The Use of Acid-Activated Montmorillonite as a Solid Catalyst for theProduction of Fatty Acid Methyl EstersLeandro Zatta,† Eduardo Jose Mendes Paiva,†,‡ Marcos Lucio Corazza,‡ Fernando Wypych,†

and Luiz Pereira Ramos*,†,§

†CEPESQResearch Center in Applied Chemistry, Department of Chemistry, Federal University of Parana, P.O. Box 19081,Curitiba, PR 81531-980, Brazil‡Department of Chemical Engineering, Federal University of Parana, Curitiba, PR, Brazil§INCT Energy & Environment, Federal University of Parana, Curitiba, PR, Brazil

ABSTRACT: The esterification of lauric and stearic acids, tall oil fatty acid, and a commercial oleic acid with methanol wasinvestigated using an acid-activated standard montmorillonite (AASM) as a catalyst. Reaction variables such as the methanol:fattyacid molar ratio, catalyst content, and temperature were evaluated. Comparative reactions were performed with K10 catalyst, andsimilar or even better results were obtained with AASM, indicating that this could be employed as a suitable Lewis/Bronstedesterification catalyst. The experimental results obtained for the esterification of lauric acid with methanol were compared to athermodynamic model. For this purpose, the UNIFAC model was used for the activity coefficient calculation for components inthe nonideal mixture. This model has shown that the catalytic system was able to drive the reaction to equilibrium within 2 h, andthis was confirmed by comparing the experimental results with thermodynamic predictions.

1. INTRODUCTION

In the past few decades, several studies have been orientedtoward the use of oils and fats as a source of fatty esters for theproduction of biodiesel and other chemicals. Most of theindustries around the world still use alkaline catalysts inhomogeneous media to produce alkyl esters via trans-esterification.1−3 Although economically viable, this processrequires a starting material with low contents of water and freefatty acids, in order to avoid saponification, since this processconsumes the catalyst, decreases the selectivity towardbiodiesel, makes the separation of the alkyl esters and glycerolvery difficult, and contributes to emulsion formation duringwashing.4−6

The use of heterogeneous catalysts may overcome many ofthe above-mentioned problems. Although having variableselectivity due to the existence of multiple active sites, solidcatalysts normally have high thermal stability, are easy torecover and reuse, and facilitate the downstream processing ofboth fatty acid alkyl esters and diglycerol.5,7−10

Biodiesel can also be produced via the esterification of fattyacids (FA) with low-molecular-weight alcohols and thisconversion technology has been considerably improved dueto its relevance for the production of fatty ester from acid oilsand their low-cost feedstock. In addition, hydrolysis of oils andfats can be used to produce free fatty acids whose esterificationmay lead to a more complete conversion in the feedstock.6

Based on a technological assessment of different continuousprocesses, Zhang et al.11 showed that acid catalysis using wasteoils is technically feasible and less complex than a conventionaltwo-step process involving pre-esterification of oils and fatswith an homogeneous catalyst followed by an alkali-catalyzedtransesterification.Esters produced from biomass can provide high value-added

materials such fuels additives, pharmaceuticals, emulsifiers, and

chemical industries.12−16 On the other hand, biomass can beused as raw material in the production of biofuels. However,some precautions must be taken to ensure sustainability. Thelife cycle assessment (LCA) and life cycle impact assessment(LCIA) are valuable tools to ensure the correct land use for theproduction of biomass, avoiding rain forest and permanentgrassland clearings as reported by Soimakallio and co-workers.17

Most of the recent studies involving fatty acid esterificationhave been focused on the kinetics of this chemicalreaction.10,13,18−23 However, a theoretical model that is basedon the thermodynamics and equilibrium conditions of thereaction can be an useful tool to understand its technicalfeasibility and limitations. Theoretical thermodynamics likethese are scarcely available for the esterification of fatty acidswith methanol. In many cases, difficulties arise when no reliabledata are available for the enthalpy and Gibbs free energy offormation of reagents and products. Also, nonideality of liquidphase must be taken into account, normally using an activitycoefficient model such as UNIFAC, UNIQUAC, NRTL, orothers, therefore increasing the complexity of the chemicalequilibrium calculations. In addition, many studies do notaddress the self-catalytic behavior of esterification (thermalconversion) and the influence of phase separation on thereaction displacement.21−24

In this context, the AASM-catalyzed methyl esterification ofdifferent fatty acids and fatty acid mixtures were investigated. Astandard clay mineral from the 2:1 group (MontmorilloniteSTx-1) was activated with phosphoric acid as previouslyreported.25 The influence of the methanol:fatty acid molar

Received: April 24, 2014Revised: August 6, 2014

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© XXXX American Chemical Society A dx.doi.org/10.1021/ef500935q | Energy Fuels XXXX, XXX, XXX−XXX

ratio, catalyst content, and temperature were studied with twodifferent fatty acids (lauric and stearic) and two fatty acid mix(tall oil and an oleic acid commercial preparation). Also, thechemical equilibrium calculation for the lauric acid esterificationwith methanol was provided using a nonideal mixture approachto drive the discussion on the theoretical chemical equilibriumfor this reaction type.

2. MATERIAL AND METHODS2.1. Materials. The catalyst was prepared as reported

previously.25 This catalyst was obtained by reacting rawmontmorillonite (STx-1) with dilute phosphoric acid(H3PO4, Nuclear, 85%) in a flat-bottomed flask connected toa reflux condenser. The acid activation was performed bymixing the solid sample in a 1:4 ratio (mass per volume) usingan acid concentration of 0.5 mol L−1 and activation for 2 h.After the reaction, the solid was washed until pH close to 7,dried first at 110 °C for 24 h and then at 250 °C for 2 h(MMT-PO4). Activation with other mineral acids required thefollowing adjustments to the reaction procedure: nitric acid at0.14 mol L−1 with a MMT solid ratio of 1:10.15; hydrochloricacid at 0.14 mol L−1 with a MMT solid ratio of 1:10.57; andsulfuric acid at a 0.06 mol L−1 with a MMT solid ratio of 1:10.The esterification was performed with methanol (Erich,

99.8%), lauric acid (C12H24O2, Vetec, 98%), stearic acid(C18H40O2, Sigma−Aldrich, 95%), and a commercial oleicacid mixture (Vetec). Tall oil fatty acids were also used foresterification and these were kindly provided by Metachem(Curitiba, PR, Brazil).For comparison, a standard commercial Lewis acid catalyst

(K10) was also used (Sigma−Aldrich). This catalyst is obtainedby thermal and acid activation of the clay mineralmontmorillonite. This process destroys most of the mon-tomorillonite structure, preserving the contaminant mineralslike muscovite and quartz. According to the supplier, thecatalyst has an average surface of 240 m2 g−1 and a microporevolume of 0.1 mL g−1.2.2. Experimental Section. In this study, the methanol:-

fatty acid molar ratio (MR), the amount of catalyst in relationto the fatty acid mass (CAT), and the reaction temperature (T)were systematically varied to reveal the best condition foresterification. These process variables were evaluated in twolevels (MR values of 6:1 and 12:1; 8 and 12 wt % of CAT; T =140 and 160 °C) with three replicates at the central point.All experiments were performed in a 50-mL Cyclone

Buchiglasuster reactor, miniclave drive model (Buchi, Switzer-land). The pressure inside the reaction vessel corresponded tothe vapor pressure of methanol at temperature found inside thereactor. The reaction time began at the moment in which thedesired temperature was reached, which was ∼25 min after theinitiation of heating (isothermal conditions). After reactioncompletion, the used catalyst was separated from the reactionmedium by centrifugation. The excess of methanol wasrecovered by rotary evaporation and the solid material waswashed three times with 15 mL of ethanol:hexane 1:1 (v/v),then centrifuged and dried in an oven at 90 °C for 12 h. Otherconditions included a reaction time of 2 h and 500 rpm, andthese values were obtained from experiments previouslyoptimized.25 The catalyst was added in percentage by mass inrelation to the dry mass of the fatty acids used for conversion.The conversion of FA to methyl esters (ME) was evaluated

by determining changes in acid number, according the AOCSCa-5a-40 official method (1998).26 A good correlation between

this method and hydrogen nuclear magnetic resonance (1HNMR) has been already demonstrated elsewhere.27

2.3. Chemical Equilibrium Calculations. Briefly, thereaction equilibrium for the esterification of lauric acid wascalculated considering a nonideal approach for the mixture inthe liquid phase, and the activity coefficients were determinedby the UNIFAC model.28 Thermodynamic parameters for purecomponents were either obtained from the literature orestimated from group contribution methods, as shown in

Tables 1 and 2. The equilibrium constant was calculated usingthe following equation:

∏ γ=−Δ

= ν⎛⎝⎜

⎞⎠⎟K

GRT

xexp ( )T

i

c

i iai

(1)

where c is the number of components, T the temperature, ΔGTthe Gibbs free energy of the reaction at a given T, γi the activitycoefficient of component i, Ka the equilibrium constant, xi themolar fraction of component in the mixture, and νi thestoichiometric coefficient of component i.Since the standard Gibbs free energy values are available in

the literature under standard conditions (298.15 K and 1 atm),corrections to the reaction temperatures had to be made by thefollowing equation:

Δ= − +

Δ− +

Δ⎜ ⎟ ⎜ ⎟⎛⎝

⎞⎠

⎛⎝

⎞⎠

GRT R

aT

bH

R T T

G

RT1 1 1T T T

T

ref ref

ref (2)

where,

= Δ − + Δ − + Δ −

+ Δ −

a a T Tb

T Tc

T T

dT T

( )2

( )3

( )

4( )

ref2

ref2 3

ref3

4ref

4

(3)

= Δ + Δ − + Δ −

+ Δ −

⎛⎝⎜

⎞⎠⎟b a

TT

b T Tc

T T

dT T

ln ( )2

( )

3( )

refref

2ref

2

3ref

3

(4)

The standard enthalpy and the Gibbs free energy offormation at reference temperature Tref (denoted as ΔHTref

and ΔGTref, respectively) were obtained using the properties

of each component by ΔHTref= ∑iviΔHf,i

0 and ΔGTref=

∑iviΔGf,i0 . Equations 3 and 4 were obtained by using the

standard heat capacity of component i (at the ideal gas state),32

= + + +Cp a b T c T d Tiig

i i i i2 3

(5)

Table 1. Thermodynamics Properties of Pure CompoundsFormation under Standard Conditions (Ideal Gas, 298.15 K,1 atm)a

component ΔGf,i0 (kJ/mol) ΔHf,i

0 (kJ/mol)

lauric acid −293.10 −640.00methyl laurate −240.00 −612.30methanol −162.32 −200.94water −228.59 −241.81

aAccording to DIPPR (2000).29

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where ΔCp = ∑i=1nc viCpi

ig, Δa = ∑iviai, Δb = ∑ivibi, Δc = ∑ivici,and Δd = ∑ividi.

3. RESULTS AND DISCUSSION

3.1. Esterification of Lauric Acid with Methanol. Adetailed study was carried out to verify the effect of processvariables for the esterification of lauric acid with methanol usingMMT-PO4 as the reaction catalyst.25 The variables involved inthis study were the methanol:fatty acid molar ratio (MR), thetemperature (T), and the amount of catalyst (CAT) (see Table3).The effect of the catalyst is clearly demonstrated by the

conversion gain relative to the noncatalyzed conversion, whichis due to the autocatalytic activity under the applied reactionconditions. The best conversions were obtained at 160 °C (Run8) and, when this temperature was maintained and the contentof the catalyst was reduced from 12% (Run 8) to 8% (Run 4),

the conversion was only slightly affected (Table 3). This alsoapplies to the MR of 6:1 in Runs 6 and 2.By comparing the difference between the variable effects and

their corresponding t8 × SD values (Student’s t-distributiontimes the standard deviation) (see Table 4), all effects weredemonstrated to be statistically significant. At 160 °C, changesin the MR caused a small increase in reaction conversion (Runs2 and 4). Increased molar ratios should produce an increase inconversion by displacing the equilibrium toward the formationof products; however, in this case, there is also the antagonisticeffect of dilution. This effect is clearly observed at lowerreaction temperatures, when an increase in MR leads to lowerreaction conversions (Runs 1 and 3).The variables that contribute the most to the highest

conversion of lauric acid to methyl laurate are the temperatureand the amount of catalyst, relative to the amount of fatty acids,which led to an increase of 2.84 and 2.12 points percent (p.p.)in the conversion values, respectively. The second-orderinteractions that showed the same trend are T × MR (3.02

Table 2. Vapor Pressure and Heat Capacities of Pure Components

component model A B C D E T range (K)

Vapor Pressurelauric acid ref 29 201.56 −20454.0 −24.33 8.055 × 10−18 6 316.9−743.0methyl laurate ref 29 155.18 −14494.0 −18.91 7.421 × 10−6 2 278.1−712.0methanol ref 30 −8.112 0.7698 −3.108 1.54481 50−1000water ref 30 −7.764 1.4583 −2.775 −1.23303 50−1000

Heat Capacitylauric acida b −3.42 1.1946 0.7 × 10−3 2.0 × 10−7 273−1000methyl lauratea b 3.7808 1.2418 0.6 × 10−3 6.0 × 10−8 273−1000methanol ref 32 19.038 9.14 × 10−2 −1.218 × 10−5 −8.034 × 10−9 273−1000water ref 32 32.218 0.192 × 10−2 1.055 × 10−5 −3.593 × 10−9 273−1800

aEquation 6 was fitted using values obtained from group contribution method of Joback and Reid.31 bEquation 7 was used as Cpiig(J mol−1 K−1) = ai

+ biT + ciT2 + diT

3.

Table 3. Factorial Design (23) for the Esterification of Lauric Acid with Methanol, Using MMT-PO4 as the Reaction Catalyst for2 h

Experimental Conditions Results (Titration)

testatemperature, T

(°C)bmethanol:lauric acid molar

ratio,b MRbulk catalyst content,b,c

CATacidity(%)d

conversion(%)e

conversion gain compared to theblank (%)

Blank 1 140 6:1 0 31.1 ± 1.0 68.9Blank 2 160 6:1 0 20.8 ± 1.0 79.2Blank 3 150 9:1 0 31.6 ± 0.9 68.4Blank 4 140 12:1 0 29.8 ± 0.3 70.2Blank 5 160 12:1 0 30.2 ± 0.4 69.8Run 1 140 (−) 6:1 (−) 8 (−) 6.9 ± 0.3 93.1 24.2Run 2 160 (+) 6:1 (−) 8 (−) 6.0 ± 0.4 94.0 14.8Run 3 140 (−) 12:1 (+) 8 (−) 13.5 ± 0.6 86.5 16.3Run 4 160 (+) 12:1 (+) 8 (−) 3.8 ± 0.0 96.2 26.4Run 5 140 (−) 6:1 (−) 12 (+) 5.8 ± 0.2 94.2 25.3Run 6 160 (+) 6:1 (−) 12 (+) 7.1 ± 0.5 92.9 13.7Run 7 140 (−) 12:1 (+) 12 (+) 5.6 ± 0.6 94.4 24.2Run 8 160 (+) 12:1 (+) 12 (+) 3.4 ± 0.0 96.6 26.8Run 9 150 (0) 9:1 (0) 10 (0) 5.0 ± 0.5 95.0 26.6Run 10 150 (0) 9:1 (0) 10 (0) 4.6 ± 0.3 95.4 27.0Run 11 150 (0) 9:1 (0) 10 (0) 4.3 ± 0.5 95.7 27.3Run 3f 140 (−) 12:1 (+) 8 (−) 5.5 ± 0.5 94.5 24.3Run 4f 160 (+) 12:1 (+) 8 (−) 3.6 ± 0.3 96.4 26.6aRuns given in this table represent the reactions that were carried out under the corresponding conditions of the experimental design. “Blank”represents the absence of an added catalyst. bThe symbols (+), (−), and (0) represent the maximum, minimum, and center point of the factorialdesign. cCAT = bulk catalyst content, relative to lauric acid (in terms of % (w/w)). dMeasured in three replicates. eOnly one experiment wasperformed under each experimental condition. fRun was carried out using standard K10 as the reaction catalyst.

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p.p.) and MR × CAT (2.13 p.p.). The third-order interaction(−1.39 p.p.) was also statistically significant, but its value wasvery close to the standard deviation of the model, meaning thatit does not contribute very much toward higher reactionconversions. Hence, the most valuable interaction effects of thefactorial design were those of the second order.The catalytic performance of the MMT-PO4, as a function of

time, is shown in Figure 1, in which the data were generatedunder the following conditions: MR = 12:1, T = 160 °C, and10% CAT, relative to the lauric acid mass.

Regarding the time range studied in this work, the catalyzedreactions always had higher conversions than those carried outwithout a catalyst (Figure 1). The maximum conversion of96.58% was obtained after 120 min and, as described in thenext section (item 3.3), these catalytic conversions were ingood agreement with calculated values from the thermody-namic equilibrium calculations.To address the relative importance of phosphate anions in

the catalytic activity of MMT-PO4, the clay mineral wassubjected to activation with three other mineral acids:hydrochloric acid, nitric acid, and sulfuric acid. The temperatureand time used for these syntheses were the same as those usedfor MMT-PO4, but the ratio between the amount ofmontmorillonite and the volume of the acid solution wasadjusted to obtain the same pH employed in the preparation ofthe phosphoric acid catalyst.The systematic characterization of these acid-activated

montmorillonites has been included in a previous publication.25

Such materials were tested as catalysts in the esterification oflauric acid at 160 °C, using a molar ratio of 12:1 and 12 wt % ofcatalyst. Under these conditions, lauric acid to methyl laurateconversions of 93.4% ± 0.4%, 95.1% ± 0.7%, and 94.8% ± 0.3%

were obtained with materials prepared with hydrochloric acid,nitric acid, and sulfuric acid, respectively. These values arerelatively close to each other but sufficiently different from theperformance of MMT-PO4 (96.6%) to suggest that the catalyticactivity of this class of materials is dependent on the type ofacid used for activation.

3.2. Esterification of Stearic Acid with Methanol. Theresults concerning the esterification of stearic acid withmethanol are shown in Table 5. The thermodynamiccalculations were not performed in this case, because reliabledata on methyl stearate cannot be found in the literature andsimulations using the group contribution method31,33 presentedhigh standard deviations (data not shown). From athermodynamic point of view, because of the proximity ofthe enthalpy and entropy energies of different saturated fattyacids, similar trends are expected, with regard to the reactionconversion under the same experimental conditions.The effects of temperature, molar ratio, and catalyst content

in the esterification of stearic acid with methanol were 10.20,−0.66, and 5.56 p.p., respectively. Hence, likewise for theesterification of lauric acid with methanol, the variables thatmost contributed to the highest conversions to methyl stearatewere the temperature and the amount of catalyst (Table 5).The only second-order effect that contributed positively wasthe simultaneous increase of temperature and molar ratio (0.72p.p.), while the simultaneous increase of the three variables hada positive effect (4.96 p.p.).Run 19 (see Table 5) resulted in the highest conversion of

stearic acid to methyl stearate (94.1%, see Table 5). Suchreaction conditions involved a molar ratio of 12:1 at 160 °Cwith 12 wt % of catalyst, in which the highest conversion oflauric acid to methyl laurate was observed (see Table 3).

3.3. Esterification of Fatty Acid Mixtures withMethanol. Table 6 shows the decrease in acid number as aresult of the esterification of both tall oil fatty acids and thecommercial oleic acid mix.All variables had a significant effect in the esterification of tall

oil fatty acids with methanol (not shown). The first ordereffects were of 12.93, 2.90, and 2.38 p.p. for the reactiontemperature, molar ratio, and amount of catalyst, respectively.Hence, the temperature had the greatest isolated effect onreaction conversion. The second-order effects were alsosignificant and had a positive contribution to the system,while the third-order interaction between variables had an effectclose to zero, indicating little statistical significance.The highest conversion of the tall oil fatty acids to methyl

esters occurred at 160 °C using a molar ratio of 12:1 and acatalyst content of 12 wt %. These conditions led to a 93.5%reduction of the tall oil fatty acids (TOFA) acid number.The catalytic activity of the MMT-PO4 catalyst was also

tested against a commercial preparation of oleic acid. Analysisby gas chromatography showed that this commercialpreparation contained a mixture of myristic, palmitic, linoleic,and oleic acids (data not shown). As in the previous situation, afactorial design was built to investigate the performance of theMMT-PO4 catalyst in the esterification of these fatty acids withmethanol. These results were very similar to those obtainedwith TOFA, as shown in Table 7, meaning that the sameprocess considerations also apply in this case.

3.3. Chemical Equilibrium Calculation for Lauric AcidEsterification with Methanol. As described in section 2.3,chemical equilibrium calculations of lauric acid esterificationwith methanol were performed using a thermodynamic

Table 4. Effects Calculated for the 23 Factorial Design ofTable 3

factor effect standard deviation (SD) t8a × SD

global average 93.50 0.49 1.13temperature, T 2.84 1.14 2.63molar ratio, MR −0.07 1.14 2.63% catalyst, CAT 2.12 1.14 2.63T × MR 3.02 1.14 2.63T × CAT −2.50 1.14 2.63MR × CAT 2.13 1.14 2.63T × MR × CAT −1.39 1.14 2.63

at8 = Student’s t-distribution.

Figure 1. Comparison of the noncatalytic and catalytic conversions oflauric acid to methyl laurate using 10 wt % of MMT-PO4 at 160 °Cand a methanol:lauric acid molar ratio of 12:1.

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approach where the reactant mixture was considered a nonidealsolution, and the activity coefficients of each compound in themixture were calculated using the UNIFAC model. Besides, thetemperature dependence was taken into account using a

shortcut derivation (eqs 2−4). The equilibrium conversion oflauric acid as a function of MR and T are presented in Figures 2and 3, respectively. Comparing the predicted values (chemicalequilibrium calculations) with the results presented in Figure 1,when MMT-PO4 is used at 160 °C with a RM of 12:1, theequilibrium conversion can be reached within 120 min.The theoretical data of Figures 2 and 3 showed that the

equilibrium conversion for the esterification of lauric acid withmethanol is influenced by the temperature when low MR valuesare employed. Considering the MR condition of 3:1 in Figure1, the equilibrium conversions were 80% at 100 °C and 95% at200 °C, while at MR = 12:1, the predicted conversions were90% at 100 °C and 95% at 200 °C. Another important findingof this thermodynamic simulation is that, at constanttemperature, the equilibrium conversion is considerablyenhanced when the MR value is increased up to 3:1 but itlevels off when the MR values are above 6:1. As predicted bythis model, there is no reason to employ MR values higher than9:1, because no further conversions will be achieved at thispoint (Figure 3). In general, the maximum allowed conversionsare within 97%−98% at MR ≥ 9:1 and T = 180 °C. Also, thesesimulated results support that the catalyst is able to drive thereaction to equilibrium within 120 min.

4. CONCLUSIONS

Acid-activated clay minerals of the 2:1 group were catalyticallyactive in the esterification of fatty acids and fatty acid mixtureswith methanol. Acid activation was better when performed withphosphoric acid but other cheaper minerals acids could also beemployed by simply adjusting the variables used during theactivation process.The thermodynamic simulations indicated that the temper-

ature and the methanol:lauric acid molar ratio are the mostimportant variables to achieve high conversions of lauric acid tomethyl laurate. Hence, these theoretical calculations are useful

Table 5. Factorial Design for Esterification of Stearic Acid with Methanol, Using MMT-PO4 as the Reaction Catalyst

Experimental Conditions Results (Titration)

testatemperature, T

(°C)bmethanol:lauric acid molar

ratio,b MRbulk catalyst content,b,c

CATacidity(%)d

conversion(%)e

conversion gain compared to theblank (%)

Blank 6 140 6:1 0 38.7 ± 0.4 61.6Blank 7 160 6:1 0 19.4 ± 0.5 80.6Blank 8 150 9:1 0 34.7 ± 0.2 65.3Blank 9 140 12:1 0 46.2 ± 0.4 53.8Blank 10 160 12:1 0 27.9 ± 0.7 72.1Run 12 140 (−) 6:1 (−) 8 (−) 25.9 ± 0.7 74.1 12.5Run 13 160 (+) 6:1 (−) 8 (−) 7.5 ± 0.4 92.6 12.0Run 14 140 (−) 12:1 (+) 8 (−) 18.1 ± 0.3 81.9 28.1Run 15 160 (+) 12:1 (+) 8 (−) 8.1 ± 0.6 91.9 19.8Run 16 140 (−) 6:1 (−) 12 (+) 7.1 ± 0.5 92.9 31.3Run 17 160 (+) 6:1 (−) 12 (+) 6.6 ± 0.6 93.4 12.8Run 18 140 (−) 12:1 (+) 12 (+) 17.7 ± 0.2 82.3 28.5Run 19 160 (+) 12:1 (+) 12 (+) 5.9 ± 0.6 94.1 22.0Run 20 150 (0) 9:1 (0) 10 (0) 7.0 ± 0.6 93.0 27.7Run 21 150 (0) 9:1 (0) 10 (0) 6.9 ± 0.5 93.1 27.8Run 22 150 (0) 9:1 (0) 10 (0) 6.7 ± 0.3 93.3 28.0Run 15*f 160 (+) 12:1 (+) 8 (−) 2.8 ± 0.1 97.2 25.0Run 19*f 160 (+) 12:1 (+) 12 (+) 4.2 ± 0.2 95.8 23.6aRuns given in this table represent the reactions that were carried out under the corresponding conditions of the experimental design. “Blank”represents the absence of an added catalyst. bThe symbols (+), (−), and (0) represent the maximum, minimum, and center point of the factorialdesign. cCAT = bulk catalyst content, relative to lauric acid (in terms of % (w/w)). dMeasured in three replicates. eOnly one experiment wasperformed under each experimental condition. fRun was carried out using standard K10 as the reaction catalyst.

Table 6. 23 Factorial Design for the Esterification of Tall OilFatty Acids with Methanol Using MMT-PO4 as the ReactionCatalyst

Experimental Conditions Titration Results

testa T MR CATdecrease inacidity (%)

standarddeviation(p.p.)

Blank11

140 6:1 0 56.3 0.7

Blank12

160 6:1 0 75.2 0.6

Blank13

150 9:1 0 65.5 0.5

Blank14

140 12:1 0 49.1 0.4

Blank15

160 12:1 0 69.9 1.1

Run 23 140 (−) 6:1 (−) 8 (−) 75.3 0.3Run 24 160 (+) 6:1 (−) 8 (−) 85.7 0.3Run 25 140 (−) 12:1 (+) 8 (−) 75.1 0.6Run 26 160 (+) 12:1 (+) 8 (−) 89.5 0.7Run 27 140 (−) 6:1 (−) 12 (+) 76.0 0.8Run 28 160 (+) 6:1 (−) 12 (+) 87.5 0.7Run 29 140 (−) 12:1 (+) 12 (+) 78.1 0.5Run 30 160 (+) 12:1 (+) 12 (+) 93.5 0.4Run 31 150 (0) 9:1 (0) 10 (0) 80.3 0.2Run 32 150 (0) 9:1 (0) 10 (0) 79.9 0.2Run 33 150 (0) 9:1 (0) 10 (0) 81.4 0.8aRuns given in this table represent the reactions that were carried outunder the corresponding conditions of the experimental design.“Blank” represents the absence of an added catalyst.

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to save time and reagents while searching for the most adequatemethanol:fatty acid molar ratio for esterification. The resultsobtained with this model supported that this catalytic system isable to drive the reaction to the equilibrium in 120 min. Hence,this model can be a useful tool to compare the catalyticperformance of different esterification catalysts and the same

calculations can be applied to other fatty acids just by feedingthe model with reliable thermodynamic data about bothreagents and products (esters and fatty acids).Clay minerals of the 2:1 group are natural, inexpensive, and

widely available materials whose acid activation is simple andinexpensive as well. Hence, this water-tolerant catalytic systemseems to be a promising alternative for the direct esterificationof fatty acids and for reducing the acid number of acid oils andspent greases prior to their conversion to biodiesel by alkalinetransesterification. Further research is currently underway toimprove the robustness of the process model by addressingquestions such as the reaction kinetics as well as the reuse andrecycling of the solid catalyst.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +55 41 3361 3175. E-mail address: luiz.ramos@ufpr.br.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to CNPq (Grant No. 558836/2010-0),MEC/Reuni, CAPES, and Fundacao Araucaria for the financialsupport and scholarships.

■ REFERENCES(1) Sharma, Y. C.; Singh, B.; Upadhyay, S. N. Fuel 2008, 87, 2355−2373.(2) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. RenewableSustainable Energy Rev. 2007, 11, 1300−1311.(3) Taravus, S.; Temur, H.; Yartasi, A. Energy Fuels 2009, 23, 4112−4115.(4) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1−15.(5) Lapuerta, M.; Rodríguez-Fernandez, J.; Oliva, F.; Canoira, L.Energy Fuels 2009, 23, 121−129.(6) Santacesaria, E.; Tesser, R.; Serio, M. D.; Guida, M.; Gaetano, D.;Agreda, A. G.; Cammarota, F. Ind. Eng. Chem. Res. 2007, 46, 8355−8362.

Table 7. 23 Factorial Design for the Esterification of Commercial Oleic Acid Preparation with Methanol Using MMT-PO4 as theReaction Catalyst

Experimental Conditions

testa temperature, T (°C)b methanol:lauric acid molar ratio, MRb CATb,c conversion (%) standard deviation (p.p.)

Blank 16 140 6:1 0 62.2 2.4Blank 17 160 6:1 0 81.4 1.2Blank 18 150 9:1 0 60.5 1.0Blank 19 140 12:1 0 75.6 1.1Blank 20 160 12:1 0 73.3 1.3Run 34 140 (−) 6:1 (−) 8 (−) 82.6 1.1Run 35 160 (+) 6:1 (−) 8 (−) 91.3 1.1Run 36 140 (−) 12:1 (+) 8 (−) 81.0 1.1Run 37 160 (+) 12:1 (+) 8 (−) 93.2 0.5Run 38 140 (−) 6:1 (−) 12 (+) 84.4 0.8Run 39 160 (+) 6:1 (−) 12 (+) 90.4 0.6Run 40 140 (−) 12:1 (+) 12 (+) 84.5 0.7Run 41 160 (+) 12:1 (+) 12 (+) 94.1 0.2Run 42 150 (0) 9:1 (0) 10 (0) 91.6 0.3Run 43 150 (0) 9:1 (0) 10 (0) 90.7 0.5Run 44 150 (0) 9:1 (0) 10 (0) 92.1 0.6

aRuns given in this table represent the reactions that were carried out under the corresponding conditions of the experimental design. “Blank”represents the absence of an added catalyst. bThe symbols (+), (−), and (0) represent the maximum, minimum, and center point of the factorialdesign. cCAT = bulk catalyst content, relative to lauric acid (in terms of % (w/w)).

Figure 2. Equilibrium conversion as a function of temperature anddifferent methanol:lauric acid molar ratio.

Figure 3. Equilibrium conversion as a function of the methanol:lauricacid molar ratio at different temperatures.

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