Biodiesel production from waste cooking oil A review on production methods recycling models material

8.09. MPa and 239°C, and 8.09 MPa, respectively [68]. As

a result, this process is energy consuming and needs high temperatures, which makes it economically inefficient for industrial applications. The lack of oxygen in the thermal cracking process also decreases the oxygen content in the final fuel [69]. Figure 6, presents the production of biodiesel via supercritical methanol process from WCO [70].

Figure 6. Schematic of biodiesel production from WCO by supercritical methanol process.

Microemulsification Microemulsions, as colloidal dispersion, could be conducted by mixing of oil, water, and surfactant. They are transparent and thermodynamically stable with the capability of eliminating concerns about the viscosity of vegetable oil. The microemulsion droplets are in the range of 100 to 1000 Å. Oil microemulsions could be obtained by the addition of alcohol, surfactant, cetane improver, and co-solvent. Microemulsions, made by hexanol, butanol, and octanol can satisfy the required viscosity for fuel. Microemulsification is a straightforward method and has advantages such as stability, high viscosity and low volatility [71, 72]. Their nonionic and ionic characteristics are similar to those of diesel fuel. In short-term operation, they showed acceptable spraying behavior with improved combustion properties due to their explosive evaporation. In long-term operation, they also showed no considerable performance failure [73, 74]. Carbon deposition on the injector nozzle chocking and piston head as well as gum formation are some consequences of high viscosity [27]. An example of microemulsification method is to produce biodiesel fuel through the extraction of vegetable oil using diesel-based reverse-micellar microemulsions as a solvent. In this technique, peanut oil is extracted directly into the oil phase of the microemulsion based on the principle of “likes dissolve likes,” resulting in a blend of peanut oil and diesel [75]. Pyrolysis method A substance can be converted to other products by heating, with or without a catalyst. This process is known as thermal cracking or pyrolysis. In this process, the substance is heated in the absence of air or oxygen to break the chemical bonds, leading to molecules with smaller sizes as shown in Figure 7 [76]. Vegetable oil can be converted

Figure 7. Schematic of biodiesel production by pyrolysis of WCO.

to biofuel via pyrolysis, producing various amounts of carboxylic acids, alkadienes, alkenes, alkanes, and aromatics [77]. Especially in developing countries, thermal cracking is a costly process for producing biodiesel. The removal of oxygen throughout the thermal cracking process eliminates the environmental advantages of consuming an oxygenated fuel. Another disadvantage of thermal cracking is the need for distillation equipment in order to separate the fractions. Parawira [78] claimed that, similar to the gasoline, the product of pyrolysis process is less eco-friendly because it has sulfur in its composition. The pyrolytic chemistry can be quite difficult to distinguish, since various reaction products may be obtained from different reaction paths. Recently, petroleum has been produced from vegetable oil via pyrolysis method [79]. Transesterification method During biodiesel production, methyl esters and glycerin are produced from triglycerides due to the exchange of esters by methanol. This results in methyl esters with lower viscosity than triglycerides and better efficiency as diesel fuel. In addition, the reaction between the triglyceride molecule and a low molecular weight alcohol results in the formation of three monoalkyl ester molecules and a by-product, i.e. glycerin molecule [72]. Strong alkaline or acidic catalysts can considerably increase the reaction rate. Generally, alkaline catalysts can decrease the reaction time. However, high concentration of free fatty acid in the oil results in an unwanted soapy reaction in the presence of alkaline catalysts. In this cases, acidic catalysts are usually used to solve the problem [80]. The best method for biodiesel production can be ester exchange. Generally, biodiesel is commercially produced by using alkaline catalysts under mild conditions, since they provide maximum production efficiency at the shortest reaction times with minimum side reactions. Transesterification is extensively used to obtained products with lower viscosity than triglycerides [81]. The reversible reaction between oil and alcohol results in the formation of glycerol alkyl esters and fatty acid. In the stoichiometric reaction, the required alcohol/oil molar ratio is 3:1, however more alcohol is used to shift the reversible reaction toward the products [82]. In the transesterification reaction, although both ethanol and methanol can be used as alcohol, the latter is less expensive and more available, has better physical and chemical properties, shows better polarity, and contains shorter hydrocarbon chain [83]. In addition, more biodiesel fuel can be obtained from oil in the presence of methanol which has higher affinity for oil. This may be due to the fact that methanol requires less energy for the separation of OH- ion in comparison with ethanol and other long hydrocarbon chain alcohols [84]. It is worth mentioning that alcohol purity significantly affects the transesterification reaction rate. In the other words, high purity avoids the mixture from waxing, resulting in higher biodiesel production efficiency. In addition, increasing the alcohol/oil molar ratio leads to higher solubility of

glycerin in alcohol, making the separation of glycerin more difficult. The residual glycerin in the solution shifts the reaction toward the reactants, hence the amount of ester is reduced [8]. In the beginning, alcohol has low solubility in the oil which limits the transesterification reaction. Oil immiscibility in alcohol can limit the reaction rate due to the insufficient oil mass transfer. Figure 8 shows the schematic of transesterification process. Furthermore, because of the inherent reversibility of the transesterification reaction, the conversion efficiency may be reduced if the product is not separated. The transesterification reaction rate may be significantly increased at shorter times in the presence of a proper solvent due to the formation of a single-phase system [85]. In such a case, the solvent increases the solubility, hence decreases the mass transfer resistance [86, 87]. On the other hand, the solvent recycling can be almost complicated. This problem can be solved by using a solvent (such as diethyl ether, tetrahydrofuran, etc.) with a boiling point similar to that of the alcohol. Tetrahydrofuran forms an oily single-phase system and considerably increases the methanolysis rate. Additionally, tetrahydrofuran can be easily recycled after transesterification because its boiling point is close to that of the methanol (67°C and 65°C, respectively) [85, 87]. Since oil has low miscibility in alcohol, vigorous stirring is required during transesterification reaction to completely disperse the alcohol in the triglyceride phase. Under these conditions, the mass transfer rate is improved which results in higher contact area between the oil and alcohol, thus increases the reaction rate and biodiesel efficiency. Otherwise, the reaction occurs only at the interface of the phases. Increasing the mixing intensity leads to higher reaction rate, hence decreases the reaction time [88]. Water content of raw materials is also affecting biodiesel production since esters are produced as a result of triglycerides hydrolysis in the presence of water and the formation of FFAs. In addition, FFAs create a soapy reaction medium which makes the product separation more difficult (due to formation of emulsion from biodiesel and soap) and hinders the catalytic activity. The free fatty acids and water contents in the oil have significant effect on the efficiency and catalytic activity [89]. Esterification reaction converts the free fatty acids to biodiesel by using an acidic catalyst. RCOOH + CH3OH → RCOOCH3 + H2O

Under these circumstances, water can both decrease the catalytic activity and biodiesel production efficiency. However, if an alkaline catalyst is used during the esterification of free fatty acid (>0.5 wt%), the catalyst converts to soap [90, 91]. RCOOH + KOH → RCOOK + H2O FFA + Base → Soap + Water

Triglyceride + Base → Glycerol + Soap CH3COOR + H2O → CH3OH + COOHR

Methyl Ester + Water → Methanol + FFA Reactions 2, 3 and 4 describe free fatty acid soaping, triglyceride soaping, and methyl ester hydrolysis during the esterification reaction, respectively. Soaping reaction results in catalyst consumption and the need for removal of the impure soap. On the other hand, water formation also decreases the transesterification reaction rate [92]. Soap dissolution in the glycerol enhances biodiesel dissolution in the glycerol, makes the separation and purification of the product more difficult. The problem regarding free fatty acids can be avoided via a two-step process; acid transesterification step for removing free fatty acids, and a transesterification step for converting triglycerides to biodiesel [93, 94]. The oil type has significant effect on selecting the appropriate temperature of transesterification reaction which in turn affects the reaction rate. As reported in literature, transesterification reaction is generally performed at 60°C, but the reaction efficiency at different temperatures depends on the catalyst type. High reaction temperature can decrease the reaction time [95]. Transesterification via electrolysis In recent years, researchers have focused on alternative methods for the production of biodiesel from recyclable materials. Catalyst-based electrolysis procedure could be a proper way for enhancing biodiesel yield [96]. Electrolysis may enhance the efficiency of biodiesel production from waste oils containing high concentrations of water and free fatty acids [89, 97]. In this method, transesterification reactions occur in one step due to the formation of H+ and OH- ions. Thus, in contrast to conventional biodiesel production methods, no soap reaction or destructive side reactions take place in electrolysis reaction. In addition, catalytic degradation does not occur at high FFA contents [98]. After addition of catalyst (low concentrations based on the oil weight) and H2O to the electrolysis cell, hydroxyl and hydrogen are formed on the cathode while oxygen is oxidized on the anode. Reaction between hydroxyl ion and methanol leads to the formation of strongly nucleophilic methoxide ions which facilitate glyceride reaction with methanol, creating methyl esters and glycerol [99].

Figure 8. Schematic of biodiesel production via transesterification process.

Figure 9. The schematic representation of the electrolysis process for biodiesel production using waste cooking oil (WCO) transesterification.

Electrolysis process has been performed for the transesterification of WCO in the presence of KOH catalyst. Biodiesel production efficiency of about 96% has been obtained after 2 h at room temperature at 50 v using methanol/oil molar ratio of 6:1, 2 wt% water, 0.5 wt% catalyst, and 10 wt% acetone (as solvent aid). The reactions were carried out at 25°C in order to reduce solvent and alcohol evaporation and decrease energy consumption [89]. Figure 9, schematically shows the electrolysis process and its reactions. Since the reaction is reversible, it is possible that the concentration of fatty acid methyl ester decreases over time. Thus, the time may have considerable effect on the concentration of fatty acid esters in the products [100]. In the electrolysis method, methanol is decomposed to methoxide ions in the presence of catalyst, which subsequently convert to reactive species for the transesterification reaction. Transesterification reaction occurs between triglycerides and short chain alcohols (such as methanol) in the following three consecutive steps: from triglycerides to diglycerides, monoglycerides, and finally to methyl esters and glycerol [101, 102]. Three moles of alcohol are consumed and one mole of ester is produced in transesterification [103]. Water molecules are removed rapidly from the reaction system by producing more active methoxide molecules. They also continuously decompose under the effect of electricity. Therefore, water plays a determining role in the electric charge transfer between the electrodes [104]. The catalyst concentration in the electrolysis cell influences the required energy or voltage for the initiation of chemical reactions at the electrode surface as well as the electric current in the reaction mixture. By increasing the applied voltage, the biodiesel production efficiency also increases, resulting in the formation of more hydroxide ions at the cathode which instantaneously react with the adjacent methanol molecules [105]. In the electrolysis process, biodiesel production performance increases at higher concentrations of OH- ions and decreases at lower voltages [104]. Increasing the intensity of the mixture increases the level of contact between the oil and the methoxide and helps to create easy conditions for the reaction to begin. Without mixing equipment, the reaction occurs only at the interface of the phases. Therefore, with increasing mixing intensification with the help of solvent, the reaction rate increases and the reaction time decreases. The primary electrical conductivity is inversely proportional to the cosolvent/methanol molar ratio. Accordingly, the optimal cosolvent/alcohol molar ratio plays an important role in the production of biodiesel fuel via electrolysis method. Electrolysis in conjunction with heterogeneous catalysts facilitates some processing operations such as neutralization and separation. For example, in other biodiesel production methods, the product is washed three to four times, while in the electrolysis method, the required washing steps is decreased to one to two times. Biodiesel production efficiency of about 93% has been obtained via electrolysis for 3 h at 40 v using 1 wt% zeolite/chitosan/

KOH composite as heterogeneous catalyst, graphite electrode, 2 wt% water, methanol/oil molar ratio of 7:1, and 10 wt% acetone as cosolvent aid at room temperature [50]. Homogeneous Alkaline and Acidic Catalysts for Biodiesel Production In order to produce biodiesel, homogeneous acidic or alkaline catalysts are commonly used depending on the oil type and FFA content of the oil (Tables 2 and 3). Homogeneous alkaline catalysts (such as sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium methoxide (CH3ONa) are superior to acidic ones for biodiesel production from low-FFA oils (generally, <0.5 wt% FFA) [15, 41]. Free fatty acids, in the presence of alkaline catalysts produce soaps, cause catalyst consumption, low FAAE yield, and more difficult separation. The FAEE production strongly depends on the initial concentration of alkaline catalysts. The optimum catalyst concentration, reported in the literature, is 1.0 wt%; however, in the case of NaOH, even lower (0.5%, 0.7% or 0.8%) and higher concentrations (6% or 3.3%) have also been used [106-109]. The maximum FAAE yield of 55% has been obtained with 3.3 wt% NaOH concentration [110]. Under similar operating conditions, a less amount of KOH catalyst (0.5 wt.%) gave the same yield as a higher amount of KOH catalyst (1.0 wt.%) [106]. The reported reaction times for transesterification, in the presence of alkaline catalysts, is usually in the range of 1-2 h; however, in some cases, it may last up to 5 h [111]. It has also been shown that the efficiency of biodiesel production from virgin oils is higher than that of waste frying oils (97% and 92%, respectively). Homogeneous alkaline catalysts are very sensitive to free fatty acid content. As a result, the free fatty acid content must be less than 0.5 wt%, to diminish soap formation during the transesterification in the presence of homogeneous alkaline catalyst [41]. In contrast, homogeneous acidic catalysts have less sensitivity to FFA in the raw material, because they can simultaneously catalyze both FFA and triglycerides. Some of the drawbacks of acidic catalysts are their low reaction rate, long reaction time, and high alcohol consumption, making them inappropriate for one-step biodiesel production methods [130]. The problems concerning biodiesel synthesis using high-FFA oils and homogeneous alkaline or acidic catalysts have been minimized by a two-step method (esterification and transesterification) [41, 131]. The first step is the esterification process using an acidic catalyst, to decrease the FFA content to less than 1%. The second step is the transesterification using an alkaline catalyst, to obtain biodiesel with high efficiency at short reaction times [132]. The effects of different homogeneous acidic catalysts (like H3PO4, sulfuric acid, BF3, HCl, and organic sulfonic acids), oil type, and reaction conditions on one-step FAAE production are shown in Table 3 [133]. The unsatisfactory conversion yields have been reported using sulfuric acid and different alcohols (i.e. methanol, ethanol, and butanol)

Table 2. Effect of different homogeneous alkaline catalysts, oil type, and operating conditions on biodiesel production yield Catalyst (wt%)

with alcohol/oil ratios of 6:1 and 20:1and the reaction times of 3 h and 18 h, respectively. In contrast, alcohol/oil molar ratio of 30:1 resulted in high conversion yield [134]. It can be seen form Table 3 that the high biodiesel yields (<90%) via transesterification in the presence of homogeneous acidic catalysts need higher temperatures, much longer reaction times, high quantity of alcohols, and more catalyst concentration [135]. Thus, transesterification of low-FFA oils using homogeneous acidic catalysts suffers from the following disadvantages: high alcohol consumption, low catalytic activity, low reaction rate, and high reaction temperature. Therefore, homogeneous acidic catalysts are best suited for biodiesel production via esterification of low-cost high-FFA oils. The conversion rate and yield of FFA into biodiesel significantly depend on the reaction temperature, the amount and type of alcohol, the concentration and type of catalyst, and the initial FFA content. Also, it is found that, for the same catalyst (H2SO4) and its amount (1 wt.%), oil (Castor oil), and reaction temperature (50°C), the biodiesel yield is higher (90.83%) for the case where the molar ratio of alcohol/oil is (20:1) [136, 137].

Heterogeneous Alkaline and Acidic Catalysts for Biodiesel Production It seems that solid catalysts can be a promising alternative to homogeneous catalysts in transesterification reactions, since they are reusable and less corrosive. In addition, they can be more easily separated from the reaction medium and produce less toxic residuals. Similar to homogeneous catalysts, heterogeneous catalysts are also influenced by factors such as water content, FFA levels, and the type of oil used as feed [146, 147]. Heterogeneous catalysts can prevent undesirable soaping in transesterification reaction and tolerate more aggressive operating conditions (high pressure and temperature). They also eliminate some operating steps, which are performed for homogeneous catalysts; thus, simplify the transesterification process and reduce the production costs [148]. Heterogeneous catalysts show better performance than homogeneous ones for high-FFA oils. Typically, solid heterogeneous catalysts can be alkaline, acidic, or bifunctional (acid-base). Solid acidic catalysts can catalyze esterification reaction, while alkaline solid catalysts can catalyze transesterification reaction. On the other side, bifunctional solid

Table 3. Effect of different homogeneous acidic catalysts, oil type, and operating conditions on biodiesel production yield Catalyst (wt%)

catalysts can concurrently catalyze both the esterification and transesterification processes [41, 149]. In the case of biodiesel production, using heterogeneous catalysts, more catalyst is needed at higher alcohol/oil molar ratio, which increases the required temperature and pressure. In addition, three phases are formed with oil and alcohol, which make the diffusion more difficult; hence, decrease the reaction rate. This problem can be solved by using an auxiliary cosolvent to improve the miscibility of oil and alcohol, thus increasing the reaction rate [150, 151]. Most of alkaline heterogeneous catalysts consist of alkalis and alkaline oxides with high specific surface area. Therefore, similar to homogeneous catalysts, solid heterogeneous alkaline catalysts have higher activity than solid heterogeneous acidic catalysts. As an example, CaO has a long catalytic life and activity, but its reaction rate for biodiesel production is low. In addition, it has low solubility in alcohol, which could be prepared by inexpensive sources such as CaO and limestone; hence, it is more environmentally friendly [152]. Granados et al. [153] performed methanolysis of sunflower oil using activated calcium oxide as catalyst. They also studied the effect of water and CaO on the catalytic activity at different time intervals. They reported that hydration and carbonation of calcium oxides occurs rapidly in air, and no CaO peak could be detected after 20 days. This means that the surface of calcium oxide is saturated with water and carbon dioxide molecules. For this reason, calcium oxide should be heated at 700°C before use, to remove the adsorbed carbon dioxide from the surface of the catalyst; hence, improving its catalytic activity. Figure 10 shows the transesterification steps for the production of biodiesel using CaO. The first step is the CaO dissociation with methanol to produce protonated (H+) catalyst and methoxide anion (CH3O-). The

generated CH3O- nucleophilically attacks carbonyl carbon of triglyceride, resulting in the generation of a tetrahedral intermediate. Upon the rearrangement of molecules in the intermediate compound, a mole of methyl ester is formed together with the diglyceride anion. The deprotonation of catalyst causes the regeneration of active species for the next catalytic cycle, i.e. reaction with the second methanol molecule. The CH3O- would attack another carbonyl carbon atom in the generated diglyceride and forms another mole of methyl ester together with monoglyceride. Finally, the monoglyceride is similarly converted to alkyl ester and glycerol [20, 154]. Nanocrystalline calcium oxides have higher efficiency than the conventional calcium oxides. For example, 100% conversion of soybean oil has been achieved using 20 nm calcium oxide nanocrystals and specific surface area of 90 m2/g at the ambient temperature after 12 h. While the conversion rate for the commercial calcium oxide was 100%. With a crystal size of 43 nm and a specific surface area of 1 m2/g, it was only 2% [155]. It has been reported that the catalytic activity and biodiesel efficiency of calcium oxide could be improved in the presence of water, since the presence of water and O2on the surface removes H+ from the water molecule. Then, OH- is produced, which removes H+ from methanol, and the methoxide anion, which is the real catalyst, is produced for the ester exchange reaction. Addition of 2.03% water to the reaction medium containing alcohol/oil molar ratio of 12: 1 and 8 wt% catalyst resulted in 95% methyl ester yield after 3 h; while in the absence of water, it reached 80%. No active phase output was observed in the reaction medium and the catalyst could be reused up to 20 times [156]. Strontium oxide is an efficient solid alkaline catalyst for the conversion of soybean oil. In spite of its low specific surface area (1.05 m2/g), 90% of the soybean oil could be converted to methyl ester after 30

Figure 10. Triglyceride transesterification mechanism using CaO as catalyst. min at 65°C using an alcohol/oil molar ratio of 12 and 3 wt% catalyst [157]. Xie et al. [158] investigated the catalytic activity of potassium-deposited alumina as solid alkaline catalyst for the production of biodiesel from soybean oil. They produced the catalyst via an aqueous solution impregnation of potassium nitrate on alumina. The best activity was obtained for 35 wt% potassium nitrate catalyst calcined at 500°C. Xie and Li [159] utilized alumina coated with potassium iodide as an alkaline catalyst for methanolysis of soybean oil. They investigated the dependency of conversion rate on potassium iodide concentration and calcination temperature. The best catalytic activity was reported for 35% potassium iodide and calcination temperature of 500°C for 3 h. Kim et al. [160] used alumina coated with sodium and sodium hydroxide as a solid alkaline catalyst, to convert oil to biodiesel using a mixture of methanol and hexane. First,

they mixed γ-alumina, sodium metal, and sodium hydroxide, and then, heated the mixture in a stainless steel reactor at 320°C. This method led to the reduction of γ-alumina specific surface area from 143.1 to 83.2 m2/g. Yang and Xie [161] investigated the catalytic activity of alkaline earth metals for the conversion of soybean oil to biodiesel and also the dependency of oil conversion on the amount of active phase loading. They reported that the concentration of surface sites on the catalyst significantly affected the catalyst efficiency. Among the synthesized catalysts, ZnO/Sr(NO3)2 exhibited the best catalytic activity. The maximum catalytic activity was obtained with 2.5 mmol strontium nitrate loaded on zinc oxide, corresponding to the basicity of 10.8 mmol/g. Xie and Huang [162] used potassium-coated ZnO as solid alkaline catalyst. Based on the Hammett indicator method, the surface basicity of 1.47 mmol/g was obtained

for 15 wt% potassium fluoride loaded on the zinc oxide, and 87% of soybean oil was converted after 9 h using 3 wt% catalyst and alcohol/oil ratio of 10:1. Sayed et al. [163] performed transesterification of sunflower WCO in the presence of a novel geopolymer, which was synthesized using natural silica resources and synthetic MgAl-layered double hydroxide (MgAl-LDH) nanostructures as a promising heterogeneous potassium-loaded catalyst. The suggested mechanism for transesterification of sunflower oil for the production of biodiesel is depicted in Figure 11. After the adsorption of methanol onto the surface

of catalyst, the hydrogen in methanol reacts by the potassium. Then, the ion exchange between potassium and methanol results in the formation of CH3O– alkoxide anion. On the other side, the terminated –OH groups physically bond with part of the fatty acid on the surface of the catalyst and form a protonated carbonyl group, which is attacked by the CH3O– ion, producing the corresponding ester. Methanol is highly acidic, thus, the presence of methoxide moieties on the surface of the catalyst is indicative of the formation of alkoxide anions. Meloni et al. [164] proposed that the basic site could catalyze the transesterification reaction,

Figure 11. The transesterification mechanism for biodiesel production from sunflower oil using MgAl-LDH based potassium/geopolymer.

providing that the presence of some acidic site stabilizes the abstracted proton from methanol to get stable methoxy species. According to the test results, the best experimental conditions for achieving 94.6% biodiesel yield were the reaction time of 4 h, reaction temperature of 120°C, catalyst concentration of 4 wt%, and methanol/oil ratio of 15:1. It was also reported that zeolites are alkaline due to the neutralization of negative lattice charge by alkali metals, which increases the electron density of the lattice oxygen, the strength of which depends on the chemical composition, network structure, and the nature of interactions. The adsorption of charged alkali metal clusters of Na increases the zeolite basicity through increasing the negative charge of the lattice oxygen. The species are inserted into the zeolite pores via wet impregnation using an aqueous solution containing precursor. The adsorbed compounds are decomposed during the calcination step, leaving alkali metal oxides on the substrate [165]. Xie et al. [166] used NaX zeolite, impregnated with KOH as solid alkaline catalyst, for transesterification reaction of oil in the presence of methanol. The catalyst was impregnated using an aqueous solution with different potassium hydroxide loadings for 24 h, in order to homogeneously disperse the potassium hydroxide on the surface of zeolite. The samples were dried in a rotary drier, and then, calcined at 120°C for 3 h. Under these circumstances, the best efficiency (85.6%) was obtained at 65°C for 8 h using 3 wt% catalyst, 10 wt% potassium hydroxide loading on the zeolite, and alcohol/oil molar ratio of 10:1. After catalyst recycling, it was revealed that potassium hydroxide has been removed from the zeolite structure and a relatively homogeneous reaction has occurred, leading to a decrease in the performance of the recycled catalyst. A similar investigation has been carried out by Wang and Chen [167] who synthesized beta zeolite with high Si/ Al ratio as heterogeneous catalyst via hydrothermal method for transesterification of triolein into biodiesel. They suggested that Na cations in the cages and defects of the beta zeolite could act as catalyst during transesterification reaction; thus, increasing the production efficiency. In addition, the lattice oxygen atoms together with the surface sodium result in the surface electronegativity of zeolite. Figure 12 schematically shows the transfer of Na+ from the defect sites or the intrapores of the Na-BEA to the surface of the zeolite support and silicate [167]. As a result, the absence of active sodium species on the surface of catalyst could be the main reason for catalyst deactivation. Ramos et al. [168] performed transesterification of sunflower oil on different zeolites like mordenite, beta zeolite, and X zeolite to investigate the influence of zeolite type on the methyl ester production efficiency. They also examined the effect of alkaline ions incorporated via ion exchange and impregnation approaches. They found that the impregnation is slightly superior to ion exchange method. In the case of X zeolite agglomerated with sodium bentonite, the production efficiency was 93.5%, while that of X zeolite without sodium bentonite was 95.1%. Kim et al. [169] synthesized

Figure 12. Schematic representation of triolein transesterification for biodiesel production using Na-treated zeolite catalyst. ZnO-Al2O3/ZSM-5 and SnO-Al2O3/ZSM-5 catalysts for transesterification of soybean oil with methanol. They also studied the effect of sodium on the catalytic activity of the synthesized catalysts. ZSM-5 zeolite substrate was prepared via hydrothermal method, and then, calcined at 650°C for 2 h. Metal oxides were deposited on the ZSM-5 substrate through impregnation of metal salt solutions. They added Zn(NO3)2 and Al(NO3)3 solutions, and a solution containing tin chloride and aluminum nitrate to ZSM-5 zeolite The as-synthesized SnO-Al2O3/ZSM-5 catalyst showed 45% fatty acid methyl ester (FAME) efficiency after the reaction time of 9 h. However, after two washing steps, the catalytic activity significantly reduced to 10% FAME. The catalyst showed no activity after three washing steps. In the case of unwashed ZnO-Al2O3/ZSM-5 catalyst, high catalytic activity (98% FAME) was achieved after 9 h. However, after two washing steps and 9 h reaction time, the efficiency reduced to 39.3% FAME. More washing steps resulted in further decrease in the catalytic activity. Sedaghat-Hoor and Anbia [170] synthesized MgAl(O) catalyst via co-precipitation method followed by ZnO impregnation and calcination. They used the synthesized catalyst for biodiesel production through transesterification of WCO. According to Figure 13, in the case of co-precipitated product, only the layered platelet structure is available for the transesterification. However, in the case of Zn/MgAl(O), synthesized via co-precipitation followed by impregnation, both layered and nanoparticle structures take part in transesterification, resulting in a higher efficiency (78.45% FAME) at 65°C using 9:1 methanol/oil ratio for 3 h.

Figure 13. Schematic representation of active sites in (a) co-precipitated ZnMgAl, and (b) Zn/MgAl(O) [170] [Open Access].

Solid acidic catalysts have less sensitivity to free fatty acids and moisture in comparison with liquid acidic catalysts. As a result, low quality feeds can be used for biodiesel production in the presence of such catalysts. In addition, it is possible for esterification and transesterification reactions to be carried out concurrently using acidic solid catalysts. Recycling and reusability of heterogeneous acidic catalysts result in less wastewater production. Medium to strong acidity, large cavities, and hydrophobic surface are crucial for biodiesel production by using solid acidic catalysts. Esterification efficiency strongly depends on crystalline phases, size of cavities, and acidity. Solid acidic catalysts have also less environmental pollution and corrosion problems than strong liquid acids [171]. In spite of their low activity, solid acidic catalysts are widely used in several industrial applications because they have more acidic sites with different Brønsted and Lewis acidity strengths in comparison with homogeneous acidic catalysts. There are numerous reports on the application of zirconia as solid acidic catalyst instead of other transition metal oxides for biodiesel production from different oils. This may be due to the high acidity of the zirconia and the fact that the acidity enhances in the presence of anions like sulfate (SO42-) and tungstate (WO42-) on the surface of metal oxides. López et al. [172] compared the catalytic activity of highly acidic ZrO2/SO42- and ZrO2/WO32- catalysts for the methanolysis of triacetin. Their findings indicate that the specific surface area and the concentration of active sites play a significant role in catalytic activity. The results showed that ZrO2/SO42- has higher activity than ZrO2/ WO32- after the reaction time of 8 h at 60°C. The reason for this observation is the higher specific surface area of ZrO2/ SO42- compared with ZrO2/WO32- (134 and 89 m2/g, respectively). In addition, ZrO2/SO42- had more active sites than

ZrO2/WO32- (94 and 54 µmol/g, respectively). In another study carried out by Chen et al. [173], catalytic activity of TiO2/ SO42- and ZrO2/ SO42- as strong solid acidic catalysts were examined for transesterification of cottonseed oil. The results showed that the catalytic activity of TiO2/ SO42- is higher than that of ZrO2/ SO42- mainly due to its higher specific surface area (99.5 and 91.5 m2/g, respectively). They also found that new Lewis and Brønsted acid sites have been formed on the surface of catalyst in the presence of sulfate anions. Methyl ester yield after 8 h reaction using 2 wt% of TiO2/ SO42- and ZrO2/SO42- catalysts and alcohol/oil molar ratio of 8:1 was 90% and 85%, respectively. Ramu et al. [174] investigated the effect of calcination temperature and WO3 loading on the catalytic activity of ZrO2/WO32- for esterification of palmitic acid in the presence of methanol. They observed the highest catalytic activity for the catalyst calcined at 500°C owing to the formation of tetragonal ZrO2 phase. Calcination above 500°C resulted in the transformation of tetragonal to monoclinic phase. For example, 98% palmitic acid conversion efficiency reduced to 8% after calcination at 900°C. On the other hand, acidic sites concentration of 1.04 mmol/g was obtained by loading 5 wt% WO3. However, at higher WO3 loadings, the catalytic activity decreased because of more coverage of ZrO2 surface by WO3. Conversion of soybean oil using tungstated zirconia (WO3/ZrO2) has been also studied by Furuta et al. [175]. They achieved 94% oil conversion after 8 h reaction at 300°C using alcohol/oil ratio of 40:1. The catalyst was active up to 100 h reaction. In addition, WO3/ZrO2 (WZO) showed higher activity for soybean conversion in comparison with sulfated zirconia (SO4/ZrO2) (SZO) and sulfated tin oxide (SO4/SnO2) (STO) which yielded 70% and 80% conversion efficiency, respectively.

Fe-Zn double-metal cyanide (DMC) complexes have been applied as solid acidic catalyst for methanolysis of sunflower oil [176]. It has been reported that the oil conversion efficiency of 97% is achievable via transesterification at 170°C for 8 h using 0.3 wt% catalyst and alcohol/ oil ratio of 15:1. They found that the catalytic activity may arise from the Lewis acid active sites owing to the presence of Zn2+ ions on the surface of the catalyst. In addition, the catalyst was able to convert oil with 20 wt% water which implies that the catalyst has hydrophobic surface. The catalyst also showed reasonable performance for esterification of oils containing high amounts of free fatty acids even after several cycles. Jacobson et al. [177] utilized various solid acidic catalysts including MoO3/SiO2, TPA/ZrO2, WO3/ZrO2-Al2O3, WO3/ZrO2, MoO3/ZrO2, zinc stearate/ SiO2, and zinc ethanoate/SiO2 for the production of biodiesel from WCO. The reactions were carried out at 200°C for 10 h using 3 wt% catalyst and alcohol/oil molar ratio of 18:1. The maximum production efficiency of 98% was achieved for zinc stearate catalyst on silica gel (SiO2) support which was synthesized via sol-gel method. The catalyst was reusable even after several catalytic cycles. Chung et al. [178] employed a variety of zeolite catalysts including mordenite (MOR), beta (BEA), faujasite (FAU), and ZSM-5 (MFI) zeolites as well as with different Si/Al molar ratios for removing free fatty acids from WCOs via esterification in the presence of methanol. The reaction was conducted at 60°C for 3 h using 1 g catalyst and alcohol/oil ratio of 30:1. The maximum FFA removal efficiencies of 78% and 80% were obtained for ZSM-5 and MOR catalysts, respectively, while silicalite showed the lowest efficiency mainly due to its very weak acidic sites. Although HMOR(10) has more acidic sites and higher acid strength than HMFI(25), they showed similar removal efficiency. The reason for this is that MFI has finer pores with zigzag structure, leading to the cracking of the reactant with long chain, while MOR has straight channels with wide pore opening. Furthermore, at higher Si/Al ratios, the acidic sites decreased in the zeolite, resulting in lower FFA removal efficiency. Karmee and Chadha [179] produced biodiesel from inedible Pongamia pinnata oil via transesterification in the presence of methanol and KOH catalyst. The reaction was performed at 60°C for 1.5 h using alcohol/oil molar ratio of 10:1 and 1 wt% KOH. They also employed solid acidic catalysts (i.e. Hβ zeolite, K-10 montmorillonite, and zinc oxide) for transesterification reaction at 120°C for 24 h using alcohol/oil molar ratio of 10:1 and 10 wt% catalyst. The conversion efficiency for zinc oxide, Hβ zeolite, and K-10 montmorillonite catalysts was 83%, 59%, and 47%, respectively, while that of potassium hydroxide was 92% which increased to 95% by the addition of tetrahydrofuran cosolvent. Shu et al. [180] synthesized lanthanum-modified beta zeolite catalyst via ion exchange method by stirring 10 g beta zeolite in 250 ml of 0.1 M La(NO3)3 solution for 3 h at room temperature followed by filtering, washing with deionized water, drying at 20 ˚C, and at last calcination at

523°C for 4 h. The synthesized catalyst was employed for biodiesel production at 60°C for 4 h using alcohol/oil ratio of 14.5 and catalyst/oil weight ratio of 0.011, resulting in

48.9. wt% triglyceride conversion. Due to the small pore size

of beta zeolite, triglyceride molecules have limited access to the internal acidic sites. Thus, the catalytic activity mainly arises from the external acidic sites on the surface of the beta zeolite. On the other hand, lanthanum creates a lot of Brønsted acid sites which are available for the reactants, resulting in an increase in triglyceride conversion efficiency by 10 wt%. Recovery of Biodiesel Purification By-products In order to increase the productivity of the biodiesel process, it is possible to convert the by-products into more valuable products [181]. One of these by-products is crude glycerol because of its high concentration (8-10%) in transesterification process. It has been predicted by Rastegari et al. [182] that about 41.9 billion liters of glycerol will be globally produced by 2020. Monteiro et al. [183] reported that pure and crude glycerol (80% purity) cost 0.60-0.91 and 0.09-0.20 USD/kg, respectively. A number of industries such as food, cosmetics, pharmaceuticals, textile, fuel additives, and medicine can utilize pure glycerol as raw materials, hence increasing the economic efficiency of biodiesel production [184]. Garlapati et al. [185] proposed biological conversion of crude glycerol to other valuable products using microbes. Purification of the produced biodiesel (i.e. separation of excess alcohol, remaining oil, catalysts, and glycerol) after transesterification is crucial to increase the performance and protection of the engine by using high quality biodiesel. As reviewed by Veljkovićet al. [186], there are a range of methods for biodiesel purification in addition to reactive distillation and centrifugal contractor methods such as dry washing, wet washing, precipitation, membrane filtration, liquid–liquid extraction, vacuum distillation, and column chromatography. The selection of which mainly depends on the type of catalyst. In wet washing method, biodiesel produced by homogeneous alkaline catalysts is first washed with acidic and then with neutral water to remove water-soluble impurities such as glycerol and residual methanol. Due to the formation of stable calcium soap, this method is not recommended for the production of biodiesel using heterogeneous catalysts (such as CaO) [187]. In precipitation method, different precipitant agents such as citric acid and oxalic acid are used to precipitate calcium ions (in the case of reactions heterogeneously catalyzed by CaO). Each gallon of biodiesel generates almost one pound of glycerol, thus there are extra glycerol at low prices produced by biodiesel industries [188]. On the other hand, the impurities in the crude glycerol (for example, glycerides, solvents (methanol), soap, etc.) prevent its direct exploitation. Bhatia et al. [133] applied neutralization, exchange resin and microfiltration methods for purification of crude glycerol. Filtration, saponification, evaporation, phase separation and neutralization methods have been also

employed to obtain glycerol with 86% purity. Other impurities like color variation of biodiesel could be adjusted via reaction parameters and the catalysts [189]. Crude glycerol has been recovered in laboratory up to 93.3% purity via combination of repeated physical and chemical steps including phase separation, acidification, solvent extraction and neutralization [190]. Conversion of the recovered glycerol into other value-added bioproducts can be conducted via different microbial processes or physicochemical reactions. Yuksel et al. [191] employed hydrothermal electrolysis for the conversion of glycerol into lactic acid under aqueous alkaline conditions at high temperature and pressure. Samudrala et al. [192] activated montmorillonite with sulfuric acid and used it as support for platinum catalyst for hydrogenolysis of glycerol into 1-3 propanediol. Other biochemicals such as lactic acid, citric acid, 1-3 dihydroxyacetone, 1-3 propanediol, biohydrogen, ethanol, etc. can be produced via microbial fermentation of glycerol c. Challenges and Limitations of Biodiesel Production In spite of intense research on new raw materials, oil conversion technologies, biodiesel processing, there are many limitations and barriers which should be overcome for cost-effective biodiesel production on industrial scale. Some of these challenges and the corresponding solutions are discussed below [193]. Feedstock limitations At present, the production cost of biodiesel is higher than that of fossil fuels. Biodiesel is mostly produced from crops because they can be easily processed, but there is a growing market competition between food and fuel. The annual crop productivity is strongly affected by the environmental conditions and other parameters. It is not possible to grow all types of feedstock in all regions of the world. Thus, transportation of feedstock is costly. Since oil extraction faces some challenges, utilization of animal fats, WCO, and microalgal oil has not sufficiently developed. Moisture and FFA content may cause some problems; water may increase microbial growth, and FFA should be removed by pretreatment [194]. Furthermore, effective risk management is not possible due to the insufficient development of feedstock market. Accordingly, the price of biodiesel is not as manageable as that of fossil fuels. Continuous production of biodiesel needs locally available feedstock all over the year with easy collection and transportation and minimum processing requirements. Technological and financial limitations Reusable catalysts, management of by-products, and downstream biodiesel processing are other technical challenges for cost-effective production of biodiesel. In addition, more processing plants with acceptable performance and novel technologies are required. New technology requires extensive field and demonstration to make it acceptable to investors and users. Biodiesel production has also financial risks because of the lag time between investment and

return. The profit is highly dependent on the government policies. Biodiesel production principles are well-known theoretically, but its execution on industrial scale needs further information and novel strategies for cost-effective biodiesel production. Infrastructure The new field of biodiesel production needs heavy investment for new infrastructure, skilled labor, distributors, storage, and transportation. Unlike developed industries, the insufficient development of biodiesel industry prevents its integration with other processes. For these reasons, investors only will to invest in fast-growing industries with good returns, thus limiting biodiesel industry to small scale plants [195]. Customer demand and approval Lack of enough knowledge on the performance and benefits of biodiesel, its high cost, and the lack of government policies and support are some reasons that have led to low demand for biodiesel. The economic conditions of rural people can be improved by making them aware of the role of biodiesel in energy security and environmental protection. For gradual development and commercialization of biodiesel, people should be convinced to use biodiesel. Government support Different countries have set policies to promote and adapt biofuel applications. Government has tried to increase biofuel consumption and production by applying mandate and tax exemption. It is planned that the USA will use 36 billion gallons of renewable fuel in its transportation sector by 2022, for instance. The climate and energy objectives to 2030 set by European Union (EU) includes reducing up to 40% greenhouse gases and increasing the renewable energy share in all sectors to 32%. The Indian National Policy on Biofuels in 2008 necessitates blending of 20% biodiesel and bioethanol with gasoline by 2017. Additionally, it was intended to plant Jatropha on 11.2 million hectares of wasteland. In 2005, Malaysia’s National Biofuel Policy mandated that 5% of palm oil be used for biodiesel. It is important to consider a number of factors, including infrastructure, emission, feedstock availability, performance, compatibility with automobiles engine, etc. are some of the key parameters that should be considered for future biodiesel production [196].