Current advances in catalysis toward sustainable biodiesel production

Journal of the Energy Institute xxx (2015) 1e11

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Current advances in catalysis toward sustainable biodiesel production Faizan Ullah a, b, *, Lisha Dong a, **, Asghari Bano b, Qingqing Peng c, Jun Huang a a

Laboratory for Catalysis Engineering, Chemical Engineering Building J01, School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia b Department of Plant Sciences, Quaid-i-Azam University, Islamabad, Pakistan c School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu Province, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 September 2013 Accepted 9 July 2014 Available online xxx

Transesterification of vegetable oil/animal fat catalyzed by acid/base catalysts has been widely performed for biodiesel production and the utilization of catalysts directly affects the yield and quality of biodiesel. Based on the up-to-date literatures, the recent applications of various catalysts in biodiesel production processes are described in this work and the popular liquid acid/base catalysts used in biodiesel industry are discussed as well. However, the solid acid/base catalysts with advantages of easy separation and less waste are paid more attention on. Although the biodiesel production process carried out with heterogeneous catalysts is an alternative, the relatively low activity limits their applications in the industry compared to liquid catalysts. To better understand the formation of active sites and develop novel catalysts for the development of biorefinery, the structural properties of various solid catalysts are described since the catalytic properties of solid acid/base catalysts vary from their structures. © 2015 Published by Elsevier Ltd on behalf of Energy Institute.

Keywords: Biodiesel Transesterification Acid and base catalysts Catalytic process

1. Introduction Majority of the world transportation fuel requirements are supplied through petrochemical sources which are non-sustainable, depleting rapidly and causing environmental pollution. Moreover, energy requirements only for transportation are expected to increase by 2% every year. Therefore, it is estimated that the energy requirements in 2030 will be 80% higher than that of 2010 which will subsequently increase the greenhouse gas emission level [1]. Currently, sustainable energy management has attracted interest in both developed and developing countries. In such a situation, biofuels emerge as potential candidates to replace petroleum based fuels. Predictions indicate that by 2030, 7% of world transport fuels will come from renewable sources, like biofuels. Brazil has the highest penetration of biofuels (21% in 2010, rising to 39% by 2030), while the U.S. leads the Organization for Economic Co-operation and Development (OECD) in incentivizing biofuels (4% in 2010, rising to 15% by 2030). Biofuels will meet more than half of the incremental demand for alternative fuels in transportation in the future [2]. Four types of alternative fuels, such as pure vegetable oils, biodiesel, FischereTropsch diesel, and dimethyl ether can be used in conventional compress ignition (CI) engines [3]. Since the beginning of 1900s, Rudolf Diesel tested the utilization of pure vegetable oil in his first compression type internal combustion engine [4]. However, the direct utilization of vegetable oil in diesel engines was problematic due to (a) high viscosity of oils, (b) high molecular weight of triacylglycerols resulted in incomplete combustion due to low volatility, (c) polymerization of unsaturated fatty acids, (d) formation of carbon deposits due to incomplete combustion [5]. To overcome these problems, several physical and chemical modifications, such as pyrolysis, microemulsification, dilution and transesterification, were investigated for vegetable oils. The later was found as more efficient in reducing the viscosity of vegetable oils and the resulting product of the reaction was termed as biodiesel [6].

* Corresponding author. Laboratory for Catalysis Engineering, Chemical Engineering Building J01, School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia. Tel.: þ61 2 9351 7483; fax: þ61 2 9351 2854. ** Corresponding author. Tel.: þ92 519064 43096. E-mail addresses: [email protected] (F. Ullah), [email protected] (L. Dong). http://dx.doi.org/10.1016/j.joei.2015.01.018 1743-9671/© 2015 Published by Elsevier Ltd on behalf of Energy Institute.

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Biodiesel is a kind of renewable, biodegradable and sustainable fuel derived from vegetable oil or animal fats. It is usually produced through transesterification reaction by reacting long chain fatty acids with short chain alcohols (like methanol and ethanol) in the presence of catalysts resulting in a mixture of long chain alkyl esters and a byproduct glycerol, the reaction equation of which is shown in Fig. 1 [7]. Biodiesel has lower sulfur content and nearly neutral with respect to carbon dioxide emissions [8,9]. In addition, the utilization of biodiesel can reduce the emission of exhaust gases like carbon monoxide and unburned hydrocarbons compared with diesel fuel [10]. Furthermore, the production of biodiesel reduces dependence of a country on imported crude petroleum oil and thus helps to maintain the price of the fuel market. Due to the property of easily scaling up and growing market for renewable energy, the biodiesel production around the world has increased in the recent decades (shown in Fig. 2) [11]. In 2008, biodiesel production was increased so much that the share of biodiesel in diesel fuel was about 1.5% because of the rapid fluctuation of petroleum prices. European Union was the major producer of biodiesel, producing nearly 60% of the world biodiesel production [12]. In spite of all these benefits associated with biodiesel production in a commercial scale, the cost of biodiesel production is higher compared with petroleum based diesel. The major costs are due to feedstock used in the process (vegetable oil), alcohol (usually methanol), catalyst and operating costs [13]. Therefore, for making biodiesel more sustainable and economical, the major areas of research revolve around the (1) improvement in currently available feedstock resources, (2) development of advanced and efficient biodiesel technologies, (3) and socioeconomic benefits [14]. The major cost comes from the source of feedstock [15]. From this perspective, efforts have been made to improve the availability and quality of feedstock resources for biodiesel production [16,17]. There are a vast majority of feedstock resources for biodiesel production like vegetable oil (both edible and non-edible), animal fats (lard or beef tallow), trap grease (restaurant grease traps), etc. Majority of the biodiesel producers use edible vegetable oil resources which vary from country to country depending upon their economical and climatic conditions. Soybean, rape seed, canola and sunflower are used for biodiesel production in the USA, Germany, Canada, Argentina and Brazil. China is the major biodiesel producer from waste cooking oil, while India is producing biodiesel from nonedible Jatropha seed oil [18]. The biodiesel production process which always requires a high conversion rate and product selectivity is directly affected by the type and amount of catalysts utilized, and which further influences the cost of biodiesel production process [19]. Therefore, the catalysis process in biodiesel production has been extensively studied. A various range of homogeneous and heterogeneous catalysts, such as acid, base and biocatalysts have been studied for biodiesel production. Both of the catalyst types have their own advantages and disadvantages. Among many homogeneous reactions, liquid catalysts showed high activity in biodiesel production process. However, the separation of biodiesel products from reaction solutions results in a higher cost, which is a serious challenge. From the green chemical process perspective, more and more solid acid and base catalysts have been utilized in the biodiesel production process. A brief description for homogeneous catalysis and more details for heterogeneous catalysis processes were displayed in this review. 2. Homogeneous catalysts Transesterification is one kind of acid or base catalyzed intermolecular reaction. Liquid mineral acid and base have been firstly used for the production of liquid biodiesel. 2.1. Homogeneous acid catalysts The most widely accepted and applied theories of acid are BrønstedeLowry theory [20] and Lewis theory [21]. According to Brønsted and Lowry [20,22,23], an acid is a molecule or ion that is able to lose, or “donate”, a proton (Hþ), while with respect to Lewis [21e23], an acid substance is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms. Acid catalysts are more suitable for biodiesel production from waste vegetable oil due to the fact that the waste vegetable oil has a great number of free fatty acids. In transesterification reactions carried out with homogeneous acid catalysts, there are two reagents, like alcohol and a free acid (FFA), reacting to form an ester as the product of reaction. Therefore, acid catalysts were preferred in cases when FFA content of vegetable oil is greater than 1 wt% [24,25]. The most commonly used homogeneous acid catalysts in biodiesel production process include HCl, BF3, H2SO4, H3PO4 and FeSO4 [26e29]. According to Bhatti et al. [30], biodiesel production from the transesterification of animal fats (dairy cow and beef) was carried out with homogeneous acid catalysts under varying experimental conditions, such as different catalyst amount, catalyst nature, reaction time and temperature. The maximum biodiesel yields were 94.1 ± 2.43 and 98.4 ± 2.3% for dairy cow and beef tallow, respectively. The optimum conditions for biodiesel production with homogeneous acid catalysts were: 2.5 g of concentrated (conc.) H2SO4, 24 h of reaction time and 50  C for dairy cow fat and 2.5 g of conc. H2SO4, 6 h of reaction time and 60  C for beef fat. Based on the research work of Chongkhong et al. [27], production of fatty acid methyl ester (FAME) from palm fatty acid distillate (PFAD) which contains high free fatty acids (FFA) was investigated. Batch esterifications of PFAD were carried out to study the influence of reaction variables including reaction temperatures of

Fig. 1. Transesterification reaction. Adapted from Puna et al. [7].

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Fig. 2. Yearly biodiesel production in the world [11].

70e100  C, molar ratios of methanol to PFAD of 0.4:1e12:1, quantity of catalysts of 0e5.502% (wt of sulfuric acid/wt of PFAD) and reaction time of 15e240 min. The optimum condition for the continuous esterification process (CSTR) was molar ratio of methanol to PFAD at 8:1 with 1.834 wt% of H2SO4 at 70  C under its own pressure with a retention time of 60 min. The amount of FFA was reduced from 93 wt% to less than 2 wt% at the end of the esterification process. However, the acid catalyzed transesterification proceeds several times slower than the base catalyzed one [31]. The possible reason is that acid catalyzed transesterification involves intermediate steps like formation of intermediate molecules which are susceptible to nucleophilic attack. Whereas, the base catalyzed reaction proceeds on straight route at which alkoxide ion is created initially and directly acts as a strong nucleophile [32,33]. Acid catalyzed esterification reactions, the protonation of the carbonyl group of the ester leads to the carbocation which after nucleophilic attack of alcohol produces the tetrahedral intermediate. This in turn eliminates glycerol to form new ester and to regenerate the catalyst. The mechanism which is described in Fig. 3 can be further extended to di and triglycerides [34,35]. The utilization of Brønsted acids like H2SO4 has some common drawbacks, such as their corrosive nature, low reaction rates, and difficult laborious removal from the mixture by neutralization which further leads to a higher cost of biodiesel production process. Therefore, more attention has been paid on replacing the mineral Brønsted acids with metal Lewis acids recently. The catalytic activity of these metal catalysts is closely correlated with the Lewis acid strength of the metals and also to the anion molecular structure [36]. The complexes of Sn, Pb and Zn in the form of M (3-hydroxy-2-methyl-4-pyrone)2 (H2O2) have been used in homogeneous Lewis acid catalyzed transesterification for biodiesel production of vegetable oils with a biodiesel yield of nearly 37% [37]. In addition, Cardoso et al. [38] showed that SnCl2$2H2O was highly comparable with its Brønsted acid counterpart (H2SO4) in esterification of oleic acid. More than 90% of oleic acid was converted into ethyl esters and the gradual increase in the yield of ethyl esters appeared by increasing the reaction time of reaction. 2.2. Homogeneous base catalysts Based on BrønstedeLowry theory [20,22,23], Brønsted bases are defined as that a base is a species with the ability to gain, or accept a proton. While according to Lewis theory [21e23], a base is one that can provide an electron lone pair to help other molecule form the stable group of atoms. Homogeneous base catalysts have been widely used in biodiesel production on industrial scales because they are faster, reliable, less corrosive and more effective compared with homogeneous acid catalysts [39]. The most commonly used base catalysts in biodiesel production include alkaline metal hydroxides and carbonates, such as sodium hydroxide, potassium hydroxide, barium hydroxide and potassium carbonate and so on [40,41]. The utilization of homogeneous basic catalysts for biodiesel production from vegetable oil and animal fats was investigated by many researchers [25,42e48]. The transesterification reaction was carried out with homogeneous base catalysts in four consecutive steps, the mechanism of which is displayed in Fig. 4 [10,34]. In the first step, base reacts with alcohol to produce an alkoxide and the protonated catalyst. In the next step, alkoxide attacks the carbonyl group of the triglyceride and results in the formation of a tetrahedral intermediate. The third step is based on the formation of alkyl ester and diglyceride anion. In the fourth step, the catalyst is deprotonated and thus the active species which react with another molecule of the respective alcohol are regenerated and then another catalytic cycle is started. Currently, alkali catalysts, like

Fig. 3. Mechanism of acid catalyzed transesterification reaction.

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Fig. 4. Mechanism of base catalyzed transesterification.

NaOH, KOH and/or their methoxides were utilized by a majority of biodiesel producers. These homogeneous base catalysts are very effective and with better activity in conversion of fatty acids into their alkyl esters compared with homogeneous acid catalysts. However, sometimes, free fatty acids present in the vegetable oil might react with those catalysts and result in formation of soap that causes a loss in biodiesel yield. Moreover, during preparation of the methoxide by dissolving NaOH or KOH pellets into methanol (Fig. 5), water molecule was produced and interfered with the transesterification reaction [45]. The NaOH or KOH catalyzed transesterification reaction is also affected by reaction temperature and time, molar ratio of oil to methanol, catalyst type and concentration, and the stirring intensity [49,50]. Vicente et al. [51] made comparative evaluations of various homogeneous base catalysts like NaOH, KOH and their methoxides for biodiesel production of sunflower seed oil. Maximum biodiesel yield (near 100%) was achieved only with the sodium and potassium methoxides. The major loss in biodiesel yield was due to saponification of triglycerides and dissolution of alkyl esters in glycerol phase. To avoid this saponification of triglycerides in base catalyzed transesterification reaction, a two-step reaction was investigated [33]. In the first phase, the triglycerides were treated with acid catalysts to produce methyl esters of FFAs in the vegetable oil and in the second phase the same base catalyzed reaction was proceeded [33,52]. However, the utilization of liquid acid catalysts along with base catalysts further leads to a higher cost of biodiesel production and the process becomes further complicated because the homogeneous acid catalyst has to be removed prior to base catalyzed transesterification.

3. Biocatalysts for biodiesel production Keeping in view of the limitations associated with homogeneous acid and base catalysts, and to make biodiesel production process more economical and sustainable, enzyme lipase was focused on transesterification of vegetable oil. Both extra cellular and intra cellular lipases were utilized for methanolysis of vegetable oil [53,54]. Tan et al. [55] and Gog et al. [56] have published detailed reviews on enzymatic transesterification of vegetable oils. Lipases are more suitable for transesterification of waste vegetable oil because they can also work with free fatty acids. The mechanism of lipase mediated alcoholysis of triglycerides involves two steps: in the first step, ester bond is hydrolyzed with the release of an alcohol moiety. In the second step, esterification of the second substrate takes place [53,57,58]. However, the cost and deactivation of enzyme by impurities in the feedstock has made the process not suitable for industrial implication. Some researchers have tried the immobilization of lipases for avoiding this deactivation of enzyme. Yagiz et al. [59] successfully carried out transesterification of waste oil by lipase immobilized on hydrotalcite which was prepared by coprecipitation. The sustainability of enzyme can be enhanced by the immobilization because of its recovery and further reutilization. However, further researches in this direction of immobilization of lipases on different materials are still needed in order to make these enzymes as more suitable catalysts for future industrial applications in more economical and sustainable ways.

4. Heterogeneous solid catalysts Majority of the heterogeneous catalysts are solids and catalyzed reactions normally occur on the surface [60]. Compared with classical homogeneous catalysts, heterogeneous solid catalysts are gaining more interests among biodiesel producers due to its easier separation and less wastes. However, the yield of methyl esters is lower compared with commonly utilized homogeneous catalysts catalyzed biodiesel production reactions [61]. Another common problem with heterogeneous catalysts is their deactivation with passage of time due to many reasons, like poisoning, leaching and coking [62]. Therefore, it is essential for the development of heterogeneous catalysts with better stability, selectivity, activity at a low temperature and pressure during the reaction, which is also economical and sustainable. The classification of solid acid and base catalysts utilized in biodiesel production is presented in Fig. 6.

4.1. Heterogeneous solid acid catalysts The solid acid catalysts are more suitable for vegetable oil with higher free fatty acid content, like waste vegetable oil because they do not facilitate saponification during transesterification. However, the activity of solid acid catalysts is highly dependent on the temperature during the transesterification reaction. They are normally less active at a lower temperature, and to obtain higher conversion rates, reaction temperature above 170  C is needed [63]. Please cite this article in press as: F. Ullah, et al., Current advances in catalysis toward sustainable biodiesel production, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.01.018

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Fig. 5. Reaction formula for preparation of methoxide. Adapted from Ullah et al. [45].

4.1.1. Strong acid resins The acid resins are composed of exchangeable Hþ cations which impart the resins to become insoluble but highly acidic. They are composed of cross linked polystyrene matrix with active sites for esterification as sulfonic acid groups which is shown in Fig. 7. These resins easily get ionized not only in acidic but also salt form of the sulfonic acid group, and are therefore nowadays utilized as strong acid catalysts in a number of reactions in place of common homogeneous acids like H2SO4 [64]. The biodiesel production on acidic resins has been studied by various researchers. Feng et al. [65] investigated the utilization of cation exchange resins like NKC-9(Hþ), 001  7(Naþ) and D61(Naþ) for transesterification of waste cooking oil with a high acid value and water content. The utilization of NKC-9 with large pore diameter (56 nm) favoring the access of reactants toward active sites, resulted in a high conversion rate. Shibasaki-Kitakawa et al. [66] carried out the transesterification of triolein by using anion-exchange resins like PA308 (1.0  103 mol/m3, ion-exchange capacity), PA306 (0.8  103 mol/m3, ion-exchange capacity), PA306s (0.8  103 mol/m3, ion-exchange capacity) and HPA25 (0.5  103 mol/m3, ion-exchange capacity) as well as a cation-exchange resin (PK208 1.2  103 mol/m3, ion-exchange capacity) as sources of heterogeneous catalysts. The conversion rate of the cation exchange resin was not higher compared with its anion exchange counterparts. The main reason was the efficient adsorption of alcohol on anion-exchange resins which was not achieved for cation exchange resins.
pez et al. [67] in the transesterification The activity of various solid acid catalysts compared with resins has been investigated by Lo reaction of triacetin and methanol with a 1:6 ratio of oil to methanol at 60  C. It is observed that the maximum activity was shown in the experiment carried out with Amberlyst 15 (a styrene based sulfonic acid) with a 79% conversion followed by sulfated zirconia with a 57% conversion and Nafion NR50 with a 33% conversion, respectively. The conversion rate was low for tungstated zirconia. However, the Amberlyst 15 was poisoned in the presence of water in waste cooking oil which reduced the access of reactants toward active sites. Similarly, Kiss et al. [68] tested Nafion-NR50 (a copolymer of tetrafluoroethene and perfluoro-2-(fluorosulfonylethoxy) propyl vinyl ether) and Amberlyst 15 for esterification of dodecanoic acid with 1-propanol, methanol and ethylhexanol. Initially both catalysts showed great esterification potential. However, after 2 h, the Amberlyst 15 got deactivated, whereas, the Nafion-NR50 was deactivated after 4.5 h. The deactivation of catalysts rendered them not suitable for industrial applications. 4.1.2. Heteropoly acids Heteropoly acids (HPAs) for the operations of liquid phase esterification reactions are important solid acids [69]. It is observed that strong Brønsted acidity was exhibited compared with conventional acid catalysts, like acidic resins and H2SO4 [70,71]. HPAs are considered as effective catalysts for all types of acidic reactions taking place in both homogeneous and heterogeneous phases [72]. During last decade, more than 100 heteropoly acids varying in composition and structure are investigated [71,73] and the HPAs with the Keggin structure (Fig. 8) were well documented and studied for their physicochemical and catalytic properties [74e80]. The catalytic effect of heteropoly acids mainly depends on three factors, the acidity, structure of heteropolyanion and the nature of reagents used in the reaction. Moreover, their acidity can be controlled by the change of the heteropolyanion charge [79]. The heteropolyacid Cs2.5H0.5PW12O40 was studied by Chai et al. [80] for the transesterification reaction of Eruca sativa oil. 100 g vegetable oil, 28 ml methanol, 10 ml tetrahydrofuran (THF) and 0.02 mmol catalyst were added to the preheated reactor at 60  C. The achieved yield of biodiesel was 99%. The activity of the catalyst was not affected by the moisture present in the vegetable oil, the content of free fatty acids and the catalyst was easily separated from the mixture. Moreover, the catalyst was still highly effective after the sixth run. Cao et al. [78] carried out the transesterification of waste frying oil with high acid value and water content by using strong heteropoly acid H3PW12O40$6H2O(PW12), the structure of which is depicted in Fig. 8. In their work, 8 g of waste frying oil, 58 ml methanol and 0.1 mmol PW12 catalyst were added to the preheated reactor at 65  C. An 87% yield of esters after the completion of transesterification reaction was achieved. The catalyst was still effective for five consecutive reactions (Fig. 9). Elemental analysis showed that negligible loss (4.6%) of W occurred from PW12 in the fatty acid methyl esters after five runs.

Fig. 6. Classification of solid acidic and basic catalysts used in biodiesel production.

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Fig. 7. Strong acid cation resin and sulfonated polystyrene. Adapted from Harmer et al. [64].

4.1.3. Acidic zeolites It is obvious from current researches that inorganic solid catalysts are more suitable for biodiesel production than ion exchange resins. Among these inorganic solid acid catalysts, zeolite is highly important as it has been widely used in current chemical industry. Zeolite is microporous (<2 nm) material composed of silicon and aluminum linked by oxygen atoms in crystal framework. When one aluminum (Al3þ) replaces one silicon (Si4þ) in the framework, the nearby framework oxygen shows negative charge. When one proton contacts this framework oxygen, the Brønsted acid site is produced [81] which is shown in Fig. 10. The acidity of zeolite including acidic strength and density can be controlled by its SiO2/Al2O3 ratio [82]. The advantages of zeolites for catalytic reactions are the shape and size of pores, acid site strength and their distribution as well as surface hydrophobicity, which can be obtained by the synthesis of a variety of crystal structures, Si/Al ratios and various proton exchange levels [83]. Due to the large molecular size of fatty acids, zeolites with large pores have been used successfully for transesterification of vegetable oil [84]. Based on zeolites Y, beta, mordenite, and ZSM-5, the reaction pathways have been significantly changed due to their various pore sizes. Zeolites with large pores give significant amounts of bicyclic reaction products, whereas the medium pore size ZSM-5 gives almost no bicyclic products. The formation of undesirable byproducts also happens because zeolites catalyzed reactions occur at high temperatures. Moreover, the efficiency of zeolites in transesterification is highly dependent on strength of acid sites and hydrophobicity. Other properties such as adsorption characteristics, geometrical factors, dimensionality of the channel system [85] and aluminum content of the zeolite framework [86] affect the catalytic activity and performance for esterification. For the reaction of the large molecular size oil on small pore zeolites, the transesterification was limited and the cracking started. Leng et al. [81] detected the cracking of palm oil on HZSM-5 catalyst and maximum gasoline range hydrocarbons were observed at 400  C. The conversion of palm oil was low (40e70%) as compared with canola which showed 100% conversion under similar conditions [87]. The reason behind was that palm oil contains more stable saturated fatty acids as compared with canola oil which posses higher quantities of unsaturated fatty acids. 4.2. Heterogeneous solid basic catalysts Heterogeneous solid base catalysts present higher activity in transesterification reactions, so they are more suitable for vegetable oils with low FFA content. The mechanism of reaction in the conversion of fatty acids into alkyl esters is the same as that of the reaction carried out with homogeneous basic catalysts [34]. These types of solid catalysts are categorized as alkaline earth metal hydroxides, alkaline metals carbonates, alumina loaded with different compounds, hydrotalcites, basic zeolites and many other compounds with a high basic properties [88e90], like SnO, the synthesis process of which is shown in Fig. 11. 4.2.1. Metal oxides According to Romero et al. [91], for the production of 8000 t of biodiesel, nearly 88 t of NaOH pellets would be required. However, only 5.6 t of supported MgO are required for the production of 1million tones of biodiesel. Currently, a detailed review has been published by Refaat [60] on the utilization and efficiency of metal oxides in biodiesel production.

Fig. 8. The Keggin structure of PW12 O3 40 . Adapted from You et al. [76] and Keggin [77].

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Fig. 9. The catalyst activity of transesterification of waste frying oil in five reaction cycles under the condition that molar ratio of methanol/oil is 70:1 at temperature 65  C with a 14 h period, and the catalyst used is 0.1 mmol. Adapted from Cao et al. [78].

Among the available metal oxide catalysts, CaO is widely used in biodiesel production which is due to its low toxicity, easy availability and cheap price [92]. The manufacturing cost of CaO was lower compared with traditional basic catalyst KOH [93]. Viriya-empikul et al. [94] used industrial waste egg shells, golden apple snails, and meretrix venus as sources of solid oxide catalysts for the conversion of palm oil into biodiesel. The waste materials calcined in air at 800  C for 2e4 h transformed Ca species into active CaO catalysts which resulted in the conversion of >90% fatty acid into methyl esters in 2 h. CaO was utilized [95] for the transesterification of sunflower oil and after 2 h, a 98% yield of biodiesel at 60  C was observed. Similarly, CaO was studied by Kawashima et al. [96] for the biodiesel production from rapeseed oil. After activating CaO by treating with methanol prior to methanolysis for 1.5 h, a 90% yield of biodiesel was achieved in 3 h of reaction at 60  C (3.9 g methanol, 0.1 g CaO and 15 g vegetable oil). Ibrahim et al. [97] prepared CaO catalysts from the hydrated lime for the conversion of Jatropha oil into biodiesel. They observed that the maximum yield of biodiesel (99.08%) was achieved at 60  C after 1 h of reaction initiation. However, there are some limitations associated with calcium-based catalysts since they are not highly stable and leaching of Ca occurs in the reaction mixture [98]. Therefore, more attention on improving the stability of calcium-based catalysts has been paid by the researchers whose projects were related to biodiesel production. The CaO catalyst was either supported or utilized in mixture with other metal oxides, like MgO. The combined effect of CaO and MgO on transesterification was more encouraging with great conversion of vegetable oil into esters and this was possibly due to the presence of bi-catalytic sites of CaO and MgO. However, it was observed that the byproducts, like glycerol, were contaminated by the production of soap catalyzed by MgO and the dissolution of surface species into the reaction mixture [99]. In another experiment, the CaO and La2O3 mixed metal oxide catalyst showed quite interesting and promising results, but structural changes of the catalyst happened after its exposure to air [100]. The application of nano-crystalline CaO and MgO in transesterification as heterogeneous catalysts attracted much interest these days. Reddy et al. [101] successfully employed CaO nano-crystalline catalyst for the transesterification of soybean oil into methyl esters and a yield of 99% was achieved with oil/methanol molar ratio of 1:27 at room temperature after 12 h. These nano-crystalline CaO and MgO catalysts can be a better choice for future biodiesel industry due to their high surface areas and thermal stability. The oxide support of nano-crystalline CaO and MgO played an important role on the basicity, surface area, mechanical strength and cost of the catalyst [102]. Recently, Thitsartarn and Kawi [98] prepared CaOeCeO2 catalyst for transesterification of palm oil. Among various CaOeCeO2 catalysts they prepared, the best result was obtained with the 1Ca1Ce coded catalyst (calcined at 650  C) with a methyl ester yield (>90%) after 2 h of reaction. The catalyst possessed high base strength with a negligible leaching of catalyst components into the reaction mixture. The negligible leaching of catalyst components into reaction mixture was due to the great interaction between calcium and cerium species in the solid catalyst. The leaching of catalyst components was further reduced by increasing the calcination temperature. The stability of catalyst was still very high and the catalyst was successfully reused up to 18 cycles without any significant decrease in yield of methyl esters. However, the catalyst lost its basicity at higher calcination temperatures. Lee et al. [103] achieved >90% yield of palm biodiesel and >80% yield of Jatropha biodiesel by using calcium based mixed metal oxide catalysts with a binary metal system (CaMgO and CaZnO) of 3% (w/w), reaction time of 3 h, methanol to oil ratio of 15:1 under 65  C. The synthesized catalysts were effective for 3 continuous runs. Mohadesi et al. [104] used different alkali earth metal oxides (CaO, MgO and BaO) doped with SiO2 as catalysts for the conversion of corn oil

Fig. 10. Brønsted acid site on zeolite. Adapted from Leng et al. [81].

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Fig. 11. Schematic representation of tin oxide synthesis. Adapted from Sharma et al. [89].

into biodiesel. Solegel method was used for the catalyst synthesis. A transesterification reaction was studied after 8 h by mixing the corn oil, methanol (methanol to oil molar ratio of 16:1), and 6% (w/w) of the catalyst (based on the oil) at 60  C and 600 rpm. The catalyst loading was studied for different catalysts ranging in amounts from 40, 60 to 80%. The purity and yield of the produced biodiesel for 60% CaO/SiO2 catalyst were higher than other catalysts and they are 97.3% and 82.1%, respectively. 4.2.2. Hydrotalcites Hydrotalcites with general formula ½M2þ ð1xÞ M3þ xðOHÞ2 xþ ðAx=nÞn $yH2 O.yH2O are an interesting class of solid base catalysts whose acidic/basic properties can be easily altered by varying their composition [105]. Due to its environmentally friendly nature, hydrotalcite Mg6Al2(OH)16CO3$4H2O which displays a high activity in the transesterification reaction has gained much interest in the development of environmentally friendly catalysts. Mg/Al hydrotalcites were investigated as a source of heterogeneous catalyst for biodiesel production of soybean oil by Silva et al. [106]. An increase in catalyst activity was observed when Al/(Mg þ Al) ratio was increased from 0.20 to 0.25 and 0.33, further increasing the ratio, a sudden decrease occurred in the activity which was closely associated with the alteration in the catalyst basicity. When the Al/(Mg þ Al) ratio reached 0.33 and a 5% (w/w) catalyst concentration, a 90% yield of biodiesel was achieved when carried out with a methanol to oil molar ratio of 13:1 at 230  C after 1 h. The catalyst with medium basic strength was highly effective in transesterification. Moreover, it was also observed that the catalytic activity of the catalysts depends on their calcination temperature. Maximum catalytic activity was observed for catalysts calcined at 400  C, which is shown in Fig. 12. Interestingly, the samples calcined at this temperature had lower basicity and possessed a bigger surface area and pore volume than those calcined at 200  C. _ The MgeAl hydrotalcites were examined by Ilgen et al. [107] for the production of canola oil methyl esters. A 71.9% conversion of vegetable oil to methyl esters was obtained when carried out with a 6:1 molar ratio of methanol to oil, 9 h of reaction and a 3% (w/w) catalyst concentration at 60  C. The particle size of catalysts was 125e150 mm. The activity of this MgeAl hydrotalcite catalyst seems much lower than conventional homogeneous basic catalysts with conversion rates of 90e99% under different experimental conditions and with feedstocks of various qualities. Therefore, there is need for the development of modified hydrotalcites with better activity for maximizing the yield of transesterification and commercial application. It has been observed that it was suitable for catalysts with strongest basic sites to be operated at low temperatures (100  C) in transesterification reaction. In contrast, those catalysts with basic sites in medium strength were operated at higher temperatures to promote the same type of reaction [108]. Martins et al. [109] synthesized hydrotalcites using the co-precipitation method with a Mg/Al molar ratio of 3.0 and the precursor was calcined at 450  C for 6 h. Transesterification reactions were carried out with magnetic stirring at 64  C under atmospheric pressure in a jacketed reactor coupled to a condenser, by varying the molar ratio of methanol/oil and the reaction time. The maximum yield of soybean biodiesel (94.8%) was achieved with a methanol/oil molar ratio of 20:1, 5.0% catalyst (w/w), for 10 h. 4.2.3. Basic zeolites The physical and chemical properties of basic zeolites, like adsorption, ionic interchange and catalytic activity also render their commercial application for biodiesel production. The activity of zeolites in transesterification have been studied by many researchers since

Fig. 12. Effect of calcinations temperature on the activity of the HT0.33 catalyst for transesterification of soybean oil with methanol (methanol/oil molar ratio of 13) at 230  C, using 5 wt% of catalyst [102].

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F. Ullah et al. / Journal of the Energy Institute xxx (2015) 1e11

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Fig. 13. Schematic bonding model of basic zeolite. Adapted from Hattori [107].

zeolites can serve as basic catalysts and generate weak active centers by decreasing the silica/alumina ratio or interchanging with alkaline cations [110e112], which is shown in Fig. 13. The basicity of zeolites is improved by the occlusion of sodium metal clusters which increase the negative charge of the framework oxygen atoms. The supported species are introduced into the zeolite pores by wet impregnation method, typically from a precursor solution [113]. It was observed that there is a positive correlation between the number of basic sites on zeolites and the yield of methyl esters. Similarly, the calcination temperature is also important for determining the activity of basic zeolites in a transesterification reaction. A maximum yield (91.58%) of methyl esters was obtained by Xue et al. [114] when the transesterification reaction was carried out with a catalyst calcined at 550  C. However, further increasing the calcination temperature to above 650  C, a gradual decrease in the yield of methyl esters occurred even though the basicity was higher. Various zeolites, such as mordenite, beta and X with different metal loadings were investigated for the transesterification of sunflower seed oil [115]. The zeolite X was agglomerated with sodium bentonite as a binder, to investigate how the catalytic performance of zeolites is changed with a binder. The 93.5 and 95.1% (w/w) yields of methyl esters were achieved at 60  C by utilizing zeolite X in the presence or absence of sodium bentonite, respectively. It was observed that stronger basic sites were more important for maximizing the conversion of sun flower oil into methyl esters compared with the surface area and pore size of the catalyst. Experimental results of this work are tabulated in Table 1. Zeolite MCM-22 with colloidal silica and sodium aluminate was successfully synthesized [116] and the comparison of efficiency with commercial Zeolite HY(CBV-780) was studied as well in transesterification of triolein. The CBV-780 was cube-like particulate with a dimension less than 1 mm, whereas the zeolites MCM-22 possessed an average size of 12 mm. Both of the catalysts were microporous with large BrunauereEmmetteTeller (BET) surface area. It was observed that the maximum biodiesel yield (98% and 99%) was achieved when Na was exchanged with the ions on the surface of these zeolites irrespective of the great loss in BET surface area and deterioration of the crystalline structure. The transesterification reactions of soybean oil with various catalysts, like NaOx occluded in NaX (faujasite zeolite), K occluded in ETS-10 (titanosilicate structure-10 zeolite) and ETS-10 (titanosilicate structure-10 zeolite) were studied [117] with an oil to methanol molar ratio of 1:6 at different temperatures for 24 h. ETS-10 catalysts were more effective than Zeolite-X type catalysts with a more than 90% yield of methyl esters. This increased conversion ratio of vegetable oil to methyl esters was due to the greater basicity of ETS-10 zeolites and their larger pore structures. The amount of basic sites was increased in the NaX zeolites by the introduction of alkali earth oxides and also depended on the molecular size of the metal oxide [118], which led to the utilization of nanocrystalline metal oxides as heterogeneous basic catalysts for biodiesel production. In this context, the activity of CaO nanoparticles supported on NaX zeolites with methanol as solvent for transesterification of sunflower oil was investigated [102]. It was observed that methyl ester content is greatly dependant on basicity of catalysts. The most effective catalyst was the one with 16% (w/w) CaO nanoparticles and a more than 93.5% conversion rate was achieved. However, the catalytic activity of catalyst was lost on reuse and only 5% of methyl esters content was achieved in the third cycle. This confirmed the hypothesis that metal species leach in the reaction mixture with a similar mechanism of homogeneous ones. 4.3. Limitations of heterogeneous solid catalysts In a heterogeneous solid catalyst catalyzed reaction system, problems related to mass transfer always occur since the catalyst appeared in a solid phase and the reactants in gaseous and/or liquid phases. Therefore, the rate of reaction is dependent on the diffusion between these phases. In order to overcome this mass transfer problem in transesterification, catalyst support or structure promoters were introduced to provide large specific surface areas and pores for active species [119]. Alumina with a high thermal and mechanical stability has been extensively utilized as catalyst support. The NaOH/Al2O3 catalyst which was prepared with different concentrations of aqueous sodium hydroxide solution by impregnation method and calcined for 3 h, was studied [120]. When this catalyst was applied for transesterification of palm oil, a 99% conversion of vegetable oil into biodiesel was obtained with a 3% (w/w) catalyst concentration, 15:1 molar ratio of methanol to oil and 3 h of reaction at 60  C. Based on the work of Boz and Kara [121], it was observed KF/Al2O3 was an efficient solid base catalyst for Table 1 Catalytic performance of zeolite X on the sunflower oil transesterification occluded with sodium oxide. Catalysts

BET surface area (m2/g)

Pore volume (cm3/g)

Strong basicityb (mmol CO2/g)

Methyl ester (wt%) UNE-EN 14103

NaX Bentonite 0.3NaX 1NaX 3NaX 3NaXB

711 37 654 540 186 197

0.29 0.09 0.28 0.24 0.10 (0.03a) 0.07 (0.04a)

1 810 8 97 260 209

1.6 0.0 5.3 38.4 95.1 93.5

NaX for the parent zeolite in the sodium form; numbers in samples 0.3NaX, 1NaX and 3NaX refer to the excess Na per supercage; the suffix “B” indicates that the sample has been agglomerated with sodium bentonite. a Mesoporous volume (cm3/g). b Desorption temperature ¼ 700  C.Adapted from Ramos et al. [110].

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F. Ullah et al. / Journal of the Energy Institute xxx (2015) 1e11

the conversion of canola oil into methyl esters. The optimum yield (99%) was observed with a 15:1 molar ratio of methanol to oil, 3% (w/w) catalyst concentration and 8 h of reaction at 60  C. Similarly, a KI/Al2O3 catalyst was prepared [122] for the conversion of soybean seed oil to biodiesel. The 96% conversion was obtained with a 2.5% (w/w) catalyst concentration, methanol to oil molar ratio of 15:1 and 8 h of reaction. Other limitations of heterogeneous catalysts include high temperature requirements, low conversion rate of reaction and the production of some unwanted byproducts whose removal further adds the cost of biodiesel production technology. The utilization of metal oxides results in the leaching of metal into methyl esters, and then decreases the quality of glycerol which is an important side byproduct in biodiesel industry. 5. Conclusion The future of heterogeneous catalysts is bright and further researches will make them more applicable on industrial scales due to their environmentally friendly and economical characteristics compared with those homogeneous catalysts. The heterogeneous solid basic catalysts seem to be more effective, economical and alternative catalysts for biodiesel industry compared with heterogeneous solid acid catalysts. The nano-scale metal oxides may be highly potent for future biodiesel industry because a high surface area and well-dispersed active sites were possessed, and the catalytic activity was approximately fully recovered during the regeneration process. 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Please cite this article in press as: F. Ullah, et al., Current advances in catalysis toward sustainable biodiesel production, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.01.018