Applied Clay Science 87 (2014) 272–277
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Esteriﬁcation of oleic acid and high acid content palm oil over an acid-activated bentonite catalyst Sirima Jeenpadiphat, Duangamol Nuntasri Tungasmita ⁎ Materials Chemistry and Catalysis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 15 November 2013 Accepted 18 November 2013 Available online 15 December 2013 Keywords: Esteriﬁcation Bentonite Acid activation Catalyst Biodiesel
a b s t r a c t The esteriﬁcation of oleic acid with methanol and that of a high acid content palm or Jatropha oil with methanol or ethanol were evaluated with different acid-activated bentonite catalysts. Na-bentonite was acid-activated by either H2SO4 at varying concentrations from 0.25 to 2.0 M or with 0.5 M HNO3. The characterization of the raw and acid-activated bentonite was then conducted by nitrogen adsorption, XRD, XRF, SEM and acid–base titration analysis. The 0.5 M H2SO4 acid-activated bentonite exhibited the best catalytic activity with 100% methyl oleate yield and 99% free fatty acid conversion in the esteriﬁcation of pure oleic acid and oleic acid in palm oil with methanol, respectively. Both conversions were higher than that obtained from esteriﬁcation via commercial Amberlyst-15 catalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Because of the limited supply of non-renewable fossil fuel and the corresponding increase in diesel oil price, an increasing trend in current research has centered upon developing alternative fuels to petroleum. Among the different possible resources, biodiesel is one popular alternative as a renewable replacement of the non-renewable petroleum diesel. Biodiesel is comprised of the alkyl esters of fatty acids and is produced from either the transesteriﬁcation of triglycerides or the esteriﬁcation of free fatty acids (FFA) with short chain alcohols in the presence of a catalyst to produce fatty acid alkyl esters (biodiesel) and glycerol (Ma and Hanna, 1999; Pintoa et al., 2005). The advantages of biodiesel are that it is biodegradable, non-toxic to nature and has a low emission proﬁle aided by being essentially free from sulfur (Dorado et al., 2003; Labeckas and Slavinskas, 2006; Ulusoy et al., 2004). Esteriﬁcation has been performed using conventional homogeneous acidic catalysts, such as H2SO4, HCl and HNO3 (Koono et al., 1993) since it gives a high conversion level of alkyl esters in a short time. However, the catalyst is difﬁcult to remove from the product and is inconvenient to regenerate. Therefore, it would be desirable to perform the esteriﬁcation reaction with a heterogeneous acidic catalyst. These catalysts can be separated easily from the system at the end of the reaction and can also be reused. A large number of heterogeneous acidic catalysts have been reported in the literatures, including ion-exchange resins (Russbueldt and Hoelderich, 2009), heteropolyacids (Xu et al., 2008), sulfated zirconia (Nuttapol et al., 2011), polymers with sulfonic acid groups (Caetano et al., 2009; Melero et al., 2010), heteropolyacids ⁎ Corresponding author. Tel.: +66 2 2187622; fax: +66 2 2541309. E-mail address: [email protected]
(D.N. Tungasmita). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.11.025
immobilized on silica (Caetano et al., 2008), zeolites (Carmo et al., 2009) and clays (Moraes et al., 2011; Nascimento et al., 2011; Wypych et al., 2011). The use of various clays as a catalyst has expanded rapidly in recent years due to their advantageous properties, such as their mechanical and thermal stabilities, their high speciﬁc surface area and ion-exchange capacity (Lagaly and Bergaya, 2001). Major clay minerals in bentonite (Bent) are smectites such as montmorillonite (Mt), beidellite, saponite, nontronite, and hectorite. Bent might also contain other clay minerals and nonclay minerals (Grim and Guven, 1978; Murray, 2000). Each smectite is comprised of a 2:1 layer, containing two silica tetrahedral (T) sheets bonded to a central alumina octahedral (O) sheet (Grim and Guven, 1978). The net negative electric charge of the 2:1 (TOT) layers, which arise from the isomorphic substitution of Al3+ with Fe2+ or Mg2+ in the octahedral sites and of Si4+ with Al3+ in the tetrahedral sites, is balanced by the presence of cations, such as Na+ and Ca2+, located between the layers and surrounding the edges. Bent may be used either in its natural form or after some physicochemical treatments, such as acid-activation, ion-exchange and calcination, according to the desired application (Breen and Watson, 1998; Lagaly and Bergaya, 2001). Acidactivated Bent powders have been used in many applications, such as adsorbents, catalysts and bleaching clay, also in the preparation of pillared clay, organoclay and nanocomposites (Hart and Brown, 2004). Some physicochemical properties of Bent, such as the crystallinity of its smectite, chemical composition, ion-exchange capacity, adsorption capacity, selectivity, surface acidity and catalytic power change, depend on the activation conditions (Komadel et al., 1990). The ﬁrst aim of this study was to investigate the variation in some of these properties, namely the chemical composition, morphology, speciﬁc surface area, pore volume and pore diameter of Bent with
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different acid activations. The second aim was to study the resultant catalytic activity of the acid-activated Bent in the esteriﬁcation of oleic acid or the FFA in Jatropha curcas oil or palm oil with methanol and to compare their catalytic activities with that of the commercial Amberlyst-15 catalytic resin.
2. Experimental 2.1. Chemicals Oleic acid (Aldrich), reﬁned palm oil (Olein Co., Ltd., Thailand), Jatropha curcas oil (Lam Soon (Thailand) Public Company Limited, Thailand), and methanol or ethanol (Merck), were used as the reactants for the esteriﬁcation reaction. H2SO4 (Merck) and HNO3 (Merck) were used for the acid activation. The commercial Amberlyst-15 in dry form (Rohm & Hass, France) was dried in an oven at 100 °C for 24 h before use.
2.2. Catalyst preparation 2.2.1. Clay source The Bent source of this study was obtained from Siam Valclay Co., Ltd. (Thailand) in the Na form, and had a chemical composition of (mass/mass) SiO2 68.5%, Al2O3 17.0%, TiO2 1.1%, Fe2O3 4.9%, Na2O 3.0%, K2O 1.3%, CaO 0.9% and MgO 2.8%. In addition, kaolin was obtained from the Industrial Mineral Development Ltd. (Thailand), and had a chemical composition of (mass/mass) SiO2 48.5%, Al2O3 38.5%, TiO2 0.9%, Fe2O3 0.7%, Na2O 0.1%, K2O 1.1%, CaO 0.1% and MgO 0.1%, as determined by X-ray ﬂuorescence spectrometry (XRF) analysis. 2.2.2. Clay acid activation The Na+-Bent sample was prepared by acid-activation with either H2SO4 at varying concentrations from 0.25 to 2.0 M or with 0.5 M HNO3. Acid-activated Bent was prepared by reﬂuxing dried Bent powder with H2SO4 or HNO3, as appropriate, at 120 °C for 1 h at a Bent:acid solution ratio of 1.0 g:30 ml. At the end of each run, the mixture was separated by centrifugation and then the harvested Bent was reﬂuxed twice more as above with a fresh sample the same acid type/concentration. Finally, the acid-activated Bent was dried in an oven at 100 °C overnight. The activated-Bent samples with 0.25, 0.5, 1.0 and 2.0 M H2SO4 were denoted as TBS-0.25, TBS-0.5, TBS-1.0 and TBS-2.0, respectively, whilst that activated with 0.5 M HNO3 was denoted as TBN-0.5. Acid-activated kaolin samples were prepared in the same way except only with 0.5 M H2SO4 or 0.5 M HNO3, and were denoted as TKS-0.5 and TKN-0.5, respectively.
2.2.3. Catalyst characterization The X-ray diffraction (XRD) pattern of each catalyst preparation was determined by a Rigaku, Dmax 2200/Ultima+ diffractometer equipped with a monochromator and Cu Kα radiation. The speciﬁc surface area was calculated using the BET method, and the pore volume and pore size distribution using the BJH method, based on the desorption data obtained by a BEL Japan BELSORP-mini 28SP adsorptometer. Philips, PW 2400 model X-ray ﬂuorescence by dispersive wavelength spectrophotometer (XRF) was used to investigate the chemical composition of clay. The morphology of the catalyst particles was identiﬁed by scanning electron microscopy (SEM) using a JSM-5410 LV. The acidity of activated Bent samples was identiﬁed the chemical band variations by using Nicolet 6700 FT-IR model, Thermo Scientiﬁc and was quantiﬁed using standard acid-base titration, where a known mass (~ 0.05 g) of the sample was added to 15 ml of 2.0 M NaCl solution and allowed to equilibrate for 30 min. Thereafter, it was titrated by the dropwise addition of 0.01 M NaOH (aq) (Mbaraka et al., 2003).
2.3. Esteriﬁcation 2.3.1. Esteriﬁcation of oleic acid The reaction was carried out in a 100 mL stainless steel autoclave reactor with a stirrer using oleic acid and methanol as starting materials with 10% (mass/mass) of catalyst (based on the reactant mass) and stirred at 200 rpm. After completion of the reaction, the used catalyst was separated from the liquid phase by centrifugation. The reaction mixture was analyzed using a Varian CP-3800 gas chromatography with a CP-8 column, following the EN 14105:2003 procedure. The content of the methyl ester yield was calculated based on the eicosane internal standard. The methyl oleate yield is reported as the mean ± 1 standard deviation (SD), derived from four independent repeats. 2.3.2. Esteriﬁcation of vegetable oil The esteriﬁcation of 85% (mass/mass) reﬁned palm oil mixed with 15% (mass/mass) oleic acid, Jatropha oil or waste cooking palm oil, was performed in a 100 mL round-bottom ﬂask equipped with a magnetic stirrer and water cooling condenser at various molar ratios of oil to methanol (1:9, 1:23, 1:30, 1:50 and 1:70), reaction times (15, 30, 45 and 60 min) and catalyst amounts (0.125, 0.25, 0.5, 1, 5 and 10% (mass/mass)). After reaction, the FFA content in the sample was determined by the titration technique following the ASTM D5555-95. The FFA conversion is shown as the mean ± 1 SD, derived from four independent repeats. 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. XRD analysis From the obtained XRD patterns (Fig. 1), the raw Bent consisted of Mt (d001-value = 1.29 nm) and a substantial amount of the crystalline quartz (Q), similar to that previously reported by Noyan et al. (2007). The crystallinity of Mt decreased as the Bent was activated with increasing concentration of H2SO4, being essentially lost after activation with 1 and 2 M H2SO4. In addition, the intensity of the quartz reﬂection decreased with increasing H2SO4 concentration. In contrast, the XRD pattern of TBN-0.5 still revealed the Mt and quartz phases that were almost the same as in the raw Bent, except that the lazulite (L) phase
Fig. 1. Representative (of four independent repeats) XRD patterns of (a) raw Bent, (b) TBS-0.25, (c) TBS-0.5, (d) TBS-1.0, (e) TBS-2.0, (f) TBN-0.5, (g) raw kaolin, (h) TKN-0.5 and (i) TKS-0.5. Mt, Q, L and K represent montmorillonite, quartz, lazulite and kaolinite, respectively.
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was found. The acid-activated kaolin samples (TKN-0.5 and TKS-0.5) both displayed the same kaolinite phase (K) as in the raw kaolin with a small amount of Mt and quartz phases. 3.1.2. Chemical composition A decrease in the RxOy% composition, where R was one of Na+, K+, Ca2+, Mg2+, Al3+ or Fe3+ cations, with increasing H2SO4 concentration was observed (Table 1). However, this was not linear, but was more marked (especially for Na+, Mg2+ and Al3+) with the initial increase from 0 to 0.25 M H2SO4 than with subsequent further increases in the H2SO4 concentration up to 2 M. This was because as the concentration of H2SO4 was increased from zero to 0.25 M, the exchangeable cations between the crystal layers were replaced by protons. The K+, Mg2+, Fe3 + and Al3 + cations were more easily dissolved from the Mt structure. As the concentration of H2SO4 was increased to 0.25, 0.5 1.0 and 2.0 M, a decrease in amount of RxOy% occurred because the H+ and –SO3H cations could be easily replaced in the Mt structure, as suggested by Onal et al. (2002) and Salem and Karimi (2009). Thus, a low residual cation level still remained in the Mt crystal layers after acid activation. The observed increase in the –SO3H was simply from the increasing concentration of H2SO4 in the activation. 3.1.3. Nitrogen adsorption and acidity of catalyst analysis The speciﬁc surface area (A) and pore volume (V) of all acidactivated Bent samples were higher than those for the raw Bent, and increased with increasing H2SO4 concentration to maximal values at 0.5 M H2SO4, and then decreased thereafter with higher H2SO4 concentration, which corresponded to the acidity of catalyst (Table 2). The maximum obtained value of A and V was due to the electrical layer around the particles, where, as the concentration of H2SO4 was increased from 0 to 0.25 M, the exchangeable Na+, K+ and Ca2+ cations at the interlayer were replaced by protons, and so the A and V values were slightly increased. Increasing the H2SO4 concentration from 0.25 M to 0.5 M caused the Mg2 +, Fe2 + and Al3+ cations to dissolve more easily from the octahedral structure and so the increase in the A and V values were larger (Salem and Karimi, 2009). That the A and V values then decreased with increasing H2SO4 concentrations above 0.5 M was because of the decomposition of the Mt structure. Thus, the concentration of H2SO4 used in the acid-activation process plays an important role in obtaining the maximum A and V values. The acidity of the samples was higher for all the acid activated-Bent samples than the raw Bent (Table 2), which is consistent with the previously reported increase in the IR spectrum of the O\H band at 2980 cm−1 (broad) after acid activation (Xinfeng et al., 2009). It may be due to exchangeable cations between layers and then H+ ions replacement after acid-activated (Komadel and Madejová, 2006). That the acidity of TBS-0.5 was higher than that for TBN-0.5 was due to the divalent proton nature of H2SO4 compared to HNO3 (1.0 vs. 0.5 M). Furthermore, the IR spectrum of TBN-0.5 showed good evident weaker O\H band than TBS-0.5 resulting in lower acidity correspond to titration result as shown in Table 2. That the acidity of TBS-0.5 was also signiﬁcantly higher than that for TKS-0.5 was because the H+ ion could be more easily replaced in the Mt layer of TBS-0.5 than in the TKS-0.5. Moreover, the acidity of the TBS-0.5 catalyst was higher than
Table 1 The chemical composition of the raw and acid-activated Bent samples. Sample
Raw Bent TBS-0.25 TBS-0.5 TBS-1.0 TBS-2.0
Mass % SiO2
68.5 58.6 42.3 38.7 36.0
17.0 13.3 10.0 9.1 8.0
4.9 4.1 3.1 2.6 2.3
1.1 1.1 0.8 0.8 0.7
2.8 1.7 1.4 1.3 1.1
0.9 0.5 0.3 0.2 0.1
3.0 0.9 0.5 0.4 0.3
1.3 1.1 0.8 0.9 0.9
0.2 18.6 40.7 45.9 45.7
Table 2 BET surface area, total pore volume, average pore diameter and the acidity of the raw and acid-activated Bent and acid-activated kaolin samples, compared to the commercial Amberlyst-15. Catalyst
BET surface area (m2/g) (A)
Total pore volume (cm3/g) (V)
Average pore diameter (nm)
Raw Bent TBS-0.25 TBS-0.5 TBS-1.0 TBS-2.0 TBN-0.5 Raw kaolin TKN-0.5 TKS-0.5 Amberlyst-15a
20 29 42 32 25 51 13 13 8 53a
0.07 0.10 0.16 0.16 0.14 0.09 0.07 0.06 0.05 0.40a
12 12 21 21 21 24 12 12 12 30a
0.2 1.6 2.5 2.2 2.1 1.1 0.6 1.1 1.2 3.0a
Data were obtained from the suppliers (Rohm & Hass, France).
TBS-1.0 and TBS-2.0 catalysts because the Mt phases of both TBS-1.0 and TBS-2.0 samples disappeared (from XRD patterns Fig. 1d and e) due to the stronger acid activation. 3.1.4. SEM analysis SEM analysis was used to probe the change in the morphological features of raw Bent and acid-activated Bent samples. From the SEM images (Fig. 2), the surface morphology of the activated Bent was clearly different to that of the raw Bent. The corrosion of the Bent surface by the acid activation resulted in a higher speciﬁc surface area (Salem and Karimi, 2009). The TBS-0.5 sample showed the best smectite layer, in terms of having an even dense array of aggregated thin sheets, followed by the TBS-1.0 and TBS-2.0 samples, which corresponds to their high speciﬁc surface area (Table 2). 3.2. Catalytic activity 3.2.1. Esteriﬁcation of oleic acid and methanol The catalytic activity of each of the acid-activated Bent catalysts was far superior to the essentially ineffective raw Bent, and so acid activation clearly increased the number of active sites of Bent (Table 3). The highest methyl oleate (MO) yield was obtained using TBS-0.5, followed by TBS-1.0. That the TBS-0.5 catalyst gave the highest MO yield is because it contained a higher acidity and speciﬁc surface area than the other catalysts. By the way, mineral cations between smectite layers were replaced by H+, which became to the acid active site in TBS-0.5 catalyst. The TBS-0.5 also exhibited a higher MO yield than TKN-0.5 and TKS-0.5. Furthermore, the TBS-0.5 provided the same maximal yield (100%) as the Amberlyst-15. Therefore, the esteriﬁcation reaction was further evaluated using TBS-0.5 and, for comparison, Amberlyst-15 as the catalysts. 3.2.2. Esteriﬁcation of vegetable oil with methanol The catalytic activity of TBS-0.5 was equal to or higher than that for the Amberlyst-15 in all tested reactions (Fig. 3). Moreover, the TBS-0.5 and Amberlyst-15 catalysts also showed higher FFA conversion than reaction without catalyst in all tested reactions. The low FFA conversion as 9%, 7%, 5% and 8% were obtained from esteriﬁcation of the mixed reﬁned palm oil and oleic acid with methanol, mixed reﬁned palm oil and oleic acid with ethanol, Jatropha oil with methanol and wasted cooking oil with methanol, respectively. For the esteriﬁcation of the mixed reﬁned palm oil and oleic acid, both catalysts yielded over a 99% FFA conversion with methanol, but whilst the yield with ethanol was lower, that obtained with TBS-0.5 was slightly higher than that with the Amberlyst-15. With respect to the esteriﬁcation of Jatropha oil or waste cooking oil with methanol, the TBS-0.5 catalyst gave a signiﬁcantly higher % FFA conversion in both cases than with the Amberlyst-15 catalyst. The comparison between methanol and ethanol
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Fig. 2. Representative SEM images (6000× magniﬁcation) of the (a) raw Bent, (b) TBS-0.25, (c) TBS-0.5, (d) TBS-1.0, (e) TBS-2.0 and (f) TBN-0.5.
in the esteriﬁcation of the mixed palm oil and oleic acid was performed in order to study the stearic effect of the alcohol structure. As expected and mentioned above, a higher % FFA conversion (about 12%) was
obtained with methanol than with ethanol, and this is likely to be due to the sterical hindrance of ethanol that impeded or limited the access of reactants to active site (Stavarache et al., 2005).
Table 3 Catalytic activity of different clays and acid-activated clays for the esteriﬁcation of oleic acid and methanol at a 9:1 methanol: oleic acid molar ratio, 60 °C, 3 h and 10% (mass/mass) catalyst. Catalyst
Methyl oleate yield (mass%)a
Non Raw Bent TBS-0.25 TBS-0.5 TBS-1.0 TBS-2.0 TBN-0.5 Raw kaolin TKN-0.5 TKS-0.5 Amberlyst-15
5 + 0.34 8 + 0.28 85 + 0.49 100 + 0 96 + 0.70 90 + 0.84 80 + 0.91 7 + 0.91 80 + 0.77 23 + 0.98 100 + 0
Data were shown as the mean + 1SD, derived from independent repeats.
Fig. 3. Comparison of the catalytic activity of TBS-0.5 and Amberlyst-15 in the esteriﬁcation of different oils at a methanol:oil molar ratio of 23:1, in a 60 °C, 1 h reaction with 10% (mass/mass) catalyst (based on total mass of reactants).
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3.2.3. Optimal reaction condition for esteriﬁcation 188.8.131.52. Oil to methanol molar ratio. The oil:methanol molar ratio in the esteriﬁcation reaction was found to signiﬁcantly affect the FFA conversion obtained in a 1 h reaction (Fig. 4). The stoichiometric molar ratio for the esteriﬁcation reaction was 1:1 (methanol:FFA), but in practice an excess amount of alcohol was needed in order to shift the reaction equilibrium to the formation of the fatty acid methyl ester products. At a molar ratio of methanol:oil of 9:1, the % FFA conversion was 82%, and this was increased to 99% by increasing the molar ratio to 23:1. Further increases in the methanol:oil molar ratio above 23:1 slightly decreased the resultant % FFA conversion, presumably since the excess methanol might reduce the catalytic efﬁciency of the sulfonic group. 184.108.40.206. Reaction time. The relationship between the obtained % FFA conversion and the reaction time was evaluated at a methanol:oil molar ratio of 23:1. The reaction time had a clearly marked effect upon the esteriﬁcation efﬁciency (Fig. 4), where it was clear that a reaction time of 15 to 45 min was insufﬁcient to complete the esteriﬁcation of the acidiﬁed oil at 60 °C. At a reaction time of 60 min (longest tested time) the % FFA conversion reached about 99%, and so under these conditions the reaction was essentially completed within 60 min. 220.127.116.11. Catalytic amount. The effect of varying the amount of the catalyst revealed that the reaction efﬁciency increased considerably with increasing amounts of the TBS-0.5 (Table 4). However, whilst dramatic increases in the reaction efﬁciency were noted with increases in the catalyst level from 0.125 to 0.5% (mass/mass) (21 to 82% FFA conversion), further 5- or 10-fold increases in the catalyst level from 1% to 5% or 10% (mass/mass) resulted in a less marked increase in the yield of biodiesel. Nevertheless, to obtain the commercially desired N99% FFA conversion a 10% (mass/mass) catalyst addition was required. The TBS-0.5 catalytic activity was also determined from the % FFA conversion with respect to the number of acidic sites, in terms of the turnover number (TON). Here, the highest TON at 309 was obtained with 0.25% (mass/mass) of TBS-0.5 (Table 4). Therefore, a small amount of catalyst exhibited the highest effectiveness of the TBS-0.5 active sites. 4. Conclusion This work indicated that Bent could be considered as a promising support material for catalyst preparation, by simple acid activation, for the esteriﬁcation of FFA and oil with a high acid content. This provides an alternative route for the production of sustainable fuels because Bent is natural clay, and this method had no need for any sophisticated modiﬁcation methods. The simple acidic catalyst preparation was
Fig. 4. The effect of the methanol:oil molar ratio and the reaction time on the efﬁciency of the esteriﬁcation reaction of the mixed palm oil and oleic acid with methanol at 60 °C and with 10% (mass/mass) TBS-0.5 as the catalyst.
Table 4 The obtained % FFA conversion and TON using varying amounts of TBS-0.5 catalyst at a 23:1 methanol: acidiﬁed oil molar ratio, 60 °C and 1 h reaction time. Amount of catalyst (mass%)
% FFA conversiona
0 0.125 0.25 0.5 1 5 10
9 + 0.26 21 + 0.98 57 + 0.77 82 + 0.49 94 + 0.35 96 + 0.70 99 + 0.14
– 224 309 220 126 25 13
Independent repeat data were reported as the derived mean + 1SD.
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