Biodiesel production using heterogeneous catalysts

Bioresource Technology 102 (2011) 2151–2161

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Review

Biodiesel production using heterogeneous catalysts Surbhi Semwal a, Ajay K. Arora b, Rajendra P. Badoni a, Deepak K. Tuli b,⇑ a b

College of Engineering, University of Petroleum & Energy Studies, Dehradun 248007, India Research & Development Centre, Indian Oil Corporation Limited, Sector-13, Faridabad 121007, India

a r t i c l e

i n f o

Article history: Received 5 May 2010 Received in revised form 21 September 2010 Accepted 18 October 2010 Available online 23 October 2010 Keywords: Biodiesel Homogeneous catalyst Heterogeneous catalyst

a b s t r a c t The production and use of biodiesel has seen a quantum jump in the recent past due to benefits associated with its ability to mitigate greenhouse gas (GHG). There are large number of commercial plants producing biodiesel by transesterification of vegetable oils and fats based on base catalyzed (caustic) homogeneous transesterification of oils. However, homogeneous process needs steps of glycerol separation, washings, very stringent and extremely low limits of Na, K, glycerides and moisture limits in biodiesel. Heterogeneous catalyzed production of biodiesel has emerged as a preferred route as it is environmentally benign needs no water washing and product separation is much easier. The present report is review of the progress made in development of heterogeneous catalysts suitable for biodiesel production. This review shall help in selection of suitable catalysts and the optimum conditions for biodiesel production. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Bio-based alternative fuels such as ethanol, biodiesel have been in focus for the reasons which are by now well understood. Heavy consumption of fossil resources, effect on global warming and concerns of energy security are main drivers for growth of biofuels. Recent studies on life cycle analysis (LCA) of biodiesel have shown a very appreciable reduction of greenhouse gas (GHG) by their use as a blend component of transport fuel. Biodiesel produced by transesterification of vegetable oils and animal fats using homogeneous base catalyst (Fig. 1) has seen several folds increase in last few years for their commercial production and use as a blending component in transport fuels. Fatty acid methyl esters (FAME) have found favour for use as a blend component of petro-diesel fuel due to lack of aromatics, negligible sulfur content, higher lubricity and very high cetane values (Dorado et al., 2003). FAME (Biodiesel) mixes freely in all proportions with petro-diesel and its use has been approved by almost all the major automotive manufactures. Biodiesel can be used in conventional compression ignition engines, which need almost no modification. Though biodiesel has been approved for use in automotives as a blend with normal petro-diesel, there are very stringent quality norms prescribed by several countries. ASTM specifications listed and detailed by Sarin et al. (2007), which any biodiesel must meet before it can be used as an auto fuel component. There are very low limits on Na/K, organic/inorganic acids,

⇑ Corresponding author. Tel.: +91 129 22 94 273; fax: +91 129 22 85 340. E-mail address: [email protected] (D.K. Tuli). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.10.080

phosphorous, glycerides and water content. Therefore, biodiesel production processes need to have in-built capability to meet these specifications. The most widely used industrial method for the commercial production of biodiesel from vegetable oils/fats is a base catalyzed transesterification process using KOH or NaOH as the homogeneous catalyst and MeOH as the lower alcohol (Fig. 1). The advantage of this process is production of methyl esters at very high yields under mild conditions and reaction generally takes about an hour for completion (Meher et al., 2006). Several oils, both edible and non-edible such as sunflower oil (Arzamendi et al., 2008), palm (Li and Xie, 2006) and jatropha (Tiwari et al., 2007) have been transesterified for biodiesel production. However, major quality related problems were encountered and it was main hindrance for large scale industrial production of biodiesel by homogeneously catalyzed transesterification. Production costs were rather high (Ma and Hanna, 1999) as the process involved number of washing and purification steps in order to meet the stipulated quality. It was quite difficult to remove the traces the K/Na remaining in the product and separation of glycerin also posed technical challenges. The higher amount of water used in washing and consequent treatment of the resulting effluent added to the overall process cost. Any commercial biodiesel plant must have the inbuilt capability to handle a variety of different feedstocks which may differ very widely in quality. The vegetable oils may be from edible sources, non-edible sources, waste cooking oils, animal fats, algae, fungi etc. (Canakci, 2007; Granados et al., 2007; Ji et al., 2006; Karmee and Chadha, 2005). In Europe and US, the primary sources for producing biodiesel are edible oils like rapeseed, sunflower, and soybean. In countries

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S. Semwal et al. / Bioresource Technology 102 (2011) 2151–2161

ROCOR' H2C OCOR' HC OCOR'' H2C OCOR''' triglyceride

catalyst 3 ROH

ROCOR'' ROCOR'''

alcohol

mixture of alkyl esters (FAME)

H2C OH HC OH H2C OH glycerol

R', R'', R''' = Hydrocarbon chain ranging from 15 to 21 carbon atoms Fig. 1. Transesterification of vegetable oil.

like India, non-edible oils like jatropha and karanjia are being promoted on a very large scale, as these can be grown on marginal and waste lands (Azam et al., 2005). Several other non-edible seeds like Guizotia abyssinica (Sarin et al., 2009a) have also been evaluated for their biodiesel potential. The conventional biodiesel production process of base catalyzed homogeneous transesterification face difficulties to handle multiple feed stocks. Oils (nonedible) with higher fatty acid content lead to formation of soap and consequent loss of oil and problems of product separation (Kwiecien et al., 2009). Due to these issues a large number of alternative methods were developed. These include supercritical process (Minami and Saka, 2006) and enzymatic process (Shimada et al., 2002). Supercritical process is also one of the promising methods for biodiesel production as this process is very fast and is carried out without catalyst. Some production plants in Europe use this technology, but due to high temperature and pressure requirement of this process, it translates to higher capital costs and that restricts its commercial utilization. Sharma et al. (2006) explored a single pot process for transesterification of jatropha oil. Enzyme based transesterification is also one of the option for biodiesel production and is generally carried out at moderate temperature with high yields. Lipase enzymes (used with different supports by immobilization or encapsulation etc.) are used for transesterification reaction (Caballero et al., 2009; Macario et al., 2007, 2009). This process can tolerate free fatty acid and water without soap formation and thereby making separation of biodiesel and glycerol easier. Enzyme cost and its deactivation due to feed impurities are major hindrance for commercial viability of this process (Dizge et al., 2009). Biodiesel synthesis using solid catalysts instead of homogeneous liquid catalyst could potentially lead to economical production costs because of reuse of the catalyst (Suppes et al., 2004) and offer the possibility for carrying out both transesterification and esterification simultaneously (Furuta et al., 2004). Additional benefit with solid based catalyst is the lesser consumption of catalyst. As per studies, for production of 8000 tonnes of biodiesel, 88 tones of sodium hydroxide may be required (Mbaraka and Shanks, 2006), while only 5.7 tonnes of solid supported MgO is sufficient for production of 100,000 tonnes of biodiesel (Dossin et al., 2006). One disadvantage with use of solid catalyst is the formation of three phases together with oil and alcohol, which leads to diffusion limitations thus decreasing the rate of the reaction (Mbaraka and Shanks, 2006). This mass transfer difficulty is overcome by using a co-solvent such as tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), n-hexane and ethanol, which assist miscibility of oil and methanol leading to increase in the rate of reaction. Use of catalyst supports, which can provide more specific surface area and pores for active species, where they can anchor and react with large triglyceride molecules is another solution for encountering the poor mass transfer (Zabeti et al., 2009). Some researchers (Di Serio et al., 2008) have reviewed the significance of solid catalysts for

biodiesel production. However, till date few detailed kinetic studies and the mechanism of acid and base catalysts using solid catalysts have been reported in the literature (Furuta et al., 2004; Suppes et al., 2004). Chemistry of heterogeneous catalyst reported, includes metal hydroxides (Dalai et al., 2006), metal complexes (Abreu et al., 2003), metal oxides such as calcium oxide (Granados et al., 2007), magnesium oxide (Wang and Yang, 2007), zirconium oxide (Jitputti et al., 2006), zeolites, hydrotalcites and supported catalysts (Xie and Huang, 2006). These types of catalysts have been investigated as solid catalysts which overcome some of the drawback on use of homogeneous catalysts. The order of activity among alkaline earth oxide catalysts was observed to be BaO > SrO > CaO > MgO (Cantrell et al., 2005). The present review discusses the use of acid, base, acid–base solid catalysts such as metal oxides, supported catalysts and zeolites etc., and enzymatic catalysts for biodiesel synthesis. This review paper presents a comparative description of continuous biodiesel production processes, through transesterification reaction using acid, base, acid–base heterogeneous catalysts and enzymatic catalysts so that proper catalyst and optimum reaction conditions can be selected.

2. Heterogeneous catalysts 2.1. Basic solid catalysts Various basic metal oxide type catalysts have been reported in literature for biodiesel synthesis. Some of the high performing catalyst preparations and their application in biodiesel synthesis are summarized here. Liu et al. (2007) studied SrO metal oxide for transesterification of soybean oil. Catalyst preparation was carried out by calcinations of SrCO3 at 1200 °C for 5 h. SrO has strong basicity H = 26.5 and have BET surface area of 1.05 m2/g. The conversion obtained was 95% at temperature of 65 °C, catalyst content of 3 wt.%, molar ratio of methanol to oil of 12:1 and reaction time of 30 min. Further, biodiesel yield was only slightly reduced when the SrO catalyst is subsequently reused for 10 cycles. Mechanism given by authors is as described in Fig. 2. The main step is formation of ionic complex by SrO with methanol. Liu et al. (2008) studied transesterification of soybean oil to biodiesel using CaO as a solid catalyst. The BET surface area of the catalyst was 0.56 m2/g. The reaction was carried out using 12:1 M ratio of methanol to oil, 8 wt.% catalyst concentration at 65 °C. Biodiesel yield (95%) was obtained when reaction was carried out for 3 h. The authors also reported comparative activity of CaO with K2CO3/cAl2O3 and KF/cAl2O3 catalysts. Preparation of these catalysts was carried out by an impregnation method with the help of aqueous solution of potassium carbonate/potassium fluoride and then calcination of impregnated catalysts at 550 °C for 5 h. It was observed that CaO maintained sustained activity for longer time (20 cycles) after repeated use and biodiesel yield was also not affected, while K2CO3/cAl2O3 and KF/cAl2O3 catalysts were not able to maintain activity and biodiesel yield also got affected after every use. This was because that the alkali metal compounds dissolved in methanol, which reduced the active ingredients and thereby decreasing biodiesel yield in the subsequent experiments. It was also observed in this study that the presence of water, if in small amount of about 2.8 by wt.% of soybean oil, act as promoter, but if amount of water increases (more than 2.8 by wt.% of soybean oil) it hydrolyzed FAME under basic conditions and also induced soap formation. The catalytic activity of activated calcium oxide was also evaluated by Granados et al. (2007) for production of biodiesel by

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CH3-OH CH3O H Sr O

Sr O O

CH3O H Sr O

R1 OR

R1

OCH3 O OR

R1

H Sr O

OCH3 O OR

CH3OH

CH3O R1 O ROH

R1

OCH3 O OR

H Sr O

CH3O R1 O ROH

Sr O

CH3O R1 O ROH

CH3O

OCH3 R1

ROH O

Fig. 2. Mechanism of SrO catalyst transesterification.

transesterification of sunflower oil in batch reactor at 13:1 methanol to oil molar ratio, 3 wt.% catalyst content at 60 °C. Under these conditions, reaction was complete in 100 min giving 94% conversion. The specific surface area of catalyst was 32 m2/g and mean pore size diameter (MPS) was approx. 25–30 nm. The authors observed poisoning of active surface site of CaO by the atmospheric H2O and CO2. Therefore, to improve catalytic activity of CaO, it was subjected to an activation treatment at high temperature (P700 °C) before the reaction and as a result of this, the main poisoning species (the carbonate group) from the surface was removed. When catalyst was activated at high temperature, some leaching of the active species was observed. However, leaching amount did not result in significant reduction of catalyst activity and the catalyst was reusable for 8 cycles. However, yield of FAME reduced from more than 90% in the first cycle to 80% in the second cycle and thereafter the performance stabilized. Veljkovic et al. (2009) described the kinetics of CaO heterogeneously catalyzed methanolysis of sunflower oil. The optimal CaO calcination temperature was 550 °C. They observed 98% yield in the transesterification with 6:1 M ratio of sunflower oil to methanol, 1 wt.% catalyst (based on oil wt.) at 60 °C within 2 h reaction time. Sarin et al. (2009b) reported use of seashell and eggshells as a catalyst for production of biodiesel from various feed stocks such as jatropha, castor, sunflower, soybean, rapeseed, cotton, corn, coconut oils etc., in a batch and continuous reactor. The catalyst combination of seashell and eggshells contained between 10–90% and 90–10% respectively. The reaction was performed using 1 mol of vegetable oil and 6 mol of methanol and 4 wt.% of catalyst composition. 98% conversion was achieved within 2 h. Kawashima et al. (2009) studied catalytic activity of calcium oxide (CaO) as a heterogeneous catalyst for biodiesel production by the transesterification of rapeseed oil. The author pretreated CaO with methanol for activation. CaO was activated with methanol at 25 °C for 1.5 h so that small amount of CaO could be converted into Ca(OCH3)2, which exhibits a higher catalytic activity than non-activated CaO. Rapeseed oil was thus transesterified using Ca(OCH3)2 to produce FAME and glycerine. During the transesterification reaction, the produced glycerin reacted with CaO at 60 °C, and a CaO-glycerin complex was formed as secondary

catalyst, which then accelerated the transesterification reaction. While this CaO-glycerin complex exhibited a high catalyst activity, the reaction advanced further and generated more glycerin. To determine the exact pattern of catalytic activity, XRD measurements of activated CaO, non-activated CaO, Ca(OH)2, and Ca(OCH3)2 were performed and it was observed that XRD spectrum of activated CaO was similar to that of non-activated CaO but exhibiting small diffraction peak attributed to Ca(OCH3)2 and Ca(OH)2. This was responsible for the observed differences in the catalytic activity and the basic strengths of non-activated CaO, Ca(OH)2. The activated CaO has basic strength in the range of 10.1–11.1. While Ca(OCH3)2 had a high basic strength in the range of 11.1–15.0 and these results explained the reasons why Ca(OCH3)2 exhibited a higher catalytic activity for the transesterification reaction than CaO and Ca(OH)2. Kouzu et al. (2008) studied CaO catalyst for transesterification of soybean oil at 12:1 M ratio of methanol to oil at 500 rpm and at reflux temperature for 2 h in glass batch reactor and achieved 93% biodiesel yield. CaO was obtained after calcination of pulverized lime stone at 900 °C for 1.5 h. Calcium diglyceride and calcium methoxide were used as reference samples. Further on comparison it was observed that the BET surface area of fresh CaO is 13 m2/g whereas surface area of CaO collected after conversion was 11 m2/g. While the BET surface area of reference samples such as calcium diglyceroxide and calcium methoxide were 11.3 m2/g and 44 m2/g, respectively. Catalytic activity of calcium based metal oxides such as CaTiO3, CaMnO3, Ca2Fe2O5, CaZrO3 and CaO–CeO2 in the methanolysis of rapeseed oil was studied by Kawashima et al. (2008). The authors also studied the change of activity on replacement of Ca with barium, magnesium, or lanthanum. The reaction was carried out in a batch reactor at 60 °C with 6:1 M ratio of methanol to rapeseed oil for 10 h, resulting in yield of 79–92%. It was found that CaZrO3 and CaO–CeO2 show high durability, ester yields greater than 80% and has the potential to be used in biodiesel production processes as heterogeneous base catalysts. For synthesis of CaTiO3, an equimolar mixture of TiO2 and CaCO3 was milled in an agate mortar then mixture calcined in air to 500 °C and subsequently at 1050 °C for 2 h. For preparing Ca2Fe2O5, Fe2O3 and CaCO3 were milled with molar ratio of 1:2 and calcined in air to 900 °C and then

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at 1050 °C for 4 h. Due to calcination step at high temperature, the surface area of each of the catalysts was small and varied from 7.7 m2/g for MgCeO3 to 0.71 m2/g for Ca2Fe2O5. The basic strengths of CaTiO3 were in the range of 6.8–7.2. CaMnO3, Ca2Fe2O5, CaZrO3, and CaCeO3 showed the highest basic strength, while Ba, Mg, and La series catalysts had weaker basic strength. Hence, Ca series catalysts exhibit the high catalytic activity for the transesterification reaction. MgO-catalyzed transesterification reaction, at industrially relevant conditions was reported by Dossin et al. (2006) in batch and continuous stirred tank reactors. A kinetic model based on the three steps ‘Eley–Rideal’ type mechanism assuming methanol adsorption as rate-determining step was proposed. Two processes were simulated, first for transesterification of ethyl acetate with methanol in a batch slurry reactor and second, transesterification of triolein with methanol to form methyl oleate in a continuous slurry reactor and results were used to simulate biodiesel production from rapeseed oil. In a continuous stirred reactor volume of 25 m3 containing 5700 kg of MgO catalyst continuous production of 100,000 tonnes of biodiesel per year can be achieved. These results were compared by the author with homogeneously catalyzed transesterification processes. K2CO3 supported on MgO catalyst was prepared by mixing K2CO3 and MgO as carrier in a mortar. The mixture calcined at 600 °C for 3 h, thus forming catalyst for synthesis of biodiesel from soybean oil with the yield of 99.5% (Liang et al., 2009). These results indicate that carriers increased the reaction yield and basic carriers have higher activities than acidic carriers. The catalytic activity of the K2CO3/MgO was higher than that of K2CO3 due to the interaction between K2CO3 and MgO and because of the high degree of dispersion of the active sites on the surface of MgO. The maximum activity of catalyst was obtained when the loading ratio was 0.7 and after 2 h reaction time the maximum conversion was achieved in transesterification when operating parameters are set at 70 °C, 6:1 M ratio methanol to oil with 50 mg (0.01 wt.% of oil) catalyst. The MgO supported K2CO3 catalyst was most efficient among all the catalyst from different carriers. After 6 cycles, the catalytic activity decreased minutely but activity was regained after calcination. The loss of the active sites on the catalyst was also investigated. Transesterification of different edible and non-edible oils (such as sunflower, soybean, ricebran and jatropha) using Mg/Zr catalyst (catalyst ratio 2:1 wt/wt.%) have been reported by Sree et al., 2009. Mg/Zr was prepared by co-precipitation method by dissolving Mg(NO3)2 and ZrO(NO3)2 in deionised water. pH was controlled at 10 by mixing of two precursors like KOH and K2CO3. The precipitate was filtered then washed and calcined at 650 °C for 4 h. The XRD results indicated that ZrO2 was in tetragonal phase, while MgO was in rocksalt form. The catalyst showed small Zr and large Mg crystallite sites, making Zr strongly interacted with MgO. However the high transesterification activity of Mg/Zr catalyst might be due to the presence of higher number of total basic sites. Total basicity of catalyst was 1204 lmol/g while surface area was 47 m2/g. The transesterification reaction was carried out at 65 °C with a 53:1 M ratio of methanol to oil and a catalyst amount of 0.1 g (0.1 wt.% of oil), to achieve the conversion of about 98% in 50 min. Due to higher number of total basic sites; the high transesterification activity of catalyst was achieved. Insignificant decrease of yield up to 5% was observed during transesterification of sunflower oil after fourth cycle. Samart et al. (2009) utilized 15 wt.% KI loaded on mesoporous silica as a solid base catalyst for transesterification of soybean oil with optimum reaction conditions of 16:1 methanol to oil ratio, 5 wt.% catalyst at 70 °C in 8 h. Conversions of 90% were obtained. The maximum activity of catalyst was obtained when KI solution got impregnated on mesoporous silica by incipient wetness

impregnation with concentration of 15 wt.%. The X-ray diffraction (XRD) patterns of KI/mesoporous silica after calcinations showed that the characteristic peaks of potassium oxide (K2O) face-centered cubic crystal at 2h equal 25.3°, 41.9°, 51.9°, and 66.9°, while the characteristic peaks of silicate hydrate phase (SiO2 x H2O) is at 21.8° and 35.7° but there was no characteristic peak of KI in the XRD pattern because all of KI phases were transformed into K2O phase. Alumina-supported potassium iodide catalyst was applied for biodiesel synthesis from soybean oil. The catalyst was prepared by impregnation of powdered alumina with an aqueous solution of KI, 35 wt.% KI loaded on Al2O3 and calcined at 500 °C for 3 h has best catalytic activity and highest basicity (1.5607 mmol/g) (Xie and Li, 2006). The catalyst activity was dependent on strength of basic sites as well as upon their amount. On comparison of alumina loaded with KI, KF, KOH, K2CO3, KBr and KNO3, the order of conversion reported by the authors was KI/Al2O3 > KF/Al2O3 > KOH/Al2O3 > KNO3/Al2O3 > K2CO3/ Al2O3 > KBr/ Al2O3. NaX zeolite loaded with 10% KOH (KOH/NaX) was reported as a base catalyst in soybean oil transesterification performed by Xie et al. (2007). NaX zeolite was first dried at 110 °C for 2 h then impregnated with aqueous solution of KOH for 24 h followed by drying and by heating at 120 °C for 3 h. The reaction was performed at reflux temperature (65 °C), 10:1 M ratio of methanol to oil and 3 wt.% catalysts. 85.6% conversion was achieved within 8 h. The results obtained by X-ray diffraction analysis showed striking similarity in XRD pattern between KOH/NaX samples and parent zeolites. Further, it was observed by SEM results that NaX zeolite and KOH/NaX catalysts have nearly spherical shape crystal with size of 2–4 lm. The basic strengths of the catalyst as observed was 15.0 < H_ < 18.4 and it’s very likely that higher percentage of KOH (>10%) resulted in agglomeration of active sites and hence lowering the surface areas for active components and resulting lower the catalytic activity. The regenerated catalyst provides the conversion rate of 84.3%. Faria et al. (2008) utilized tetramethylguanidine, which is covalently bonded onto silica gel surface (SiG), as solid catalyst for transesterification of soybean oil with methanol. SiG catalyst was prepared by suspending activated silica gel in dry xylene and the new agent silylant (Silylant was prepared by reacted SiCl with triethylamine). The SEM image showed that SiG catalyst particles had spherical morphology with average size of about 1 lm. It was also observed that in SiG catalyst covalent immobilization of tetramethylguanidine onto the silica gel surface existed, which was confirmed by FTIR, 29Si and 13C NMR spectrums. The pore size and BET surface area was 8.78 ± 0.73 nm and 216.14 ± 34 m2g 1 respectively. The reaction was performed with 1.5 g methanol, 10 g oil and catalyst content of 0.5 g (0.05 wt.% of oil) at 80 °C and after 3 h, 86% of soybean oil was converted to biodiesel. The yield decreased continuously from 86% to 62% after catalyst is recycled for 9 times due to activity drop by loss of the weakly attached tetramethylguanidine to the silica surface. Georgogianni et al. (2009a) studied conversion of used soybean frying oil over Mg MCM-41, Mg–Al hydrotalcite and K+ impregnated zirconia catalysts and found that the Mg–Al hydrotalcite has the greater activity due to higher basicity. After 25 h under operation conditions of 60 °C, 5 g oil, 65 ml methanol and 0.5 g of catalyst (0.1 wt.% of oil), 97% of oil was converted to biodiesel. Mg/Al hydrotalcite catalyst was synthesized by using mixture of Mg(NO3)26H2O, Al(NO3)39H2O and (NH4)2CO3 at 65 °C for 1 h. The pH was controlled at 5 by concentrate ammonia solution. The mixture was agitated for 3 h at 65 °C and precipitate was filtered, dried and the calcined at 500 °C for 3 h. Georgogianni et al. (2009b) also tested Mg MCM-41, Mg–Al hydrotalcite and K+ impregnated zirconia catalysts and found that

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Mg–Al hydrotalcite was more active catalyst due to its highly basic nature for transesterification of rapeseed oil giving quantitative yield. MCM-41 also gave high yields of methyl esters in the transesterification reaction (conversion 87%). Authors also compared these results with that of homogenous catalyst (NaOH) in the transesterification reaction under identical reaction conditions and found that the homogenous catalyst accelerated the transesterification reaction significantly and gave the equivalent conversion only within 15 min. KF/ZnO catalyst has been reported as solid base catalyst for the transesterification of palm oil with methanol to produce biodiesel (Hameed et al., 2009). The KF/ZnO was synthesized by impregnation of the ZnO support with aqueous of KF2H2O followed by overnight drying at 110 °C and calcination at 600 °C for 5 h. A loading of 35 wt.% of KF was done on ZnO and good conversions were achieved. Yan et al., 2009 studied the ZnO-La2O3 catalyst for transesterification of unrefined and waste oil. ZnO-La2O3 was prepared by homogeneous co-precipitation method where 2 M Zn(NO3)2 and 1 M La(NO3) solutions were prepared in distilled water. These solutions, with various Zn–La ratios, were mixed with a 2 M urea solution and the resulting mixture calcined at 450 °C for 8 h. The catalyst with 3:1 ratio of Zinc to lanthanum was found to exhibit highest activity in the transesterification of unrefined or waste oil. A strong interaction between Zn and La species was observed with enhanced catalytic activities. The catalyst was active in both transesterification and esterification reactions, and with no hydrolytic activity. Sr(NO3)2/ZnO catalyst has been reported by Yang and Xie, 2007 for soybean oil transesterification with methanol at 65 °C. Sr(NO3)2/ZnO was prepared by an impregnation method with an aqueous solution of an alkaline earth metal nitrate and calcined at 600 °C for 5 h. The optimum catalytic activity was obtained by loading 2.5 mmol of strontium nitrate Sr(NO3)2/g on ZnO. The basicity of catalyst was 10.8 mmol/g and 94.7% soybean oil conversion was achieved after 5 h with 5 wt.% of Sr(NO3)2/ZnO and 12:1 methanol/oil molar ratio. However, Sr(NO3)2/ZnO after recovery exhibited lower catalytic activity with a conversion of soybean oil of 15.4% and it’s basicity decreased from 10.32 to 6.79 mmol/ g. This was due to decomposition of reactants and products on the active sites and their interactions during the reaction. But the catalytic activity of reused catalyst was regained by impregnating it in an aqueous solution of Sr(NO3)2. They also observed that the co-solvent such as dimethyl sulfoxide (DMSO), n-hexane and tetrahydrofuran (THF) overcomes the mixing problems of transesterification system and among them THF was the most effective cosolvent which increased the conversion rate of soybean oil up to 96.8%. The activity and selectivity of NaOH/c-Al2O3 catalyst for the transesterification of sunflower oil with methanol was investigated by Arzamendi et al. (2007). NaOH/cAl2O3 was synthesized by incipient wetness impregnation of the 212–300 lm size fraction of alumina. Prior to use alumina support was calcined at 500 °C for 12 h. Subsequently, the required amount of NaOH solution were slowly added to the support, dried for 12 h at 120 °C followed by calcination at 400 °C for 12 h. NaOH contents of the final solids were 10.7 and 19.3 wt.% and this was referred as 10-Al and 19-Al respectively. XRD analysis revealed that a calcined and non-calcined 19-Al catalyst was very similar with the presence of NaOH and Na2O2 diffraction peaks, both as hydrated compounds, as well as sodium aluminate (NaAlO2). These results indicate that NaOH has reacted with the support giving rise to the formation of aluminate. For calcined sample, the reaction was performed at 50 °C, 12:1 M ratio of methanol/oil with 19-Al at 0.4 wt.% of NaOH which gave the conversion rate of about 86% for 24 h. While the conversion for the non-calcined 19-Al sample increased up to 99%. Thus

the result indicated that calcination of NaOH/cAl2O3 catalyst had a negative effect on their activity. Benjapornkulaphong et al. (2009) compared the catalytic performance of Al2O3–supported alkali, alkali earth metal oxides and effect of calcination temperature on activity of different catalyst for transesterification of palm kernel oil and crude coconut oil with methanol. They found that Ca(NO3)2/Al2O3 calcined at 450 °C was the most suitable catalyst giving 94.3% conversion, however when the calcination temperature was increased the methyl ester formation dropped due to the formation of inactive metal aluminates. On the other hand NaNO3/Al2O3 and KNO3/Al2O3 improved methyl ester formation tendency at the calcination temperature of above 550 °C but LiNO3/Al2O3 catalyst was active with conversion of 91.6% at 450 °C calcination and 93.4% at 550 °C. Mg(NO3)2/Al2O3 catalyst was not active at any calcination temperature (conversion 10.4% at 450 °C). These catalysts were prepared by the incipient wetness impregnation of aqueous solution of the corresponding metal salt precursors (nitrate salt of alkali and alkali earth metals) on an aluminum oxide support followed by calcination at 450 °C for 2 h. XRD results indicated that after calcination at 450 °C of Ca(NO3)2/Al2O3 catalyst, mainly CaO species were formed. But when the calcination was performed at higher temperature, crystalline CaO peak decreased. After 3 h of reaction time, at 60 °C with 65:1 M ratio of alcohol/oil and 10 wt.% catalyst content, the maximum conversion achieved was 94.3% from palm kernel oil whereas only 85% conversion was obtained in the case of crude coconut oil due to high acid value and moisture content of crude coconut oil than palm kernel oil. But when catalyst amount was increased from 15 to 20 wt.%, the conversion of crude coconut oil also increased from 94% to 99.8%. Results of experiment are tabulated in Table 1. Kumar et al. (2010) reported an effective catalyst composition which contained major amount of nickel zinc aluminate supported on clay and alumina. On a continuous reactor system, this catalyst gave conversion in the range of 40–60% at 200 °C and 40 bar pressure. Abdullah et al., 2009 reported use of SBA-15 as a neutral material made up of Si-O-Si network as a catalyst for biodiesel production. The incorporation of potassium into mesoporous SBA-15 (K/ SBA-15) imparted basicity to make it suitable for base catalyzed reaction like transesterification of palm oil. The authors used a composite experimental design for optimization of biodiesel yield so that this mathematical model could predict the biodiesel yield at any point of time in the experimental domain as well as the determination of the optimal biodiesel conditions with sufficient degree of accuracy. K/SBA-15 was prepared by impregnating the high surface area material (mesoporous SBA-15) with 20 wt.% KOH solution for 24 h. After that, catalyst was dried followed by calcination at 350 °C for 3 h. The specific surface area of the catalyst was high and has relatively easy diffusion of reactants in the mesopores and thereby allowing the use of the mesoporous catalyst at high temperature without suffering major structure modifi-

Table 1 Methyl ester content from palm kernel oil over supported metal oxides calcined at different temperature (**Reaction conditions: 60 °C, 10 wt.% of catalyst amount, methanol: oil molar ratio 65:1 and 3 h of reaction time). Catalyst

Optimum calcination temperature (°C)

Methyl ester content (wt.%)**

Al2O3 LiNO3/cAl2O3 NaNO3/cAl2O3 KNO3/cAl2O3 Mg(NO3)2/cAl2O3 Ca(NO3)2/cAl2O3

450 550 650 550 450 450

0 93.4 95.1 94.7 10.4 94.3

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cation. 93% conversion was achieved in 5 h at 70 °C with methanol/ oil molar ratio of 11.6 and catalyst content of 3.91 wt.%. 2.2. Acidic solid catalysts Di Serio et al. (2007) investigated application of vanadyl phosphate (VOP) as catalyst in the transesterification of soybean oil. They found that the catalyst was active in transesterification reaction with 80% methyl ester yield obtained only after 1 h reaction time even though the specific surface area of catalyst was low (2–4 m2/g). Vanadyl phosphate was prepared from the suspension of V2O5 in diluted phosphoric acid and then calcined at 500 °C for 2 h. The catalytic activity was increased by the increasing the calcination temperature, which helped in removing the hydration water of the sample and thus, increasing the concentration of the coordinatively unsaturated VO group and resulted in increased Lewis acidity of solids. XRD results indicated that uncalcined VOPO42H2O has crystallographic a form of peaks while another crystallographic aII peaks was observed in the case of calcined VOPO4 (VOP500). Shu et al. (2009) studied carbon based solid acid catalyst for transesterification of cottonseed oil with methanol. The carbon based solid acid catalyst was prepared by the sulfonation of carbonized vegetable oil asphalt. The catalyst was prepared from carbonized vegetable oil asphalt and concentrated H2SO4 solution. Both these ingredients were heated at 210 °C in an oil bath for 10 h. The suspension was diluted by de-ionized water and dried at 120 °C for 4 h to obtain the sulfonated vegetable oil asphalt catalyst. The authors also compared transesterification efficiency of asphalt-based catalyst with sulfonated multi-walled carbon nanotubes (s-MWCNTs) and observed that the asphalt-based catalyst showed higher activity than the s-MWCNTs with 90% conversion. This is because of high Bronsted acid site density (2.21 mmolg 1), loose irregular network and large pores (43.90 nm), which provide more acid sites for the reactants. The low surface area (7.48 m2 g 1) and high SO3H density (2.21 mmol g 1) of the dry sulfonated carbon catalysts indicated that most of the SO3H groups were in the interior of the catalyst. The sulfonated polycyclic aromatic hydrocarbons provided an electron-withdrawing function to keep the acid site stable. The morphology of carbon catalyst (SEM) indicated that after the sulfonation treatment by concentrated H2SO4, the particle agglomerates had disintegrated to some extent and the pores have became larger. The disintegration of the agglomerated particles also implied that the prepared carbon material catalyst had a high quantity of external acid sites which can be made available to the reactant. The catalyst was able to catalyze the transesterification reaction for two cycles without any treatment, but from third cycle, activity decreased due to swelling effect. In recycling experiments combined influence of catalyst swelling and deactivation due to leaching of SO3H can be seen and further more it was observed that activity improved when swelling exceeded the effect of the leaching of SO3H group. Sulfated zirconia solid acid catalyst was studied for transesterification reaction of soybean oil and simultaneous esterification of oleic acid with methanol and ethanol in a high pressure reactor by Garcia et al. (2008). Sulfated zirconia was prepared by either solvent free method (S-ZrO2) or standard precipitation method (SZ). In solvent free method ZrOCl28H2O and (NH4)2SO4 are mixed in molar ratio of 1:6 for 20 min at room temperature and calcined at 600 °C for 5 h. Whereas, in standard precipitation method, SZ was prepared by precipitation of zirconium oxychloride hydrate (ZrOCl2.8H2O) with ammonium hydroxide at pH 8.5 and then washed, dried and after that powder was sulfated by impregnated H2SO4 and then calcined at 650 °C for 4 h. They found that sulfated zirconia prepared by solvent free method was very active in the transesterification as well as esterification reaction. The conversion

in alcoholysis catalyzed by S-ZrO2 obtained under optimized conditions at 120 °C, 5 wt.% of catalyst (w/w) was 98.6% (methanolysis) and 92% (ethanolysis) respectively after 1 h. The performance of ethanolysis was not as good as in methanolysis due to the higher water content of ethanol (0.44%) compared to methanol (0.08%). SZrO2 was an amorphous material while SZ are crystalline (tetragonal and monoclinic phases of zirconia) as depicted by XRD. Carma et al. (2009) studied the Al-MCM-41 mesoporous molecular sieves with Si/Al ratio of 8 for esterification of palmitic acid with methanol, ethanol and isopropanol. The catalyst Al-MCM-41 with ratio of 8 of Si/Al, produced the highest conversion at 130 °C, 0.6 wt.% catalysts with alcohol to acid molar ratio of 60 and reaction time of 2 h. The conversion rates for catalyst were 79%, 67%, and 59% for methanol, ethanol, and isopropanol, respectively. Catalyst Al-MCM-41 was synthesized by dissolving aluminum chloride hexahydrate (AlCl3.6H2O) in a cetyltrimethylammonium (CTABr) and sodium hydroxide (NaOH) under intense agitation and by adding tetraethylorthosilicate (TEOS). The final product was then dried in an oven at 105 °C for 24 h, followed by thermal treatment in oven at 550 °C for 7 h and thus, eliminating the surfactant residue (CTABr) from the pores of the aluminosilicate. The catalyst was calcined at 480 °C for 3 h. XRD result showed that the samples have crystallographic patterns characteristic of the mesoporous solid aluminosilicate Al-MCM-41. An increase in the amount of aluminum incorporated into the mesopores leads to disorder in the structural arrangement of Al- MCM41. Zeolite beta modified with La (La/zeolite beta) had been tested as a solid acid catalyst for methanolysis of soybean oil (Shu et al., 2007). Zeolite beta has a high silica zeolite, containing an intersecting three dimensional structure of 12 member ring channels. Due to this relatively voluminous channel structure, it is possible to carry out numerous acid catalyzed reactions effectively. The La/ zeolite beta catalyst was prepared by an ion exchange method by the suspension of zeolite beta in lanthanum nitrate (La(NO3)2 aqueous solution under vigorous stirring at room temperature for 3 h and dried at 100 °C for 24 h and finally calcined at 250 °C for 4 h. The conversion of triglyceride of 48.9 wt.% was observed. The SEM result indicated that crystal structure of La/zeolite beta became considerably less agglomerated than zeolite beta due to the modification because of La3+ (Na+ is exchanged with La3+ so that instable framework aluminum will be stabilized). The higher Si/Al ratio can be related to higher structure stability of the tetrahedral framework aluminum. FTIR result indicated La/Zeolite beta shows higher conversion and stability than zeolite beta for the production of biodiesel, which may be correlated to the higher quantity of external Bronsted acid sites available for the reactants (the increment of the intensity of Bronsted acid sites can be attributed to the presence of Si–OH–La groups and La–OH groups in the La/ zeolite beta after ion exchange). Karmee and Chadha (2005) used Hb-zeolite, montmorillonite K10 and ZnO catalysts for transesterification of non-edible oil of crude Pongammia Pinnata with 1:10 M ratio of oil/methanol, 0.575 g (0.115 wt.% of oil) catalyst in 5 g oil at 120 °C. They found that ZnO gave the highest conversion rate of 83%, while Hb-zeolite, montmorillonite K-10 catalyst gave low conversion rates of 59% and 47% respectively after 24 h of reaction time. Jitputti et al. (2006) compared the catalyst activities of several acidic and basic solids catalysts such as ZrO2, ZnO, SO24 =SnO2 , SO24 =ZrO2 , KNO3/KL zeolite and KNO3/ZrO2 for transesterification of crude palm kernel oil (PKO) and crude coconut oil (CCO) with methanol. Among the catalysts, sulfated zirconia (SO24 =ZrO2 ), a super acid gave the highest amount of methyl ester yield due to its high acid strength. The reaction was carried out at 200 °C with 1 wt.% catalyst content, 50 bar pressure and 6:1 of methanol/oil ratio in a high pressure reactor. After 1 h reaction time, 90.3 and 86.3


wt.% of methyl ester was obtained in the crude palm kernel oil and crude coconut oil respectively. While in comparison ZrO2 gave the lower amounts of methyl esters content and yield. Crude palm kernel oil yielded higher methyl ester than crude coconut oil due the higher free fatty acid and water content of crude coconut oil which reduced the methyl ester yield. The activity of solid catalysts for crude palm kernel oil transesterification was SO24 =ZrO2 > SO24 =SnO2 > ZnO > KNO3/ZrO2 > KNO3/KL zeolite > ZrO2. In the case of crude coconut oil the catalysts activity was in order of SO24 =ZrO2 > SO24 =SnO2 > ZnO > KNO3/KL zeolite > KNO3/ZrO2 > ZrO2. However as compared with SO24 =SnO2 and SO24 =ZrO2 , the basic ZnO catalyst gave higher methyl ester contents (98.9%) but a lower methyl ester yield (86.1%) in PKO. In addition, spent SO24 =ZrO2 , was not directly reused for transesterification (yield only 27.7 wt.%), as catalyst deactivated due to combination of catalyst leaching and blocking of active sites by the products or unreacted starting materials. Application of sodium molybdate (Na2MoO4) was reported by Nakagaki et al. (2008) for the methanolysis of different types of lipids derived from soybean oil such as refined soybean oil (0.7 mg KOH/gm acid value), degummed soybean oil (1.0 mg KOH/gm acid value, 180 ppm of phosphorous as phosphatides) and used frying oil (1.5 mg KOH/gm acid value). The reaction was carried out at 65 °C with 54:1 of methanol/oil ratio, 5 wt.% catalyst contents in 3 h. The conversion achieved for refined soybean oil, degummed soybean oil and used frying oil were 95.6 wt.%, 92.6 wt.% and 94.6% respectively. The catalytic activity of the compound was attributed to the presence of the sites of molybdenum (VI) that has high Lewis acidity and can polarize at the alcohol O–H bond leading to a transient species, which has high nucleophilic character. Na2MoO4 was synthesized by treating MoO3 at 550 °C for 2 h with NaOH solution. Subsequently MeOH was added and Na2MoO4H2O filtered, washed by methanol and acetone and dried at 120 °C for 3 h. Furuta et al. (2004) studied solid superacid catalysts such as sulfated tin oxide (STO), tungstated zirconia-alumina (WZA) and sulfated zirconium-alumina (SZA) for transesterification of soybean oil with methanol at 300 °C with 4.0 g of catalyst: molar ratio of methanol to oil was 40:1. For the preparation of tungstated zirconia-alumina (WZA), a mixture of hydrated zirconia powder (amorphous), hydrated alumina (pseudo-boehmite), aqueous ammonium metatungstate solution and de-ionized water were put into a kneader with stirring for 25 min and there after extruded in cylindrical pellets shape and followed by drying at 130 °C and calcination at 800 °C for 1 h. The authors observed that among the catalysts, tungstated zirconia-alumina catalyst showed highest activity for transesterification with 94% conversion in 8 h reaction time while sulfated tin oxide and sulfated zirconium-alumina gave 80% and 70% conversions respectively. Peng et al. (2008) prepared SO24 /TiO2–SiO2 solid for the production of biodiesel from low cost feedstocks (50% oleic acid + 50% refined cotton seed oil) with high FFAs in autoclave reactor at 200 °C, with molar ratio of methanol to oil 9:1 and 3 wt.% catalyst concentration. The 92% conversion was obtained within 70 min reaction time. SO24 /TiO2–SiO2 catalyst was synthesized when SiO2 powder was slowly added to tetraisopropyl titanate solution of isopropyl alcohol under reflux for 4 h and dried at 110 °C for 2 h and then calcined at 450 °C for 4 h. The subsequent TiO2–SiO2 particles were soaked in H2SO4 for 1 day and then dried. SO24 /TiO2–SiO2 was finally obtained after calcination at 500 °C for 4 h. The authors observed that the large specific surface area of catalyst (258 m2/g) and the average pore diameter (10.8 nm) of the catalyst was big enough for reactant and product molecules to pass through the channels. The effect of FFA amount on the yield of esters was studied by adding 10, 30, 50 and 80 wt.% oleic acid to refined cottonseed oil under similar reaction conditions and it was observed

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that the FFA content increased the yield of methyl ester and the rate of esterification of oleic acid was higher than the rate of transesterification of cottonseed oil due to the better solubility of FFAs of cottonseed oil in methanol. Zirconia supported tungsten oxide (WO3/ZrO2) has been tested as a solid acid catalyst for esterification of palmitic acid with methanol (Ramu et al., 2004). The catalyst was prepared by impregnation of zirconium hydroxide gel with ammonium meta tungstate with 2.5–25 wt.% WO3 loading. The catalyst was dried and finally calcined at 500 °C. The maximum conversion of 98% was obtained at 5 wt.% WO3/ZrO2 catalyst in 6 h reaction time. The presence of crystalline WO3 and monoclinic phase of zirconia appeared to reduce the catalytic activity. The acidity of 5 wt.% WO3/ZrO2 catalyst was 1.04 mmol/g, which decreased by increasing the amount of WO3 due to excess coverage of WO3 species on ZrO2. Lopez et al., 2007 studied tungstated zirconia (ZrO2 =WO23 ) as strong solid acid catalyst for both esterification and transesterification with methanol as a reactant. The authors evaluated the effect of calcination temperature (400–900 °C) on the catalytic properties of tungusted zirconia. Catalytic activities of esterification and transesterification were increased with the formation of polymeric W species in the presence of the tetragonal phase of the ZrO2 support. They concluded that the optimum calcination temperature 800 °C was efficient to activate tungstated zirconia for both transesterification and esterification reactions. They examined the transesterification of liquid-phase of triacetin at 60 °C and esterification of acetic acid at 60 °C (liquid phase) and 120 °C (gas phase) with methanol. The maximum catalytic activity was obtained with catalyst calcined at 800 °C due to the Bronsted acid sites which contribute most of the activity. Sreeprasanth et al. (2006) reported Fe–Zn double–metal cyanide (DMC) complex as a solid acid catalyst for esterification/ transesterification of sunflower oil. Double-metal cyanide complexes have zeolite- like cage structures (Graverau and Garnier, 1984). The catalyst was synthesized by mixing three solutions; aqueous solution of K4Fe(CN)63H2O, a solution of ZnCl2 in mixture of distilled water and tert-butanol and a solution of tri-block copolymer, in mixture of water and tert-butanol. The catalyst was hydrophobic and contained only Lewis acidic sites. This is because of coordinatively unsaturated Zn2+ ions in the structure of the Fe–Zn complex. The Fe–Zn complexes had a spherical morphology as shown by scanning electron microscopy (SEM). The transesterification reaction took place at temperature of 170 °C, with methanol/oil ratio of 15:1 and 3 wt.% of catalyst and after 8 h of reaction time and 98.3 wt.% conversions was obtained. The catalyst was compatible for both esterification (high amount of FFA in the oil) and transesterification. The water content did not influence the FAME yield due to hydrophobicity of surface. The catalyst was reused without any purification and no significant drop of activity was detected in the transesterification reaction. 2.3. Acid–base solid catalysts Cheaper feedstocks like waste oils, animal fats cannot be converted to biodiesel using the conventional base mediated process, as the FFA of oils creates the problems of saponification. Acids can esterify FFA but the slow rates and limitation of using expensive metallurgy makes it less accepted. Heterogeneous catalysts having both acidic and basic sites have been investigated which could esterify FFA and at the same time transesterify triglycerides to biodiesel. Lin et al. (2006) reported synthesis of mixed metal oxide mesoporous silica material for TG transesterification and simultaneous esterification of FFA. They prepared these mesoporous calcium silicate mixed metal catalysts having different amount of calcium oxide. A co-condensation method was used for preparation in

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which cetyltrimethyl ammonium bromide (CTAB) provided the micelles template in a NaOH catalyzed reaction of tetraethylorthosilicate (TEOS) and the metal oxide. The catalyst after isolation was freed from surfactant CTAB by calcination at 600 °C for 6 h. SEM/ TEM showed increased structural disorder with increasing content of calcium oxide. XRD analysis showed total absence of peaks associated with CaO and solid state NMR showed the structure similar to crystalline calcium silicate. These catalysts could esterify soybean oil in methanol in 24 h (80 °C) and could also esterify the free acids. The recovered catalysts could be reused 30 times for transesterification and 8 times for esterification without significant loss of catalyst activity. Lin et al. (2008) obtained a patent for preparation of mesoporous calcium, magnesium silicate and barium silicate by co-condensation method. By forming a mixed oxide from strong basic metal oxide and weak acidic silica, the acidity of silica was significantly enhanced. In calcium silicates mixed oxide, silica sites were lewis acidic, Ca sites as basic and hydroxyl group on surface acted as Brönsted acids. The co-condensation procedure adopted were similar to the one reported by authors earlier (Lin et al., 2006). The three catalysts, having different Ca/Si ratios, were able to transesterify soybean oil in 90–100% conversion level. The effective temperature range was claimed to be 80 °C and complete conversion took more than 26 h. Under similar conditions complete esterification of poultry fat acids could be achieved in 24 h. All the catalysts were evaluated for recyclability’s and no loss of activity was noticed in 20 cycles. Very recently Macario et al. (2010) reported a biodiesel production process by homogeneous/ heterogeneous catalyst system of acid–base type. First the acid catalyst, both strong acid type USY, BEA and weak acid catalyst of the type MCM-41 were prepared by hydrothermal synthesis procedures. Later, for preparation of acid–base type catalyst, potassium (K) was loaded on different materials by ionic exchange methods. For K loading, the calcined catalyst materials were treated with 1 M KCl solution at 80 °C and the ratio of solid/solution was kept at 0.01 g/mL. These K loaded samples were calcined again at 300 °C for 8 h. Transesterification reactions were carried out at 100 to 180 °C, molar ratio of oil to methanol at 1:20, and using 5 wt.% of catalyst. At the end of reaction, the catalyst was separated by centrifugation, washed with water and dried overnight at 120 °C. It was observed that strong acid catalysts like USY, BEA were not good for triglyceride conversion and commercial potassium silicate was found to be much better. The K loading of MCM-41 increased the conversion of triglyceride to a great extent but biodiesel production was low as the main products were FFA (32%), mono-glycerides (42%). The K loaded delaminated zeolites (K ITQ-6) gave 97% triglyceride conversion and biodiesel yield of 80%, under the similar reaction conditions. However, when the recovered catalyst was recycled, a sharp decrease in biodiesel yield was observed and this has been attributed to leaching of K from the catalyst. The authors proposed a conceptual flow sheet of a continuous process in which two fixed bed reactors were employed, one for transesterification and the other for the catalyst regeneration. 2.4. Enzymatic catalysts Considering the problems of saponification during the transesterification process, of oil having FFA, by adopting the basic catalyst and slow reaction rate in acid catalyzed reactions, large efforts have been made to investigate the enzymatically catalyzed transesterification of oils. Enzymatic transesterification avoids soap formation, works at neutral pH, lower reaction temperatures and thus can be economical. The reusability of enzymes by immobilizing these on solid supports have provided a new window of opportunity. Several methods for enzymatic immobilization like

covalent bonding, cross-linking and micro-encapsulation have been reported. Lipase has been the main enzyme used for transesterification, as these are cheaper and are able to catalyze both hydrolysis and transesterification of triglycerides at very mild conditions and thus are considered for biodiesel production (Goncalves et al., 1996; Huge-Jensen et al., 1988; Oliveira et al., 1997). Catalytic behavior of the Rhizomucor miehei lipase (RML) immobilized on zeolite materials has been studied by Macario et al. (2007) for biodiesel synthesis with olive oil, containing 76 wt.% of oleic acid, and methanol. The result shows that biocatalysts have high capabilities to transesterify fatty acids in olive oil for several cycles with higher total biodiesel productivity compared to using free enzyme. The results indicated that the zeolitic materials, having a large number of Si-OH groups, are able to adsorb the lipase enzyme in its open conformation. Silicalite-1 obtained by different synthesis routes (synthesized in alkaline system and fluorine media i.e. S1 and F-S2) and delaminated zeolite ITQ-2 has been prepared as lipase-supports. The results were compared with free enzyme and lipase covalently attached to the functionalized sepiolite/AlPO4. For the synthesis of enzyme immobilization, the RML enzyme and the calcined support (wt. ratio: free enzyme/support equal to 2.5) were mixed in 0.2 M phosphate buffer pH 7, and stirred at 250 rpm for 24 h at 0 °C. The support with immobilized lipase was separated by filtration, washed with de-ionized water and dried at 25 °C overnight. The total protein concentration was calculated by UV absorption at 280 nm. The transesterification reaction were carried out at 40 °C temperature, 5:1 ratio of methanol to oil (Ma and Hanna, 1999), 0.6 g for lipase/S1 and lipase / ITQ-2 catalysts, 0.4 g for lipase/sepiolite/AlPO4 or 100 mg of free lipase for 3 h reaction time. It was evaluated that the oleic acid conversion of Lipase/ S1 and Lipase/ITQ-2 and free lipase were about 100%, 91% and 100%. Whereas, the lower oleic acid conversion (about 65%) and methyl oleate content (about 43%) was obtained of Lipase/sepiolite/AlPO4, but the stability of the lipase/sepiolite/ AlPO4 biocatalyst was higher than that of the lipase/S1 and lipase/ TQ-2 biocatalysts. The authors also examined the productivity (mg of methyl oleate/mg of enzyme/h of reaction) of catalysts and observed that catalysts prepared by adsorption show the highest productivity (more than twice) than free enzyme. However, the lipase/ sepiolite–AlPO4 catalyst had a lower productivity than free lipase as the covalent binding forces reduced the catalytic activity of the immobilized lipase. The enzyme immobilized on zeolites was recycled several times but it gradually leached from the support. The methyl oleate content of biocatalysts was drastically decreased from first cycle to third cycle. The methyl oleate content decreased from 79% to 51% for lipase/S1, and from 70% to 43% for lipase/ ITQ2. However, the lipase/sepiolite/AlPO4 catalyst does not show any enzyme leaching due to covalent binding of enzyme and support. The recovered catalyst was washed with n-hexane, dried at room temperature and stored at 0 °C until subsequent use. Macario et al. (2009) reported that encapsulation of lipase enzyme (Rhizomucor miehei lipase) in highly ordered mesoporous matrix by a sol–gel method that involves the hydrolysis/ polycondensation of a silica precursor at neutral pH and room temperature. The enzyme is encapsulated within the micellar phase of the surfactant that is self-assembled with silica. The encapsulated biocatalyst has been used for the transesterification reaction of triolein with methanol under solvent free conditions. The highest fatty acid methyl esters yield (77%) was obtained after 96 h at 40 °C, with triolein:methanol molar ratio of 1:3 and 5 wt.% of catalyst (1.5 wt.% of enzyme). For the preparation of heterogeneous lipase enzyme, two different immobilization procedures such as encapsulation and the adsorption procedure were studied. In encapsulation procedure, lipase solution was added to the cetyltrimethylammonium bromide (CTMABr) solution and stirred for 1 h at room temperature. The silica precursor was then introduced


into the solution and, subsequently, ethanolamine (20 wt.%) was added. The gelation was slow and the sol–gel was stirred for 24 h at room temperature and pH 7.2 followed by the filtration. After that the liquid was analyzed by UV-adsorption at 280 nm to determine the degree of enzyme encapsulation and the enzyme/silica weight ratio in the final solid catalyst (immobilization yield). In adsorption-immobilization procedure, 50 ml of a 0.2 M phosphate buffer solution (pH 7.0) and RML was added to the support in powder form. The mixture was stirred (250 rpm) for 24 h at room temperature. The support with adsorbed lipase was washed twice with de-ionized water and dried at 25 °C overnight and then the total protein concentration was measured by UV absorption method at 280 nm. Various enzyme contents of MCM-41 were prepared by this method and the authors observed that the immobilization yield of lipase turns out to be higher than 95% for all the samples. The order of the mesoporous structure moderately decreases when the enzyme content increases, with molar ratio of enzyme to silica ranging from 0.005 to 0.020. The pore diameter and BET surface area and pore diameter were increased by increasing the enzyme amount that produces the swelling of the micelle of surfactant. The yield of methyl esters and the enzyme activity increases with the amount of lipase loaded. It was observed that the FAME yields of free lipase at 18 h of reaction are lower than those obtained with the encapsulated lipase. But after 70 h, the FAME yield is close to 80% for the encapsulated and 50% for free lipase. Therefore, due to interaction of hydrophobic chains of the surfactant with the hydrophobic patches of the enzyme, enzyme structure gets opened and accessibility of the lipase catalytic centre to the reactants gets increased. Hence, even after the presence of surfactant, enzyme activities are not inhibited. Total productivity of the immobilized enzyme is almost six times higher than the one obtained using free lipase. Caballero et al. (2009) studied the free and immoblized Pig pancreatic lipase (PPL) enzyme on sepiolite for transesterification of sunflower oil and alcohol. The optimum reaction conditions of free and immobilized Pig pancreatic lipase were: reaction temperature of 40 °C, oil to ethanol ratio of 2:1 v/v, pH of 12 and catalyst contents of 0.01 g (0.1 wt.% of total substrate) for free PPL and 0.5 g of demineralised sepiolite containing 0.01 g of immobilised PPL (0.1 wt.% of total substrate) for immobilized PPL. The PPL activity was increased on increasing pH value (12) and the maximum biodiesel yield found after 10 h reaction time was around 57.7% and 26.9% for free PPL and immobilized PPL respectively. The enzyme PPL was immobilized on sepiolite, which is a natural silicate having fibrous structure, after acid treatment to remove Mg atoms. The Mg free sepiolite was treated with PPL enzyme in ethanol at 0 °C for 24 h, centrifuged to remove the non-immobilized enzyme. Through the immobilized enzyme was less efficient as compared to the free enzyme, but the ease of recyclability and retention of initial enzyme activity were major factors favour of its use for biodiesel production. Immobilized Lipase (Thermomyces lanuginosus) on novel microporous polymer matrix (MPPM) has been tested for the transesterification reaction of sunflower, soybean and waste cooking oils with methanol as a low cost biocatalyst (Dizge et al., 2009). Poly HIPE using styrene, divinylbenzene, and polyglutaraldehyde was used to synthesize Microporous polymeric matrix (MPPM) containing aldehyde functional group. Thermomyces lanuginosus lipase was covalently attached onto MPPM with 80%, 85%, and 89% immobilization efficiencies using bead, powder, and monolithic forms, respectively. MPPM synthesis (monolithic, bead, and powder forms), microporous polymeric biocatalyst (MPPB) preparation by immobilization of lipase onto MPPM and biodiesel production by MPPB are the three aspect of the process on which research is focused. MPPM was prepared by polymerizing the continuous phase of a high internal phase emulsion consisting of organic and water

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phases. The organic phase was composed of styrene, divinylbenzene, and Span 80. While potassium persulphate and polyglutaraldehyde solution were contained in water phase. Immobilization of lipase was carried out by reaction of powder (4–8 mesh in size) or bead matrix with enzyme in calcium acetate buffer (25 mM, pH 6) at 26 °C for 25 h with gentle shaking (250 rpm). Then, to remove the unbound enzyme the immobilized enzyme was washed with acetate buffer. SEM micrographs and FTIR spectrum showed that copolymer can be produced as a porous structure having aldehyde functional groups. The immobilization efficiencies obtained using bead and powder forms were 80% and 85%, respectively. The transesterification reaction was carried out in accordance with design of experiment based on Taguchi methodology at 65 °C, 1:6 M ratio of oil and methanol, 250 rpm of stirring and 0.0108 wt.% of immobilized lipase (powder or bead form) for 24 and 5 h in batch reactor. Methanol was added to the mixture in three-steps to avoid strong methanol inhibition. In 5 h reaction time, biodiesel yields for sunflower oil was 63.8%, 81.1%, and 86.9% using monolithic, bead, and powdered MPPB, respectively. It was observed that the most effective biocatalyst was powdered MPPB for the production of biodiesel as it get efficiently mixed with reactants during reaction. It was also observed that the immobilized enzyme retained the activity during 10 repeated batch reactions. 3. Conclusions Several solid acidic catalysts have been investigated for biodiesel synthesis but their uses have been limited due to lower reaction rates and unfavorable side reactions. Basic heterogeneous catalysts have also been investigated but their activity gets degraded in the presence of water. Acid–base catalysts are one of the potential catalysts because they catalyze both esterification and transesterification simultaneously. Enzymatic catalysts though highly promising but are rather slow. For a successful commercial catalyst, catalyst life, recyclability and lower cost are extremely important as these have a direct effect on overall cost of the process. Only few reports indicate the commercial level production of biodiesel by adopting the heterogeneous catalyst route. Acknowledgements The authors would like to express their gratitude to the University of Petroleum and Energy Studies, Dehradun and to Management of R&D Centre, Indian Oil Corporation Limited, Faridabad for the permission to publish this work. One of the author (S. Semwal) would like to thank University of Petroleum and Energy Studies for award of research fellowship. References Abdullah, A.Z., Razali, N., Lee, K.T., 2009. Optimization of mesoporous K/SBA-15 catalyzed transesterification of palm oil using response surface methodology. Fuel Process. Technol. 90, 958–964. Abreu, F.R., Lima, D.G., Hamu, E.H., Einloft, S., Rubim, J.C., Suarez, P.A.Z., 2003. New metal catalysts for soybean oil transesterification. J. Am. Oil Chem. Soc. 80, 601– 604. Arzamendi, G., Campo, I., Arguinarena, E., Sanchez, M., Montes, M., Gandia, L.M., 2007. Synthesis of biodiesel with heterogeneous NaOH/alumina catalysts: comparison with homogeneous NaOH. Chem. Eng. 134, 123–130. Arzamendi, G., Arguinarena, E., Campo, I., Zabala, S., Gandia, L.M., 2008. Alkaline and alkaline-earth metals compounds as catalysts for the methonolysis of sunflower oil. Catal. Today, 133–135. Azam, M.M., Waris, A., Nahar, N.M., 2005. Prospects and potential of fatty acid methyl esters of some non-traditional seed oils for use as biodiesel in India. Biomass Bioenergy 29, 293–302. Benjapornkulaphong, S., Ngamcharussrivichai, C., Bunyakiat, K., 2009. Al2O3supported alkali earth metal oxides for transesterification of palm kernel oil and coconut oil. Chem. Eng. J. 145, 468–474. Caballero, V., Bautista, F.M., Campelo, J.M., Luna, D., Marinas, J.M., Romero, A.A., Hidalgo, J.M., Luque, R., Macario, A., Giordano, G., 2009. Sustainable preparation

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