Esterification of Fatty Acids Using a Bismuth-Containing Solid Acid

Mar 25, 2013 - Fabiano Rosa da Silva, Marcos Henrique Luciano Silveira, Claudiney Soares ... Shirley Nakagaki, Fernando Wypych, and Luiz Pereira Ramos...
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Esterification of Fatty Acids Using a Bismuth-Containing Solid Acid Catalyst Fabiano Rosa da Silva, Marcos Henrique Luciano Silveira, Claudiney Soares Cordeiro, Shirley Nakagaki, Fernando Wypych, and Luiz Pereira Ramos* Research Center in Applied Chemistry, Department of Chemistry, Federal University of Paraná, P.O. Box 19081, Curitiba, PR 81531-990, Brazil ABSTRACT: The conversion of lauric acid and fatty acid mixtures to methyl esters was investigated using bismuth-containing solid catalysts obtained from Bi2O3. When the reaction was carried out at 140 °C for 2 h using a methanol:fatty acid molar ratio of 2:1 and 5 wt % of Bi2O3 in relation to the mass of fatty acids, conversions up to 87 wt % were achieved even after four consecutive reaction cycles. Besides the observed catalytic activity, the bismuth-containing solid catalyst was also amenable to recycling for at least four reaction cycles without significant loss of its catalytic performance. However, the analysis of the recovered solids by X-ray diffraction and Fourier transform infrared spectroscopy demonstrated that Bi2O3 was converted to layered bismuth carboxylates, which acted as the actual esterification catalyst. On the basis of these, layered bismuth carboxylates unfold as a suitable catalytic system for the esterification of fatty acids and lipid sources of high acid number.

1. INTRODUCTION Liquid biofuels derived from renewable oils and fats represent one of many possible alternatives to support the increasing demand for energy and fuels without compromising the environment inasmuch depth as the uncontrolled use of fossil fuels.1,2Biodiesel is a suitable diesel fuel substitute that can be produced by alcoholysis of vegetable oils or animal fats and/or by esterification of fatty acids in the presence of short chain monohydroxylated alcohols in a catalytic process that can be either homogeneous or heterogeneous.1−4The process most commonly used for biodiesel production in industrial scale is the alkaline transesterification in homogeneous medium, which is based on the use of commercially available metal alkoxides or alkaline metal hydroxides that are able to generate the alkoxides “in situ”.5,6 Acid catalysis in homogeneous medium can also be used, but its practical application has been restricted to the neutralization of acid oils prior to classical methods of alkaline transesterification. However, as we move toward the use of low cost feedstocks such as industrial soapstocks, spent greases, oil sludges, and used frying oil, acid catalysts may become as important for biodiesel production as the alkaline catalysts used today.7 Another option to improve the sustainability and competitiveness of the entire biodiesel production chain is the use of heterogeneous rather than homogeneous conversion catalysts. However, ideal heterogeneous catalysts must be reusable and/ or recyclable, active at relatively low temperatures, not easily leachable, and capable to produce relatively pure alkyl monoesters and glycerin, avoiding the complexity and cost of the subsequent stages of downstream processing.4,8−12 Layered compounds having high Lewis acid character were described as efficient heterogeneous catalysts for the productions of alkyl esters. For example, Quintinite-3T (Q3T), a naturally occurring layered double hydroxide (LDH) of the general formula Mg4Al2(OH)12CO3·3H2O, was successfully © 2013 American Chemical Society

used as a heterogeneous bifunctional catalyst for the conversion of free fatty acids and triacylglycerols into alkyl esters. Ester yields beyond 96 wt % were obtained at 75 °C using a methanol-to-oil molar ratio (MR) of 12:1 and a catalyst loading of 10 wt % in relation to mass of oil. Q-3T was also amenable to recycling without any significant loss in ester yields, with the Al:Mg ratio remaining close to 2.1 even after five reaction cycles.13 Layered zinc hydroxide nitrate [ZHN, Zn5(OH)8(NO3)2·2H2O] was used in a pressurized steel reactor for the conversion of lauric acid to methyl and ethyl laurate.14 Conversions of 97.4% in methyl esters were obtained using a methanol:lauric acid MR of 4:1 and 4 wt % of catalyst in relation to the mass of fatty acids at 140 °C for 2 h, whereas 77.2% of ethyl esters were produced using a MR of 6:1 and otherwise identical experimental conditions. However, after reaction completion, the solids were recovered by filtration, washed extensively with organic solvents, and characterized by X-ray diffraction (XDR) and Fourier-transform infrared spectroscopy (FTIR). By doing so, these authors were able to prove that ZHN was converted “in situ” to zinc laurate [ZL, (C12H23O2)2Zn] and that this was the actual active phase in the catalytic conversion of lauric acid to alkyl esters. ZL was also synthesized as a pure phase and used in the esterification of lauric acid with methanol and ethanol, with conversions higher than 97.0% being obtained with the former for as many as eleven consecutive reaction cycles. In another study, Zieba et al.15 achieved monoester yields above 70 wt % when ZHN was used as catalyst in the methanolysis of castor oil. These reactions were typically carried out at 50 °C for 3 h using 5 wt % of catalyst and a methanol:oil MR of 29:1. The reuse of the catalyst was also Received: January 1, 2013 Revised: March 25, 2013 Published: March 25, 2013 2218

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2. EXPERIMENTAL SECTION

Bi2O3. The reaction mixture was kept under stirring at 500 rpm for 2 h at the desired reaction temperature, agitation under which no mass transfer limitations are expected to occur in biphasic systems such as those observed during biodiesel synthesis.20 Then, after reaction completion, the solids were separated from the reaction mixture by centrifugation and the excess alcohol was removed from the supernatant by evaporation under reduced pressure at 50 °C. The acid number was determined by titration according to the AOCS Ca-5a-40 standard method. A known mass of the sample was neutralized with a standard solution of aqueous NaOH (0.1 mol·L−1) using an automatic buret (Brand) with a total capacity of 25.0 mL. The acid number of the reaction product was always measured in two replicates and expressed in relation to the acid number of the original fatty material. 2.6. Experimental Design. The preliminary optimization of the reaction conditions was carried out using a typical 23 experimental design with three replicates at the center point, in which the methanol:fatty acid MR ranged from 2:1 to 10:1, the reaction temperature (T) from 100 °C to 140 °C, and the solid loading (CAT) from 1 wt % to 5 wt % in relation to the mass of fatty acids used for conversion.6,14 The response variable used to build the model corresponded to the conversion of the ester phase that was obtained in each of the selected experimental conditions. Both primary and secondary effects of the main reaction variables were calculated and plotted as a Pareto chart using the Matlab R2012B software. The statistical significance of the model was evaluated by analysis of variance (ANOVA) using the same software. Furthermore, a graph confronting the experimental and the modeled data was also built to evaluate the robustness of the model.21,22 2.7. Catalysts Reuse. To investigate the extent of catalyst recovery and reuse, pure Bi2O3 was initially used in one reaction cycle with a MR of 2:1 and 1 wt % of catalyst in relation to the mass of fatty acids at 500 rpm and 140 °C for 2 h. Then the solids were separated from the reaction mixture, washed with ethanol:hexane 1:1 (v/v), dried at 65 °C for 12 h, and analyzed by XRD and FTIR. After characterization, the recovered solids were reused in a new reaction cycle under the same experimental conditions used previously, with the variables being adjusted according to the recovered mass of the catalyst.

2.1. Materials. Lauric acid (C12H24O2 98 wt %) and bismuth oxide (98 wt %) were purchased from Vetec (Rio de Janeiro, RJ, Brazil). Methanol (99 wt %) as well as both fatty acid mixtures (FAM) were obtained from Labsynth (São Paulo, SP, Brazil). FAM1 and FAM2 were rich in saturated and unsaturated fatty acids, respectively. Other reagents were of analytical grade and used as received. 2.2. X-ray Diffraction (XRD). XRD patterns were recorded with a Shimadzu XRD-6000 instrument using CuKα radiation (λ = 1.5418 Å), a dwell time of 2°·min−1 from 2° to 60° 2θ, 30 mA, and 40 kV. The samples were placed on neutral glass sample holders and gently handpressed prior to analysis. 2.3. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra were recorded with a Bomem Michelson MB1000 instrument using 1 wt % of sample in 100 mg of spectroscopic grade KBr pellets. The measurements were performed in the transmission mode from 4000 and 400 cm−1 with accumulation of 32 scans at a nominal resolution of 4 cm−1. 2.4. Chemical Analysis of the Fatty Acid Mixture. The composition of the fatty acid mixtures was determined through the Ce1F-96 method of the American Oil Chemists’ Society (AOCS). In this method, esterification of fatty acids is carried out using methanol and BF3 as the reaction catalyst, and the resulting methyl esters are subsequently analyzed by quantitative capillary gas chromatography (GC). 2.5. Synthesis of Fatty Esters. Bi2O3 was used as-received in the catalytic conversion of both lauric acid and free fatty acid mixtures to their corresponding methyl esters. The experiments were carried in a pressurized stainless steel reactor (Büchiglass miniclave drive) with the internal pressure controlled by the vapor pressure of methanol. A known mass of fatty acids and a predetermined volume of methanol were transferred to the reactor chamber, followed by the addition of

3. RESULTS AND DISCUSSION 3.1. Stability of the Solid Catalyst. Table 1 shows the catalytic performance of Bi2O3 in the esterification of lauric acid with methanol. In general, the resulting catalytic conversions were always higher than that of the thermal conversion at 140 °C (see Table 1 for details), and this was expected to occur because the fatty acids themselves are Brönsted acids that have the ability to autocatalyze their esterification under appropriate reaction conditions. Once the catalytic activity of Bi2O3 was demonstrated, the structure of the solid catalyst was investigated before and after a single reaction course in order to confirm the stability of this heterogeneous catalytic system. Figure 1 shows the XRD patterns and the FTIR spectra of the solids recovered after one reaction cycle compared to those of the pristine Bi2O3. Experiments L1, L3, L6, L9, and L11 of Table 1 were chosen for this purpose because they are representative of the entire range of experimental conditions used in this work. The recovered solids presented a typical Xray diffraction pattern of layered metal carboxylates in which bismuth atoms are located in the center of the layers and the carboxylate groups are coordinated in tetrahedral geometry through a bridged bidentade mode, connecting two different metallic centers.13,23 No apparent contaminations with Bi2O3 and lauric acid were observed in the X-ray diffraction pattern of the recovered solids. In addition, a mean value of 37.7 Å, calculated in relation to the peak of highest order using the Bragg equation, was obtained

demonstrated for several reaction cycles, but there were situations in which the structure of the layered hydroxide salt was partially converted to Zn3(OH)4(NO3)2 or ZnO, depending on the reaction conditions. Jacobson et al.16 applied zinc stearate immobilized on silica gel in the methanolysis of waste cooking oil containing 15 wt % of free fatty acids. Ester yields of 98 wt % were obtained when the reaction was carried out at 200 °C for 10 h with an oil-toalcohol MR of 18:1 and a catalyst loading of 3 wt % . More recently, zinc carboxylates of different chain lengths [Zn(CnH2n+1COO)2 with n = 1, 11, 15, 17, and Zn(C17H33COO)2] were utilized in the transesterification of soybean oil and in the esterification of oleic acid. Conversions between 88% and 94% with yields between 71% and 74% were obtained after soybean oil methanolysis at 100 °C for 2 h with an oil-to-alcohol MR of 30:1 and a catalyst loading 3 wt % in relation to the oil mass. By contrast, conversions between 60% and 80% were obtained when oleic acid was esterified with methanol at 140 °C with an oleic acid-to-methanol MR of 1:30 and a catalyst loading of 6 wt % .17 In the present work, the catalytic activity of a bismuthcontaining solid catalyst was investigated in the esterification of saturated and unsaturated fatty acids with methanol. Differently from many heavy metals, bismuth is a noncorrosive metal of low toxicity that is relatively easy to handle. Also, bismuth compounds present from moderate to strong Lewis acidity due to the deficient nucleus shielding by the f electrons shell (Bi = [Xe] 4f14, 5d10, 6s2, 6p3).18 As a result, the catalytic behavior of bismuth compounds has been investigated in different chemical reactions, but their application in the esterification of fatty acids is still unknown. Also, bismuth carboxylates are used in the synthesis of different bismuth oxide materials such as superconductors and drugs.19

2219

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the X-ray diffraction pattern reveals the formation of a well arranged layered structure in which a good stacking of layers is observed, despite not being as crystalline as Bi2O3 (Figure 1g).19,24 The two diffraction peaks, at 33° and 47° in 2θ, corresponded to nonbasal crystallographic directions, and the broad band observed at 28° can be attributed to (1) the scattering of the X-ray diffraction beam through the imperfectly organized interlayer hydrocarbon chains, (2) the amorphous contribution from crystals of a very small particle size, and (3) the liquid/crystalline state of matter, which is frequently observed in smectic crystals during their transition from crystalline to mesomorphic state25 (Figure 1a−e). The conversion of Bi2O3 into bismuth laurate was only observed when the experimental conditions were changed from methanol reflux (data not shown) to a closed system in which the internal pressure corresponded to the vapor pressure of methanol at the selected reaction temperature. Therefore, relatively high temperatures and pressures were required to synthesize layered carboxylates of high crystallinity. Saturated free fatty acids such as lauric acid seemed to be ideal to produce crystalline-layered carboxylates because their linear structure facilitates the organized packing of the organic anions into a fully extended all-trans configuration with a bilayer arrangement within the adjacent layers.23 Another important consequence of performing the reaction at higher pressures is the increase of the amount of methanol readily available for esterification. The FTIR analysis of all recovered solids confirmed the formation of bismuth laurate because their spectra were typical of this class of compounds (Figure 1).14,19,23 The bands at 2850, 2918, and 2954 cm−1 were attributed to the symmetric and asymmetric axial deformation of C−H bonds in the CH2 and CH3 groups, respectively, with their angular deformation being observed at 1467 cm−1. On the other hand, the bands at 1529 and 1406 cm−1 were attributed to the asymmetric and symmetric axial deformations of the carboxylate groups of bismuth laurate. The difference of 123 cm−1 between these two bands suggests that the carboxylate anions are coordinated to the metal atoms through a bridged bidentade form, providing further evidence for the synthesis of a new layered compound

Table 1. Results Obtained in the Esterification of Lauric Acid with Methanola experiment

MRb

CAT (%)c

T (°C)d

acidity (%)e

TC (%)f

TCFAME (%)g

control Ah control Bh L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13

6:1 14:1 2:1 2:1 4:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1 10:1 20:1 30:1

− − 2 5 2 1 2 2 2 3 5 10 3 5 10

140 140 140 140 140 140 100 120 140 140 140 140 140 140 140

38.0 41.0 15.4 14.5 9. 7 17.4 62.3 35.4 10.6 11.5 8.4 7.4 16.2 25.9 32.5

62.00 59.00 84.19 85.12 90.10 83.14 36.80 63.73 90.13 88.18 91.34 92.45 84.43 74.79 68.76

62.00 59.00 79.85 74.14 85.45 80.99 34.90 60.44 85.48 81.36 79.56 68.60 77.90 65.14 51.02

a

All the experiments were performed at 500 rpm for 2 h. Methanol:fatty acid molar ratio. cPercentage of Bi2O3 in relation to the mass of lauric acid. dInternal temperature of the reaction chamber. e Acidity of the reaction products, expressed as grams of lauric acid per 100 g of the sample specimen. fTotal conversion of fatty acids in relation to changes in the sample acidity, as determined by titration according to the AOCS Ca-5a-40 standard method. gConversion of fatty acids in fatty acid methyl esters (FAME) which was derived from TC by deducting the amount of fatty acids that was theoretically consumed in the formation of bismuth carboxylates. hObtained from Lisboa et al.27 b

for the basal distance of the recovered solids, and this was consistent with the laurate anions being arranged as tilted bilayers between the bismuth layers, forming layered carboxylates.19,23,24 Therefore, Bi2O3 was converted “in situ” into a typical layered carboxylate, here identified as bismuth laurate. The layered structure of bismuth laurate showed five basal peaks, but the first peak of this series cannot be seen in Figure 1 because data acquisition began only at 4° in 2θ. Hence,

Figure 1. X-ray diffraction patterns and FTIR spectra of the solids recovered from experiments L1 (a), L3 (b), L6 (c), L9 (d), and L11 (e), with lauric acid (f) and Bi2O3 (g) being displayed as reference materials. See Table 1 for details of reaction conditions. 2220

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in the reaction mixture.14,23,26 Such observation also suggested that bismuth laurate was the active phase in the esterification of lauric acid with methanol, as already demonstrated for other metal carboxylates.14,17,27,28 On the basis of these findings, it was clear that, when Bi2O3 was used as the reaction catalyst or catalyst precursor, part of the fatty acids were consumed to convert Bi2O3 to the respective bismuth carboxylates. Hence, assuming complete conversion of Bi2O3 (5 wt % in relation to the fatty acid mass) into bismuth laurate, 0.0225 mol of the lauric acid would be consumed, and this represents nearly 13% of the total amount of lauric acid that was available at the beginning of the reaction (0.1747 mol). For this reason, the total conversion values (TC) shown in Table 1 were recalculated to express the total conversion to fatty acid methyl esters (TCFAME), giving a better view of the actual catalytic activity. With this, a decrease in the conversion values was observed, and this was proportional to the amount of Bi2O3 that was used as the catalyst precursor. Even so, the bismuth catalyst caused an increase of ca. 19 points percent (p.p.) in TCFAME, confirming the activity performance of this catalytic system. However, in recycling experiments, these calculations were only applied to the conversion of the first reaction cycle, when Bi2O3 was initially added to the reaction medium. 3.2. Esterification of Lauric Acid. Table 1 shows the catalytic performance of Bi2O3 in the esterification of lauric acid with methanol. Experiments L5, L6, and L7 indicated that an increase in the reaction temperature reduces the acid number of the final product, evidencing that high temperatures favor the conversion of lauric acid into methyl laurate (TCFAME). On the other hand, L1 and L2 as well as L9 and L10 show that, under the same MR and temperature, an increase in the Bi2O3 loading does not cause a significant increase in TCFAME because this led to a higher consumption of fatty acids to produce the solid bismuth carboxylates. Experiments L1 and L3 revealed that an increase in the methanol excess displaces the reaction equilibrium toward methyl ester synthesis (Table 1). However, this effect leveled off in the presence of slightly higher amounts of methanol under the same initial Bi2O3 loading (see L7 in Table 1). On the other hand, experiments L11, L12, and L13 suggest that lower conversions are obtained when exceedingly high amounts of methanol are used, regardless of the initial Bi2O3 loading. Hence, methanol in excess apparently interferes with the structure and performance of this catalytic system. 3.3. Esterification of Fatty Acid Mixtures (FAM). The catalytic performance of the bismuth-containing solid catalyst was also tested against two free fatty acid mixtures (FAM1 and FAM2) whose chemical composition was initially characterized by gas chromatography (Table 2). FAM1 was primarily composed of saturated free fatty acids (93.52%), with palmitic and stearic acids predominating as its major components (87.84%). On the other hand, FAM2 consisted mainly of monounsaturated fatty acids (72.88%), with 85.65% of these corresponding to oleic acid (62.42%). Two esterification reactions were performed with FAM1 and methanol under the same experimental conditions used in experiments L7 (MR of 6:1, 2 wt % of Bi2O3, 140 °C) and L11 (MR of 10:1, 3 wt % of Bi2O3, 140 °C) of Table 1. TCFAME values of 79.60 wt % and 65.86 wt % were achieved in each case, respectively, based on the observed decrease in acid number. Compared to L7 (85.48 wt %) and L11 (77.90 wt %), the use of FAM1 rather than lauric acid led to lower

Table 2. Chemical Composition of the Fatty Acid Mixtures (FAM) Used in This Study fatty acid

name

FAM1 (% m/m)

FAM2 (% m/m)

C 12:0 C 14:0 C 15:0 C 16:0 C 17:0 C 18:0 C 14:1 C 16:1 C 17:1 C 18:1n9c C 18:1n-t9 C 18:2n6c C 18:3n3 C 20:1 others

lauric myristic pentadecanoic palmitic heptadecanoic stearic myristoleic palmitoleic heptadecenoic oleic elaidic linoleic linolenic eicoseinoic −

0.13 3.48 0.58 40.57 1.49 47.27 − 0.44 − 0.16 − − − − 5.88

0.39 5.37 0.53 5.96 0.28 2.03 1.50 5.08 1.12 62.42 2.25 7.46 0.52 0.51 4.58

conversions of fatty acids into methyl esters. Considering that FAM1 was primarily composed of stearic (C18:0) and palmitic (C16:0) acids, the longer the hydrophobic chain of the saturated fatty acids used for esterification, the lower the activity of the catalytic system, and this was probably due to the stability of the catalyst formed “in situ”. Moreover, FAM1 consisted of a mixture of free fatty acids, and this may have led to a mixture of bismuth carboxylates whose layered structure would not be as well organized as that obtained when pure lauric acid was used as a sole fatty acid source. After esterification of the FAM1 mixture with methanol, the residual solids were recovered as described above and characterized by XRD and FTIR (Figure 2). Once again, both techniques revealed that Bi2O3 underwent significant structural changes after reaction completion. The resulting XRD pattern and FTIR spectrum were typical of carboxylates whose structure was similar to those obtained when Bi2O3 was used in the esterification of lauric acid with methanol (Figure 1).14,23 However, in the present case, a mixture of layered carboxylates was obtained. This observation was confirmed by the average basal distance of the recovered solids, which was in the order of 50.5 Å (Figure 2), consistent with the “in situ” insertion of stearate and/or palmitate anions during the experiments.29 However, due to the increase of the hydrocarbon chain length and probable mixture of different carboxylates in the same phase, these materials were less crystalline than those obtained by the intercalation of laurate anions (Figure 1). The esterification of FAM2 with methanol was initially carried out with a MR of 6:1, 2 wt % of Bi2O3, and 500 rpm at 140 °C for 2 h, conditions in which a monoester conversion of 86.07 wt % was obtained. This conversion was considerably higher than that obtained when FAM1 (79.60 wt %) was used as the fatty acid source, suggesting that, under the same experimental conditions, unsaturated fatty acids resulted in a better catalytic response, even when the fatty material presents a higher complexity in terms of fatty acid composition. This was probably due to effect of double bonds on the ease with which zinc carboxylates assembled themselves into an organized layered structure. The presence of double bonds modifies the linear structure of saturated fatty acids, decreasing their melting point and leading to lower crystallization speed and smaller crystals that are more difficult to settle down. However, smaller 2221

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Figure 2. X-ray diffraction patterns and FTIR spectra of the solids recovered after esterification of FAM1 under the same condition found in Table 1 for L7 (a) and L11 (b), with FAM1 (c) and Bi2O3 (d) being displayed as reference materials.

of 69.08%, which was indicative of a good statistical significance for the entire experimental procedure. All three individual factors and two binary interaction (T × MR and CAT × T) were statistically significant at the 95% confidence level, while the effect of MR by CAT had no statistical significance (Figure 3). The temperature presented a

crystals are likely to have higher surface areas and a greater availability of catalytically active acid sites for interaction with the chemical reactants. On the other hand, saturated fatty acids led to carboxylates with larger particle sizes and higher crystallinity that were much easier to recover and reuse. 3.4. Preoptimization Studies. The catalytic performance was further investigated through a factorial design using the FAM2 mixture as a fatty acid source for esterification (Table 3). Table 3. Preoptimization of the FAM2 Esterification with Methanol experiment

MRa

O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11

2:1 (−1)c 2:1 (−1) 2:1 (−1) 10:1 (+1) 6:1 (0) 6:1 (0) 6:1 (0) 10:1 (+1) 10:1 (+1) 2:1 (−1) 10:1 (+1)

CATa (%)

Ta (°C)

1 1 5 1 3 3 3 1 5 5 5

100 140 100 100 120 120 120 140 100 140 140

(−1) (−1) (+1) (−1) (0) (0) (0) (−1) (+1) (+1) (+1)

(−1) (+1) (−1) (−1) (0) (0) (0) (+1) (−1) (+1) (+1)

acidity (%)b

TCFAMEa (%)

65.8 17.2 49.9 83.5 23.4 21.8 23.3 21.5 74.3 8.6 10.1

31.16 79.55 40.69 13.45 68.57 70.01 68.65 75.29 19.63 76.43 75.17

Figure 3. Pareto chart describing the primary and secondary effects of the main reaction variables.

a

See Table 1 for abbreviations. bProduct acidity was expressed in grams of oleic acid per 100.0 g of the sample specimen. cCodes used in the factorial design (in brackets): −1 for the lower limit, +1 for the upper limit, and 0 for center point of the reaction variables.

well pronounced positive effect of 88.03 p.p. in the reaction conversion, as observed in O2, O8, O10, and O11, followed by Bi2O3 concentration with a lower positive effect of 5.44 p.p. (see experiments O3, O9, O10, and O11). On the other hand, MR presented a negative effect of −19.36 p.p., confirming that, under methanol excess, a much lower fatty acid conversion was obtained (see also O4, O8, and O9). As mentioned above, both T × MR and CAT × T interactions were statistically significant, but their effects on the response variable were opposite. The T × MR effect of 14.52 was probably due to the primary effect of temperature, which was able to displace the initial negative effect of MR to a positive second-order effect. On the other hand, the antagonist effect of CAT × T (−8.29 p.p.) revealed a greater complexity in

In this regard, all experiments of the factorial design were carried out for the same reaction time because this preoptimization study was not addressed to discuss kinetic issues. The three main process variables (MR, T, and CAT) were evaluated in two levels (maximum and minimum) with three replicates at the center point (MR of 6:1, CAT at 3 wt %, and T at 120 °C). A relative standard deviation of only 1.17% was achieved in these replicates for an average reaction conversion 2222

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the interaction between these two variables. A pronounced effect of the curvature (32.23 p.p.) was also observed in the proposed mathematical model, and for this reason, the data curvature was included in our attempt to adjust the model to a multiple linear regression. The coefficients obtained in the standard linear model are presented in Table 4, along with their corresponding p values. In general, these values were in good agreement with the statistical interpretation of the Pareto chart (Figure 3).

Table 5. Analysis of Variance (ANOVA) of the Experimental Data Found in Tables 3 and 4

Table 4. Coefficients of the Linear Model That Were Adjusted to the Experimental Data coefficient

value

p value

mean intercept curvature (1) molar ratio (MR) (2) catalyst (CAT) (3) temperature (T) 1 by 2 1 by 3 2 by 3

−77.7866 17.6579 −7.5980 7.9266 1.1256 −0.0058 0.0519 −0.0593

0.002810 0.000961 0.003461 0.012545 0.000887 0.886440 0.004711 0.014249

source of error

sum of squares

degree of freedom

root mean squares

regression residues lack of fit pure error total R2 adjusted R2 (R2adj)

6202.899 6.318 5.009 1.310 6209.217 0.9990 0.9998

6 4 2 2 10

1033.816 1.580 2.504 0.655

F

Ftab

654.472

6.1631

3.824

19

The solids were once again isolated and characterized by XRD and FTIR, and as originally observed in reactions involving lauric acid and FAM1, their structure was typical of layered metal carboxylates (Figure 5). However, the solids presented an average basal distance of 46.1 Å, which is consistent with the formation of layered bismuth carboxylates whose fatty acid composition differed from those obtained in other experiments. Besides, the peaks of the basal sequence were broad and partially overlapped, suggesting that the resulting layered material has a lower crystallinity and/or a smaller particle size. Hence, the intercalation of unsaturated fatty acid anions and the complexity of the raw material chemical composition must have triggered a higher structural disorder and a lower crystallinity in the recovered solids.30 Similar to the examples observed in Figure 1 and 2, both XRD peaks in the region of 33° and 47° (in 2θ) correspond to nonbasal crystallographic directions. Therefore, the resulting material maintained its basic layered structure, regardless of changes in the interlayer fatty acid hydrocarbon chains. Again, the corresponding FTIR data supported the formulated hypothesis (Figure 5). In general, layered metallic carboxylates have a melting point by about 130 °C, where the melting process corresponds to losing 30% of the “zig-zag” fatty chain conformation.31 Consequently, the structure of the metal carboxylates is partially broken during the reaction course but this moiety would reassemble from its “melted state” by cooling after reaction completion, being recoverable by filtration independently of the fatty acid composition of the starting material, as already noted by other authors.14,17,27,28 3.5. Reuse Studies. The recovery and reuse of the solid catalyst was investigated in four consecutive reaction cycles, which were always carried out with the FAM2 mixture and methanol at the best condition identified in Table 3 (experiment O2). This condition corresponded to a methanol:fatty acid MR of 2:1 at 140 °C for 2 h, starting with 5 wt % of Bi2O3 in the first reaction cycle and reusing the recovered solids for the three subsequent reaction cycles as described above. The catalyst loading in these experiments was always 5 wt % in relation to the mass of fatty acids. Considering the apparent average molecular mass of bismuth carboxylates (∼250 g·mol−1), based on the chemical analysis of FAM2 (Table 2), the number of moles of bismuth in the second reaction cycle was close to half of that observed in the first reaction cycle in the form of Bi2O3. Even so, the catalytic activity of the first reuse (or second reaction cycle) was improved by 9.11 p.p., with the TCFAME rising from 79.55 wt % to 88.66 wt %. Also, the decrease in catalytic activity from the second to the fourth reaction cycle was only 1.85 p.p., which is close to the standard

Figure 4 demonstrates the excellent correlation that was obtained between the predicted (modeled) values and the

Figure 4. Correlation between the modeled and the experimental data obtained in this study for the production of methyl esters.

experimental data, and this was more clear evidence for the goodness of fit as well as the robustness of the proposed model. The analysis of variance (ANOVA) confirmed the goodness of fit of the model, based on the values obtained for the rootmean-square error, the F value, the p value, the correlation coefficient (R2), the adjusted correlation coefficient (R2adj), and the lack of fit (Table 5). For instance, the R2adj value (0.9998) was in good agreement with the R2 value (0.9987), meaning that 99.85% of response variable could be explained by the fitted model. This was confirmed by the model F for both regression and residues, because the former (499.573) was much greater and the latter was smaller (5.295) than their corresponding critical values of F0.05,6,4 = 6.1631 and F0.05,2,2 = 19.0000, respectively. Thus, there was a good correlation between both the theoretical and the experimental data within a confidence level of 95%. Table 5 also shows that the lack of fit was not significant in relation to the pure error, and this confirmed the statistical validity of the model. 2223

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Figure 5. X-ray diffraction patterns and FTIR spectra of the solids used in the experiments O2 (a), O8 (b), O10 (c), and O11 (d) and the corresponding data of pristine Bi2O3 (e).

Figure 6. X-ray diffraction patterns and FTIR spectra of the solids successively used in the esterification of FAM2: first use (a), second use (b), third use (c), and fourth use (d).

deviation of the analysis. Hence, these results are a strong indication that bismuth carboxylates are more catalytically active than Bi2O3 and that more reuse cycles could have been achieved without any major loss in catalytic performance. The residual solids were then recovered after the first reaction cycle and characterized by XRD (Figure 6), revealing the occurrence of the same structural changes that were already demonstrated for other fatty acid sources in Figures 1, 2, and 5. Also, the XRD pattern of the recovered solids remained relatively unchanged after the three subsequent reaction cycles, indicating that layered bismuth carboxylates are structurally stable and therefore recyclable. However, after the second recovery and reuse (or the third reaction cycle), the layered solids experienced a small reduction in their basal spacing from 46.2 to 41.3 Å, which remained constant up until the last

reaction cycle. These results indicate that, as the solids are reused in several consecutive reactions, mixed layered carboxylates are formed with a chemical composition that resembles the fatty acid composition of the reaction mixture, with an apparent trend for the intercalation of anions with smaller hydrocarbon chain lengths. The FTIR spectra of all recovered solids were always typical of that of layered carboxylates (Figure 6). The formation of metal carboxylates probably proceeds by the interaction of the nonligand electron pairs of the oxygen atoms in the Bi2O3 structure with the ionizable hydrogen atoms of the free fatty acids, leading to the elimination of water in the reaction medium and to the formation of Bi3+ and the carboxylate anion. Both ions combine in an acid/base reaction to produce the layered bismuth carboxylates which, after being 2224

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generated “in situ”, perform as the actual catalyst in the esterification of fatty acids with methanol.4,14,17,27,28,32 The observed catalytic activity of Bi3+ carboxylates in the esterification of fatty acids can be explained by the Lewis acid character of Bi3+ and its interaction with the oxygen atom of the carboxylate group. The increase in the positive charge of the carbonylic carbon atom favors the nucleophilic attack by the electron pair of the alcoholic hydroxyl groups, forming a tetrahedral intermediary that eliminates water to produce one molecule of the alkyl monoester.4,32

(11) Sharma, Y. C.; Singh, B.; Korstad, J. Fuel 2011, 90, 1309−1324. (12) Kouzua, M.; Hidaka, J. S. Fuel 2012, 93, 1−12. (13) Kondamudi, N.; Mohapatra, S. K.; Misra, M. Appl. Catal., A 2011, 393, 36−43. (14) Cordeiro, C. S.; Arizaga, G. G. C.; Ramos, L. P.; Wypych, F. Catal. Commun. 2008, 9, 2140−2143. (15) Zieba, A.; Pacuza, A.; Drelinkiewicz, A. Energy Fuels 2010, 24, 634−645. (16) Jacobson, K.; Gopinath, R.; Meher, L. C.; Dalai, A. K. Appl. Catal., B 2008, 85, 86−91. (17) Reinoso, D. M.; Damiani, D. E.; Tonetto, G. M. Appl. Catal., A 2012, 449, 88−95. (18) Silva, F. R.; Brugnago, R. J.; Marangoni, R.; Cordeiro, C. S.; Nakagaki, S.; Wypych, F.; Ramos, L. P. Quim. Nova 2012, 35, 108− 113. (19) Logvinenko, V.; Mikhailov, K.; Yukhin, Y. J. Therm. Anal. Calorim. 2005, 81, 107−110. (20) El-Amin, M. Mass Transfer in Multiphase Systems and its Applications; InTech: Rijeka, 2011; pp 433−458. (21) Teófilo, R. F.; Ferreira, M. M. C. Quim. Nova 2006, 29, 338− 350. (22) Barros Neto, B.; Scarminio, I. S.; Bruns, R. E. Como fazer experimentos: pesquisa e desenvolvimento na ciência e na indústria, 3rd ed.; Editora da Unicamp: Campinas, 2007. (23) Yukhin, Y. M.; Mikhailov, K. Y.; Bokhonov, B. B.; Vorsina, I. A. Chem. Sustainable Dev. 2004, 12, 403−408. (24) Cordeiro, C. S.; Silva, F. R.; Marangoni, R.; Wypych, F.; Ramos, L. P. Catal. Lett. 2012, 142, 763−770. (25) Takahashi, T.; Kimura, T.; Sakurai, K. Polymer 1999, 40, 5939− 5945. (26) Wypych, F.; Arízaga, G. G. C.; Gardolinski, J. E. F. C. J. Colloid Interface Sci. 2005, 283, 130−138. (27) Lisboa, F. S.; Arízaga, G. G. C.; Wypych, F. Top. Catal. 2011, 54, 474−481. (28) Lisboa, F. S.; Gardolinski, J. E. F. C.; Cordeiro, C. S.; Wypych, F. J. Braz. Chem. Soc. 2012, 23, 46−56. (29) Minina, A. V.; Yukhin, Y. M.; Bokhonov, B. B.; Vorsina, I. A.; Mikhaylov, Y. I.; Danilova, L. E. Chem. Sustainable Dev. 2003, 11, 371− 378. (30) Sánchez-Muñoz, L.; Cava, S. S.; Paskocimas, C. A.; Cerisuelo, E.; Longo, E.; Carda, J. B. Cerâmica 2002, 48, 108−113. (31) Barman, S.; Vasudevan, S. J. Phys. Chem. B 2006, 110, 22407− 22414. (32) Yan, S.; Salley, S. O.; Simon, K. Y. N. Appl. Catal., A 2009, 353, 203−212.

4. CONCLUSIONS Bi2O3 was catalytically inactive in the methanolysis of soybean and palm oils under reflux or pressurized conditions (data not shown). The esterification of fatty acids was also unsuccessful under methanol reflux, but when the reaction was carried out in a pressure vessel, fatty acids were readily converted to the corresponding methyl esters. However, the analysis of the solids recovered after reaction completion demonstrated that Bi2O3 was converted to layered bismuth carboxylates, and these were shown to be catalytically active in the esterification of fatty acids. The stability and final structure of the resulting carboxylates depended on the size of the fatty acid hydrocarbon chains and their degree of unsaturation. The reaction temperature presented the largest influence in the catalytic performance, followed by the catalyst loading. Higher temperatures improved the reaction kinetics by promoting more frequent and effective collisions between the reacting molecules. Higher conversion rates were also obtained when larger amounts of catalytic sites were available in the reaction medium.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+5541) 33613175; fax: (+5541)33613186; e-mail: luiz. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the following Brazilian funding agencies for financial support: CNPq (grants 550348/2009-3 and 312362/2006-4), FINEP (grant 01.07.0480-00), and CAPES, for providing scholarships to our graduate students.

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