.1. Determination of rate constant and order of reaction

For the irreversible bimolecular-type second order reaction

Aþ B ����!CatalystProduct ð1Þ

The rate of consumption of A, (�rA) is given by

rA ¼ k½C�½A�½B� ð2Þ

The concentration of catalyst is constant through out the reaction.Therefore,

rA ¼ k0½A�½B� ð3Þ

Where k0 = k [C], [A] = Concentration of fatty acid, lauricacid.[B] = Concentration of alcohol, 1-butanol.The integrated formof Eq. (3) is

ln½aðb� xÞ=bða� xÞ� ¼ ðb� aÞk0t ð4Þ

where x is the fraction of the product formed, a the initial concen-tration of fatty acid (mmol/ml), b the initial concentration of alcohol(mmol/ml), and k0 is the second order rate constant.

Fig. 9 shows plot for Eq. (4). The linearity of the data indicatesthe validity of Eq. (4).

The increase in catalyst concentration shows noticeable effecton yields of butyl laurate. This can be attributed to higher numberof substrate molecules get activated in presence of 30% PW/MCM-

V. Brahmkhatri, A. Patel / Fuel 102 (2012) 72–77 77

41 catalyst. Thus the nucleophilic attack by 1-butanol becomesmore favorable and consequently, an increase in the formation ofester was observed. The linearity of data obtained indicates secondorder dependence (Fig. 10) with respect to catalyst concentration.

4.2. Determination of activation energy

It was verified that temperature affects both reaction rate andyields. With increase in temperature higher yields were achieved.The increase in reaction temperature caused corresponding in-crease in the reaction rate, especially in the range of 343–373 K.From this data Arrhenius plot was constructed and from the ob-tained curve pre-exponential factor (A) and activation energy(Ea) for this reaction was calculated (Fig. 11).

The pre-exponential factor (A) and activation energy (Ea) wasfound to be 27 mol�1 min�1 and 78 kJ mol�1 respectively. The rateconstant (k0) for esterification reaction of lauric acid with butanolwas found to be 2.3 � 10�3 mol�1 min�1.

It is important to recognize that the whether the reaction rate isdiffusion limited/mass transfer limited or it is truly governed bythe chemical step where the catalyst is being used to its maximumcapacity. It is reported that the activation energy for diffusion lim-ited reactions is as low as 10–15 kJ mol�1 and for reactions whoserate is governed by a truly chemical step usually show activationenergy excess of 25 kJ mol�1 [36]. In the present case the observedactivation energy is 38 kJ mol�1 and hence the rate is truly gov-erned by chemical step.

5. Conclusion

Esterification of fatty acid, lauric acid with 1-butanol was suc-cessfully carried out over a solid acid comprising 12-tungstophos-phoric acid and MCM-41. 95% yield was obtained for butyl laurate.Also the catalyst was regenerated and reused up to four cycles. Thekinetic study reveals that the reaction follows the second orderkinetic law with respect to reactants as well as the catalyst. Theinfluence of temperature on rate constant was also studied andthe activation energy was found to be 78 kJ mol�1.

Acknowledgements

One of the authors, Ms. Varsha Brahmkhatri, is thankful toBRNS, Project No. 2007/37/20/BRNS/917, BARC, Mumbai, for thefinancial assistance.

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