The effects of catalysts in biodiesel production: A review

Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

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Review

The effects of catalysts in biodiesel production: A review I.M. Atadashi, M.K. Aroua *, A.R. Abdul Aziz, N.M.N. Sulaiman Chemical Engineering Department, Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia

A R T I C L E I N F O

Article history: Received 9 May 2011 Accepted 14 July 2011 Available online 20 July 2012 Keywords: Biodiesel Production Alkaline catalyst Acid catalyst Solid catalyst

A B S T R A C T

Biodiesel fuel has shown great promise as an alternative to petro-diesel fuel. Biodiesel production is widely conducted through transesterification reaction, catalyzed by homogeneous catalysts or heterogeneous catalysts. The most notable catalyst used in producing biodiesel is the homogeneous alkaline catalyst such as NaOH, KOH, CH3ONa and CH3OK. The choice of these catalysts is due to their higher kinetic reaction rates. However because of high cost of refined feedstocks and difficulties associated with use of homogeneous alkaline catalysts to transesterify low quality feedstocks for biodiesel production, development of various heterogeneous catalysts are now on the increase. Development of heterogeneous catalyst such as solid and enzymes catalysts could overcome most of the problems associated with homogeneous catalysts. Therefore this study critically analyzes the effects of different catalysts used for producing biodiesel using findings available in the open literature. Also, this critical review could allow identification of research areas to explore and improve the catalysts performance commonly employed in producing biodiesel fuel. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Techniques for biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Feedstocks for biodiesel production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. The effects of homogeneous catalyst in biodiesel production . . . . . . . . . . . . . . . . . The effects of alkaline catalysts in biodiesel production. . . . . . . . . . . . . . . . 3.1. The effects of acid catalysts in biodiesel production. . . . . . . . . . . . . . . . . . . 3.2. The effects of homogeneous catalysts on the yield of biodiesel fuel . . . . . . 3.3. The effects of heterogeneous catalysts in biodiesel production. . . . . . . . . . . . . . . . The effects of solid alkaline and acid catalysts in biodiesel production . . . . 4.1. The effects of solid alkaline catalysts in biodiesel production. . . . 4.1.1. The effects of solid acid catalysts in biodiesel production. . . . . . . 4.1.2. The effects of enzymes catalysts in biodiesel production . . . . . . . . . . . . . . . 4.2. The effects of heterogeneous catalysts on the yield of biodiesel . . . . . . . . . 4.3. The advantages and disadvantages of homogeneous catalysts and heterogeneous Conclusions and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Energy is the prime mover for socio-economic development. The World’s economic growth is affected by climatic change, fuel

* Corresponding author. Tel.: +60 3 79674615; fax: +60 3 79675319. E-mail address: [email protected] (M.K. Aroua).

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price hike, and the gradual depletion of fossil fuel reserves. Therefore, to increase energy security for economic development, the need to search for an alternative source of energy such as biodiesel is necessary [1,2]. Biodiesel is renewable, sustainable, biodegradable, and emits low greenhouse gases [3,4]. As well, the oxygen content of 11–15% in the molecular structure speed up the combustion process in compression ignition engines and decreases pollutants such as soot, fine particles, and carbon monoxide (CO)

1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.07.009

I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

O || CH2 - O - C - R1 | O | | || CH - O - C - R2 + 3 R4OH (KOH) | | O | || CH2 - O - C - R3 Triglyceride

alcohol

O || R4 - O - C - R1 + O || R4 - O - C - R2 + + O || R4- O - C - R3

CH2 - OH | CH - OH | CH2 - OH

Alkyl Esters

glycerin

Fig. 1. Transesterification reaction of triglycerides via alkaline catalyst.

[5,6]. Thus, biodiesel is a potential substitute to replace/supplement petro-diesel fuel [7,8]. Biodiesel fuel is produced via transesterification of refined vegetable oil, waste cooking oil, and used frying oil using alkaline catalysts [9–12] as shown in Fig. 1. The nature of catalyst employed during transesterification reaction is crucial in converting triglycerides to biodiesel. As a result different catalysts have being explored for converting triglycerides to biodiesel fuel. The catalysts usually employed to catalyze transesterification reaction are homogeneous catalysts and heterogeneous catalysts. Conventionally, homogeneous alkaline catalysts such as NaOH, KOH, CH3ONa, and CH3OK are more often used in producing biodiesel [13]. The catalytic performance of these catalysts and their ability to perform under moderate conditions has led to their choice [14]. Among these homogeneous alkaline catalysts, CH3ONa is most active, providing biodiesel yield above 98 wt% in short reaction time (30 min) [15,16]. However because of low price, industrial biodiesel production process mostly employs NaOH and KOH [14]. The process involving these catalysts needs high-quality feedstocks, thus the free fatty acid (FFAs) level of the feedstocks should not exceed 3 wt%, beyond which the reaction will not occur. In addition, water content of the feedstocks is critical, as a result the feedstocks used in alkali-catalyzed transesterification have to anhydrous [14,17]. Thus, presence of water leads to hydrolysis of oils to FFAs. Fig. 2 shows water hydrolysis of fats and oils to form free fatty acid. The FFAs react with alkaline catalysts to produce soaps formation. Fig. 3 presents soaps formation in homogeneous alkali-catalyzed transesterification. Soaps formation consumes the catalyst, deactivates it and makes biodiesel purification process difficult [13,18]. Therefore, preparation of biodiesel by low quality feedstocks containing huge quantity of FFAs and water needs sound technology [19]. However, high cost of refined feedstocks result in high price of biodiesel compared to diesel fuel [15,20]. The

CH2 - O - CO - R1 | | | CH - O - CO - R2 + | | | CH2 - O - CO - R3 Triglyceride

CH2 - OH | | O | || CH - O - CO - R2 + HO - C-R1 | | | CH2 - O - CO- R3

H2O

Water

Diglyceride

+

KOH

Potassium hydroxide

R1 - COOK (Soap)

cost of refined feedstocks, account to over 70% of the overall cost of biodiesel production [21]. As a result, different kinds of low quality feedstocks such as: waste cooking oils, used cooking oil, greases (yellow and brown), and non-edible oils have being investigated [13,22]. The price of low quality feedstocks such as waste cooking oil is 2–3 times lower than refined oils. Nonetheless, the feedstocks contains higher amount of FFAs and water contents. This features makes their processing challenging [23,24]. Therefore to augment their processing difficulties, acid-catalyzed transesterification is first employed to decrease the content of FFAs before performing alkali-catalyzed transesterification [19]. Thus adopting two-step transesterification technique could provide large biodiesel conversion of up to 98% [25]. Recently heterogeneous catalysts such as solid catalysts and enzyme catalysts are employed to catalyze transesterification reaction for producing biodiesel. Heterogeneous catalysts offers many advantages over homogeneous catalysts such as; simple catalyst recovery, catalyst reusability, simple product purification, less energy and water consumption, less added cost of purification, and simple glycerol recovery. Besides, most of the heterogeneous catalysts used especially solid alkaline catalysts have provided high yields [26], though faced with problem of leaching [27]. Also, the stability of enzymes catalysts in non-aqueous media is significant to its excellent catalytic activity, this improves transesterification and esterification during biodiesel production [28], and providing high biodiesel yield (95 wt%) [29]. However, the problem mostly associated with enzymes catalysts is the cost of the enzymes, but immobilization of the catalyst could mitigate the cost [30]. Therefore, to achieve biodiesel that is economically feasible, development of active and cheap catalysts for effective transesterification of different kinds of feedstocks is necessary [31]. In this regards, this study extensively examined and reported the effects of different catalysts in producing biodiesel fuel. 2. Biodiesel production 2.1. Techniques for biodiesel production Biodiesel is usually produced through different techniques such as direct/blends [32,33], microemulsion [34,35], pyrolysis [36,37] and transesterification [38,39]. As stated earlier, alkali-catalyzed transesterification is the most adopted technique for producing biodiesel, this method usually needs refined feedstocks containing less FFAs content otherwise it will result to much soaps formation. For alkali-catalyzed transesterification, if the feedstocks contains high amount of FFAs then it has undergo pretreatment steps before transesterification [40]. Hideki et al. [14] and Ramadhas et al. [25] recommended acid value of feedstocks to be less than 4.0 mg KOH/g before performing alkaline transesterification. Although, Canakci and Gerpen [41] and Mittelbach [42] stated that before alkalicatalyzed transesterification, the acid value of a feedstock has to be 2.0 mg KOH/g. Nonetheless, the use of heterogeneous catalysts in biodiesel production has reduced the effects of using low quality feedstocks, especially enzymes catalysts that has the potential of converting FFAs into biodiesel, besides high purity by-products [14]. 2.2. Feedstocks for biodiesel production

Free fatty acid

Fig. 2. Water hydrolysis of fats and oils to form free fatty acid (FFAs).

O || R1- C -OH Free fatty acid

15

+

H2O Water

Fig. 3. Soap formation in homogeneous alkali-catalyzed transesterification.

Biodiesel production is achieved via different kinds of feedstocks. The nature of feedstock used is dependent on the geographical position and climate of the place. For instance Europe employs sunflower and rapeseed oils, palm oil predominates in tropical countries, soybean in United States and canola oil in Canada [43]. Singh and Singh [44] reported the major feedstocks employed in producing biodiesel are cotton seed, palm oil, sunflower, soybean, canola, rapeseed, and Jatropha curcas.

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I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

Table 1 Level of FFAs recommended for alkali-catalyzed transesterification.

Table 3 FFAs levels in feedstocks.

FFAs recommended (%)

Reference

Feedstocks

FFA levels

References

1 1 <3 <0.5 <3 <0.5 <0.5 2 <1 <2 0.5 1 <1

Demirbas [16] Chongkhong et al. [19] Ramadhas et al. [25] Adam Khan [35] Canakci and Van Gerpan [41] Zhang et al. [45] Martino et al. [47] Sahoo et al. [48] Ma and Hanna [49] Huang and Chang [50] Szczesna Antczak et al. [51] Marchetti et al. [52] Tiwari et al. [53]

Trap grease Refined vegetable oils Finished greases Crude soybean oil Restaurant waste grease Waste palm oil Municipal sludge Animal fat Trap grease Use cooking oil Waste oil

50–100% <0.05% 8.8–25.5% 0.4–0.7% 0.7–41.8% >20% Up to 65% 5–30% 75–100% 2–7% 46.75%

Sharma and Singh [3] Davies [71] Canakci [41] Canakci [41] Canakci [41] Balat and Balat [23] Bryan [58] Davies [71], Van Gerpen [69] Davies [71], Van Gerpen [69] Van Gerpen [69] Nie [70]

Additionally, Zhang et al. [45] remarked that employing feedstocks such as waste frying oils, non-edible oils, and animal fats, as feedstocks could be useful in producing biodiesel. Although, Banerjee and Chakraborty [46] stated that FFAs contents in the waste cooking oil should be kept within certain limit for reaction involving both acid- and alkali-catalyzed transesterification reactions. Otherwise these substances may cause severe difficulties in refining of biodiesel products. Table 1 presents the recommended FFAs values for alkali-catalyzed transesterification method. While Table 2 shows FFAs contents of different vegetable oils. In addition, Table 3 presents FFAs levels of most of the feedstocks used to produce biodiesel. The cost of feedstocks decreases as FFAs content increases. In case of industrial biodiesel production, there is need for low-cost (high FFAs) feedstocks such as used cooking oils, waste cooking oils, and non-edible vegetable Table 2 Values of FFAs content of different vegetable oils. Vegetable oils

FFA levels (%) Reference

Polanga oil Cottonseed oil Tobacco oil Spent bleaching earth Rubber oil Palm fatty acid distillate (PFAD) Pongamia pinnata Palm oil Rape seed oil Tall oil Jatropha oil Tung oil Soybean soapstock Ponagamia oil Pongamia oil Salvadora oil Moringa oleifera Karanja oil Sorghum bug oil Mahua oil Mahua oil Madhuca indica Zanthoxylum bungeanum Acid oil Karanja oil Trap grease Finished greases Crude soybean oil Restaurant waste grease Waste palm oil Municipal sludge Animal fat Trap grease Use cooking oil Waste oil

22.0 0.11 35.0 24.1 17.0 93 20 5.3 2.0 100 14.0 9.55 >90 0.64 2.69 1.76 2.9 2.53 10.5 21.0 19.0 20.0 45.5 59.3 2.53 50–100% 8.8–25.5% 0.4–0.7% 0.7–41.8% >20% Up to 65% 5–30% 75–100% 2–7% 46.75%

Sahoo et al. [48] Sahoo et al. [48] Veljkovic et al. [54] Huang and Chang [50] Ramadhas et al. [25] Chongkong et al. [19] Naik et al. [55] Balat and Balat [23] Balat and Balat [23] Demirbas [56] Tiwari et al. [54], Bryan [57] Bryan [57] Bryan [57] Sureshkumar et al. [58] Kaul et al. [59] Kaul et al. [59] Rashid et al. [60] Sharma and Singh [61] Mariod et al. [62] Puhan et al. [63] Ghadge and Raheman [64] Kumari et al. [65] Zhang and Jiang [66] Haas et al. [67] Meher et al. [68] Sharma and Singh [3] Canakci [41] Canakci [41] Canakci [41] Balat and Balat [23] Bryan [57] Van Gerpen [69] Van Gerpen [69] Van Gerpen [69] Nie [70]

oils. Since biodiesel fuels from refined oils are costly when compared to petro-diesel fuel. Besides, the application of such feedstocks in biodiesel production could minimize competition between demand of edible vegetable oils and cost of biofuel [27]. In addition, application of vegetable oils as sources of biodiesel needs great efforts to either develop more productive plant species with a high yield of oil or to increase oilseeds’ production [72]. Further, many studies have reported microalgal oil as feedstocks for producing biodiesel [73–78]. Demirbas [79] noted that microalgal oil is the only feedstock that can meet the global demand for transport fuels. The author also reported that soon, microalgal oil will become the most important feedstocks for biofuel production. Singh and Gu [80] reported that microalgae feedstocks are receiving great attention as sources of energy because of their quick growth potential coupled with reasonably high lipid, carbohydrate and nutrients contents. In addition, Demirbas [81] highlighted that microalgae possess much quicker growth-rates than terrestrial crops. The author noted the per unit area yield of oil from algae is estimated to be from 20,000 to 80,000 l per acre, per year. In deed this is 7–31 times more than the next best crop, palm oil. Table 4 presents lipid content in the dry biomass of various species of microalgae [82].

Table 4 Lipid content in the dry biomass of various species of microalgae [82]. Species

Lipid content (% dryweight)

Botyococcus braunii Chlamydomonas reinhardtii Chlorella emersonii Chlorella protothecoides Chlorella pyrenoidosa Chlorella vulgaris Crypthecodinium cohnii Cylindrotheca sp. Dunaliella primolecta Dunaliella salina Dunaliella tertiolecta Euglena gracilis Hormidium sp. Isochrysis sp. Monallanthus salina Nannochloris sp. Nannochloropsis sp. Neochloris oleoabundans Nitzschia sp. Phaeodactylum tricornutum Pleurochrysis carterae Prymnesium parvum Scenedesmus dimorphus Scenedesmus obliquus Schizochytrium sp. Spirulina maxima Spirulina platensis Tetraselmis sueica

25–80 21 28–32 57.9 46.7 14–22 20 16–37 23 6 35.6 14–20 38 25–33 >20 30–50 31–68 35–54 45–47 20–30 30–50 22–38 16–40 12–14 50–77 6–7 4–9 15–23

I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

17

Base catalysed SN2 Transesterification

O C H-O-H

CH -O-H 3

O

Gly'

K-O-H O C CH3-O

O

Gly'

-

O C

O

Gly'

H

CH3-O

O-CH3 H-O-H O C CH3-O

HO-Gly' HO-CH 3 -OH

Fig. 4. Mechanism of base-catalyzed transesterification reaction.

Also to highlight the significance of microalgae as feedstock for biodiesel production, Tran et al. [83] have reviewed biodiesel production via microalgal oil. The authors noted that catalysts such as acid, base and zeolites catalysts can used in catalyzing transesterification involving microalgal oil as feedstocks. 3. The effects of homogeneous catalyst in biodiesel production 3.1. The effects of alkaline catalysts in biodiesel production Application of alkali-catalyzed transesterification reaction provide faster rate, nearly 4000 times faster than that catalyzed by the same amount of an acid catalyst [14]. Some of the alkaline catalysts used for the transesterification reaction include among others; NaOH [19,84], KOH [85,86], and sodium methoxide [16,84]. Other alkaline catalysts include; sodium ethoxide [57], potassium methoxide [3,13], sodium propoxide [57], sodium butoxide [31] and carbonates [31,87], etc. Based on biodiesel yield, CH3ONa or CH3OK are better and more suitable catalyst than NaOH and KOH. Thus, CH3ONa and CH3OK are more suitable due to their ability to dissociate into CH3O and Na+ and CH3O and K+ respectively. Besides, the catalysts do not form water during transesterification reaction [3]. For these reason, alkaline catalyst is mostly preferred in commercial production of biodiesel fuel [13]. Transesterification of refined oils with less than 0.5 wt% FFAs via chemical catalysts could lead to high-quality biodiesel fuel with better yield within short time of 30–60 min [88]. Fig. 4 presents the mechanism of base-catalyzed transesterification reaction [71]. Vicente et al. [89] have compared different basic catalysts (sodium hydroxide, potassium hydroxide, sodium methoxide and potassium methoxide) to produce biodiesel fuel using sunflower oil. The reactions were conducted at temperature of 65 8C, methanol to oil molar ratio of 6:1 and basic catalyst by weight of vegetable oil of 1%. They

achieved 85.9 and 91.67 wt% yield of esters for NaOH and KOH and 99.33 and 98.46 wt% yields of esters for CH3ONa and CH3OK respectively. The authors recorded 98 wt% yields of esters for methoxides after separation and purification steps were completed. Further, less yields losses and negligible ester dissolution in glycerol were observed with methoxides compared to hydroxides. Umer et al. [90] used alkali catalyst to produce sunflower oil methyl esters. They reported notable yield of 97.1 wt% at 60 8C. In addition, alkaline catalysts concentrations ranging from 0.5–1 wt% could yield 94–99 wt% conversion of vegetable oils to alkyl esters. However, increase in catalyst concentration above 1 wt% does not increase the conversion but could add to extra costs of production. Since it is essential to get rid of the catalyst from the products after the reaction is completed [91,92]. Chung [93] transesterified V. fordii and C. japonica seed oils with methanol using alkaline catalysts (KOH, NaOH, and CH3ONa) to produce biodiesel. The authors noted that KOH provided higher catalytic activity to the seed oils in the reaction. The optimum reaction conditions used were: 6:1 molar ratio of methanol to the seed oils, 1 wt% loading amount of catalyst, 65 8C reaction temperature, and reaction time of 3 h. The biodiesel contents of the C. japonica and V. fordii seed oils under these reaction conditions were 97.7% and 96.1% on KOH catalyst. Figs. 5 and 6 shows the effects of alkaline catalysts on feedstocks contain high amount of water and FFAs [94]. The lesser the water and FFAs contents the better yield vice versa. In another study, Sahoo et al. [48] employed alkaline transesterification after reducing the FFAs value from 44 mg KOH/g a below 4 mg KOH/g through acid catalyzed transesterification. The author found that 1.5 wt% of KOH was adequate to obtained maximum biodiesel yield. Marchetti and Errazu [95] revealed that ethanol and sulfuric acid are suitable to carry out both direct esterification and transesterification reactions simultaneously. These processes could

I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

18

Alkaline catalyst

110

Methyl esters (%)

100 90 80 70 60 50 40

0

5

10

15

20

25

30

35

Water content (%) Fig. 5. Graph of methyl esters yield against water content in transesterification reaction.

Alkaline catakyst

105 100

Methyl esters (%)

95 90 85 80 75

separation time of 30 min. Also, Saifuddin and Chua [97] optimized transesterification of used frying oil to ethyl ester using microwave irradiation. They used a microwave oven equipped with non-contact infrared continuous feedback temperature system and magnetic stirrer to heat the oil and the alcohol at 60 8C. Twenty five percent (25%) of an exit power of 750 W was used to irradiate the reaction mixture. The authors experimented different concentrations of sodium methoxide (0.3 wt% to 0.5 wt%) and achieved maximum conversion (87 wt%) at 0.5 wt%. During transesterification process, both sodium ethoxide and potassium hydroxide provided good conversions. However, due simplicity in products phase separation, sodium ethoxide was viewed as most promising catalyst for producing biodiesel. Besides, microwave-assisted transesterification process dramatically reduced the reaction time from 75 min to 4 min at 60 8C, thus saving great time. Additionally during transesterification, irradiation times must be controlled and the levels of radiation power should not be too high, to avoid destruction of organic molecules. Moreover, to determine optimum operating parameters such as catalyst concentration, Ferella et al. [98] studied transesterification reaction of rapeseed oil for biodiesel production using response surface methodology (RSM). They employed a 500 ml jacketed stirred (at 600 rpm) reactor tank. At optimum conditions of temperature of 50 8C, KOH concentration of 0.6% (w/w), reaction time of 90 min, and 60 methanol to KOH ratio by weight, large amount of triglycerides, diglycerides and monoglycerides were converted into biodiesel. As well, the final concentrations were 0.05% triglycerides, 0.09% diglycerides and 0.36% monoglycerides and the triglyceride conversion was 98–99%. The effects of catalysts such as NaOH on transesterification of rapeseed oil under supercritical/subcritical conditions were also studied by He et al. [99]. The authors employed little quantity of NaOH as catalyst and achieved excellent biodiesel yield with no soaps formation. 3.2. The effects of acid catalysts in biodiesel production

70 65

0

1

2

3

4

5

6

FFA (%) Fig. 6. Graph of methyl esters yield against FFA in transesterification reaction.

effectively convert waste cooking oil containing high amount of FFAs ranging from 3% to 40% to biodiesel. The authors noted the FFAs content was reduced via esterification process from 10.684% to a value close to 0.54 wt%. Though, the final FFAs concentration was slightly more than the recommendable quantity. They reported minimized soaps formation during alkali-catalyzed transesterification reaction. Further, to improve biodiesel production process, Refaat et al. [96] studied microwave irradiation technique to produce biodiesel. They employed sunflower oil (used 3 times at a cooking temperature of 130 8C) and methanol to oil ratio of 6:1 in the presence of 1 wt% of potassium hydroxide at 65 8C. The authors used a normal pressure glass reactor 500 ml flask and reflux condenser. Using the microwave system, the vegetable oil was preheated to a desired temperature of 65 8C. The mixture of alcohol and catalyst then charged into the flask through the condenser. The power output adjusted to 500 W and under reflux the mixture irradiated via different reaction times of 0.5, 1, 1.5, 2, 2.5, 3 and 6 min. Using microwave irradiation technique, reaction time was reduced by 97% and the separation time reduced by 94%. They recorded biodiesel yield of 100% within 2 min and

The most notable acids commonly employed in transesterification reaction include among others; sulfuric acid [13,19,100], sulfonic acid [14], hydrochloric acid [16], organic sulfonic acid [14], ferric sulfate [13], etc. Among these acids, hydrochloric acid, sulfonic acid and sulfuric acid are usually favored as catalysts for the production of biodiesel. The catalyst and the alcohol are vigorously mixed with a stirrer in a small reactor. The oil is first charged into the biodiesel reactor and then the mixture of catalyst/alcohol is fed into the oil. Brønsted acids preferably sulfuric acid or sulfonic acid is used to catalyze the reaction. Although the catalysts give high yield of biodiesel, but the reaction rates are slow. The alcohol to oil molar ratio is the main factor influencing the reaction. Therefore addition of excess alcohol speeds up the reaction and favors the formation biodiesel products. The steps involve during acid-catalyzed transesterification are: (1) initial protonation of the acid to give an oxonium ion (2) the oxonium ion and an alcohol undergo an exchange reaction to give the intermediate (3) and this in turn can lose a proton to become an ester. Reversibility of each of the above step is possible but the equilibrium point of the reaction is displaced in the presence of excess large alcohol, by allowing esterification to advance to completion [16]. Leung et al. [40] reported that esterification by acid-catalysis makes the best use of the FFAs in the animal fats and vegetable oils and converts them into fatty acid alkyl esters. The authors noted that one-step esterification pretreatment may not reduce the FFAs efficiently, if the acid value of the oils or fats is very high. This is because of the high content of water produced during the reaction. In this case, addition of

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19

Table 5 Reaction yield as function of the homogeneous catalyst weight. Feedstock

Catalyst

Conc. (wt/v/v)

Reaction time (h)

Reaction temp (8C)

Yield/conv. (w/w%)

Molar ratios

Reference

Waste tallow (chicken) Palm fatty acid Sunflower oil Jojoba oil-wax Brassica carinata Canola oil Jatropha curcas Cottonseed oils Roselle oil Rubber seed oil (Hevea brasiliensis) Mahua oil (Madhuca indica) Mahua oil Sunflower frying oil Tobacco seed oil Rice bran oil Used frying oil Waste cooking oils Jatropha oil Karanja oil

H2SO4 H2SO4 KOH Sodium methoxide KOH KOH KOH Sodium hydroxide Potassium hydroxide NaOH NaOH H2SO4/KOH KOH NaOH Sulfuric acid KOH KOH H2SO4/KOH KOH

1.25 – 0.55 1 – 0.5

24 2 – 4 – 0.33 1 1 1 1 2 1 0.5 1.5 8 2 0.33–2 2 3

50 70 70 60 – 25 30 55 60 60 60 60 25 55 100 60 30–50 60 65

99.01  0.71 99.6 96 55 98.27 86.1 92 77 99.4 84.46 92 98 max max 98 72.5 88–90% 90–95 97–98

1:30 7.2:1 – 7.5:1 – 6:1 – – 8:1 6:1 6:1 6:1 6:1 3:1 – 12:1 7:1–8:1 6:1/9:1 6:1

[85] [19] [104] [105] [106] [107] [108] [109] [110] [111] [63] [64] [112] [113] [21] [12] [11] [114] [115]

0.5 1.5 1 1 1/0.7 1 1.5 2 1 0.75 0.25–1.5/0.5 1

mixture of alcohol and sulfuric acid into the oils or fats three times (three-step pre-esterification) is required. The time needed for this process is about 2 h and removal of water is necessary by a separation funnel before adding the mixture into the oils or fats for esterification again. Further, Palligarnai and Briggs [24] reported sulfuric acid catalyzed transesterification to provide a few advantages over base-catalyzed technique such as one-step process as opposed to a two-step transesterification. The authors also reported that feedstock with a high FFAs content could be easily handle, downstream separation is straightforward, and a high-quality glycerol byproduct is achievable. Additionally, Soriano Jr. et al. [101] studied transesterification of canola oil to produce biodiesel via homogeneous Lewis acid (AlCl3 and ZnCl2) as catalyst. The reaction occurred in a round bottom flask submerged in an oil bath equipped with a reflux condenser, temperature controller and a magnetic stirrer. The authors reported use of variable parameters such as: reaction time (6, 18, 24 h), methanol to oil molar ratio (6, 12, 24, 42 and 60), reaction temperature (75, 110 8C). And tetrahydrofuran (THF) as co-solvent (1:1 methanol to THF by weight in runs with THF), and catalyst (AlCl3 or ZnCl2). In all the runs, the catalyst amount was kept at 5% based on the weight of oil. 1H NMR monitored converting canola oil into fatty acid methyl esters. Thus the best conditions with AlCl3 were 24:1 molar ratio at 110 8C and 18 h reaction time with THF as co-solvent provided a conversion of 98%. AlCl3 was far more active compare to ZnCl2 due to its higher acidity. In another study, Cardoso et al. [102] discussed the effects of Lewis acid on the transesterification process in producing biodiesel. The authors have introduced an inexpensive Lewis acid Tin(II) chloride dihydrate (SnCl2
2H2O), and evaluated its potential as catalyst on the ethanolysis of oleic acid of fats and vegetable oils. The catalytic tests were carried out in triplicate with a molar ratio fatty acid: catalyst (100:1), reaction time of 2 h. Using these conditions, the process provided more than 90% biodiesel yield with a high selectivity of more than 93%. 3.3. The effects of homogeneous catalysts on the yield of biodiesel fuel The nature of catalyst used plays a great role during transformation of vegetable oil to biodiesel [103]. As a result, catalyst is among the key factors determining the rate and yield of biodiesel during biodiesel production process [17,46]. Thus, Table 5 presents reaction yield as function of the homogeneous catalyst weight. The data obtained shows that production of biodiesel via homogeneous catalyst could yield more than 99%.

4. The effects of heterogeneous catalysts in biodiesel production Despite the problems surrounding alkali-catalyzed transesterification, the process is still favorable in producing biodiesel fuel. The main reason is due to their faster kinetic rate and economic viability [116]. Recently several researches were conducted on heterogeneous catalysts with the aim of finding solutions to problems caused by using homogeneous catalysts in producing biodiesel. As a result a good number of heterogeneous catalysts were explored and many of the catalysts have displayed very good catalytic performances [27]. Some of these catalysts include among others; oxides [117], hydrotalcides [118], zeolites [119], etc. Currently, majority of heterogeneous catalysts used in producing biodiesel are either oxides of alkali or oxides of alkaline earth metals supported over large surface area [15]. Further, biodiesel is commercially produced using heterogeneous catalyst through the Esterfip-H process. This biodiesel process is commercialized by Axens. The process needs neither catalyst recovery nor aqueous treatment steps, which are major bottlenecks from the current homogeneous catalytic processes. Additionally the Esterfip-H process displays high biodiesel yields and directly produces saltfree glycerol at purities exceeding 98% compared to 80% glycerol purity from homogeneous catalyzed process [120]. Thus, the effects of these catalysts are discussed as follows. 4.1. The effects of solid alkaline and acid catalysts in biodiesel production Semwal et al. [121] reported that many studies on solid acidic catalysts for producing biodiesel were carried out, however lower reaction rates and unfavorable side reactions have limited their uses. The authors noted that a good number of investigations on basic heterogeneous catalysts were also conducted but their activity gets degraded in the presence of water. They stated that acid–base catalysts are among the most promising catalysts to employ in biodiesel production; this is because they can catalyze both esterification and transesterification simultaneously. Further, Lee and Shiro [122] reported simultaneous esterification and transesterification of waste cooking oil using solid catalyst ZnO– La2O3, which combines acid (ZnO) and base sites (La2O3). Although the process provided high conversion of 96% in 3 h, but like zirconia, lanthanum is a rare and expensive metal, therefore cost of the catalyst would prohibit it high use in the production of biodiesel. Further gelular resin catalysts having covalently bound

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sulfonic groups are developed and employed to simultaneously esterify and transesterify variety of feedstocks including beef tallow and soybean oil of high acid number. The authors noted the key factors influencing the feasibility and performance of solid catalysts, are durability, catalytic activity and cost of production. Therefore the main challenges to R&D in using solid catalyst in production of biodiesel are exploring the impacts of those chemical properties of supports on the catalytic activity. And designing specifically tailored deactivation–prevention and/or renewal techniques for each catalyst and developing less expensive supports. 4.1.1. The effects of solid alkaline catalysts in biodiesel production In recent times there is a great development in the preparation of solid alkaline catalysts for producing biodiesel. Solid alkaline catalysts such as CaO provides many advantages for instance higher activity, long catalyst life times, and could run under only moderate reaction conditions [123]. However, use of CaO in biodiesel production presents slow reaction rate [31]. Antunes et al. [124] noted that like homogeneous catalysts, solid alkaline catalysts present more catalytic activity than solid acid catalysts. Meher et al. [115] examined methanolysis of karanja oil via solid basic catalysts. They remarked that increased in FFAs content of karanja oil from 0.48 to 5.75% decreased the yield of biodiesel produced decrease from 94.9 to 90.3 wt%. The authors achieved an acid value of 0.36 mg KOH/g and an ester content of 98.6 wt%, after purification of the esters. The properties of the esters produced met both American and European standard specifications for biodiesel fuels. Also, Liu et al. [125] have produced biodiesel fuel from soybean oil using CaO as a solid base catalyst. The authors reported that use of CaO as a catalyst could provide a numbers of advantages such as: high activity, lengthen catalyst life and moderate condition of reaction. In a similarly study, Masato et al. [123] produced biodiesel fuel via solid base-catalyzed transesterification at a reaction time of 1 h and obtained esters yield of 93 wt% for CaO, 12 wt% for Ca(OH)2, and 0% for CaCO3. The authors stated that CaO will probably provide good productivity as NaOH as well as easy recovery of products and environmental benignity. They used CaO to transesterify waste cooking oil with an acid value of 5.1 mg KOH/g. The yield of esters was above 99% at a reaction time of 2 h. But a fraction of the catalyst transformed into calcium soap. Thus, solving this problem will entail removal of FFAs before transesterification reaction. The authors also noted that due to neutralization reaction of the catalyst, concentration of calcium in the produced biodiesel increased from 187 ppm to 3065 ppm. Conversely, Lim et al. [126] noted that transesterification reaction involving CaO needs longer reaction time. But the benefits gained from the process such as elimination of neutralization process, less waste generation and prospect of catalyst reusability compensates the delay. The authors also achieved biodiesel purity of 98.6  0.8% within 2.5 h. Besides for CaO catalyst, the process generated 90.4% biodiesel yield compared to biodiesel yield of 45.5% for NaOH and 61.0% for KOH catalysts. Further, it was observed that compared to biodiesel yield of 80% under anhydrous conditions using CaO, addition of 2.03 wt% water into the reaction medium of 8 wt% catalysts, 12:1 alcohol/oil molar ratio and at 3 h of reaction time, could provide biodiesel yield above 95%. Besides the catalyst active sites were found not to leach and the activity of the catalyst was stable after 20 cycles of the reaction [27]. Similarly, Puna et al. [127] conducted an experimental study to produce biodiesel via CaO and CaO modified with Li catalysts. Both catalysts showed good catalytic performances with high activity and stability. In fact yields of biodiesel were higher than 92% in two consecutive reaction batches without expensive intermediate

reactivation procedures. As a result, the catalysts are suitable for producing biodiesel. In another study, Sharma et al. [128] remarked that the regeneration and reuse of CaO for many times (for instance, 20 runs) makes the catalyst most favorite among the oxides. These characteristics are useful from economic analysis. Further, moderate conversion and yield could be obtained using mixed oxides as catalyst therefore oxides of calcium and magnesium are preferred. Besides presence of some amount of water/moisture in the reaction mixture has fewer effects on the activity of CaO, this reduces pretreatment cost of feedstock and alcohol. However, Hsin et al. [129] reported that the high solubility of CaO in methanol makes it a less attractive candidate for recyclable heterogeneous catalysis. Consequently the authors have produced biodiesel fuel from soybean oil (SBO) and poultry fat using new calcium containing silicate mixed oxide-based heterogeneous catalysts. The catalysts are; PME-templated calcium containing silicate mixed oxide catalysts (PMCS 1–9) and mesoporous calcium containing silicates (CS 1–9). The authors reported transesterification of SBO via PMCS-5 consisting of total surface area of 150 m2/g to provide 100% yield within 2 h. This catalyst could be recycled and reused for 8 times after regeneration by calcinations. They remarked that transesterification of soybean oil using CS-9 improved the reaction kinetics by a factor of 6. Also this catalyst could be recycled and reused for 3 times without any decrease in reactivity for poultry fat and 9 times for soybean oil. Additionally, Zabeti et al. [130] optimized biodiesel yields produced using transesterification of palm oil via CaO/Al2O3. They used a 150 ml glass jacketed reactor equipped with a water-cooled condenser and a magnetic stirrer (1000 rpm) to perform the experiments for 5 h. The estimated optimal reaction conditions achieved were; reaction temperature of 65 8C, catalyst content of 6 wt% and alcohol/oil molar ratio of 12:1, and achieved a corresponding yield of 98.64 wt%. The authors reported that the values achieved from ICP-MS showed little leaching of CaO/Al2O3 active species into the reaction medium and the catalyst successfully reused for two successive cycles. To further, examine the potential of solid alkaline catalysts, Ilgen and Akin [131] studied different heterogeneous catalysts such as K2CO3/g-Al2O3, Na2CO3/g-Al2O3, LiOH/g-Al2O3, NaOH/g-Al2O3, g-Al2O3, and KOH/ g-Al2O3and reported FAME yield of 89.40%. In recent years hydrotalcites materials are receiving great attention because they can be applied as precursors [132] and as catalyst [133]. Hydrotalcite-like compounds (HTlcs) are a category of anionic and basic clays referred as layered double hydroxides (LDH) with the formula Mg6Al2(OH)16CO3
4H2O [15]. Thus production of biodiesel was experimented using different hydrotalcites such as activated Mg–Al hydrotalcites [133,134]. Zeng et al. [135] have examined activation of Mg–Al hydrotalcite catalysts for rape oil, and achieved 90.5% conversion. The production of biodiesel from soybean oil via Mg–Al hydrotalcite was patented by Siano et al. [136]. The ratio of Mg/Al was found to affect the performance of the catalyst. And the catalytic activity was highest at 3–8 ratio of Mg/Al. The authors remarked that the catalyst is best suitable for oils containing up to 10,000 ppm quantity of water. They achieved 92% conversion at a reaction condition: reaction time of 1 h, temperature of 180 8C, catalyst concentration of 5 wt% and alcohol/oil weight ratio of 0.45. Similarly the catalytic performance of Mg–Al hydrotalcite for the transesterification of vegetable oil to biodiesel was investigated by Di Serio et al. [137]. The authors noted that at higher reaction temperatures (215– 225 8C); Mg–Al hydrotalcite displayed high catalytic activity. The transesterification was then conducted at methanol/oil weight ratio of 0.45 and catalyst concentration of 1 wt%, and esters yield of 94 wt% was achieved. Xie et al. [138] have experimented methanolysis of soybean using Mg–Al hydrotalcite as solid base catalyst. The synthesis of the Mg–Al hydrotalcite was carried out

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by employing co-precipitation technique of aqueous solution of NaOH, Na2CO3, Al(NO3)3
9H2O, and Mg(NO3)2
6H2O under intense stirring. Increased in catalytic performance and improved basicity were observed by improving temperature of calcinations to 500 8C. But the catalyst showed very low catalytic activity. And a conversion of 67% was recorded at an operating condition of 9 h, catalyst concentration 7.5 wt%, temperature of 65 8C, and alcohol/oil molar ratio of 15:1. Further, Georgogianni et al. [118] have employed heterogeneous catalyst to produce biodiesel fuel using used frying oil. The authors conducted comparison between the catalytic activities of MgO supported on MCM-41 and Mg–Al hydrotalcite base catalyst in the production of biodiesel fuel from soybean frying oil with alcohol (methanol). They reported the conversion of oil to biodiesel to be 97% and 85% respectively after 24 h under the same reaction conditions. Further the hydrotalcite showed greater activity which was attributed to its high basicity. Even though Mg-MCM 41 has higher specific surface area of 1289 m2/g compared to the low specific surface area of Mg–Al hydrotalcite of 82 m2/g. Also, Wen et al. [139] have synthesized fatty acid methyl esters from vegetable oil with methanol catalyzed by Li-doped magnesium oxide catalysts. A number of Li-doped MgO samples were prepared by the incipient wetness impregnation with the Li/Mg molar ratio in the range of 0.02–0.15. The catalyst weight was varied in the range of 3–15 wt% (methanol/oil molar ratio of 12, temperature of 338 K, residence time of 2 h). The results showed that the formation of strong base sites was principally promoted by adding Li. This results to an increase of fatty acid methyl esters synthesis. In addition the catalyst with Li/Mg molar ratio of 0.08 and calcined at 823 K exhibits the highest biodiesel yield of 93.9% at 333 K with the molar ratio of methanol/soybean oil of 12 and the catalyst amount of 9 wt%. The use of the catalyst for the second and third cycle decreased the fatty acid methyl esters yield from 79.3% and 71.0%, respectively. The biodiesel conversion decreases with further increasing Li/Mg molar ratio above 0.08, which is most likely attributed to the separated lithium hydroxide formed by excess Li ions and a concomitant decrease of BET values. The metal leaching from the Li-doped MgO catalysts was detected. The leaching from the catalyst and the agglomeration of crystallites caused the deactivation of the catalyst during the initial reaction cycles. Furthermore, Suppes et al. [119] have transesterified soybean oil with zeolites (ETS-10 zeolite, NaX faujasite zeolite) and metal catalysts. The transesterification reaction was performed at 6:1 molar ratio of alcohol and temperatures of 60, 120, and 150 8C. The conversion to esters increases from 60 to 150 8C with an average conversion of 90% achieved at 125 8C. Thus, ETS-10 gave better conversions of triglycerides than zeolite-X type catalysts. This was attributed to the higher activity of ETS-10 zeolites and larger pore structures that improved intra-particle diffusion. The catalyst activity was not affected after reused. However, FFAs loadings in excess of 25% quench the catalyst activity. Meyer et al. [140] conducted experimental studies on heterogeneous catalysts to produce biodiesel. The reaction occurred at temperature of 90 8C and a methanol to triglyceride ratio of 9:1. The authors stated that among the studied catalysts the potassium exchanged low silica zeolite of the faujasite type (K-LSX) has shown the best results and reached ester formation of over 90 wt% after 1 h. The result obtained was comparable to those of homogeneous catalysts such as alkali metal hydroxides. Another study conducted by Marı´a et al. [141] examined different zeolite catalysts such as NaX for the parent zeolite in sodium form: 0.3MNaX, 1MNaX, 3MNaX, 3MNaXB, and samples from mordenite and beta zeolite were named as 3NaM and 3Nab for the transesterification of sunflower oil with alcohol (methanol). Hence, zeolite X provided greater activity compared to other zeolites (mordenite and beta) due to its

21

higher concentration of super-basic sites. The authors reported that biodiesel contents of 93.5 and 95.1 wt% were achieved at 60 8C for zeolite X with and without sodium bentonite, respectively. Though, during the course of the reaction, the active species were observed to be leached out to the product [141]. Additionally, Supamathanon et al. [142] have transesterified jatropha oil using heterogeneous catalysts consisting of potassium supported on NaY (K/NaY) with potassium loading of 4, 8 and 12 wt% to produce biodiesel. The experiments were conducted in a 50 ml roundbottom flask equipped with a water-cooled condenser and heated with a water bath. They remarked that under reaction temperature of 65 8C, methanol to oil molar ratio of 16:1 and at a reaction time of 3 h, the catalyst with 12 wt% of potassium loading yielded optimum biodiesel yield of 73.4%. 4.1.2. The effects of solid acid catalysts in biodiesel production The replacement of homogenous catalysts by the solid acid catalysts is useful for green chemistry [143]. Helwani et al. [15] reported that compared to homogeneous acid, solid acid catalysts are preferred although their catalytic activities are low. This is due to fact that they contain a multiple sites with different strength of Bronsted or Lewis acidity. Kathlene et al. [144] reported solid acid catalysts to have strong capacity to substitute liquid acids, thus wiping out separation, corrosion and environmental problems. The authors evaluated different solid catalysts for the production of biodiesel from high FFAs feedstocks (waste cooking oil). The catalysts investigated include; zinc stearate/SiO2, MoO3/ZrO2, WO3/ZrO2, WO3/ZrO2–Al2O3, MoO3/SiO2, TPA/ZrO2, and zinc ethanoate/SiO2. They stated that zinc stearate immobilized on silica gel was most active and stable. The catalyst recycled several times at optimized conditions of reaction temperature of 470 K, 1:18 molar ratio of oil to alcohol, and 3 wt% catalyst loading without being deactivated. The authors recorded a maximum ester content of 98 wt%. Catalyst recycling reduces the biodiesel processing cost. In another study, Ngo et al. [145] have esterified FFAs in greases (12–40 wt% FFA) using a diphenylammonium triflate acid catalyst immobilized onto two robust and highly porous solid silica supports (MCM-48 and SBA-15). The authors evaluated the catalytic activities of the catalysts. The catalysts were reported to be effective for the esterification of FFAs in greases with a conversion to biodiesel of 95–99%, resulting in a pretreated grease with a final FFAs content of <1 wt%. Also, Furuta et al. [146] studied biodiesel fuel production via transesterification using soybean oil and methanol with solid superacid catalysts sulfated tin oxide (SO4/SnO2) and zirconium oxide (SO4/ZrO2) and tungstated zirconia (WO3/ZrO2) at 200–300 8C. And conducted esterification of n-octanoic acid with methanol at 175–200 8C in fixed bed reactor under atmospheric pressure. The authors noted that out of the catalysts prepared, tungstated zirconia–alumina catalyst (WZA) showed high performance, yielding over 90% conversion for esterification processes. Although detail analysis for the acidity of WZA has not yet been determine. Besides, Kitiyanan et al. [147] studied transesterification of palm and coconut oils using solid catalysts such as KNO3/ZrO2, KNO3/KL zeolite, SO42 /ZrO2, SO42 /SnO2, and ZrO2, ZnO. The authors noted that transesterification of crude palm kernel oil using SO42 /SnO2 and SO4/ZrO2catalysts provided highest yield of methyl esters (90.3 wt%). The purities of the esters were 95.4 wt% for SO42 /SnO2 and 95.8 wt% for SO42 /ZrO2, respectively. However, for ZnO the highest content of the esters was 98.9 wt%. In addition, owing to the availability of enough acid site strength, solid acid catalysts for example sulfated zirconia, tungstated zirconia and Nafion-NR50 were selected to catalyze transesterification [147,148]. However, due to its acid strength, the selectivity of Nafion-NR50 towards biodiesel and glycerol production was higher, compared to both sulfated zirconia and tungstated

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zirconia [149]. Nonetheless, Chai et al. [150] remarked that the major drawbacks of Nafion as catalyst to produce biodiesel, is it lower catalytic activity and high cost compared to liquid acids. Further, application of amberlystTM BD20 catalyst has provided more impetus for the development of biodiesel fuel. The amberlystTM BD20 process can effectively convert any feedstock containing 0.5–100% FFAs to biodiesel. Further amberlystTM BD20 presents fast solid esterification catalyst. Besides high biodiesel yield can be obtained with improved biodiesel and glycerol purity [151]. Additionally the search for a suitable solid catalyst has lead to the development of vanadium phosphate catalyst by Di Serio et al. [152] for biodiesel production. The authors reported 2–4 m2/g as specific surface area of the catalyst. However, despite the surface area of the catalyst is low, the catalyst was found to be active in producing biodiesel from soybean oil. They reported ester yield of 80% at 150 8C and observed catalysts deactivation at elevated temperatures owing to reduction of V5+ to V3+ with the alcohol (methanol). Although regenerating process of the catalyst after use is simple and could be achieved without complexities. Also, zeolites are used as a catalyst for esterification and as a support material for transesterification catalysts [2,142]. Zeolites are microporous crystalline solids with well defined structures and that they contain aluminum, silicon and oxygen in their framework and cations. As catalysts, unlike the compositionally equivalent amorphous catalysts, zeolites demonstrate substantial acid activity and shape selective features [153–155]. Chung et al. [153] employed different Si/Al molar ratio to remove FFAs in waste frying oil by esterification with methanol using different zeolite catalysts. The catalysts used include mordenite (MOR), faujasite (FAU), beta (BEA) zeolites, ZSM-5 (MFI), and silicalite. The pore structure and the acidic properties of the zeolites were particular useful in the removal of FFAs. These properties influenced the catalytic activity in FFAs removal. High conversion of FFAs was comparatively induced by strong acid sites of zeolites. The MFI zeolite induced an improvement of the FFAs removal efficiency by cracking to the FFAs in its pore structure owing to its constricted pore mouth. Converting FFAs on HMFI and HMOR zeolites provided 80% at a reaction temperature of 60 8C. In addition, HMOR zeolites showed almost a similar conversion of FFAs with different Si/Al molar ratio, but with decreased in acidity. Decreased in acid strength of the zeolites lowered the catalytic performance for FFAs removal [156]. 4.2. The effects of enzymes catalysts in biodiesel production Transesterification reaction can be catalyzed with enzymes catalysts such as Candida antarctica lipase [28], Pseudomonas cepacia [157], Candida sp. 99–125 [28,158], Pseudomonas fluorescens [157], Rhizomucor miehei and Chromobacterium viscosum [20,159] and Rhizopus oryzae lipase [160]. Casimir et al. [161] noted that biodiesel can be excellently produced via enzymatictransesterification reactions involving lipases. They suggested that large quantities of lipases can be produced using recombinant DNA technology. The authors believed that application of immobilized lipase may reduce the overall cost of biodiesel production and lower downstream processing problems. Besides, enzymatic approach is environmentally friendly [161]. Watanabe et al. [29] explored use of immobilized Candida antarctica lipase for continuous production of biodiesel fuel from vegetable oil. The transesterification of vegetable oil was conducted using 4% immobilized Candida lipase as a catalyst at 30 8C in a 20- or 50ml screw-capped vessel, shaking at 130 oscillations/min. The authors noted the activity of Candida antarctica lipase was not affected in a mixture of vegetable oil and more than 1:2 molar equivalent of methanol against the total fatty acids. They discovered inactivation of the lipase to be eradicated via three

consecutive additions of 1:3 molar equivalent of methanol. And then developed a three-step methanolysis by which over 95% of the oil triacylglycerols (TAG) transformed to their corresponding esters. Additionally, Shah et al. [30] have prepared biodiesel from jatropha oil using lipase catalyst. The authors have screened pancreas porcine, candida rugosa, Chromobacterium viscosum and in a solvent-free system for the production of biodiesel. They employed a screw-capped vial and jatropha seed oil (0.5 g) and ethanol were taken in the ratio of 1:4 (mol mol 1). Also 50 mg of enzyme preparation (tuned or immobilized) was added and incubated at 40 8C with constant shaking at 200 rpm. The immobilization of lipase (Chromobacterium viscosum) on Celite545 improved yield of esters from 62% to 71% by free tuned enzyme preparation with a process time of 8 h at 40 8C. Additionally to explore more information on the use of enzymes for the production of biodiesel, Tan et al. [28] reviewed biodiesel production using immobilized lipase. The immobilized lipase as biocatalyst draws great interest because that process is environmentally friendly. The authors noted different techniques for lipase immobilization, such as covalent bonding, cross-linking, encapsulation, entrapment and adsorption. Lipase immobilization technique is commonly used to increase the stability of lipase in biodiesel production. They stated that, for biodiesel (fatty acid methyl esters) preparation, at least a stoichiometric amount of methanol is needed for the complete conversion of triglycerides to their resultant fatty acid methyl ester. But, methanolysis is reduced considerably by adding >1/2 molar equivalent of methanol at the commencement of the enzymatic process. Usually, the polar short chain alcohols causes inactivation of enzymes and this is the major obstacle for the enzymatic-transesterification reaction. Therefore to overcome this difficulty one of these options is selected: acyl acceptor alterations, solvent engineering and methanol stepwise addition. The authors reported that biodiesel yield of 97 wt% was obtained after 24 h at temperature of 50 8C with a reaction mixture containing 13.5% methanol, 32.5% t-butanol, 54% oil and 0.017 g enzyme (g oil) 1. With the same mixture, a 95% biodiesel yield was achieved using a one step fixed bed continuous reactor with a flow rate of 9.6 ml h 1 (g enzyme) 1. The authors concluded that low cost of immobilized Candida sp. 99–125 lipases is rather competitive for industrial use [28]. More so, Hideki et al. [14] noted use of enzymatic-catalyzed transesterification to avoid problems associated with homogeneous catalysts. Therefore the production processes are compared in Figs. 7 and 8. The authors reported conversion of high FFAs feedstocks to biodiesel using immobilized antarctica lipase (Novozym-435) with ease of separation process. 4.3. The effects of heterogeneous catalysts on the yield of biodiesel As earlier stated great efforts have being made by several researchers and industries to explore and exploit use of heterogeneous catalysts in the production of biodiesel. Some of the reasons for the recent growth and development of heterogeneous catalysts include among others: biodiesel yield of 98 wt% and simplicity in catalyst separation process [130], high-purity by-products, less cost of separation and low energy consumption [31]. Table 6 presents reaction yield as function of the heterogeneous catalyst weight. 5. The advantages and disadvantages of homogeneous catalysts and heterogeneous catalysts The choice of catalyst to use in the production of economically viable biodiesel fuel is one of the most critical issues that are being discussed by futurists and industries [117]. As discussed earlier, NaOH and KOH are mostly preferred because they are cheaper than

I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

23

Alcohol for recycling Vegetable oils/Animal fats Alcohol

Crude biodiesel

Centrifuge/settler for separation of crude biodiesel products

Transesterification Reaction

Alkali

Neutralization process/water washing

Crude glycerin/ impurities Mineral acid (H2SO4)

Distillation/ Evaporation

Drying Facility

Waste water Mineral acid (H3PO4)

Finished product (biodiesel)

Pure glycerin for

Glycerin purification

Storage/recycling/sale

Free Fatty Acid (FFAs) Fig. 7. Traditional alkali biodiesel production.

Fats and oils Alcohol

Separation/purification of transesterified products

Transesterification reaction

Enzymes

Upper layer

Finished biodiesel product for consumption

Lower layer

Pure glycerin for sale

Glycerin purification Fig. 8. Enzymatic biodiesel production.

alkoxides, but are less active [17]. Leung and Guo [38] revealed that industrially, separation of esters is much easier when KOH catalyst is used compared to NaOH or CH3ONa. Because potassium soap formation is much softer and does not sink into glycerol phase. Owing to this reason, KOH is most often employed to produce biodiesel from waste recycled feedstocks [38]. Saka and Isayama [170] reported that even though the alkaline catalyst technique has the advantage of using moderate reaction conditions, several water

washings are needed to remove the catalysts. Zhang et al. [45] conducted economic analysis of four continuous processes to produce biodiesel such as alkali- and acid-catalyzed processes, with virgin vegetable oil and waste cooking oil. The authors revealed that acid-catalyzed transesterification with waste cooking oil were more economically viable. The process gave lesser total production cost, a more attractive after-tax rate of return and a less biodiesel break-even price.

Table 6 Reaction yield as function of the heterogeneous catalyst weight. Feedstock

Catalyst

Conc (wt)

Reaction time (h)

Reaction temp (8C)

Yield (w/w%)

Molar ratios

Reference

Soybean oil Vegetable oil Sunflower oil Sunflower Soybean oil Palm oil Waste bleaching earths Soybean Crude palm kernel oil Waste cooking oil Soybean oil Palm oil Canola oil Mixture of soybean and rapeseed oils Jatropha oil Soybean oil Sunflower oil

CaO M(3-hydroxy-2-methyl-4-pyrone)2(H2O)2 CaO/SBA-14 Zeolite X ETS-10 zeolite CaO/Al2O3 Rhizopus oryzae Na/NaOH/g-Al2O3 SO42 /ZrO2 ZS/Si Lipase CaO/Al2O3 Nano-g-Al2O3 Candida lipase

25

4

1 3 5 – 24 5 96 2 1 10 6.3 5 8 24

– 60 160 60 125 65 35 60 200 200 36.5 65 65 30

93 93 95 95.1 90 98.64 55 96 90.3 98 92.2 95% 97.7  2.1 93

– – 12 – – 12:1 1:4 9:1 6:1 1:18 3.4:1 12:1 15:1 1:3

[121] [162] [163] [164] [119] [26] [160] [165] [144] [166] [159] [130] [167] [29]

Chromobacterium viscosum lipase Pseudomonas cepacia lipase WO3/ZrO2

5 47.5 3

8 1 5

40 35 200

92 67 mol% 97

1:4 1:7.5 20:1

[30] [168] [169]

4.2 0.03 3.5 1 1 3 0.9 3.5

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Table 7 Advantages and disadvantages of homogeneous catalysts and heterogeneous catalysts. Type of catalyst

Example

Merits

Demerits

Ref.

Homogeneous catalyst Alkaline catalysts:

NaOH, KOH, CH3ONa, CH3OK

Less corrosive, high reaction rate

[13,17,98,153, 170,177–179]

H2SO4

Zero soap formation, the catalyst can be used to catalyze both esterification and transesterification simultaneously

Formation of saponified product, emulsion formation, high water and energy consumption, huge, high wastewater discharges, high purification cost. Feedstocks are limited to 0.5 wt FFAs, not recycle More waste as a result of neutralization, recycling difficulty, high purification cost, energy consuming, low reaction rates

MgO, CaO, ZnO KOH/NaY, CaO/MgO, Al2O3–SnO, KOH/K2CO3, Al2O3–ZnO, Ca(NO3)2/Al2O3, CaO/Al2O3, KOH/Al2O3, Al2O3/KI Sr(NO3)2/ZnO, ZrO2/SO42 TiO2/SO4 2 . ETS-10 zeolite, zeolite HY, and zeolite X Candida antarctica B lipase, Rhizomucor meihei lipase, candida rugosa Pseudonas cepacia, M. meihei (Lypozyme), M. meihei (Lypozyme IM60), Aspergillus niger.

Environmentally friendly, easily ecycle, less discharges, less separation difficulty, high purity glycerol, lower cost of separation Insignificant leaching of CaO/Al2O3

Leaching effects, catalysts preparation is complicated and expense relatively slow rates

[15,27,47,150, 152,162,172, 174,175,181, 182] [26,130]

Zero saponification products nonpolluting, easily separable, lesser separation cost, high purity glycerol and biodiesel products, environmentally benign Simple glycerol recovery

Catalysts inhibition by water

[14,40,161, 180,183]

High cost of enzymes

[29,30]

Acid catalysts:

Heterogeneous catalysts Solid alkaline catalysts and solid acid catalyst

Enzymes catalysts:

P. fluorescens, R. Oryzae

In contrast both alkali- and acid catalyzed transesterification are associated with a number of problems. Madras et al. [171] reported that alkali-catalyzed transesterification present high cost of biodiesel production and large energy consumption during down-stream biodiesel refining process. Ferella et al. [98] stated that potassium hydroxide produces soaps formation by neutralizing FFAs. Additionally, due to the tendencies to produce soaps formation and mono- and di-glycerides during transesterification, direct use of sodium and potassium alkylates as catalysts is raising serious issues in the industries. The soaps formations cause formation of gels and makes separation and purification of biodiesel hard. Also, Cardoso et al. [102] reported that the problems associated with H2SO4 catalyst include much reactor corrosion and huge wastewater discharges resulting from the neutralization of mineral acid. Another investigation conducted by Helwani et al. [15] revealed that application of homogeneous catalysts in biodiesel production presents difficulty in biodiesel product separation and makes recovery of used catalyst cumbersome. The authors reported that biodiesel production via homogeneous alkaline catalysts needs multi-steps of production and purification, since such catalysts do not tolerate presence of moisture or FFAs. In addition, there are many notable issues surrounding the existing transesterification processes such as; difficulties in processing low quality feedstocks and refining of transesterified products [40]. Further heterogeneous catalysts provide many advantages compared to homogeneous catalysts such as: catalyst re-usability, less separation and purification difficulties, high purity glycerol (above 99%), and catalyst recoverability [117,172]. Additionally no neutralization step is required [173]. Thus, in biodiesel production, the overall economy can be improved through the use or sale of by-product, glycerol [117]. Mariscal et al. [172] reported that heterogeneous catalysts are less corrosive, leading to safer, less costly, and more environmentally friendly operations. Watanabe et al. [29] reported enzymatic-transesterification reaction to overcome problems associated with chemical processes. Palligarnai and Briggs [24] found glycerol from enzymaticcatalyzed transesterification not to show negative effects on biodiesel fuel property. The authors stated that the major

[31,177,180]

advantage of employing enzyme catalysts is lack of soaps formation. They reported application of insoluble solid catalysts (immobilized enzymes) to speed up catalyst removal from both glycerol and alkyl esters. Zabeti et al. [27] showed solid-catalyzed transesterification reaction to yield biodiesel products with no catalyst impurities. Therefore absence of leaching improves separation process and lowers the cost of final refining. Martino et al. [47] noted that heterogeneous catalysts can be separated more easily, besides low processing costs and zero waste streams. Hameed et al. [174] stated that heterogeneous catalysts are searched over time to achieve environmentally benign and economically viable biodiesel production and purification processes. However, issues such as leaching of catalysts especially when solid alkaline catalysts such as CaO, MgO, La2O3, ZnO, SrO, Li-promoted CaO, CaCO3, Ba(OH)2, KxX/Al2O3 (X-halide ion or other mono/di-valent anion), Zinc aluminates, Metal salts of amino acids, Mg–Al hydrocalcites and K- and Cs-exchanged zeolites are used in producing biodiesel creates great deal of concern during separation process. The leaching of active sites of solid alkaline catalysts into liquids results from its characteristic changed turning it partly ‘‘homogeneous’’. This changed in the properties of the catalysts affects biodiesel quality, makes catalyst separation and biodiesel purification difficult, thus bringing extra cost of production [15]. Mariscal et al. [172] reported significant leaching of potassium during transesterification of triglyceride via K/c-Al2O3 catalysts. The authors reported 99 wt% biodiesel yield which was attributed to homogeneous contribution from active basic species dissolved in the methanol. In addition, the homogeneity of this catalyst complicates its separation process. Also leaching of solid acid catalyst (sulfate species) was reported to restrict its reusability [175]. Moreover, a study carried out by Lee et al. [176] remarked that leaching of base active species caused great negative effects on the degree of purity of biodiesel and glycerol. Another investigation conducted through optimization of catalytic activity of CaO/Al2O3 for producing biodiesel with response surface methodology showed insignificant leaching of catalyst into the reaction medium [26,130]. Table 7 summarizes the merits and demerits of homogeneous catalysts and heterogeneous catalysts for biodiesel production.

I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

6. Conclusions and recommendations Based on the foregoing the following conclusions and recommendations were made: 1. Use of high FFAs feedstocks to produce biodiesel is more economically viable, though it has great negative effects on the catalyst. Besides refining of crude biodiesel produced from low quality feedstock with alkaline catalysts is complicated. 2. It was found that acid-catalyzed transesterification reaction needs higher alcohol to oil molar ratios. In addition the reaction was reported to cause large corrosion effects on the production facilities. 3. It observed that biodiesel can be conveniently produced from low quality feedstocks using heterogeneous catalysts. Especially, enzymatic-catalyzed transesterification reaction that was discovered to converts FFAs to biodiesel fuel. 4. The conversion of FFAs to biodiesel was found to increase the yield of biodiesel and simplify the separation of catalyst form crude biodiesel mixture. 5. To make biodiesel production process cost effective, the chemistry of heterogeneous catalysts such as solid alkaline catalysts needs to be thoroughly explored, developed and use consistently in producing biodiesel. Further the catalysts were found provide biodiesel yield similar those obtained from homogeneous alkaline catalysts. 6. Tremendous effort should be made to exploit ways of improving the kinetic rate of CaO catalysts, since its catalytic activity is high, its catalyst life time is long and it operates under only mild reaction conditions. 7. Development of enzymes catalyst as a more environmentally benign process is necessary due to the prevailing environmental needs. Besides the process is potentially sound providing highquality biodiesel fuel that can compete favorably with petrodiesel fuel.

References [1] K.K. Oh, Y.S. Kim, H.H. Yoon, B.S. Tae, Journal of Industrial and Engineering Chemistry 8 (1) (2002) 64. [2] B.-H. Uma, Y.-S. Kim, Review: Journal of Industrial and Engineering Chemistry 15 (2009) 1. [3] Y.C. Sharma, B. Singh, Renewable & Sustainable Energy Reviews 13 (2009) 1646. [4] S.B. Lee, K.H. Han, J.D. Lee, I.K. Hong, Journal of Industrial and Engineering Chemistry 16 (2010) 1006. [5] S.B. Lee, J.D. Lee, I.K. Hong, Journal of Industrial and Engineering Chemistry 17 (2011) 138. [6] S.S. Kim, K.H. Kim, S.C. Shin, E.S. Yim, Journal of the Korean Industrial and Engineering Chemistry 18 (5) (2007) 401. [7] B. Saeid, M.K. Aroua, A. Abdul Raman, N.M.N. Sulaiman, Journal of Chemical and Engineering Data 53 (2008) 877. [8] K.R. Szulczyk, B.A. McCarl, Renewable & Sustainable Energy Reviews 14 (2010) 2426. [9] A.B. Chhetri, K.C. Watts, M.R. Islam, Energies 1 (2008) 3. [10] M.J. Haas, M.S. Karen, W.N. Marmer, T.A. Foglia, Journal of the American Oil Chemists Society 81 (2004) 83. [11] R. Bai, S. Wang, F. Mei, T. Li, G. Li, Journal of Industrial and Engineering Chemistry 17 (2011) 777. [12] J.M. Encinar, J.F. Gonza´lez, A. Rodrı´guez-Reinares, Fuel Processing Technology 88 (2007) 513. [13] Y.C. Sharma, B. Singh, S.N. Upadhyay, Fuel 87 (2008) 2355. [14] F. Hideki, K. Akihiko, N. Hideo, Journal of Bioscience and Bioengineering 92 (2001) 405. [15] Z. Helwani, M.R. Othman, N. Aziz, W.J.N. Fernando, J. Kim, Fuel Processing Technology 90 (2009) 1502. [16] A. Demirbas, Energy Conversion and Management 50 (2009) 14. [17] A. Demirbas, A Realistic Fuel for Alternative Diesel Engines, Springer, ISBN 9781-84628-994-1, e-ISBN 978-1-84628-995-8, doi:10.1007/978-1-84628-995-8. [18] J. Van Gerpen, B. Shanks, R. Pruszko, D. Clements, G. Knothe, Biodiesel Production Technology, NREL/SR-510-36244 August 2002–January 2004. [19] S. Chongkhong, C. Tongurai, P. Chetpattananondh, Renewable Energy 34 (2009) 1059. [20] K. Bozbas, Renewable & Sustainable Energy Reviews 12 (2008) 542.

25

[21] S. Zullaikah, C. Lai, S.R. Vali, Y. Ju, Bioresource Technology 96 (2005) 1889. [22] K.-W. Lee, J.X. Yu, J.H. Mei, L. Yan, Y.-W. Kim, K.-W. Chung, Journal of Industrial and Engineering Chemistry 13 (2007) 799. [23] M. Balat, H. Balat, Applied Energy 87 (2010) 1815. [24] T.V. Palligarnai, M. Briggs, Journal of Industrial Microbiology & Biotechnology 35 (2008) 421. [25] A.S. Ramadhas, S. Jayaraj, C. Muraleedharan, Fuel 84 (2005) 335. [26] M. Zabeti, W.M.A.W. Daud, M.K. Aroua, Applied Catalysis A: General 366 (2009) 154. [27] M. Zabeti, W.M.A.W. Daud, M.K. Aroua, Fuel Processing Technology 90 (2009) 770. [28] T. Tan, J. Lu, K. Nie, L. Deng, F. Wang, Biotechnology Advances 28 (2010) 628. [29] Y. Watanabe, Y. Shimada, A. Sugihara, H. Noda, F. Hideki, Y. Tominaga, JAOCS 77 (4) (2000). [30] S. Shah, S. Sharma, M.N. Gupta, Energy & Fuels 18 (2004) 154. [31] L.C. Thiam, S. Bhatia, Bioresource Technology 99 (2008) 7911. [32] A.L. Boehman, Fuel Processing Technology 86 (2005) 1057. [33] A. Keskin, M. Gu¨ru¨, D. Altiparmakc, K. Aydin, Renewable Energy 33 (2008) 553. [34] A.S. Ramadhas, S. Jayaraj, C. Muraleedharan, Renewable Energy 29 (2004) 727. [35] A. Khan, Evaluating Biodiesel Catalysts, 2007, p. 25, www.eptq.com. [36] L. Brennan, P. Owende, Renewable & Sustainable Energy Reviews 14 (2010) 557. [37] S.N. Naik, V.V. Goud, P.K. Rout, A.K. Dalai, Renewable & Sustainable Energy Reviews 14 (2010) 578. [38] D.Y.C. Leung, Y. Guo, Fuel Processing Technology 87 (2006) 883. [39] A. Salahi, M. Aoobbasi, T. Mohammadi, Desalination 251 (2010) 153. [40] D.Y.C. Leung, X. Wu, M.K.H. Leung, Applied Energy 87 (2010) 1083. [41] M. Canakci, J.V. Gerpen, American Society of Agricultural Engineers 42 (1999) 1203. [42] M. Mittelbach, B. Pokits, A. Silberholz, ASAE Publication Nashville, TN, USA, 1992, p. 74. [43] P. Cao, M.A. Dube´, A.Y. Tremblay, Biomass & Bioenergy 32 (2008) 1028. [44] S.P. Singh, D. Singh, Renewable & Sustainable Energy Reviews 14 (2010) 200. [45] Y. Zhang, M.A. Dube´, D.D. McLean, M. Kates, Bioresource Technology 89 (2003) 1. [46] A. Banerjee, R. Chakraborty, Resources Conservation and Recycling 53 (2009) 490. [47] D. Martino, R. Tesser, L. Pengmei, E. Santacesaria, Energy & Fuels 22 (2008) 207. [48] P.K. Sahoo, L.M. Das, M.K.G. Babu, S.N. Naik, Fuel 86 (2007) 448. [49] F. Ma, L.D. Clements, M.A. Hanna, Bioresource Technology 70 (1999) 1. [50] Y. Huang, J.I. Chang, Renewable Energy 35 (2010) 269. [51] M. Szczesna Antczak, A. Kubiak, T. Antcza, S. Bielecki, Renewable Energy 34 (2009) 1185. [52] J.M. Marchetti, V.U. Miguel, A.F. Errazu, Renewable & Sustainable Energy Reviews 11 (2007) 1300. [53] A.K. Tiwari, A. Kumar, H. Raheman, Biomass & Bioenergy 31 (2007) 569. [54] V.B. Veljkovic, S.H. Lakicevic, O.S. Stamenkovic, Z.B. Todorovic, M.L. Lazic, Fuel 85 (2006) 2671. [55] M. Naik, L.C. Meher, S.N. Naik, L.M. Das, Biomass & Bioenergy (2007), http:// dx.doi.org/10.1016/j.biombioe.2007.10.006. [56] A. Demirbas, Energy Sources, Part A 30 (2008) 1896. [57] R.M. Bryan, In Vitro Cellular & Developmental Biology: Plant 45 (2009) 229. [58] K. Sureshkumara, R. Velrajb, R. Ganesana, Renewable Energy 33 (2008) 2294. [59] S. Kaul, R.C. Saxena, A. Kumar, M.S. Negi, A.K. Bhatnagar, H.B. Goyal, et al. Corrosion Fuel Processing Technology 88 (2007) 303. [60] U. Rashid, F. Anwar, B.R. Moser, G. Knothe, Bioresource Technology 99 (2008) 8175. [61] Y.C. Sharma, B. Singh, Fuel (2007), http://dx.doi.org/10.1016/j.fuel.2007.08.001. [62] A. Mariod, S. Klupsch, H. Hussein, B. Ondruschka, Energy & Fuels 20 (2006) 2249. [63] S. Puhan, N. Vedaraman, G. Sankaranarayanan, V. Boppan, B. Ram, Renewable Energy 30 (2005) 1269. [64] S.V. Ghadge, H. Raheman, Biomass & Bioenergy 28 (2005) 601. [65] V. Kumari, S. Shah, M.N. Gupta, Energy & Fuels 21 (2007) 368. [66] J. Zhang, L. Jiang, Bioresource Technology 99 (2008) 8995. [67] M.J. Haas, P.J. Michalski, S. Runyon, A. Nunez, K.M. Scott, Journal of the American Oil Chemists Society 80 (2003) 97. [68] L.C. Meher, V.S.S. Dharmagadda, S.N. Naik, Bioresource Technology 97 (2006) 1392. [69] J. Van Gerpen, Fuel Processing Technology 86 (2005) 1097. [70] K. Nie, F. Xie, F. Wang, T. Tianwei, Journal of Molecular Catalysis B: Enzymatic 43 (2006) 142. [71] W. Davies, Biodiesel Technologies and Plant Design, August 2005 [email protected]. [72] B.H. Uma, Y.S. Kim, Journal of Industrial and Engineering Chemistry 15 (2009) 1. [73] T. Minowa, S.Y. Yokoya, M. Kishimoto, T. Okakura, Fuel 74 (1995) 1731. [74] H. GuanHua, F. Chen, D. Wei, X.W. Zhang, G. Chen, Applied Energy 87 (2010) 38. [75] M. Xin, J. Yang, X. Xu, L. Zhang, Q. Nie, M. Xian, Renewable Energy 34 (2009) 1. [76] S.A. Khan, Rashmi, M.Z. Hussain, S. Prasad, U.C. Banerjee, Renewable & Sustainable Energy Reviews 13 (2009) 2361. [77] S.A. Scott, M.P. Davey, J.S. Dennis, I. Horst, C.J. Howe, D.J. Lea-Smith, A.G. Smith, Current Opinion in Biotechnology 21 (2010) 277. [78] T.M. Mata, A.A. Martins, N.S. Caetano, Renewable & Sustainable Energy Reviews 14 (2010) 217. [79] A. Demirbas, Energy Conversion and Management 51 (2010) 2738. [80] J. Singh, S. Gu, Renewable & Sustainable Energy Reviews 14 (2010) 1367. [81] A. Demirbas, M.F. Demirbas, Energy Conversion and Management 52 (2011) 163. [82] A. Singh, P.S. Nigam, J.D. Murphy, Bioresource Technology 102 (2011) 10.

26

I.M. Atadashi et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 14–26

[83] N.H. Tran, J.R. Bartlett, G.S.K. Kannangara, A.S. Milev, H. Volk, M.A. Wilson, Fuel 89 (2010) 265. [84] S. Behzadi, M.M. Farid, Bioresource Technology 100 (2009) 683. [85] N.B. Haq, M.A. Hanif, M. Qasim, Ata-ur-Rehman, Fuel 87 (2008) 2961. [86] A. Casas, M.J. Ramos, A. Pe´rez, Biomass & Bioenergy (2011) 1. [87] N.A. Khan, H. el Dessouky, Renewable & Sustainable Energy Reviews 13(2009) 1576. [88] Y. Wang, S. Ou, P. Liu, Z. Zhang, Energy Conversion and Management 48 (2007) 184. [89] G. Vicente, M. Martinez, J. Aracil, Bioresource Technology 92 (2004) 297. [90] R. Umer, F. Anwara, B.R. Moser, S. Ashrafa, Biomass & Bioenergy 32 (2008) 1202. [91] A. Srivastava, R. Prasad, Renewable & Sustainable Energy Reviews 4 (2000) 111. [92] A.K. Agarwal, Progress in Energy and Combustion Science 33 (2007) 233. [93] K.-H. Chung, Journal of Industrial and Engineering Chemistry 16 (2010) 506. [94] D. Kusdiana, S. Saka, Bioresource Technology 91 (2004) 289. [95] J.M. Marchetti, A.F. Errazu, Biomass & Bioenergy 32 (2008) 892. [96] A.A. Refaat, S.T. El Sheltawy, K.U. Sadek, International Journal of Environmental Science and Technology 5 (3) (2008) 315. [97] N. Saifuddin, K.H. Chua, Malaysian Journal of Chemistry 6 (2004) 077. [98] F. Ferella, G.M. Di Celso, I. De Michelis, V. Stanisci, F. Veglio`, Fuel 89 (2010) 36. [99] L. Wang, H. He, Z. Xie, J. Yang, S. Zhu, Fuel Processing Technology 8 (2007) 8477. [100] S. Induri, S. Sengupta, J.K. Basu, Journal of Industrial and Engineering Chemistry 16 (2010) 467. [101] N.U. Soriano Jr., R. Venditti, D.S. Argyropoulos, Fuel 88 (2009) 560. [102] A.L. Cardoso, S. Cristina, G. Neves, M.J. Da Silva, Energies 1 (2009) 79. [103] S. Gryglewicz, Bioresource Technology 70 (1999) 249. [104] G. Antolın, F.V. Tinaut, Y. Briceno, V. Castano, C. Perez, A.I. Ramırez, Bioresource Technology 83 (2002) 111. [105] L. Canoira, R. Alca´ntara, M.J. Garcı´a-Martı´nez, J. Carrasco, Biomass & Bioenergy 30 (2006) 76. [106] M. Cardone, M. Mazzoncini, S. Menini, V. Roccoc, A. Senatorea, M. Seggianid, S. Vitolod, Biomass & Bioenergy 25 (2003) 623. [107] S.L. Dmytryshy, A.K. Dalai, S.T. Chaudhari, H.K. Mishra, M.J. Reaney, Bioresource Technology 92 (2004) 55. [108] N. Foidl, G. Foidl, M. Sanchez, M. Mittelbach, S. Hackel, Bioresource Technology 58 (1996) 77. [109] M.N. Nabi, R. Md Mustafizur, A. Md Shamim, Applied Thermal Engineering 29 (2009) 2265. [110] P. Nakpong, W. Sasiwimol, Fuel 89 (2010) 1806. [111] O.E. Ikwuagwu, I.C. Ononogbu, O.U. Njoku, Industrial Crops and Products 12 (2000) 57. [112] A.V. Tomasevic, S.S.S. Marinkovic, Fuel Processing Technology 81 (2003) 1. [113] N. Usta, Biomass & Bioenergy 28 (2005) 77. [114] P.D. Prafulla, S. Deng, Fuel 88 (2009) 1302. [115] L.C. Meher, M.G. Kulkarni, A.K. Dalai, S.N. Naik, European Journal of Lipid Science and Technology 108 (2006) 5389. [116] L.Y. Wang, J.C. Yang, Fuel 86 (3) (2007) 328. [117] Chementator, Biodiesel Production Using a Heterogeneous Catalyst, January 2008, http://www.axens.net/upload/news/fichier/chemical_engineering.pdf, 2004. [118] K.G. Georgogianni, A.P. Katsoulidis, P.J. Pomonis, M.G. Kontominas, Fuel Processing Technology 90 (2009) 671. [119] J.G. Suppes, M.A. Dasari, E.J. Doskocil, P.J. Mankidy, M.J. Goff, Applied Catalysis A: General 257 (2004) 213. [120] http://www.axens.net/html-gb/offer/offer_processes_104.html (assessed 3/7/2011). [121] S. Semwal, A.K. Arora, R.P. Badoni, D.K. Tuli, Bioresource Technology 102 (2011) 2151. [122] J.-S. Lee, S. Saka, Bioresource Technology 101 (2010) 7191. [123] M. Kouzu, T. Kasuno, M. Tajika, Y. Sugimoto, S. Yamanaka, J. Hidaka, Fuel 87 (2008) 2798. [124] W.M. Antunes, C.O. Veloso, C.A. Henriques, Catalysis Today 133–135 (2008) 548. [125] X. Liu, H. He, Y. Wang, S. Zhu, Fuel 87 (2008) 216. [126] B.P. Lim, G.P. Maniam, S. Abd Hamid, European Journal of Scientific Research (2009) 347. [127] J.F. Puna, J.F. Gomes, M. Joana, N. Correia, A.P. Soares Dias, J.C. Bordado, Fuel 89 (2010) 3602. [128] Y.C. Sharma, B. Singh, J. Korstad, Fuel 90 (2011) 1309. [129] T.-M. Hsin, S. Chen, E. Guo, C.-H. Tsai, M. Pruski, V.S.Y. Lin, Topics in Catalysis 53 (2010) 746. [130] M. Zabeti, W.M.A.W. Daud, M.K. Aroua, Fuel Processing Technology 91 (2010) 243. [131] O. Ilgen, A.N. Akin, Turkish Journal of Chemistry 33 (2009) 1. [132] K. Schulze, W. Makowski, R. Chyzy, R. Dziembaj, G. Geismar, Applied Clay Science 18 (2001) 59. [133] J.L. Shumaker, C. Crofcheck, S.A. Tackett, E. Santillan-Jimenez, T. Morgan, Y. Ji, M. Crocker, T.J. Toops, Applied Catalysis B: Environmental 82 (2008) 120. [134] N. Barakos, S. Pasias, N. Papayannakos, Bioresource Technology 99 (2008) 5037. [135] H.-Y. Zeng, Z. Feng, X. Deng, Y.-Q. Li, Fuel 87 (13–14) (2008) 3071. [136] D. Siano, M. Nastasi, E. Santacesaria, M. Di Serio, R. Tesser, G. Minutillo, M. Ledda, T. Tenore, WO 050925 A1 (2006).

[137] M. Di Serio, M. Ledda, M. Cozzolino, G. Minutillo, R. Tesser, E. Santacesaria, Industrial and Engineering Chemistry Research 45 (9) (2006) 3009. [138] W. Xie, H. Peng, L. Chen, Journal of Molecular Catalysis A: Chemical 246 (2006) 24. [139] Z. Wen, X. Yu, S.-T. Tu, J. Yan, E. Dahlquist, Applied Energy 87 (2010) 743. [140] O. Meyer, F. Ro¨ßner, R.A. Rakoczy, R.W. Fischer, DGMK Conf, September 29– October 1, Berlin, Germany, 2008. [141] J.R. Marı´a, A. Casas, L. Rodrı´guez, R. Romero, A. Pe´rez, Applied Catalysis A: General 346 (2008) 79. [142] N. Supamathanon, J. Wittayakun, S. Prayoonpoka, Journal of Industrial and Engineering Chemistry 17 (2011) 182. [143] S. Praserthdam, P. Wongmaneenil, B. Jongsomjit, Journal of Industrial and Engineering Chemistry 16 (2010) 935. [144] J. Kathlene, R. Gopinath, L.C. Meher, A.D. Kumar, Applied Catalysis B: Environmental 85 (2008) 86. [145] H.L. Ngo, N.A. Zafiropoulos, T.A. Foglia, E.T. Samulski, W. Lin, Journal of the American Oil Chemists Society 87 (2010) 445. [146] S. Furuta, H. Matsuhasbi, K. Arata, Catalysis Communications 5 (2004) 721. [147] J. Jitputti, B. Kitiyanan, P. Rangsunvigit, K. Bunyakiat, L. Attanatho, P. Jenvanitpanjakul, Chemical Engineering Journal 116 (2006) 61. [148] P. Wongmaneenil, B. Jongsomjit, P. Praserthdam, Journal of Industrial and Engineering Chemistry 16 (2010) 327. [149] D.E. Lopez, J.G. Goodwin Jr., D.A. Bruce, Journal of Catalysis 245 (2007) 381. [150] F. Chai, F. Cao, F. Zhai, Y. Chen, X. Wang, Z. Su, Advanced Synthesis and Catalysis 349 (2007) 1057. [151] Rohm and Hass, AMBERLYSTTM BD20 solid catalyst FFAs Esterification Technology Philadelphia, USA. www.amberlyst.com. [152] M. Di Serio, M. Cozzolino, R. Tesser, P. Patrono, F. Pinzari, B. Bonelli, E. Santacesaria, Applied Catalysis A: General 320 (2007) 1. [153] K. Chung, D. Chang, B. Park, Bioresource Technology 99 (2008) 7438. [154] H.J. Park, J.-I. Dong, J.-K. Jeon, K.-S. Yoo, J.-H. Yim, J.M. Sohn, Y.-K. Park, Journal of Industrial and Engineering Chemistry 13 (2007) 182. [155] S.J. Kim, M.J. Park, K.-D. Jung, O.-S. Joo, Journal of Industrial and Engineering Chemistry 10 (2004) 995. [156] K.-H. Chung, B.-G. Park, Journal of Industrial and Engineering Chemistry 15 (2009) 388. [157] A. Salis, M. Pinna, M. Monduzzi, V. Solinas, Journal of Molecular Catalysis B: Enzymatic 54 (2008) 19. [158] K. Nie, F. Xie, F. Wang, T. Tan, Journal of Molecular Catalysis B: Enzymatic 43 (2006) 142. [159] C.J. Shieh, H.F. Liao, C.C. Lee, Bioresource Technology 88 (2003) 103. [160] A.V.L. Pizarro, E.Y. Park, Process Biochemistry (Oxford, U.K.) 38 (2003) 1077. [161] C.A. Casimir, C. Shu-wei, L. Guan-chiun, J. Shaw, Journal of Agricultural and Food Chemistry 55 (2007) 8995. [162] F. Abreu, F.B. Alves, C.C.S. Maceˆdo, L.F. Zara, P.A.Z. Suarez, Journal of Molecular Catalysis A: Chemical 227 (2005) 263. [163] M.C.G. Albuquerque, I. Jimenez-Urbistondo, J. Santamarıa-Gonzalez, J.M. Merida-Robles, R. Moreno-Tost, E. Rodrıguez-Castellon, A. Jimenez-Lopez, D.C.S. Azevedo, C.L. Cavalcante Jr., P. Maireles-Torres, Applied Catalysis A 334 (2008) 35. [164] M.J. Ramos, C. Abraham, R. Lourdes, R. Rubı´, P. A´ngl, Applied Catalysis A: General 346 (2008) 79. [165] H.J. Kim, B.S. Kang, M.J. Kim, Y.M. Park, D.K. Kim, J.S. Lee, K.Y. Lee, Catalysis Today 93–5 (2004) 315. [166] K. Jacobson, G. Rajesh, C.M. Lekha, K.D. Ajay, Applied Catalysis B: Environmental 85 (2008) 86. [167] N. Boz, N. Degirmenbasi, D.M. Kalyon, Applied Catalysis B: Environmental 89 (2009) 590. [168] H. Noureddini, X. Gao, R.S. Philkana, Bioresource Technology 96 (2005) 769. [169] G. Sunita, B.M. Devassy, A. Vinu, D.P. Sawant, V.V. Balasubramanian, S.B. Halligudi, Catalysis Communications 9 (2008) 696. [170] S. Saka, Y. Isayama, Fuel 88 (2009) 1307. [171] G. Madras, C.R. Kolluru, Fuel 83 (2004) 2029. [172] D. Martı´n Alonso, R. Mariscal, R. Moreno-Tost, M.D. Zafra Poves, M. Lo´pez Granados, Catalysis Communications 8 (2007) 2074. [173] J. Janaun, N. Ellis, Sustainable Energy Reviews 14 (2010) 1312. [174] B.H. Hameed, L.F. Lai, L.H. Chin, Fuel Processing Technology 90 (2009) 606. [175] M. Mittelbach, A. Silberholz, M. Koncar, in: Proceedings of the 21st World Congress of the International Society for Fats Research, The Hague, October, (1996), p. 497. [176] D. Lee, Y. Park, K. Lee, Catalysis Surveys from Asia 13 (2009) 63. [177] A. Demirbas, Energy Conversion and Management 47 (2006) 2271. [178] Z.J. Predojevic, Fuel 87 (2008) 3522. [179] I.M. Atadashi, M.K. Aroua, A. Abdul Aziz, Renewable Energy 36 (2011) 437. [180] D. Casanave, D. Jean-Luc, E. Freund, Pure and Applied Chemistry 79 (2007) 2071. [181] Z. Yang, W. Xie, Fuel Processing Technology (2007) 631. [182] H. Gerard, B. Delfort, D. le Pennec, L. Bournay, C. Jean-Alain, Preprints of Papers: American Chemical Society, Division of Fuel Chemistry 48 (2) (2003) 636. [183] I.M. Atadashi, M.K. Aroua, A. Abdul Aziz, Renewable & Sustainable Energy Reviews 14 (2010) 1999.