Biodiesel production from jatropha oil by catalytic and non-catalytic approaches: An overview

Bioresource Technology 102 (2011) 452–460

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Review

Biodiesel production from jatropha oil by catalytic and non-catalytic approaches: An overview Joon Ching Juan a,⇑, Damayani Agung Kartika a, Ta Yeong Wu b, Taufiq-Yap Yun Hin c a

Laboratory of Applied Catalysis and Environmental Technology, School of Science, Monash University, Bandar Sunway 46150, Malaysia Chemical and Sustainable Process Engineering Research Group, School of Engineering, Monash University, Bandar Sunway 46150, Malaysia c Centre of Excellence for Catalysis Science and Technology, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia b

a r t i c l e

i n f o

Article history: Received 24 May 2010 Received in revised form 15 September 2010 Accepted 16 September 2010 Available online 1 October 2010 Keywords: Biodiesel Catalyst Ester Jatropha oil Transesterification

a b s t r a c t Biodiesel (fatty acids alkyl esters) is a promising alternative fuel to replace petroleum-based diesel that is obtained from renewable sources such as vegetable oil, animal fat and waste cooking oil. Vegetable oils are more suitable source for biodiesel production compared to animal fats and waste cooking since they are renewable in nature. However, there is a concern that biodiesel production from vegetable oil would disturb the food market. Oil from Jatropha curcas is an acceptable choice for biodiesel production because it is non-edible and can be easily grown in a harsh environment. Moreover, alkyl esters of jatropha oil meet the standard of biodiesel in many countries. Thus, the present paper provides a review on the transesterification methods for biodiesel production using jatropha oil as feedstock. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Energy is a basic requirement for every sector of economic development in a country. As a result, energy demands have been steadily increasing along with the growth of human population and industrialization. Common sources of energy are petroleum, natural gas and coal from fossil fuels. This growing consumption of energy has rapidly depleted non-renewable sources of energy. Rising price of fossil-based fuels and potential shortage in the future have led to a major concern about the energy security in every country (Agarwal and Agarwal, 2007; Jain and Sharma, 2010a; Jayed et al., 2009; Robles-Medina et al., 2009). Moreover, there are many disadvantages of using fossil-based fuels, such as atmospheric pollution and environmental issues. Fossil fuels emissions are major contributors of greenhouse gases which may lead to global warming. Combustion from fossil fuels is major source of air pollutants, which consist of CO, NOx, SOx, hydrocarbons, particulates and carcinogenic compounds (National Biodiesel Board, 2010; Diwani et al., 2009). Fig. 1 shows the comparison between pollutants emitted from petro-diesel engine and biodiesel engine. The disadvantages and shortages of fossil fuels have motivated many researchers to find an alternative source of renewable energy.

⇑ Corresponding author. Tel.: +60 3 5514 6106; fax: +60 3 5514 6364. E-mail address: [email protected] (J.C. Juan).

At present, diesel-powered vehicles represent about one-third of vehicles sold in Europe and United States (Jayed et al., 2009). The global consumption of petroleum diesel is 934 million tonnes per year (Kulkarni and Dalai, 2006). Biodiesel is one of the most promising alternative fuels for diesel engines. The demand of biodiesel has significantly increased from 2005 especially in USA (Pahl, 2008). Biodiesel is defined as a fuel comprising of monoalkyl esters of long chain fatty acids derived from vegetable oil or animal fat (Su and Wei, 2008). There are several advantages of biodiesel as compared to conventional diesel. Advantages of biodiesel are (1) It helps to reduce carbon dioxide and other pollutants emission from engines, (2) Engine modification is not needed as it has similar properties to diesel fuel, (3) It comes from renewable sources whereby people can grow their own fuel, (4) Diesel engine performs better on biodiesel due to a high cetane number, (5) High purity of biodiesel would eliminate the use of lubricant, (6) Biodiesel production is more efficient as compared to fossil fuels as there will be no underwater plantation, drilling and refinery and (7) Biodiesel would make an area become independent of its need for energy as it can be produced locally (Jain and Sharma, 2010a; Jayed et al., 2009; Nie et al., 2006; Robles-Medina et al., 2009; Shah et al., 2004; Su and Wei 2008; Vieira et al., 2006). There are three types of oil as potential sources for biodiesel production, which are vegetable oil, animal fat and used cooking oil. Animal fat is infeasible because it becomes solid wax at room temperature and causes problems during the production process

0960-8524/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.09.093

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Fig. 1. Comparison of exhaust gas emission between petrodiesel and biodiesel (National Biodiesel Board, 2010).

(Patil and Deng, 2009). Waste cooking oil is a mixture of animal and vegetable oils which have undergone chemical and physical changes during cooking process. Waste cooking oil is contaminated by many kinds of impurities from the cooking process, such as polymers and free fatty acids (Kulkarni and Dalai, 2006). Various vegetable oils are promising feedstock for biodiesel production since they are renewable in nature, can be mass propagated and environmentally friendly (Helwani et al., 2009; Leung et al., 2010; Parawira, 2009). Edible oils, such as palm oil, rapeseed oil, sunflower oil, and soybean oil have all become a major source of biodiesel production. However, there are serious concerns regarding the use of edible oil in biodiesel. This may cause an increase of edible oil prices and also biodiesel prices (Jain and Sharma, 2010a). The cost of raw material in biodiesel production accounts about 60–80%. Edible oil may become more expensive due to the competition between human consumption and biodiesel market. Environmental issues would likely develop as the mass propagation of plants producing edible oil would lead to deforestation (Leung et al., 2010). In order to overcome these drawbacks, researchers have focused on non-edible oil for biodiesel production. Examples of non-edible, oil-producing plants are Jatropha curcas (Jatropha), Pongamina pinnata (Karanja), Madhua indica, Calophyllum inophyllum (Polanga), Hevea brasiliensis (Rubber) and others (Pinzi et al., 2009). Among those plants, jatropha is the most advantageous for biodiesel production in terms of economical, sociological and environmental implications. 2. Jatropha plant as potential source for biodiesel production J. curcas belongs to the Euphorbiaceae family. The name jatropha derives from latin words jatros (doctor) and trophe (food) as it has many medicinal values. The jatropha plant is native to American tropics (Pramanik, 2003) but naturally grows in tropical and subtropical countries, like sub-Sahara Africa, India, South East Asia, and China (Tamalampudi et al., 2008). The seeds usually mature 3–4 months after flowering and once this plant becomes an adult, it will continue producing seeds for 50 years. The oil is covering approximately 40% of the seed content (Jain and Sharma, 2010a). The fatty acid composition of jatropha oil is listed in Table 1. Due to leaf-shedding activity, jatropha plant becomes highly adaptable in harsh environment because decomposition of the shed leaves would provides nutrients for the plant and reduces water loss during dry season. Thus, it is well adapted to various types of soil, including soils that are poor in nutrition such as sandy, saline and stony soils. Jatropha plant also has the ability to tol-

Table 1 Composition of jatropha oil (Jain and Sharma, 2010 and Pinzi et al., 2009). Fatty acid

Systemic name

Structure

Composition (%)

Palmitic acid Stearic acid Oleic acid Linoleic acid Other acids

Hexadecanoic acid Octadecanoic acid Cis-9-octadenoic acid Cis-9-cis-12-octadecadeneoic acid

C16 C18 C18:1 C18:2

13.4–15.3 6.4–6.6 36.5–41 35.3–42.1 0.8

erate a wide range of climate and rainfall (Kumar and Sharma, 2008; Makkar and Becker, 2009) but cannot grow in waterlogged land. As a drought-resistant plant, jatropha plant is a good candidate for eco-restoration in wastelands (Acthen et al., 2008; Kumar et al., 2008; Kumari et al., 2009). Jatropha cultivation in wastelands would help the soil to regain its nutrients and will be able to assist in carbon restoration and sequestration (Jain and Sharma, 2010a; Makkar and Becker, 2009). 3. Transesterification methods for jatropha oil Many types of methods have been developed to convert vegetable oil such as jatropha oil into biodiesel. The four main categories are the direct use of vegetable oil, micro-emulsion, thermal cracking and transesterification. Direct use of vegetable oil is not applicable to most of diesel engines as the high viscosity would damage the engine by causing coking and trumpet formation (Agarwal and Agarwal, 2007). Biodiesel obtained from micro-emulsion and thermal cracking methods would likely lead to incomplete combustion due to a low cetane number and energy content (Leung et al., 2010). Transesterification is the most common method for biodiesel production due to its simplicity, thus this method has been widely used to convert vegetable oil into biodiesel (Helwani et al., 2009; Jain and Sharma, 2010a). Generally, vegetable oil or jatropha oil is consisted of a series of saturated and unsaturated monocarboxylic acids with trihydric alcohol glycerides. As shown in Table 1, jatropha oil consists of four major groups of fatty acids, which are oleic acid, linoleic acid, palmitic acid and stearic acid. Those major fatty acids can be transesterified to alkyl esters in the presence of alcohol or other acyl acceptor. 3.1. Non-catalytic transesterification Transesterification reaction without using any catalyst requires a high temperature above the critical temperature of alcohol and

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this is called as supercritical method. In this method, alcohol e.g. methanol is turned into a supercritical fluid state by applying extreme pressure and temperature. The common reaction temperature is more than 250 °C, as the critical temperature of methanol is 240 °C (D’Ippolito et al., 2007). In this extreme environment, liquid methanol will reach critical point where both gas and liquid become indistinguishable fluids, in which it would exhibit properties of both liquid and gas. It is able to penetrate to solid like gas and dissolve other material into them like liquid (Leung et al., 2010). A higher molar ratio is required to push the reaction forward in this method. Rathore and Madras (2007) investigated the possibility of using the supercritical method on methanol and ethanol for biodiesel production from crude jatropha oil. In their experiment, a 50 to 1 alcohol to oil molar ratio was proven to be the best ratio under 20 MPa at 300 °C. A maximum conversion of 70% jatropha oil was successfully converted to fatty acids methyl esters in 10 min and the conversion continued increasing to 85% after 40 min under the same reaction conditions. The percentage of conversion was higher by around 2.5% when ethanol was employed under the same reaction condition. In separated reaction conditions, they managed to obtain a higher percentage of conversion up to 95% at 400 °C for both methyl and ethyl esters. Then, in a latter study by Hawash et al. (2009), a 100% yield of methyl esters can be obtained under milder conditions. The supercritical reaction was carried out within 4 min at 320 °C under 8.4 MPa and the molar ratio of methanol to jatropha oil molar was 43 to 1. The significant improvement is due to the free fatty acid (FFA) content in jatropha oil. The FFA content of the jatropha oil used by Hawash et al. (2009) is only 2%, whereas for Rathore and Madras (2007) the FFA content is more than 10%. In a simpler explanation, higher FFA content will generate more water via esterification reaction and thus the water will hydrolyze the esters that produced from transesterification. The main drawback of supercritical condition is that methyl/ethyl esters are easily degraded in an extremely high temperature (ca. 300 °C). Recently, jatropha oil was extracted using supercritical carbon dioxide and then subjected to subcritical hydrolysis (Chen et al., 2010). The hydrolyzed fatty acid obtained was further reacted with methanol under supercritical methylation (esterification). In the supercritical methylation, 99% of biodiesel from jatropha oil was obtained in 15 min only under 11 MPa at 290 °C with 33% v/v of hydrolyzed oil to methanol. Similarly, Ilham and Saka (2010) also employed a two step process to produce biodiesel from jatropha oil but dimethyl carbonate was used instead of methanol in the second step. They successfully obtained 97% of methyl esters in 15 min at 300 °C under 9 MPa. This process produces more valuable glyoxal as by-product instead of glycerol and is not affected by the high FFA content. However, the cost of dimethyl carbonate is more expensive as compared to that of methanol/ethanol. The main advantage of either one or two step supercritical methods is that there is no purification step needed to remove the catalyst. Nevertheless, the usage of very high temperature, high pressure and large amount of alcohol are disadvantageous to push the biodiesel production from laboratory to industrial scale (Kulkarni and Dalai, 2006). Thus, further investigations in the production process and economic evaluations are needed.

3.2. Catalytic transesterification Catalytic transesterification are divided into three main categories, which are acid catalyzed, base catalyzed and enzymatic catalyzed transesterification. There are two type of catalyst depending on its mobility, one is homogenous and the other one is heterogeneous. The comparison of advantages and disadvantages between homogeneous and heterogeneous are listed in Table 2. The application of homogenous catalysts, either acid or base, would generate more problems as compared to heterogeneous catalysts. It would be difficult to remove a homogenous catalyst from the biodiesel product due to its liquid form. Homogenous catalyst would generate hazardous wastewater and further treatment is necessary. Heterogeneous catalyst is a type of solid catalyst which allows the catalyst to be easily separated from the biodiesel by a simple filtration. Furthermore, the reusability of the solid catalyst is also possible after separation. Transesterification or alcoholysis of jatropha oil using different types of catalyst is shown in Fig. 2. The type of catalyst used for producing biodiesel is very much dependent on FFA content of jatropha oil. This is because mixing of jatropha oil that contains high FFA with alkaline catalyst will result in soap formation. Thus, either a two-step transesterification or enzymatic approach is applied. 3.2.1. Alkaline-catalyzed transesterification Base or alkaline-catalyzed transesterification is the most common method to produce biodiesel these days. Common alkaline catalysts are sodium hydroxide (NaOH), potassium hydroxide (KOH) and sodium methoxide (NaOCH3) (Kulkarni and Dalai, 2006). Generally base-catalyzed transesterification is a three-step reaction. Initially, the base catalyst would react with alcohol to produce alkoxide anion. In the first step, nucleophilic will attack the alkoxide anion on the carboxyl group of glyceride to form tetrahedral intermediate I. This intermediate would react with a second alcohol molecule to generate another alkoxide and form intermediate II. Those intermediates would rearrange to form fatty acid ester and glycerin (Schuchardt et al., 1998; Tapanes et al., 2008). 3.2.1.1. Homogeneous alkaline-catalyzed transesterification. The advantages of using a homogenous alkaline catalyst are that it is a cheap catalyst with a high catalytic activity and using it produces high quality of biodiesel in a short period of time (Helwani et al., 2009; Kulkarni and Dalai, 2006). Many researchers have studied the transesterification of jatropha oil using homogenous alkaline catalysts. The rate of alkaline-catalyzed transesterification could be 4000 times higher than an acid-catalyzed process. However, there are some drawbacks in this process. For instance, alkaline catalysts do not have the ability to convert FFA into alkyl esters. As mentioned earlier, any source of oil that has significant amount of FFA (>1%) would not be fully converted into biodiesel because those FFAs would undergo saponification and lead to soap formation. The soap formed prevents glycerol separation and will damage the engine in the long run; hence a further purification step is needed to separate it from biodiesel. Otherwise, the jatropha

Table 2 Comparison of homogeneous and heterogeneous catalyzed transesterification (Helwani et al., 2009). Factors

Homogeneous catalyst

Heterogeneous catalyst

Reaction rate After treatment Methodology Presence of high FFA and water Catalyst reusability Cost

Fast and high conversion Catalyst cannot be recovered and lead to chemical waste production Limited used of continuous methodology Sensitive Not possible Comparatively costly

Moderate conversion Catalyst can be recover Continuous fix bed operation is possible Not sensitive Possible Potentially cheaper

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Jatropha oil

FFA ≤ 1 %

FFA > 1 %

FFA > 1 % Acid catalyst

Alkaline catalyst

Enzyme

FFA ≤ 1 % Alkaline catalyst

Biodiesel Fig. 2. Catalytic production of biodiesel based on free fatty acid (FFA) content in jatropha oil.

Table 3 Summary of alkaline-catalyzed transesterification of jatropha oil. No.

Authors

Alkaline catalyst

Time (min)

Reaction temp (°C)

Methanol/oil molar ratio

Catalyst amount

Conv (%)

1 2 3 4 5 6

Tang et al. (2007) Tapanes et al. (2008) Chitra et al. (2005) Berchmans et al. (2010) Zhu et al. (2006) Vyas et al. (2009)

Homogeneous NaOH Homogeneous NaOH Homogeneous NaOH Homogeneous KOH Heterogeneous CaO Heterogeneous KNO3/Al2O3

28 30 90 120 150 360

250 45 60 50 70 70

24:1 9:1 ca. 5.6:1 6:1 9:1 12:1

0.8% w/w 0.8% w/v 1.0% w/w 1.0% w/w 1.5 wt.% 6 wt.%

90.5 96 98 97 93 87

oil source must not contain or have a considerable amount of FFA (Leung et al., 2010). A number of researchers have transesterified jatropha oil with methanol and ethanol, using alkaline catalysts such as KOH and NaOH despite their drawbacks (Chitra et al., 2005; Tang et al., 2007; Tapanes et al., 2008; Berchmans et al., 2010). Chitra et al. (2005) optimize transesterification of jatropha oil using a NaOH alkaline catalyst. Under the optimum reaction conditions (Table 3, No. 3), the conversion of jatropha oil to methyl esters was 98% in 90 min. It is noteworthy that the FFA content in crude jatropha oil was reduced to 0.25% from 3.1% using NaOH. It is undeniable that NaOH will induce soap formation but at the same time NaOH will also neutralize free fatty acids to an acceptable level to meet biodiesel specifications. Nevertheless, an extra step is needed to remove the sodium soap after the reaction. The catalyst amount, molar ratio of methanol and reaction time were not only investigated but also optimized using completely randomized design (CRD). Large scale production of biodiesel from 25 kg of jatropha oil has resulted in 24 kg of biodiesel (96% of yield), which is only reduced by 2% as compared to lab scale (Chitra et al., 2005). The properties of biodiesel produced from jatropha oil also fall within the limits of Bureau of Indians Standards (BIS) specification. A similar alkaline catalyst was also used by Tapanes et al. (2008), but the catalyst was mixed with the methanol or ethanol before mixing it together with jatropha oil. Sodium methoxide/ethoxide was formed when NaOH was mixed with methanol/ethanol and its act as a homogeneous catalyst. Although the yield obtained by Tapanes et al. (2008) is slightly lower as compared to that of Chitra et al. (2005), the reaction time is three times faster (Table 3, No. 2 and 3). They also investigated the effect of alcohol on transesterification of jatropha oil. In methanol, the biodiesel yield was 96% in 30 min but when ethanol was employed biodiesel yield dropped to 93% under the same reaction conditions. The reaction rate with ethanol is slightly slower than that of methanol, as from the kinetic rate constant of methanol is higher than of ethanol. It is more dif-

ficult to break down molecule of ethanol to form ethoxide anion as compared to breaking down methanol to methoxide anion (Tapanes et al., 2008). In contrast, Asakuma et al. (2009) had pointed out that transesterification process occurs via transition state, when the alkoxy group attacks the carbon of the carboxyl group, an intermediate polygonal ring is formed. They predicted that lower activation energy is acquired when a longer chain alcohol formed a larger polygonal ring (Asakuma et al., 2009). This would mean that longer chain alcohols, such as ethanol and n-butanol are more suitable to be used to produce alkyl ester biodiesel. The same alkaline catalyst (NaOH) for the transesterification of jatropha oil containing 0.42% FFA was performed under supercritical method on methanol (Tang et al., 2007). The yield of methyl esters without the presence of catalyst under the supercritical methanol was only 20% in 28 min. Under the same supercritical condition, the conversion of biodiesel was remarkably increased to 90.5% using micro-NaOH catalyst (Table 3, No. 1). The biodiesel with the best properties was obtained using a methanol to oil molar ratio of 24:1, micro-NaOH (0.8% w/w), temperature of 250 °C and pressure of 7.0 MPa. It is very obvious that the yield of biodiesel is much higher with the presence of an alkaline catalyst. Transesterification of a mixture comprised of jatropha oil (5.5% FFA) and waste cooking oil (0.45% FFA) using potassium hydroxide (KOH) was studied by Berchmans et al. (2010). The level of FFA in the mixture was adjusted to about 1% prior transesterification to avoid soap formation. The highest conversion of 97% was achieved in 2 h using methanol to oil ratio of 6:1, a stirring speed of 900 rpm, 1% w/w of KOH to oil and the reaction temperature at 50 °C. The percentage conversion is comparable to a NaOH catalyst (Table 3, No. 1–4) but the reaction time is much longer than the NaOH catalyst. Alkaline-catalyzed transesterification for biodiesel production is the common method to produce biodiesel from jatropha oil today. This method will generate high quality biodiesel from jatropha oil in a short period of time. However the disadvantages (such as generating large amount of wastewater from alkaline

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catalyst, high cost of refining jatropha oil as raw material and additional purification step is necessary to remove the alkaline catalyst) have motivated many researchers to find a better alternative method to produce biodiesel. 3.2.1.2. Heterogeneous alkaline-catalyzed transesterification. Heterogeneous alkaline catalysts have several advantages as compared to the corresponding homogeneously catalyzed reaction as mentioned earlier in Table 2. Due to these advantages, heterogeneous alkaline catalysts have been studied for biodiesel production from jatropha oil. Recently, Vyas et al. (2009) have supported potassium nitrate (KNO3) on alumina (Al2O3) as solid base catalyst for transesterification of jatropha oil. A high amount of FFA acid content in jatropha oil, which contain up to 5.3% FFA was used. The conversion of jatropha oil to biodiesel up to 87% in 6 h was obtained by using KNO3/Al2O3. After the first reaction, the catalyst was calcined again at 500 °C for 4 h prior to reuse. However, the activity decreased by 9% as compared to the first reaction and it continued decreasing to 72% on the third reaction. Thus, this catalyst only can be reused for three times due to poor reusability. Zhu et al. (2006) have studied the use of super solid base of calcium oxide (CaO) as heterogeneous catalyst for biodiesel production from jatropha oil. CaO was chosen because it was believed to exhibit strong basicity and the presence of more active sites that would able to improve the transesterification reaction of jatropha oil. The commercial CaO was immersed in ammonium carbonate solution to increase the base strength and calcined at 900 °C for 1.5 h. The FFA’s acid content of jatropha oil used was less than 0.5%, which suggested that concentration of FFA was very low. The highest conversion of jatropha oil to biodiesel was 93% in 2.5 h. When the reaction was extended for more than 3 h, formation of white gel in the product was observed. This will increase the viscosity of biodiesel, which decrease in the fuel’s ability to flow and further induces incomplete burning of the fuel with ignition delay. The catalyst could be reused at least three times without significant loss of catalytic activity. Nevertheless, Granados et al. (2007) have pointed out that the dissolution of CaO does occur even if the catalyst can be reused for several times without significant deactivation. They observed that the catalytic activity of CaO is not only contributed by the heterogeneous active sites but also the homogeneous active species due to the dissolution of CaO in methanol. Homogeneous alkaline catalyst such as NaOH and KOH have been proven to be a very good alkaline catalyst for the transesterification of jatropha oil if the FFA content is lower than 1%. The final yield product seems to be dependent on the FFA content of jatropha oil. Among those alkaline catalysts, homogeneous NaOH seemed to be the best alkaline catalyst with the highest conversion of 98% in the shortest reaction time (Table 3, No. 3). However, due to the high level of purity and low FFA content of jatropha oil, which require extra refining process are expensive. Moreover, the separation process and alkaline wastewater treatment would increase the cost of biodiesel. Although the application of heterogeneous alkaline catalyst would eliminate those post-reaction purification steps, it did not result in a high conversion as compared to homogeneous alkaline catalyst. Reusability of heterogeneous alkaline catalysts is proven to be a challenging task. Although studies have proven that the heterogeneous catalysts possesses a high catalytic activity and could be reused for at least three times, further study on the reusability is necessary. 3.2.2. Acid and alkaline catalyzed, a two-step transesterification Acid catalyzed transesterification is often chosen when there is a high amount of FFA content in the oil. Normally, an acid catalyst is used to reduce the amount of FFA content to less than 1%. Generally, acid catalyzed transesterification requires more alcohol to

oil molar ratio as compared to alkaline-catalyzed transesterification. A higher molar ratio of alcohol would shorten the reaction time but it will not necessarily increase the ester yield. Comparing electrophilic species (in acid catalyst) to the stronger nucleophile (in base catalyst), the base catalyst appears to follow a more direct route to activate the reaction of catalytically active species. Thus, base catalyst exhibits a faster catalytic activity (Helwani et al., 2009). Despite of the fact that acid catalyst can be used to transesterify oil with high FFA content, acid catalysts have been ignored due to their slow reaction rate. However, acid catalyst is still being utilized to reduce FFA content before transesterification with alkaline catalyst. The method is called two-step transesterification process and it has gained popularity due to its effectiveness. An acidic catalyst is used in the first step to reduce the FFA content in jatropha oil to less than 1% by converting them into esters (esterification reaction). Then, an alkaline catalyst would be used in the second step to transesterify jatropha oil into biodiesel. Crude jatropha oil usually has FFA content up to 15% and this value is beyond the acceptable limit of alkaline-catalyzed transesterification (Berchmans and Hirata, 2008). However, the price of crude jatropha oil is much lower than the refined deodorized jatropha oil (RDJO), which has FFA content of more than 1%. Two-step transesterification is an excellent way to utilize crude jatropha oil for biodiesel production. According to Berchmans and Hirata (2008), jatropha oil with FFA content of 15% was initially treated with sulfuric acid (H2SO4) to reduce the FFA level to less than 1%. Pre-treatment with acid catalyst was done by using 1% w/w H2SO4 to oil and 60 wt.% of methanol in oil at 50 °C in 1 h. In the second step, the maximum conversion of jatropha oil was 90% in 120 min using NaOH catalyst (Table 4, No. 1). A comparison between the pretreated jatropha oil and non-pretreated jatropha oil was also conducted. The results of the methyl esters yield were 90% and 55% for pretreated and non-pretreated jatropha oil, respectively. Thus, the results indicated that high FFA content has a significant effect on transesterification of jatropha oil and two-step transesterification is a better choice to improve biodiesel yield. Likewise, Jain and Sharma (2010b,c) also pretreated the jatropha oil with H2SO4 to reduce the FFA content from 21.5% to less than 1% and then followed by transesterification using NaOH used as alkaline catalyst. The total amount of yield obtained is 90%, which is the same with the previous study (Table 4, No. 1 and 2). Besides using NaOH as alkaline catalyst, KOH was also employed as potential alkaline catalyst for biodiesel production. Biodiesel production from crude jatropha oil with FFA content of 14% was also studied by Tiwari et al. (2007) and Patil and Deng (2009) by using H2SO4 and KOH in a two-step transesterification. A better result was obtained using KOH catalyst in the second step (Table 4, No. 3 and 4). The pretreated jatropha oil was subjected to esterification using H2SO4 to reduce the FFA content from 14% to less than 1%. Then, the pretreated jatropha oil was transesterified by alkaline KOH. In the latter process, Patil and Deng (2009) managed to achieve 90–95% conversion in 120 min but Tiwari et al. (2007) successfully achieved up to 99% conversion in 24 min, even by using a lower molar ratio of methanol to oil (4:1) and a lower amount of KOH (0.55% w/w) (Table 4, No. 3 and 4). As compared to Patil and Deng (2009); Tiwari et al. (2007) have used response surface methodology (RSM) based on the central composite rotatable design to optimize the reaction conditions. In this respect, RSM is a powerful tool for optimizing transesterification of jatropha oil. Lu et al. (2009) tested the catalytic activity of homogeneous (H2SO4) and heterogeneous (SO24 /TiO2) acid catalysts in the pretreatment process of jatropha oil. Solid acid catalyst was prepared by calcining metatitanic acid at 500 °C for 3 h. The result suggested that the solid acid catalyst (SO24 /TiO2) gave more than 97%

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J.C. Juan et al. / Bioresource Technology 102 (2011) 452–460 Table 4 Summary of two-step transesterification of jatropha oil.

a b

No.

Authors

Acid/alkaline catalyst

Time (min)a

Reaction temp (°C)b

Methanol/oil molar ratio

1 2 3 4 5

Berchmans and Hirata (2008) Jain and Sharma (2010b,c) Tiwari et al. (2007) Patil and Deng (2009) Lu et al. (2009)

H2SO4/NaOH H2SO4/NaOH H2SO4/KOH H2SO4/KOH

60/120 180/180 88/24 120/120 120/20

65 50 60 60 64

ca. 6.7:1 9.6:1 4:1 9:1 6:1

1.4% w/w 1.0% w/w 0.55% w/v 4.5% w/v 1.3% w/w

90 90 99 90–95 98

6

Corro et al. (2010)

120/120

60

6:1

1.0% w/w

99.5

(SO24 /TiO2)/KOH SiO2.HF/NaOH

b

Catalyst amount

b

Conv (%)b

Time (minutes): reaction time of acid catalyzed/reaction time of alkaline-catalyzed. The reaction is referring to second step of alkaline-catalyzed transesterification.

Table 5 Advantages and disadvantages on different types of catalysts used in biodiesel production (Leung et al., 2010). Catalyst type

Advantages

Disadvantages

Alkaline catalyst Acid catalyst

High catalytic activity, low cost, modest operation condition, non corrosive Convert FFA to biodiesel, no soap formation

Enzymes (lipases)

No soap formation, non-polluting, easier purification

Low FFA requirement, anhydrous condition, emulsion formation, wastewater formation Corrosive, wastewater formation, difficult to recycle, higher reaction temperature, longer reaction time, weak catalytic activity Expensive, denaturation problem, long reaction time

conversion of FFA to methyl esters in 120 min. The optimum molar ratio of methanol to FFA and amount of SO24 /TiO2 were 20:1 and 4 wt.% in oil, respectively. Besides using a solid SO24 /TiO2 catalyst, they also used homogeneous H2SO4 to pre-treat the crude jatropha oil. In comparison, the optimum pre-treatment process using homogeneous H2SO4 was 1% w/w of H2SO4 to oil and 12 wt.% methanol in oil (molar ratio of methanol to oil about 4:1) for 120 min at 70 °C. In both reaction conditions, the amount of FFA acid content was reduced to less than 0.5% from initial 12% before converted into biodiesel via transesterification. The yield of biodiesel by alkaline-catalyzed transesterification was more than 98% in 20 min. Lately, another type of solid acid catalyst (SiO2.HF) was used to pre-treat the jatropha oil before transesterified with methanol using NaOH (Corro et al. 2010). The SiO2.HF manages to reduce the FFA content of jatropha oil from 7.9% to 0.3% in 180 min. The reaction was conducted at 60 °C, using 10 wt.% of catalyst to oil and the molar ratio of methanol to oil was 12 to 1. Then the final biodiesel obtained via transesterification using NaOH was 99.5% but the reaction is much longer as compared to that of Lu et al. (2009) (Table 4, No. 5 and 6). KOH and NaOH can act as effective catalysts for transesterification of jatropha oil if the FFA content is reduced to less than 1% by acid catalyst. The biodiesel production from jatropha oil using two-step transesterification suggested by Tiwari et al. (2007) is the most effective because it obtained the highest conversion in a shortest time as compared to others (Table 4, No. 1–6). Two-step transesterification process is a good alternative and efficient way to transesterify crude jatropha oil that contains high FFA. Utilization of acid and alkaline catalysts in the first and second stage would help to overcome the problems of slow reaction rate with acid catalyst and eliminate the soap formation by using alkaline catalyst. However, the use of a homogeneous catalyst such as H2SO4 would also create a wastewater problem and purification method is needed to separate the catalyst from the product. The use of heterogeneous catalyst in each step of this process would help to overcome these problems. Thus, the explorations of heterogeneous catalysts are necessary to overcome the drawbacks in a two-step transesterification process. 3.2.3. Enzyme-catalyzed transesterification with alcohol The disadvantages of using chemicals of either acid or alkaline catalysts for the transesterification of jatropha oil (such as pre-treatment of feedstock, glycerol recovery, catalyst removal,

wastewater treatment and high energy requirement in the process) have drawn many researchers to seek for more environmental friendly approach. Enzymatic approach is likely to overcome the problems of a chemical-catalyzed process. Enzyme catalyzed reaction is more efficient, highly selective, involves less energy consumption, produces less waste (Akoh et al., 2007) and it is recyclable as enzymes can be immobilized onto a support medium (Robles-Medina et al., 2009). Glycerol recovery in enzymatic process is easier as it would produce high grade glycerol as compared to an alkaline process (Robles-Medina et al., 2009). It is reported that the enzymatic reactions are insensitive to FFA and water content in the raw material (Kulkarni and Dalai, 2006) (Table 5). Lipase is a class of enzymes which belongs to the serine hydrolyzes family (Haeffner et al., 1998). Lipase catalyzes hydrolysis of long chain, insoluble triglycerides and other insoluble esters of fatty acids (Pleiss et al., 1998). Due to lipase selectivity, it would give a high yield to a specific product as compared to non-enzymatic reactions (Malcata et al., 1990). Synthesis of biodiesel from jatropha oil using lipases from Candida antarctica (Novozym 435), Mucor meihei, Thermomyces lanuginosus (Lipozyme), Pseudomonas cepacia and Rhizopus oryzae have been reported in literatures (Table 6, No. 1–4). Among those lipases, lipase B from C. Antarctica (Novozym 435) has been ranked to be the most effective lipase in biodiesel production by other researchers (Akoh et al., 2007; Talukder et al., 2009). Novozym 435 is the commercial C. antarctica lipase B that has been immobilized onto micro-acrylic resins. Various alcohols can be utilized for transesterification of jatropha oil using enzymatic approaches. In most countries, methanol is still considered to be the best choice for large scale industrial production of biodiesel, due to its low cost and availability (Du et al., 2008). Su and Wei (2008) screened three lipases, namely C. antarctica lipase B (Novozym 435), Thermomyces lanuginosus lipase (Lipozyme TL IM) and Rhizomucor meihei lipase (Lipozyme RM IM) to facilitate the transesterification of jatropha oil using co-solvent to avoid lipase deactivation cause by methanol and glycerol. The reaction conditions developed were proven to be effective for the transesterification of jatropha oil. The highest conversion was up to 98% by using Novozym 435 in 24 h. The optimized reaction conditions were as follows: temperature (45 °C), stirring speed (150 rpm), enzyme amount to oil (7.5% w/w), molar ratio of methanol to oil (5:1), and a co-solvent consisted of 25% pentanol and 75% iso-octane. Lipozyme TL IM and Lipozyme RM IM gave lower

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Table 6 Summary of enzymatic transesterification of jatropha oil. No.

Authors

Lipase

Time (hrs)

Reaction temp (°C)

Alcohol/oil molar ratio

Conv (%)

Remarks

1

Shah and Gupta (2007)

2

Su and Wei (2008)

Kumari et al. (2009) Tamalampudi et al. (2008)

8 24 24 24 24 24 48 60 90

50 40 40 45 45 45 55 30 30

4:1 4:1 4:1 5:1 5:1 5:1 4:1 3:1 3:1

98 65 91 98 77 78 68 80 75

Addition of 5% w/w of water No solvent No solvent A mixture of 25% pentanol and 75% iso-octane were used as solvent

3 4

Pseudomonas cepacia on Celite Free Pseudomonas cepacia Pseudomonas cepacia on Celite Candida antarctica lipase B (Novozym 435) Thermomyces lanuginosus (Lipozyme TL IM) Rhizomocour meihei (Lipozyme RMIM) Enterobacter aurogenes on activated silica Rhizopus oryzae on polyurethane foam Candida antarctica lipase B (Novozym 435)

catalytic activity as compared to that of Novozym 435 under the same reaction condition. Yield of methyl esters was only 77% and 78% with Lipozyme TL IM and Lipozyme RM IM, respectively (Table 6, No. 2). Considering the cost of Novozym 435, which is more expensive, they tried to mix those three enzymes to reduce the lipase cost. The results indicated that at least 1% w/w of Novozym 435 to oil was needed, so that the reaction could achieve >95% conversion. The downside of this research was the use of solvent engineering method. Lipase from Enterobacter aurogenes has also been immobilized onto activated silica with ethanolamine for transesterification of jatropha oil (Kumari et al. 2009). They also used t-butanol as a solvent to eliminate lipase deactivation. However, the highest methyl esters yield that could be obtained was 68% only after 48 h. The lipase from E. aurogenes resulted in a lower yield as compared to that of Novozym 435. As mentioned by Dossat et al. (2002), using a solvent in biodiesel production would lead to several problems such as toxicity of the solvent, involves higher investment cost to meet safety requirement and necessity to perform purification to remove the solvent. Enzyme catalysts are known to be sensitive to alcohol inactivation and it would result in low conversion of alkyl esters. Stepwise addition of methanol may overcome the problem of lipase deactivation in solvent-free system in biodiesel production (Ranganathan et al., 2008). Shah and Gupta (2007) and Tamalampundi et al. (2008) have conducted the transesterification of jatropha oil via stepwise addition of alcohol. Another advantage of stepwise methanolysis is that the FFA and water in jatropha oil will have little or no effect in the efficiency of transesterification of enzyme catalyst (Kulkarni and Dalai, 2006). Shah and Gupta (2007) reported transesterification of jatropha oil with ethanol using P. cepacia lipase immobilized on Celite. Free P. cepacia lipase only gave 65% conversion whereas immobilized P. cepacia lipase on Celite gave more than 91% conversion. Thus, immobilization has enhanced the catalytic activity of the lipase. By adding an extra 5% of water, they successfully obtained 98% of conversion in 8 h using 10% w/w of immobilized P. cepacia to oil at 50 °C (Table 6, No. 1). Addition of water was able to increase the yield but the existence of water in biodiesel will cause damage to the engine. Furthermore, the addition of water is more complicated in design and tedious in large scale production (Shah and Gupta, 2007). The immobilized biocatalyst could be used four times without any loss of activity. It is shown that P. cepacia lipase is resistant towards ethanol or methanol inactivation as there is no significant difference when alcohol is added singly or stepwise. The transesterification of jatropha oil under a solvent-free system has also been investigated by Tamalampundi et al. (2008) in a stepwise addition of methanol using Novozym 435 and R. oryzae lipase. The R. oryzae lipase was immobilized onto the biomass support particle (reticulated polyurethane foam). The conversion of jatropha oil to methyl esters with R. oryzae lipase was 5% higher than that of Novozym 435 (Table 6, No. 4). The methanol to oil

t-butanol was used as solvent No solvent No solvent

molar ratio, temperature and enzyme to oil were 3:1, 30 °C and 4% w/w was used respectively. They found that R. oryzae lipase exhibit higher conversion as compared to that of Novozym 435 regardless the types of alcohol used. Among the alcohols, methanol is the most active for biodiesel production from jatropha oil. In another work, Rathore and Madras (2007) studied the effects of enzymatic activity in transesetification of jatropha oil in supercritical carbon dioxide with methanol and ethanol. They found that the optimum conversion obtained under supercritical condition in the presence of Novozym 435 was only 35% and 50% using methanol and ethanol, respectively. The optimum reaction conditions for both alcohols were as follows: 45 °C in supercritical CO2, molar ratio of 5:1 (alcohol:oil), reaction time of 8 h and enzyme amount of 30% w/w (enzyme to oil). The conversion is much lower as compared to that of under a supercritical methanol/ethanol tranesterification. The summary of enzyme-catalyzed transesterification in the arrangement of reaction time is presented in Table 6. Enzymecatalyzed transesterification is a promising method to produce biodiesel from jatropha oil in a more environmental friendly process. Enzyme catalyzed reaction requires a lower temperature and also a lower alcohol to oil molar ratio. Among the lipases used to produce biodiesel from jatropha oil, only Novozym 435 and immobilized P. cepacia lipase are capable of achieving 98% conversion (Table 6, No. 1 and 2). It is obvious that enzymatic transesterification needs a longer time to produce biodiesel as compared to other approaches such as an alkaline catalyzed reaction. Moreover, much longer reaction time is required when a solvent-free enzymatic system is used. For example, Novozym 435 was able to achieve 98% conversion in 24 h when n-pentanol and iso-octane were used but the conversion was only 75% after 90 h under a solvent-free system (Table 6, No. 2 and 4). One might argue that the comparison between these two studies was not valid because the reaction conditions were different in both studies. However, it has been proven that Novozym 435 exhibit lower catalytic activity under a solvent-free system using palm oil and methanol as a reactant (Talukder et al. 2009). It is similarly possible to embed the enzymatic activity of jatropha oil with methanol will be also slower under solvent-free system. The current problem associated with enzyme is enzyme deactivation caused by methanol and glycerol inhibition especially under a solvent-free system (RoblesMedina et al., 2009; Talukder et al., 2009). The use of solvents will solve this problem but may lead to other issues such as toxicity, higher cost and environmental problems. Besides, the cost of biodiesel production is more expensive by using enzymatic approach and a longer reaction time is required as compared to a chemicalcatalyzed reaction (Ranganathan et al., 2008; Robles-Medina et al., 2009; Shah and Gupta, 2007). Therefore, many studies need to be conducted to overcome the current problem of enzymatic transesterification before taking biodiesel production from a lab to an industrial scale.

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3.2.4. Enzyme-catalyzed transesterification with alkyl acetates The use of alkyl acetate to substitute alcohols as acyl acceptor in the transesterification process has been reported (Modi et al., 2007). The application of solvent to reduce enzyme deactivation is not applicable as it would generate more toxic waste. In addition, it is necessary to purify fatty acid esters from the solvent and the use of solvents would also increase the overall cost of biodiesel production. Low alkyl esters yield and slow reaction have been a problem in the transesterification process using alcohol in a solvent-free system. Transesterification with alkyl acetates would produce fatty acid esters and triacetylglyrerol. Thus, this method would eliminate the risks of enzyme deactivation caused by alcohol or glycerol. Modi et al. (2007) have studied the use of ethyl acetate to produce biodiesel from jatropha oil catalyzed by Novozym 435. The optimum ethyl esters yield obtained was 91% in 12 h using ethyl acetate to jatropha oil of 11:1, 10% w/w of Novozym 435 and the reaction temperature at 50 °C. The enzyme activity could be well maintained over 12 repeated cycles when ethyl acetate was used, but the enzyme activity dropped to zero on the sixth cycle when ethanol was employed. This once again proves that enzymes are easily poisoned or deactivated by alcohol or glycerol. Su et al. (2007) studied in situ transesterification of jatropha oil using alkyl acetates with Novozym 435. They investigated in situ extraction of oilseed oil from jatropha kernel using alkyl acetates and then followed by transesterification by Novozym 435 with the same alkyl acetate. The highest ester yield obtained from methyl acetate and ethyl acetate at molar ratio of 7.5:1 using 30% w/w oil Novozym 435 to oil after 36 h were 86% and 87%, respectively. The results indicated that the type of alkyl acetate (methyl or ethyl acetate) did not cause any significant difference in the yield. In situ reactive extraction and transesterification is an effective method because oil extraction and transesterification reaction are combined into a single process. This method would also reduce the reaction time and eliminate the problems associated with solvent extraction.

4. Conclusion and future work Jatropha oil is the future renewable source of non-edible oil for biodiesel production. An alkaline catalyst is more suitable for biodiesel production if the FFA content in the jatropha oil is lower than 1%, such as RDJO. If the FFA content is more than 1%, a twostep transesterification process is a better choice. However, an extra step is required and it would increase the cost of biodiesel production. Enzymatic approach is a very good option to all chemical-catalyzed transesterification of jatropha oil. Nevertheless, it must be optimized under a solvent-free system in order to become a green and commercial viable process.

Acknowledgement We are grateful to Monash University funding (Grant BCHHSS-2-05-2009) for the financial support on this project.

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