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Esterification of fatty acids using acid-activated Brazilian smectite natural clay as a catalyst
Renewable Energy 92 (2016) 171e177
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Esterification of fatty acids using acid-activated Brazilian smectite natural clay as a catalyst Michelle J.C. Rezende*, Angelo C. Pinto ria, CT, Bloco A, 21941-909, Rio de Janeiro, RJ, Brazil Universidade Federal do Rio de Janeiro, Instituto de Química, Cidade Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2015 Received in revised form 1 February 2016 Accepted 2 February 2016 Available online xxx
This work reports the use of an acid-activated Brazilian smectite natural clay-based catalyst in the esterification of fatty acids at atmospheric pressure and without a co-solvent. Conversion levels of 99%, 98%, 93% and 80% were reached for caprylic, lauric, stearic and oleic methyl esters, respectively, using 1:3 fatty acid/methanol molar ratio, heating bath at 100 C after 4 h. A conversion level of 89% was achieved for methyl esters from a fatty acid residue of the palm oil refining industry in the same reaction condition. The acid-activated clay provided better performance than the commercial catalyst K-10. The effects of catalyst amount, temperature, fatty acid/alcohol molar ratio and time were investigated. The heating activation of the catalyst immediately before the catalytic test increased the conversion from 94% to 99% using 1:1.5 stearic acid/methanol molar ratio, heating bath at 100 C after 4 h. The catalyst was reused five times. The conversion decreases less than 5% in the first three reuses. The smectite natural clay and the catalyst were characterized by X-ray fluorescence, X-ray diffraction, n-butylamine thermodesorption, nitrogen adsorption analysis, thermogravimetric analysis and differential thermal analysis. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Clay Fatty acid esterification
1. Introduction The increasing demand for biodiesel has stimulated the development of new technologies and new catalysts. The industrial production of biodiesel is performed by a transesterification process using mainly hydroxide or methoxide as a catalyst [1]. The problem involved in this process is the formation of soap. Because of this, there are studies involved in the development of new catalysts. Most of them are focused on the development of heterogeneous catalysts [2e9]. The homogeneous alkaline catalysis provides high yields in a short time and mild reaction condition, so find a promising heterogeneous catalyst is not an easy task. There are also parallel efforts to optimize the homogeneous alkaline catalytic process. Some examples include economics in terms of process conditions, studies involved in continuous process design as well as scale-up [10e12]. Although vegetable oils are the major feedstock for the production of biodiesel, there are other fatty acid sources that have been studied, such as used cooking oils, sewage sludge and animal
* Corresponding author. E-mail address: [email protected] (M.J.C. Rezende). http://dx.doi.org/10.1016/j.renene.2016.02.004 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
fats [13]. Some feedstocks have high concentrations of free fatty acids (FFA), reaching levels up to 15 wt%. This characteristic makes them inappropriate for the conventional direct base-catalyzed transesterification route to biodiesel due to soap formation. An integrated process enables the use of these materials by the combination of acid-catalyzed FFA esterification followed by basecatalyzed triacylglyceride transesterification. Despite the additional cost of production, the process is being increasingly applied to produce biodiesel from high-FFA feedstock [14]. Biodiesel can also be produced from the direct esterification of FFA feedstocks. Low-grade starting materials, such as waste from vegetable oil refining industries, can be used. To achieve a satisfactory ester conversion, it is necessary to remove the water produced or use a stoichiometric excess of alcohol. Sulfuric acid is the most widely used catalyst in esterification reactions. It is very active and has an important dehydrating effect. By contrast, it is corrosive, presents problems of storage and control and, in particular conditions, can react with the double bond present in unsaturated fatty acids [15]. Nowadays, there is intensive research of new catalysts for the esterification of fatty acids that allow milder reaction conditions and that can be reused. In an integrated process, the intermediate step of neutralizing the acid catalyst increases the volume of waste.
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The use of a heterogeneous catalyst may result in additional steps or units, but still could simplify the process, since it could be removed by filtration, without neutralization, and the product could be immediately submitted to the transesterification reaction. There are several studies in the literature for the esterification of fatty acids with short-chain alcohols and using different heterogeneous catalysts [16e30]. Regarding the use of clay as a catalyst in the esterification of fatty acids, Kantam, Bhaskar and Choudary used Fe3þ-montmorillonite in the esterification of stearic acid with methanol. A conversion rate of 98% was obtained after 5 h of reaction, using a DeaneStark apparatus and toluene as a solvent [31]. Neji, Trabelsi and Frikha investigated the use of commercial montmorillonite-based catalysts (KSF, KSF/0, KP10 and K10) in the esterification of stearic, oleic and palmitic acids with short-chain alcohols [32]. The reactions were carried out in a semicontinuous reactor at 150 C with continuous removal of water. The best catalytic activities (>90% after 3 h) were obtained with the KSF/0 catalyst. Nascimento et al. prepared catalysts for the esterification of oleic acid with methanol from two Amazon kaolins and two standard kaolins. After 4 h, a conversion rate of more than 90% was achieved using an acid/alcohol molar ratio of 1:60 at 130 C [33]. Ghiaci, Aghabarari and Gil showed that a modified bentonite with 1-benzyl-1H-benzimidazole-based Brønsted acidic ionic liquids was able to catalyze the esterification of oleic acid to its methyl ester in 6 h with yields of more than 92% [34]. Zatta, Gardolinski and Wypych studied the esterification of lauric acid with methanol using raw halloysite as a catalyst. A conversion level of 95% was achieved at 160 C for 2 h in a pressurized steel reactor [35]. The esterification of oleic acid with methanol catalyzed over sulfuric acid-activated pillar bentonite clay was performed by Jeenpadiphat and Tungasmita [36]. The use of a 1:9 molar ratio of oleic acid/ methanol at 60 C for 3 h reached 100% of yield. In 2012, we used acid-activated Brazilian clay-based catalysts in the esterification of different carboxylic acids and alcohols, including lauric acid and methanol. After 3 h, a conversion rate of 87% in methyl laurate was achieved using a carboxylic acid/alcohol molar ratio of 1:3, atmospheric pressure and heating bath at 100 C [37]. Recently, Aghabarari and Dorostkar studied the esterification of oleic acid with ethanol using bentonite modified with ionic liquids as a catalyst [38]. A conversion rate greater than 93% was observed after 6.5 h at 60 C using an oleic acid/ethanol molar ratio of 1:2. This work reports the esterification of fatty acids using acidactivated Brazilian smectite natural clay as a catalyst, at atmospheric pressure and without a co-solvent. The objective is to evaluate the potential use of a low-cost and friendly solid catalyst for the production of biodiesel based on the direct esterification of FFA. The acid treatment is very simple, and the acid-activated clay was tested in the esterification of different fatty acids, including a FFA industrial residue used as feedstock for the production of biodiesel in Brazil. The effect of the main variables was studied, and high conversions were observed using very mild reaction conditions. The reuse of the catalyst was also investigated. These results are a new contribution to the knowledge on the use of clays as catalysts in the esterification of fatty acid. 2. Materials and methods 2.1. Materials The smectite natural clay was from the Boa Vista district, Paraíba, Brazil. Sulphuric acid (95e99%), lauric acid (99%), oleic acid (P.A.), sodium sulphate (99%), ethanol (99.8%), propanol (99.5%) and butanol (99.4%) were purchased from Vetec (Brazil). Ethyl acetate (>99%) and methanol (>99%) were from Tedia (Brazil), and caprylic acid (>98%) was from Merck (USA). Stearic acid (95%), methyl
stearate (>99%), ethyl stearate (>99%), methyl oleate (>99%), methyl laurate (>99%) and methyl caprylate (>99%) were from Aldrich (USA). The FFA industrial residue was kindly donated by the palm oil refining industry, and its fatty acid composition was determined by gas chromatography-mass spectrometry. It is composed by 46% palmitic acid, 39% oleic acid, 8% linoleic acid, 6% stearic acid and 1% myristic acid (% of area in the chromatogram). 2.2. Catalyst preparation The natural clay was used as received, without any previous treatment to eliminate organic materials, quartz, other materials or minerals. It was completely crushed and passed through a 0.250 mm sieve opening. The homogenised clay, called SMEnat, was acid-activated in a round-bottom flask connected to a reflux condenser. A 10% w/v suspension of clay in 4 mol L1 sulphuric acid was stirred at 90 C for 2 h [39]. The solid was filtered under reduced pressure and washed with distilled water until the washing water had the same pH of the first. The material was dried in an oven at 110 C for 24 h and finally ground until passage through a 0.250 mm sieve opening again. The activated clay, called SMEacid, was kept in a desiccator. 2.3. Catalyst characterization The chemical compositions of SMEnat and SMEacid were determined by X-ray fluorescence (XRF) using a Bruker spectrometer (AXS S4 Explorer) equipped with a Rhodium tube. The sample, previously dried, was fused with lithium tetraborate at 1100 C at a ratio of 1:6 sample/lithium tetraborate. The cation exchange capacity (CEC) of the natural clay was determined according to the procedure proposed by Jackson [40], and 1 g of SMEnat was exchanged with 1 mol L1 potassium acetate solution through centrifugation. The suspension was washed with 95% ethanol, and the aqueous phase was discarded. Then, the Kþ 1 ion was exchanged by NHþ ammonium acetate 4 ion with a 1 mol L solution. The Kþ solutions were collected in a volumetric flask, and the volume was completed to 100.00 mL with ultrapure water. The procedure was performed in triplicate, and the amount of Kþ ion was determined by flame spectrometry, using a flame photometer Micronal B262. X-ray diffraction (XRD) analysis was performed on a Bruker-AXS D5005 at 35 kV and 40 mA. A 2q range from 5 to 80 was scanned at 0.02 s1. The BET-specific surface area was measured by nitrogen adsorption data in a relative pressure range from 0.05 to 0.98, employing a Micromeritics A.S.A.P. 2010. The average distribution of pore sizes was calculated using the BJH method. The adsorption and desorption isotherms were obtained at 196 C. Prior to each measurement, all samples were degassed at 120 C for 5 h under vacuum. The number of weak, moderate and strong acid sites was estimated by thermodesorption of n-butylamine. Each sample was heated to 120 C at a rate of 10 C min1 under helium flow at 40 mL min1. After 30 min at 120 C, the material was kept under helium flow saturated with n-butylamine for 10 min. Next, pure nitrogen percolated the sample during 20 min to remove any physisorbed n-butylamine molecules. Thermogravimetric results were obtained under nitrogen flow at 20 mL min1 from 20 up to 800 C at a rate of 10 C min1, on a TGA-51 Shimadzu. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were done on a 2960 STD V.3.0.F TA instrument under air flow at 35 mL min1. The samples were analyzed from 25 to 1000 C at a rate of 10 C min1.
M.J.C. Rezende, A.C. Pinto / Renewable Energy 92 (2016) 171e177
on the esterification of free fatty acids.
The required amounts of fatty acid, alcohol and catalyst were transferred to a 5 mL round-bottom flask that was immediately connected to a reflux condenser and immersed in a heating bath at a constant temperature. The suspension was kept under constant magnetic stirring at atmospheric pressure. After the reaction, the mixture was cooled, and ethyl acetate was added to extract the fatty acid, alcohol and ester. The catalyst was filtered, and the organic layer was dried over anhydrous sodium sulphate. Ethyl acetate and alcohol were removed by evaporation under reduced pressure, and the final product was composed of fatty acid and ester. All of the reactions were performed in duplicate and the replicates presented a difference from 1% to 3% of conversion. 2.5. Catalyst recovery The product was dissolved in ethyl acetate, and the catalyst was removed from the organic layer by filtration. It was then taken to the oven at 120 C for 2 h, crushed and passed through a 0.250 mm sieve opening. The recovered catalyst was kept in a desiccator to be reused in the next batch. 2.6. Conversion analysis The conversion to ester was determined by the Proton Nuclear Magnetic Resonance (1H NMR) method as described by Gelbard et al. [41]. It consisted of the direct analysis (Bruker DPX-200) of 10 mg of the reaction product in 0.6 mL of CDCl3 using TMS as the internal standard. The relative areas of signals were obtained by integration using the program Win-NMR1D 5.1. The analysis was based on the relative ratio of specific signals integration. For methyl esters, the relevant signals chosen for integration were the three hydrogens of methoxy groups (eOCH3) at 3.7 ppm and the two hydrogens of a-carbonyl methylene groups present in both fatty acid and ester at 2.3 ppm. For ethyl, propyl and butyl esters, the relevant signals chosen for integration were the two hydrogens of the alcoxy groups (eOCH2R) at 4.1 ppm and the two hydrogens of a-carbonyl methylene groups present in both fatty acid and ester at 2.3 ppm. Calibration graphs were built with known amounts of ester and fatty acid. The curves showed a linear relationship between the instrumental response and the analyte concentration. The standard calibration parameters are summarized in Table 1. 3. Results and discussion In this work, natural Brazilian clay was acid-activated without any previous treatment to eliminate organic materials, quartz, other materials or minerals, although these treatments are widely used in studies involving the evaluation of clay as catalyst. The objective was to simplify the preparation of the clay-based catalyst to evaluate its potential for use in the production of biodiesel based
Table 1 Equations for correction of conversion values provided by NMR. Equation
Stearic acid (C18:0) Stearic acid (C18:0) Oleic acid (C18:1) Lauric acid (C12:0) Caprylic acid (C8:0)
% % % % %
methyl stearate ¼ % NMR/0.9174 ethyl stearate ¼ % NMR/0.9354 methyl oleate ¼ % NMR/1.0136 methyl laurate ¼ % NMR/0.9629 methyl caprylate ¼ % NMR/0.9828
R2 0.9995 0.9968 0.9990 0.9972 0.9972
The methyl stearate equation was used to calculate the conversion for the FFA industrial residue. The ethyl stearate equation was also used to calculate the conversion on propyl and butyl stearate.
3.1. Evaluation of the acid-activated clay as catalyst The catalytic performances of the natural clay SMEnat and acidactivated clay SMEacid were investigated in the esterification of stearic acid (C18:0) with methanol. The montmorillonite K-10, a commercial catalyst produced by the acid treatment of raw clay [42], was also tested. The reactions were performed at atmospheric pressure, under magnetic stirring, and a co-solvent was not used. The duplicates presented a difference from 1% to 3% of conversion. The average results are showed in Fig. 1. The acid activation treatment markedly increased the catalytic activity of the natural clay SMEnat. The acid-activated clay SMEacid provided a better catalytic performance than K-10. The superiority of acid-activated clays over K-10 has already been demonstrated by Zatta, Ramos and Wypych [43]. The SMEnat was acid-activated three times to evaluate the repeatability of the preparation of the SMEacid catalyst. The three SMEacid catalysts were also tested in the esterification of stearic acid with methanol and showed a difference from 1% to 3% of conversion. A mechanistic study on the esterification using clay as catalysts was not done. However, we assume that the mechanism involved is probably similar to those proposed by other works involving Bronsted or Lewis acid esterification [44,45]. 3.2. Characterization of SMEnat and SMEacid catalyst The chemical composition analysis of the SMEnat and SMEacid catalysts shows that the levels of structural elements, such as aluminum, iron and magnesium, presented a reduction after the acid treatment. The results of chemical analysis are shown in Table 2. The values of R1 and R2 for the SMEacid catalyst were higher than for the natural clay. This result suggests that the acid treatment of the natural clay affects its structural composition. The absence of sodium and the reduced amounts of calcium and potassium in SMEacid indicates that the interlayer cationic species of these elements were replaced by H3Oþ ions. The cation exchange capacity (CEC) showed the value of 66 meq/ 100 g. This value is slightly below the range of 80e150 meq/100 g expected for smectite. However, this deviation may be related to the presence of quartz and other impurities. The XRD diffraction pattern of the natural clay presented reflection at 13.5 Å (2q ¼ 6.5 ) is typical of smectite. The XRD analyses indicated that smectite is the dominant mineral. The peaks at 4.0 Å (2q ¼ 22.2 ) and 3.4 Å (2q ¼ 25.4 ) suggest the presence of quartz in the raw material. The characterization of smectite was confirmed by the expansion of the interlayer distance after the treatment with ethylene glycol. A heat treatment at 550 C was also
Conversion (%)
2.4. Esterification reactions
Fatty acid
173
93
100
70
80 60 40
12
20 0 -20
SMEnat
SMEacid
K-10
Fig. 1. Conversion in methyl stearate using clay-based catalysts. Reaction condition: stearic acid (4 mmol), methanol (12 mmol), catalyst (200 mg), heating bath at 100 C, 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
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Table 2 Chemical composition by X-ray fluorescence.
Table 4 Textural properties of the clays.
Analite
Oxide
% SMEnat
% SMEacid
Clays
SBET (m2 g1)
Pore volume (cm3 g1)
Pore diameter (Å)
P Cl S K Na Ti Ca Mg Fe Al Si R1 R2
P2O5 Cl SO3 K2O Na2O TiO2 CaO MgO Fe2O3 Al2O3 SiO2
0.054 ± 0.003 e e 0.26 ± 0.01 0.40 ± 0.02 0.80 ± 0.04 0.84 ± 0.04 2.1 ± 0.1 6.6 ± 0.3 14.4 ± 0.7 75 ± 4 5.2 3.2
0.046 ± 0.002 0.048 ± 0.002 0.085 ± 0.004 0.160 ± 0.008 e 0.84 ± 0.04 0.077 ± 0.004 0.74 ± 0.04 2.6 ± 0.1 10.9 ± 0.5 84 ± 4 7.7 5.9
SMEnat SMEacid
137 167
0.21 0.27
59.2 56.6
R1: SiO2/Al2O3. R2: SiO2/(Al2O3 þ Fe2O3 þ MgO).
summarized in Table 5. K-10 presented almost the same number of acid sites of SMEacid in the range of 50e150 C (0.60 mmol/g). On the other hand, the commercial catalyst has 0.31 and 0.46 mmol/g in the ranges 150e350 C and 350e500 C, respectively. The lowest amount of moderate and strong acid sites may be an explanation for the lower conversion values observed for K-10 when compared to those obtained by SMEacid. 3.3. Esterification of different fatty acids with methanol
used to check the collapse of the structure. The XRD analyses indicated a peak at 17.2 Å (2q ¼ 5.1 ) after the first treatment and a breakdown to 10.3 Å (2q ¼ 8.5 ) after heating. The XRD analyses of the acid-activated clay SMEacid revealed that the d(001) smectite peak at 15.8 Å (2q ¼ 5.5 ) had a lower intensity compared to the natural sample, suggesting a partial destruction of the crystalline phase of the clay after the acid treatment [46]. The DTA curves of the natural clay SMEnat and the catalyst SMEacid showed a single and intense peak of loss of adsorbed water from 40 to 200 C, with a maximum at approximately 60 C. This peak can be attributed to water molecules intercalated between the layers. The second peak observed, with a maximum at 460 C for SMEnat and 430 C for the catalyst SMEacid, can be attributed to the loss of structural hydroxyls. The loss of hydroxyl groups in the lower temperature for the catalyst SMEacid compared to the natural clay SMEnat indicates that the material became thermally less stable after the acid treatment. At 900 C, a soft double endoexothermic peak appeared. The endothermic peak represents the destruction of the crystalline framework, whereas the exothermic peak represents the formation of quartz. A thermogravimetric analysis corroborates the results observed in the DTA curves. Two regions of mass loss were observed for SMEnat and SMEacid. Table 3 shows the percentage of mass loss for the two regions. The reduction of mass was less pronounced in the region of dehydroxylation. For the catalyst SMEacid, a lower mass loss was observed. This result is due to the destruction of the octahedral sheet caused by the acid treatment [39]. The nitrogen adsorption isotherms exhibit stepwise adsorption and desorption (type IV isotherms), indicating a typical mesoporous structure. The textural data are displayed in Table 4 and indicate that the surface area increased 22% after the acid treatment. The corresponding pore size distributions indicate the presence of mesoporosity in the clays. The hysteresis loop observed in the isotherm can be classified as H4 type in the IUPAC system. This H4 hysteresis loop indicates that the structures are mesoporous, reinforcing the isotherm data. The thermodesorption of n-butylamine showed that the acid treatment of SMEnat increased the weak, moderate and strong acid sites. The number of acid sites per range of temperature is
Table 3 Loss of mass from the TGA curves.
The catalyst SMEacid was used in the esterification of caprylic acid (C8:0), lauric acid (C12:0) and oleic acid (C18:1). The objective was to evaluate the catalytic performance of the acid-activated clay in the esterification of carboxylic acids with different sizes of hydrocarbon chain and with the presence of double bond. These fatty acids are usually found in FFA feedstocks. The catalyst was also evaluated in the esterification of an FFA industrial residue used as feedstock for the production of biodiesel in Brazil. The FFA mixture was from the palm oil refining industry, and it was composed of 46% palmitic acid (C16:0), 39% oleic acid (C18:1), 8% linoleic acid (C18:2), 6% stearic acid (C18:0) and 1% myristic acid (C14:0). The montmorillonite K-10 was also tested, and Fig. 2 shows the average results. Based on the conversion values obtained for the two catalysts, we can see a reduction in the formation of the ester as the hydrocarbon chain increases. This is due to the hydrocarbon chain flexibility resulting from the rotation of the carbonecarbon bonds. With the increase of the hydrocarbon chain, some conformers may hinder the access to the carbonyl, blocking its activation by the catalyst, and may hinder the nucleophilic attack by the alcohol. Comparing the values obtained for stearic and oleic acids, we can observe that the presence of a double bond resulted in an even more pronounced reduction of the conversion. The chain of oleic acid is less flexible, and the steric conformation may be hindering its spread. 3.4. Esterification of stearic acid with different alcohols The catalytic performances of SMEacid and K-10 were also tested in the esterification of stearic acid with ethanol, propanol and butanol. The objective was to evaluate the effect of the alcohol hydrocarbon chain in the conversion to ester. Fig. 3 shows the results. The catalyst SMEacid had a higher conversion in methyl ester when compared to the commercial clay. The results indicated a reduced conversion to ethyl, propyl and butyl esters, and the performances of the two catalysts were similar in the production of these esters. The difference between the conversion obtained using methanol and the other alcohols can be attributed to the sterically
Table 5 Number of acid sites (mmol/g clay) per range of temperature.
Clays
30e200 C
400e500 C
Clays
50e150 C
150e350 C
350e500 C
SMEnat SMEacid
15% 9%
3% 2%
SMEnat SMEacid
0.27 0.57
0.26 0.53
0.46 0.73
M.J.C. Rezende, A.C. Pinto / Renewable Energy 92 (2016) 171e177
SMEacid
99
Conversion (%)
100
K-10
98
85
93 84
74
80
3.5. Effect of variables
52 60
40 20
0 caprylic acid
lauric acid
stearic acid
oleic acid industrial feedstock
Fig. 2. Esterification of different fatty acids with methanol using SMEacid and K-10 as catalyst. Reaction condition: fatty acid (4 mmol), methanol (12 mmol), catalyst (200 mg), heating bath at 100 C, 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
SMEacid
Conversion (%)
K-10
93
100
67
71
80
58
59
64
60
52
60 40 20 0
methyl ester ethyl ester propyl ester butyl ester Fig. 3. Esterification of stearic acid with different alcohols using SMEacid and K-10 as catalyst. Reaction condition: stearic acid (4 mmol), alcohol (12 mmol), catalyst (200 mg), heating bath at 100 C, 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
hindered access of the alcohol to the electrophilic site of the stearic acid, as the size of the alcohol hydrocarbon chain increased. Increasing the temperature of the heating bath to 110 C did not improve the conversion to ethyl and propyl esters using commercial clay K-10 as the catalyst (Fig. 4). On the other hand, an increase in conversion to butyl stearate was observed. For both temperatures tested (100 and 110 C), ethanol and propanol were at reflux, and then a part of the molecules was in the vapor phase. In this case, the increased energy supplied to the system did not affect the conversion. Butanol was not at reflux in the two tested
100 °C 110 °C
Conversion (%)
100 80 60
temperatures, so this alcohol was predominately in the reaction medium. By providing more energy to the system, it was possible to increase the conversion to butyl ester.
89
80
71
175
This work evaluated the effect of the main variables in the conversion to methyl stearate. The effect of the amount of the catalyst SMEacid and reaction time was investigated in the conversion to methyl stearate. The results (Table 6) showed that the conversion reached equilibrium in about 4 h of reaction. Reducing the amount of catalyst, the conversion decreased dramatically. The hydrocarbon chain of the stearic acid is a barrier to accessing the catalytic sites, making it necessary to use a larger mass of the catalyst. This explanation is supported by the results presented in the esterification of different fatty acids with methanol, for which we observed higher values of conversion to fatty acids with a smaller hydrocarbon chain. This indicates that a smaller mass of catalyst may be employed as the acid chain is smaller. Regarding economic issues, the use of clay-based catalyst amount a little over the usual, does not seem to be a limiting factor, since it is a low-cost material and reusable. The effect of the stearic acid/methanol molar ratio and temperature of the heating bath was also evaluated. The results are shown in Table 7. The temperature of the heating bath is an important variable, clearly changing the result of the reaction. In contrast, the reduction of methanol may be employed without prejudice to the conversion. Finally, the effect of the heating activation of the catalyst SMEacid immediately before the catalytic test was evaluated. The catalyst SMEacid was used without pre-activation, pre-activated by microwave oven at 760 W for 5 min and also by oven at 120 C for 2 h. The conversion rates were 94%, 99% and 99%, respectively. The activation by microwave oven does not modify the crystalline structure and the catalytic properties of clays [47]. The results show that the developed catalyst can be used without previous activation. However, a conversion close to 100% is achieved by heating to remove the water adsorbed on its surface. 3.6. Reuse of the catalyst SMEacid One of the great advantages of using clay as a catalyst is the possibility of reuse. The activated clay SMEacid was recovered and reused in consecutive batches. All reactions were carried out in the same condition. The first batch was carried out on a larger scale (10). Thus, the catalyst mass required to perform the subsequent batches was completed by the recovered catalyst in the first reaction. The conversion to methyl stearate and the percentage recovery of the catalyst are showed in Fig. 5. We can see that the conversion was maintained almost unchanged in the first three reuses. Even after the fifth recycle, it showed about a 70% conversion rate to methyl estearate. This result is a clear indication that
74 58
55
59
61
60
Table 6 Effect of the amount of SMEacid and the reaction time in the conversion.
40
Entry
SMEacid
Time
Conversion (%)
20
1 2 3 4 5 6 7
200 mg 200 mg 200 mg 200 mg 200 mg 150 mg 50 mg
6 4 3 2 4 4 4
96% 93% 81% 66% 93% 64% 27%
0 ethyl ester propyl ester butyl ester Fig. 4. Esterification of stearic acid with different alcohols using K-10 as catalyst. Reaction condition: stearic acid (4 mmol), alcohol (12 mmol), catalyst (200 mg), 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
h h h h h h h
Condition: stearic acid (4 mmol), methanol (12 mmol), heating bath at 100 C.
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Table 7 Effect of the stearic acid/methanol molar ratio and the temperature of heating bath in the conversion. Stearic acid/methanol molar ratio
Heating bath temperature
Conversion (%)
1:3 1:3 1:1.5 1:1.5
100 C 80 C 100 C 80 C
93% 76% 94% 72%
Condition: 200 mg of SMEacid, 4 h.
Conversion (%) Recovery (%)
98 100
86
80
83 77
82 76
72
76
69
76
60 40 20
0 1st
2nd
3rd
4th
5th
Batch Fig. 5. Recovery (wt%) and conversion of the reused SMEacid. Reaction condition: stearic acid (4 mmol), methanol (12 mmol), 200 mg of SMEacid, heating bath at 100 C, 4 h.
the catalyst is acting by a heterogeneous route. 4. Conclusions In summary, smectite clay is abundant, has a low cost and is non-toxic and environmentally friendly. The acid treatment of a Brazilian smectite natural clay generated a material with catalytic activity for the esterification of free fatty acids. The conversions vary from good to excellent. Conversion levels of 99%, 93% and 80% were reached for caprylic, stearic and oleic methyl esters, respectively, using 1:3 fatty acid/methanol molar ratio, heating bath at 100 C after 4 h. The catalyst proved to be very promising in the esterification of different fatty acids. It can be used at atmospheric pressure, without an inert atmosphere and without a co-solvent. These conditions are relevant and simplify an industrial process. The results showed that a deeply study deserves to be done in order to evaluate the use of smectite-based catalysts in a fatty acid esterification industrial process. The viability of a continuous process, a scale-up and the impact of smectite from different mining sites in the catalytic activity are some of the challenges. Acknowledgments The authors would like to thank CNPq, CAPES (Brazilian Research Councils), FAPERJ (Rio de Janeiro State Research Sponsor Foundation) and ANP (Brazilian National Agency of Petroleum, Natural Gas and Biofuels) for financial support. References [1] Z. Helwani, M.R. Othman, N. Aziz, W.J.N. Fernando, J. Kim, Technologies for production of biodiesel focusing on green catalytic techniques: a review, Fuel Process Technol. 90 (2009) 1502e1514. [2] A.C. Pinto, L.L.N. Guarieiro, M.J.C. Rezende, N.M. Ribeiro, E.A. Torres, W.A. Lopes, P.A.P. Pereira, J.B. de Andrade, Biodiesel: an overview, J. Braz. Chem. Soc. 16 (2005) 1313e1330. [3] C.S. Cordeiro, F.R. da Silva, F. Wypych, L.P. Ramos, Heterogeneous catalysts for
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Esterification of fatty acids using acid-activated Brazilian smectite natural clay as a catalyst Michelle J.C. Rezende*, Angelo C. Pinto ria, CT, Bloco A, 21941-909, Rio de Janeiro, RJ, Brazil Universidade Federal do Rio de Janeiro, Instituto de Química, Cidade Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2015 Received in revised form 1 February 2016 Accepted 2 February 2016 Available online xxx
This work reports the use of an acid-activated Brazilian smectite natural clay-based catalyst in the esterification of fatty acids at atmospheric pressure and without a co-solvent. Conversion levels of 99%, 98%, 93% and 80% were reached for caprylic, lauric, stearic and oleic methyl esters, respectively, using 1:3 fatty acid/methanol molar ratio, heating bath at 100 C after 4 h. A conversion level of 89% was achieved for methyl esters from a fatty acid residue of the palm oil refining industry in the same reaction condition. The acid-activated clay provided better performance than the commercial catalyst K-10. The effects of catalyst amount, temperature, fatty acid/alcohol molar ratio and time were investigated. The heating activation of the catalyst immediately before the catalytic test increased the conversion from 94% to 99% using 1:1.5 stearic acid/methanol molar ratio, heating bath at 100 C after 4 h. The catalyst was reused five times. The conversion decreases less than 5% in the first three reuses. The smectite natural clay and the catalyst were characterized by X-ray fluorescence, X-ray diffraction, n-butylamine thermodesorption, nitrogen adsorption analysis, thermogravimetric analysis and differential thermal analysis. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Clay Fatty acid esterification
1. Introduction The increasing demand for biodiesel has stimulated the development of new technologies and new catalysts. The industrial production of biodiesel is performed by a transesterification process using mainly hydroxide or methoxide as a catalyst [1]. The problem involved in this process is the formation of soap. Because of this, there are studies involved in the development of new catalysts. Most of them are focused on the development of heterogeneous catalysts [2e9]. The homogeneous alkaline catalysis provides high yields in a short time and mild reaction condition, so find a promising heterogeneous catalyst is not an easy task. There are also parallel efforts to optimize the homogeneous alkaline catalytic process. Some examples include economics in terms of process conditions, studies involved in continuous process design as well as scale-up [10e12]. Although vegetable oils are the major feedstock for the production of biodiesel, there are other fatty acid sources that have been studied, such as used cooking oils, sewage sludge and animal
* Corresponding author. E-mail address: [email protected] (M.J.C. Rezende). http://dx.doi.org/10.1016/j.renene.2016.02.004 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
fats [13]. Some feedstocks have high concentrations of free fatty acids (FFA), reaching levels up to 15 wt%. This characteristic makes them inappropriate for the conventional direct base-catalyzed transesterification route to biodiesel due to soap formation. An integrated process enables the use of these materials by the combination of acid-catalyzed FFA esterification followed by basecatalyzed triacylglyceride transesterification. Despite the additional cost of production, the process is being increasingly applied to produce biodiesel from high-FFA feedstock [14]. Biodiesel can also be produced from the direct esterification of FFA feedstocks. Low-grade starting materials, such as waste from vegetable oil refining industries, can be used. To achieve a satisfactory ester conversion, it is necessary to remove the water produced or use a stoichiometric excess of alcohol. Sulfuric acid is the most widely used catalyst in esterification reactions. It is very active and has an important dehydrating effect. By contrast, it is corrosive, presents problems of storage and control and, in particular conditions, can react with the double bond present in unsaturated fatty acids [15]. Nowadays, there is intensive research of new catalysts for the esterification of fatty acids that allow milder reaction conditions and that can be reused. In an integrated process, the intermediate step of neutralizing the acid catalyst increases the volume of waste.
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The use of a heterogeneous catalyst may result in additional steps or units, but still could simplify the process, since it could be removed by filtration, without neutralization, and the product could be immediately submitted to the transesterification reaction. There are several studies in the literature for the esterification of fatty acids with short-chain alcohols and using different heterogeneous catalysts [16e30]. Regarding the use of clay as a catalyst in the esterification of fatty acids, Kantam, Bhaskar and Choudary used Fe3þ-montmorillonite in the esterification of stearic acid with methanol. A conversion rate of 98% was obtained after 5 h of reaction, using a DeaneStark apparatus and toluene as a solvent [31]. Neji, Trabelsi and Frikha investigated the use of commercial montmorillonite-based catalysts (KSF, KSF/0, KP10 and K10) in the esterification of stearic, oleic and palmitic acids with short-chain alcohols [32]. The reactions were carried out in a semicontinuous reactor at 150 C with continuous removal of water. The best catalytic activities (>90% after 3 h) were obtained with the KSF/0 catalyst. Nascimento et al. prepared catalysts for the esterification of oleic acid with methanol from two Amazon kaolins and two standard kaolins. After 4 h, a conversion rate of more than 90% was achieved using an acid/alcohol molar ratio of 1:60 at 130 C [33]. Ghiaci, Aghabarari and Gil showed that a modified bentonite with 1-benzyl-1H-benzimidazole-based Brønsted acidic ionic liquids was able to catalyze the esterification of oleic acid to its methyl ester in 6 h with yields of more than 92% [34]. Zatta, Gardolinski and Wypych studied the esterification of lauric acid with methanol using raw halloysite as a catalyst. A conversion level of 95% was achieved at 160 C for 2 h in a pressurized steel reactor [35]. The esterification of oleic acid with methanol catalyzed over sulfuric acid-activated pillar bentonite clay was performed by Jeenpadiphat and Tungasmita [36]. The use of a 1:9 molar ratio of oleic acid/ methanol at 60 C for 3 h reached 100% of yield. In 2012, we used acid-activated Brazilian clay-based catalysts in the esterification of different carboxylic acids and alcohols, including lauric acid and methanol. After 3 h, a conversion rate of 87% in methyl laurate was achieved using a carboxylic acid/alcohol molar ratio of 1:3, atmospheric pressure and heating bath at 100 C [37]. Recently, Aghabarari and Dorostkar studied the esterification of oleic acid with ethanol using bentonite modified with ionic liquids as a catalyst [38]. A conversion rate greater than 93% was observed after 6.5 h at 60 C using an oleic acid/ethanol molar ratio of 1:2. This work reports the esterification of fatty acids using acidactivated Brazilian smectite natural clay as a catalyst, at atmospheric pressure and without a co-solvent. The objective is to evaluate the potential use of a low-cost and friendly solid catalyst for the production of biodiesel based on the direct esterification of FFA. The acid treatment is very simple, and the acid-activated clay was tested in the esterification of different fatty acids, including a FFA industrial residue used as feedstock for the production of biodiesel in Brazil. The effect of the main variables was studied, and high conversions were observed using very mild reaction conditions. The reuse of the catalyst was also investigated. These results are a new contribution to the knowledge on the use of clays as catalysts in the esterification of fatty acid. 2. Materials and methods 2.1. Materials The smectite natural clay was from the Boa Vista district, Paraíba, Brazil. Sulphuric acid (95e99%), lauric acid (99%), oleic acid (P.A.), sodium sulphate (99%), ethanol (99.8%), propanol (99.5%) and butanol (99.4%) were purchased from Vetec (Brazil). Ethyl acetate (>99%) and methanol (>99%) were from Tedia (Brazil), and caprylic acid (>98%) was from Merck (USA). Stearic acid (95%), methyl
stearate (>99%), ethyl stearate (>99%), methyl oleate (>99%), methyl laurate (>99%) and methyl caprylate (>99%) were from Aldrich (USA). The FFA industrial residue was kindly donated by the palm oil refining industry, and its fatty acid composition was determined by gas chromatography-mass spectrometry. It is composed by 46% palmitic acid, 39% oleic acid, 8% linoleic acid, 6% stearic acid and 1% myristic acid (% of area in the chromatogram). 2.2. Catalyst preparation The natural clay was used as received, without any previous treatment to eliminate organic materials, quartz, other materials or minerals. It was completely crushed and passed through a 0.250 mm sieve opening. The homogenised clay, called SMEnat, was acid-activated in a round-bottom flask connected to a reflux condenser. A 10% w/v suspension of clay in 4 mol L1 sulphuric acid was stirred at 90 C for 2 h [39]. The solid was filtered under reduced pressure and washed with distilled water until the washing water had the same pH of the first. The material was dried in an oven at 110 C for 24 h and finally ground until passage through a 0.250 mm sieve opening again. The activated clay, called SMEacid, was kept in a desiccator. 2.3. Catalyst characterization The chemical compositions of SMEnat and SMEacid were determined by X-ray fluorescence (XRF) using a Bruker spectrometer (AXS S4 Explorer) equipped with a Rhodium tube. The sample, previously dried, was fused with lithium tetraborate at 1100 C at a ratio of 1:6 sample/lithium tetraborate. The cation exchange capacity (CEC) of the natural clay was determined according to the procedure proposed by Jackson [40], and 1 g of SMEnat was exchanged with 1 mol L1 potassium acetate solution through centrifugation. The suspension was washed with 95% ethanol, and the aqueous phase was discarded. Then, the Kþ 1 ion was exchanged by NHþ ammonium acetate 4 ion with a 1 mol L solution. The Kþ solutions were collected in a volumetric flask, and the volume was completed to 100.00 mL with ultrapure water. The procedure was performed in triplicate, and the amount of Kþ ion was determined by flame spectrometry, using a flame photometer Micronal B262. X-ray diffraction (XRD) analysis was performed on a Bruker-AXS D5005 at 35 kV and 40 mA. A 2q range from 5 to 80 was scanned at 0.02 s1. The BET-specific surface area was measured by nitrogen adsorption data in a relative pressure range from 0.05 to 0.98, employing a Micromeritics A.S.A.P. 2010. The average distribution of pore sizes was calculated using the BJH method. The adsorption and desorption isotherms were obtained at 196 C. Prior to each measurement, all samples were degassed at 120 C for 5 h under vacuum. The number of weak, moderate and strong acid sites was estimated by thermodesorption of n-butylamine. Each sample was heated to 120 C at a rate of 10 C min1 under helium flow at 40 mL min1. After 30 min at 120 C, the material was kept under helium flow saturated with n-butylamine for 10 min. Next, pure nitrogen percolated the sample during 20 min to remove any physisorbed n-butylamine molecules. Thermogravimetric results were obtained under nitrogen flow at 20 mL min1 from 20 up to 800 C at a rate of 10 C min1, on a TGA-51 Shimadzu. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were done on a 2960 STD V.3.0.F TA instrument under air flow at 35 mL min1. The samples were analyzed from 25 to 1000 C at a rate of 10 C min1.
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on the esterification of free fatty acids.
The required amounts of fatty acid, alcohol and catalyst were transferred to a 5 mL round-bottom flask that was immediately connected to a reflux condenser and immersed in a heating bath at a constant temperature. The suspension was kept under constant magnetic stirring at atmospheric pressure. After the reaction, the mixture was cooled, and ethyl acetate was added to extract the fatty acid, alcohol and ester. The catalyst was filtered, and the organic layer was dried over anhydrous sodium sulphate. Ethyl acetate and alcohol were removed by evaporation under reduced pressure, and the final product was composed of fatty acid and ester. All of the reactions were performed in duplicate and the replicates presented a difference from 1% to 3% of conversion. 2.5. Catalyst recovery The product was dissolved in ethyl acetate, and the catalyst was removed from the organic layer by filtration. It was then taken to the oven at 120 C for 2 h, crushed and passed through a 0.250 mm sieve opening. The recovered catalyst was kept in a desiccator to be reused in the next batch. 2.6. Conversion analysis The conversion to ester was determined by the Proton Nuclear Magnetic Resonance (1H NMR) method as described by Gelbard et al. [41]. It consisted of the direct analysis (Bruker DPX-200) of 10 mg of the reaction product in 0.6 mL of CDCl3 using TMS as the internal standard. The relative areas of signals were obtained by integration using the program Win-NMR1D 5.1. The analysis was based on the relative ratio of specific signals integration. For methyl esters, the relevant signals chosen for integration were the three hydrogens of methoxy groups (eOCH3) at 3.7 ppm and the two hydrogens of a-carbonyl methylene groups present in both fatty acid and ester at 2.3 ppm. For ethyl, propyl and butyl esters, the relevant signals chosen for integration were the two hydrogens of the alcoxy groups (eOCH2R) at 4.1 ppm and the two hydrogens of a-carbonyl methylene groups present in both fatty acid and ester at 2.3 ppm. Calibration graphs were built with known amounts of ester and fatty acid. The curves showed a linear relationship between the instrumental response and the analyte concentration. The standard calibration parameters are summarized in Table 1. 3. Results and discussion In this work, natural Brazilian clay was acid-activated without any previous treatment to eliminate organic materials, quartz, other materials or minerals, although these treatments are widely used in studies involving the evaluation of clay as catalyst. The objective was to simplify the preparation of the clay-based catalyst to evaluate its potential for use in the production of biodiesel based
Table 1 Equations for correction of conversion values provided by NMR. Equation
Stearic acid (C18:0) Stearic acid (C18:0) Oleic acid (C18:1) Lauric acid (C12:0) Caprylic acid (C8:0)
% % % % %
methyl stearate ¼ % NMR/0.9174 ethyl stearate ¼ % NMR/0.9354 methyl oleate ¼ % NMR/1.0136 methyl laurate ¼ % NMR/0.9629 methyl caprylate ¼ % NMR/0.9828
R2 0.9995 0.9968 0.9990 0.9972 0.9972
The methyl stearate equation was used to calculate the conversion for the FFA industrial residue. The ethyl stearate equation was also used to calculate the conversion on propyl and butyl stearate.
3.1. Evaluation of the acid-activated clay as catalyst The catalytic performances of the natural clay SMEnat and acidactivated clay SMEacid were investigated in the esterification of stearic acid (C18:0) with methanol. The montmorillonite K-10, a commercial catalyst produced by the acid treatment of raw clay [42], was also tested. The reactions were performed at atmospheric pressure, under magnetic stirring, and a co-solvent was not used. The duplicates presented a difference from 1% to 3% of conversion. The average results are showed in Fig. 1. The acid activation treatment markedly increased the catalytic activity of the natural clay SMEnat. The acid-activated clay SMEacid provided a better catalytic performance than K-10. The superiority of acid-activated clays over K-10 has already been demonstrated by Zatta, Ramos and Wypych [43]. The SMEnat was acid-activated three times to evaluate the repeatability of the preparation of the SMEacid catalyst. The three SMEacid catalysts were also tested in the esterification of stearic acid with methanol and showed a difference from 1% to 3% of conversion. A mechanistic study on the esterification using clay as catalysts was not done. However, we assume that the mechanism involved is probably similar to those proposed by other works involving Bronsted or Lewis acid esterification [44,45]. 3.2. Characterization of SMEnat and SMEacid catalyst The chemical composition analysis of the SMEnat and SMEacid catalysts shows that the levels of structural elements, such as aluminum, iron and magnesium, presented a reduction after the acid treatment. The results of chemical analysis are shown in Table 2. The values of R1 and R2 for the SMEacid catalyst were higher than for the natural clay. This result suggests that the acid treatment of the natural clay affects its structural composition. The absence of sodium and the reduced amounts of calcium and potassium in SMEacid indicates that the interlayer cationic species of these elements were replaced by H3Oþ ions. The cation exchange capacity (CEC) showed the value of 66 meq/ 100 g. This value is slightly below the range of 80e150 meq/100 g expected for smectite. However, this deviation may be related to the presence of quartz and other impurities. The XRD diffraction pattern of the natural clay presented reflection at 13.5 Å (2q ¼ 6.5 ) is typical of smectite. The XRD analyses indicated that smectite is the dominant mineral. The peaks at 4.0 Å (2q ¼ 22.2 ) and 3.4 Å (2q ¼ 25.4 ) suggest the presence of quartz in the raw material. The characterization of smectite was confirmed by the expansion of the interlayer distance after the treatment with ethylene glycol. A heat treatment at 550 C was also
Conversion (%)
2.4. Esterification reactions
Fatty acid
173
93
100
70
80 60 40
12
20 0 -20
SMEnat
SMEacid
K-10
Fig. 1. Conversion in methyl stearate using clay-based catalysts. Reaction condition: stearic acid (4 mmol), methanol (12 mmol), catalyst (200 mg), heating bath at 100 C, 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
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Table 2 Chemical composition by X-ray fluorescence.
Table 4 Textural properties of the clays.
Analite
Oxide
% SMEnat
% SMEacid
Clays
SBET (m2 g1)
Pore volume (cm3 g1)
Pore diameter (Å)
P Cl S K Na Ti Ca Mg Fe Al Si R1 R2
P2O5 Cl SO3 K2O Na2O TiO2 CaO MgO Fe2O3 Al2O3 SiO2
0.054 ± 0.003 e e 0.26 ± 0.01 0.40 ± 0.02 0.80 ± 0.04 0.84 ± 0.04 2.1 ± 0.1 6.6 ± 0.3 14.4 ± 0.7 75 ± 4 5.2 3.2
0.046 ± 0.002 0.048 ± 0.002 0.085 ± 0.004 0.160 ± 0.008 e 0.84 ± 0.04 0.077 ± 0.004 0.74 ± 0.04 2.6 ± 0.1 10.9 ± 0.5 84 ± 4 7.7 5.9
SMEnat SMEacid
137 167
0.21 0.27
59.2 56.6
R1: SiO2/Al2O3. R2: SiO2/(Al2O3 þ Fe2O3 þ MgO).
summarized in Table 5. K-10 presented almost the same number of acid sites of SMEacid in the range of 50e150 C (0.60 mmol/g). On the other hand, the commercial catalyst has 0.31 and 0.46 mmol/g in the ranges 150e350 C and 350e500 C, respectively. The lowest amount of moderate and strong acid sites may be an explanation for the lower conversion values observed for K-10 when compared to those obtained by SMEacid. 3.3. Esterification of different fatty acids with methanol
used to check the collapse of the structure. The XRD analyses indicated a peak at 17.2 Å (2q ¼ 5.1 ) after the first treatment and a breakdown to 10.3 Å (2q ¼ 8.5 ) after heating. The XRD analyses of the acid-activated clay SMEacid revealed that the d(001) smectite peak at 15.8 Å (2q ¼ 5.5 ) had a lower intensity compared to the natural sample, suggesting a partial destruction of the crystalline phase of the clay after the acid treatment [46]. The DTA curves of the natural clay SMEnat and the catalyst SMEacid showed a single and intense peak of loss of adsorbed water from 40 to 200 C, with a maximum at approximately 60 C. This peak can be attributed to water molecules intercalated between the layers. The second peak observed, with a maximum at 460 C for SMEnat and 430 C for the catalyst SMEacid, can be attributed to the loss of structural hydroxyls. The loss of hydroxyl groups in the lower temperature for the catalyst SMEacid compared to the natural clay SMEnat indicates that the material became thermally less stable after the acid treatment. At 900 C, a soft double endoexothermic peak appeared. The endothermic peak represents the destruction of the crystalline framework, whereas the exothermic peak represents the formation of quartz. A thermogravimetric analysis corroborates the results observed in the DTA curves. Two regions of mass loss were observed for SMEnat and SMEacid. Table 3 shows the percentage of mass loss for the two regions. The reduction of mass was less pronounced in the region of dehydroxylation. For the catalyst SMEacid, a lower mass loss was observed. This result is due to the destruction of the octahedral sheet caused by the acid treatment [39]. The nitrogen adsorption isotherms exhibit stepwise adsorption and desorption (type IV isotherms), indicating a typical mesoporous structure. The textural data are displayed in Table 4 and indicate that the surface area increased 22% after the acid treatment. The corresponding pore size distributions indicate the presence of mesoporosity in the clays. The hysteresis loop observed in the isotherm can be classified as H4 type in the IUPAC system. This H4 hysteresis loop indicates that the structures are mesoporous, reinforcing the isotherm data. The thermodesorption of n-butylamine showed that the acid treatment of SMEnat increased the weak, moderate and strong acid sites. The number of acid sites per range of temperature is
Table 3 Loss of mass from the TGA curves.
The catalyst SMEacid was used in the esterification of caprylic acid (C8:0), lauric acid (C12:0) and oleic acid (C18:1). The objective was to evaluate the catalytic performance of the acid-activated clay in the esterification of carboxylic acids with different sizes of hydrocarbon chain and with the presence of double bond. These fatty acids are usually found in FFA feedstocks. The catalyst was also evaluated in the esterification of an FFA industrial residue used as feedstock for the production of biodiesel in Brazil. The FFA mixture was from the palm oil refining industry, and it was composed of 46% palmitic acid (C16:0), 39% oleic acid (C18:1), 8% linoleic acid (C18:2), 6% stearic acid (C18:0) and 1% myristic acid (C14:0). The montmorillonite K-10 was also tested, and Fig. 2 shows the average results. Based on the conversion values obtained for the two catalysts, we can see a reduction in the formation of the ester as the hydrocarbon chain increases. This is due to the hydrocarbon chain flexibility resulting from the rotation of the carbonecarbon bonds. With the increase of the hydrocarbon chain, some conformers may hinder the access to the carbonyl, blocking its activation by the catalyst, and may hinder the nucleophilic attack by the alcohol. Comparing the values obtained for stearic and oleic acids, we can observe that the presence of a double bond resulted in an even more pronounced reduction of the conversion. The chain of oleic acid is less flexible, and the steric conformation may be hindering its spread. 3.4. Esterification of stearic acid with different alcohols The catalytic performances of SMEacid and K-10 were also tested in the esterification of stearic acid with ethanol, propanol and butanol. The objective was to evaluate the effect of the alcohol hydrocarbon chain in the conversion to ester. Fig. 3 shows the results. The catalyst SMEacid had a higher conversion in methyl ester when compared to the commercial clay. The results indicated a reduced conversion to ethyl, propyl and butyl esters, and the performances of the two catalysts were similar in the production of these esters. The difference between the conversion obtained using methanol and the other alcohols can be attributed to the sterically
Table 5 Number of acid sites (mmol/g clay) per range of temperature.
Clays
30e200 C
400e500 C
Clays
50e150 C
150e350 C
350e500 C
SMEnat SMEacid
15% 9%
3% 2%
SMEnat SMEacid
0.27 0.57
0.26 0.53
0.46 0.73
M.J.C. Rezende, A.C. Pinto / Renewable Energy 92 (2016) 171e177
SMEacid
99
Conversion (%)
100
K-10
98
85
93 84
74
80
3.5. Effect of variables
52 60
40 20
0 caprylic acid
lauric acid
stearic acid
oleic acid industrial feedstock
Fig. 2. Esterification of different fatty acids with methanol using SMEacid and K-10 as catalyst. Reaction condition: fatty acid (4 mmol), methanol (12 mmol), catalyst (200 mg), heating bath at 100 C, 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
SMEacid
Conversion (%)
K-10
93
100
67
71
80
58
59
64
60
52
60 40 20 0
methyl ester ethyl ester propyl ester butyl ester Fig. 3. Esterification of stearic acid with different alcohols using SMEacid and K-10 as catalyst. Reaction condition: stearic acid (4 mmol), alcohol (12 mmol), catalyst (200 mg), heating bath at 100 C, 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
hindered access of the alcohol to the electrophilic site of the stearic acid, as the size of the alcohol hydrocarbon chain increased. Increasing the temperature of the heating bath to 110 C did not improve the conversion to ethyl and propyl esters using commercial clay K-10 as the catalyst (Fig. 4). On the other hand, an increase in conversion to butyl stearate was observed. For both temperatures tested (100 and 110 C), ethanol and propanol were at reflux, and then a part of the molecules was in the vapor phase. In this case, the increased energy supplied to the system did not affect the conversion. Butanol was not at reflux in the two tested
100 °C 110 °C
Conversion (%)
100 80 60
temperatures, so this alcohol was predominately in the reaction medium. By providing more energy to the system, it was possible to increase the conversion to butyl ester.
89
80
71
175
This work evaluated the effect of the main variables in the conversion to methyl stearate. The effect of the amount of the catalyst SMEacid and reaction time was investigated in the conversion to methyl stearate. The results (Table 6) showed that the conversion reached equilibrium in about 4 h of reaction. Reducing the amount of catalyst, the conversion decreased dramatically. The hydrocarbon chain of the stearic acid is a barrier to accessing the catalytic sites, making it necessary to use a larger mass of the catalyst. This explanation is supported by the results presented in the esterification of different fatty acids with methanol, for which we observed higher values of conversion to fatty acids with a smaller hydrocarbon chain. This indicates that a smaller mass of catalyst may be employed as the acid chain is smaller. Regarding economic issues, the use of clay-based catalyst amount a little over the usual, does not seem to be a limiting factor, since it is a low-cost material and reusable. The effect of the stearic acid/methanol molar ratio and temperature of the heating bath was also evaluated. The results are shown in Table 7. The temperature of the heating bath is an important variable, clearly changing the result of the reaction. In contrast, the reduction of methanol may be employed without prejudice to the conversion. Finally, the effect of the heating activation of the catalyst SMEacid immediately before the catalytic test was evaluated. The catalyst SMEacid was used without pre-activation, pre-activated by microwave oven at 760 W for 5 min and also by oven at 120 C for 2 h. The conversion rates were 94%, 99% and 99%, respectively. The activation by microwave oven does not modify the crystalline structure and the catalytic properties of clays [47]. The results show that the developed catalyst can be used without previous activation. However, a conversion close to 100% is achieved by heating to remove the water adsorbed on its surface. 3.6. Reuse of the catalyst SMEacid One of the great advantages of using clay as a catalyst is the possibility of reuse. The activated clay SMEacid was recovered and reused in consecutive batches. All reactions were carried out in the same condition. The first batch was carried out on a larger scale (10). Thus, the catalyst mass required to perform the subsequent batches was completed by the recovered catalyst in the first reaction. The conversion to methyl stearate and the percentage recovery of the catalyst are showed in Fig. 5. We can see that the conversion was maintained almost unchanged in the first three reuses. Even after the fifth recycle, it showed about a 70% conversion rate to methyl estearate. This result is a clear indication that
74 58
55
59
61
60
Table 6 Effect of the amount of SMEacid and the reaction time in the conversion.
40
Entry
SMEacid
Time
Conversion (%)
20
1 2 3 4 5 6 7
200 mg 200 mg 200 mg 200 mg 200 mg 150 mg 50 mg
6 4 3 2 4 4 4
96% 93% 81% 66% 93% 64% 27%
0 ethyl ester propyl ester butyl ester Fig. 4. Esterification of stearic acid with different alcohols using K-10 as catalyst. Reaction condition: stearic acid (4 mmol), alcohol (12 mmol), catalyst (200 mg), 4 h. Experiments were carried out in duplicate. The calculated confidence interval was 95%.
h h h h h h h
Condition: stearic acid (4 mmol), methanol (12 mmol), heating bath at 100 C.
176
M.J.C. Rezende, A.C. Pinto / Renewable Energy 92 (2016) 171e177
Table 7 Effect of the stearic acid/methanol molar ratio and the temperature of heating bath in the conversion. Stearic acid/methanol molar ratio
Heating bath temperature
Conversion (%)
1:3 1:3 1:1.5 1:1.5
100 C 80 C 100 C 80 C
93% 76% 94% 72%
Condition: 200 mg of SMEacid, 4 h.
Conversion (%) Recovery (%)
98 100
86
80
83 77
82 76
72
76
69
76
60 40 20
0 1st
2nd
3rd
4th
5th
Batch Fig. 5. Recovery (wt%) and conversion of the reused SMEacid. Reaction condition: stearic acid (4 mmol), methanol (12 mmol), 200 mg of SMEacid, heating bath at 100 C, 4 h.
the catalyst is acting by a heterogeneous route. 4. Conclusions In summary, smectite clay is abundant, has a low cost and is non-toxic and environmentally friendly. The acid treatment of a Brazilian smectite natural clay generated a material with catalytic activity for the esterification of free fatty acids. The conversions vary from good to excellent. Conversion levels of 99%, 93% and 80% were reached for caprylic, stearic and oleic methyl esters, respectively, using 1:3 fatty acid/methanol molar ratio, heating bath at 100 C after 4 h. The catalyst proved to be very promising in the esterification of different fatty acids. It can be used at atmospheric pressure, without an inert atmosphere and without a co-solvent. These conditions are relevant and simplify an industrial process. The results showed that a deeply study deserves to be done in order to evaluate the use of smectite-based catalysts in a fatty acid esterification industrial process. The viability of a continuous process, a scale-up and the impact of smectite from different mining sites in the catalytic activity are some of the challenges. Acknowledgments The authors would like to thank CNPq, CAPES (Brazilian Research Councils), FAPERJ (Rio de Janeiro State Research Sponsor Foundation) and ANP (Brazilian National Agency of Petroleum, Natural Gas and Biofuels) for financial support. References [1] Z. Helwani, M.R. Othman, N. Aziz, W.J.N. Fernando, J. Kim, Technologies for production of biodiesel focusing on green catalytic techniques: a review, Fuel Process Technol. 90 (2009) 1502e1514. [2] A.C. Pinto, L.L.N. Guarieiro, M.J.C. Rezende, N.M. Ribeiro, E.A. Torres, W.A. Lopes, P.A.P. Pereira, J.B. de Andrade, Biodiesel: an overview, J. Braz. Chem. Soc. 16 (2005) 1313e1330. [3] C.S. Cordeiro, F.R. da Silva, F. Wypych, L.P. Ramos, Heterogeneous catalysts for
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