Bio-Derived Catalysts: A Current Trend of Catalysts Used in Biodiesel Production

Biodiesel is a promising alternative to fossil fuels and mainly produced from oils/fat through the (trans)esterification process. To enhance the reaction efficiency and simplify the production process, various catalysts have been introduced for biodiesel synthesis. Recently, the use of bio-derived catalysts has attracted more interest due to their high catalytic activity and ecofriendly properties. These catalysts include alkali catalysts, acid catalysts, and enzymes (biocatalysts), which are (bio)synthesized from various natural sources. This review summarizes the latest findings on these bio-derived catalysts, as well as their source and catalytic activity. The advantages and disadvantages of these catalysts are also discussed. These bio-based catalysts show a promising future and can be further used as a renewable catalyst for sustainable biodiesel production.


Introduction
Global industrialization leads to the extensive use of fuel-based energy for transportation, which consequently causes the depletion of fossil fuels and global warming. Therefore, renewable fuels are considered an alternative energy to solve the problem of fuel depletion and environmental pollution. Biodiesel, a biomass-derived fuel, is a promising bioenergy, which is increasingly produced worldwide to replace fossil fuels because of its combustion efficiency, compatibility with diesel engines, and low carbon dioxide emissions [1,2]. As a result, studies have been focusing on developing an efficient approach for biodiesel production.
Biodiesel is mainly synthesized from oils/fat (derived from plants, animals, and microorganisms) through the (trans)esterification process [3,4]. The efficiency of this process is mainly affected by several factors including the quality of feedstock (level of free fatty acids), the type of acyl acceptor (e.g., alcohols or methyl acetate), and the type of reactions (e.g., noncatalytic reaction, chemical-catalyzed reaction, and enzymatic reaction) [5,6]. To enhance the reaction efficiency, most studies focused on developing efficient catalysts for the (trans)esterification reaction. Consequently, different types of catalysts, such as alkali catalysts, acid catalysts, and enzymes have been studied [7,8]. Commonly, chemical catalysts (such as KOH, NaOH, H 2 SO 4 , and HCl) are used for biodiesel production processes [9]. Although these chemical catalysts efficiently catalyze the reaction, they retain several limitations regarding their reusability, negative effect on the environment, and     RT: room temperature.

Bio-Derived Acid Catalysts for Biodiesel Production
Alkali-catalyzed transesterification is efficient for producing biodiesel from refined oils (containing a low level of free fatty acids (FFA)). However, the biodiesel yield is significantly reduced when the oil contains a high level of FFA (>1%, w/w) because alkali catalysts cannot convert FFA into biodiesel and the liquid alkali catalysts can react with FFA to form soap [2]. Therefore, acid-catalyzed esterification/transesterification is commonly proposed to produce biodiesel from high FFA-containing oils. Acid catalysts simultaneously catalyze the esterification of FAA and transesterification of oil (triglyceride) into biodiesel; therefore, they are insensitive to the quality of the raw material. In addition, the use of the acid catalysts for biodiesel production prevents the saponification reaction, which is commonly found in the homogenous alkali-catalyzed transesterification reaction. Homogenous acid catalysts (such as HCl, H 2 SO 4 , H 3 PO 4 ) are widespread in biodiesel production because they efficiently convert FFA and triglyceride into biodiesel [9,10]. However, there are lots of associated problems in the downstream process, which is costly and requires complicated steps for product purification and separation of the catalyst. In addition, the use of these homogenous acid catalysts causes corrosive damage to the equipment and negatively affects the environment. These liquid catalysts are also difficult recover and reuse. To address these obstacles, heterogenous/solid acid catalysts have been increasingly considered as promising alternative catalysts to facilitate a cleaner, safer, simpler, and cheaper process for biodiesel production [101,102].
In recent years, biomass-derived acid catalysts have gained much interest in biodiesel production due to their ecofriendly properties, potential reusability, and the availability and low cost of materials used for catalyst synthesis. Recently, several forms of heterogeneous acid carbon-based catalysts have been developed for biodiesel production from high-FFA oils. The carbonization followed by sulfonation method is commonly used to synthesize various solid acid catalysts such as sulfonated carbon from corn cobs [103], sulfonated starch [104], sulfonated carbon from vegetable oil asphalt [105], sulfonated carbon from cacao shell [106], sulfonated rice husk [107], sulfonated bamboo [108], sulfonated sugarcane bagasse [109], sulfonated biochar derived from cassava peel [110], and sulfonated biochar derived from sugarcane bagasse, corncob, coconut shell, and peanut shell [111]. Different materials result in different catalytic activities of the synthesized catalysts. The catalysts prepared from these materials demonstrated good catalytic efficiency towards esterification of high-FFA oils, with FFA conversions ranging from 71% to 98% [109,110]. Among the materials used, waste shells, such as cacao shell [106], wing shell [112], and coconut shell [113], show promise for the synthesis of solid acid catalysts. More acid catalysts used for biodiesel production are shown in Table 2. In comparison with alkali catalysts, the bio-based acid-catalyzed reaction commonly requires a longer reaction time and higher temperature for biodiesel production. Therefore, the acid-catalyzed reaction is only suggested for producing biodiesel from feedstock containing a high level of FFA.

Enzyme
With an increasing demand for environmental protection, green processes have been rapidly developed for chemical production. Consequently, various ecofriendly processes have been proposed for producing biodiesel to reduce the adverse environmental effects [5,149]. Particularly, the enzyme-catalyzed reaction is one of the most promising processes for biodiesel production due to the ecofriendly and reusable nature of the enzyme. Notably, the enzymatic process proceeds at mild reaction temperature and pressure, thus lowering the energy consumption [150]. For this approach, biodiesel can be produced via lipase-catalyzed transesterification or lipase-catalyzed hydroesterification processes (hydrolysis of oils into FFA followed by esterification of the produced FFA with short-chain alcohols). The lipase catalyzes the esterification and transesterification simultaneously; therefore, the enzymatic process is insensitive to high-FFA oil [150]. Because of such benefits, enzymatic processes have been widely developed for biodiesel production from various feedstocks [150].
The efficiency of the enzymatic process mainly depends on the activity of lipases. Therefore, a great effort has been made to use lipase from different sources (microorganisms, plants, animals) for biodiesel production [151,152] (Table 3). The most common source of the lipase is microorganisms such as Candida antarctica [153,154], Thermomyces lanuginosu [155,156], Rhizomucor miehei [157,158], Pseudomonas cepacia [159,160], Candida rugosa [161,162], Aspergillus oryzae [163], Burkholderia cepacia [164,165], Adansonia grandidieri [166], Rhizopus oryzae [167], Pseudomonas fluorescens [168], Lactobacillus plantarum [169], and Aspergillus terreus [170]. Lipases from microorganisms are mainly used for biodiesel production due to the availability of sources and rapid growth rate of microorganisms for enzyme production [171]. Lipase activity depends not only on the source of the enzyme, but also the type of enzyme used (immobilized form or liquid form) [171]. Immobilizing lipase on the support material can enhance the stability of the enzyme, making the enzyme less susceptible to the pH, temperature, and impurities of reactants [171]. Notably, the supports and/or immobilization protocols can greatly modulate the specific activity of lipase, affecting biodiesel yield. Tacias-Pascacio et al. [172] immobilized different lipases on different supports and used them for biodiesel production. They found that the specific activity of lipases and biodiesel yield greatly depended on the support, solvent used, and media [172]. In addition, the immobilized enzyme is easy to reuse. Consequently, lipase immobilized on various supporting materials has been studied for biodiesel production. Recently, Iuliano et al. [173] reported that lipase from C. rugosa was physically attached to Mg modified Fe 2 O 4 nanoparticles and used to turn brewers' spent grains into biodiesel. After 48 h at 45 • C, a remarkable yield of 98% was achieved using a 1:4 oil/methanol molar ratio. In addition, lipases were immobilized on other materials such as graphene oxide [174], polyhydroxyalkanoate [175], alginate-polyvinyl alcohol (PVA) [167], polydopamine coated iron oxide (Fe 3 O 4 _PDA_lipase) [170], modified polyporous magnetic cellulose support [153], Co 2+ -chelated magnetic nanoparticles [168], core-shell structured Fe 3 O 4 @MIL-100(Fe) composites [162], Fe 3 O 4 /Au nanoparticles [176], waste-derived activated carbon support [177], genipin cross-linked chitosan [178], and other materials [160]. Several immobilized lipases have been commercialized and used for biodiesel production such as Novozym ® 435 (lipase B from C. antarctica) [179][180][181] and Lipozyme TL IM (lipase from T. lanuginosus) [182]. Nevertheless, the immobilized lipase-catalyzed reaction rate is relatively low due to the mass transfer limitation between the substrate and enzyme [183]. Notably, the immobilized lipases are expensive, thus limiting their industrial applications.    Liquid lipase formulations or free lipases have been considered as a substitute for immobilized lipase for biodiesel production due to their high catalytic activity and significantly low cost (30 to 50 times lower) as compared to immobilized lipase [183,210]. Recent studies have demonstrated a promising use of several liquid lipases for biodiesel production such as C. antarctica lipase A [154] and liquid lipase formulations from T. lanuginosus (Eversa ® Transform, Eversa ® Transform 2.0, and NS-40116) [195,205,211]. The use of liquid lipase facilitates the homogenous reaction, thus overcoming the mass transfer limitation presented in the immobilized lipase-catalyzed reaction. However, liquid lipase is sensitive to the reaction environment. Studies have reported that high water content (from the feedstock and/or generated from the esterification of alcohol and fatty acid) not only promotes the reverse reaction but also negatively affects the lipase activity (including the formation of lipase-lipase aggregates in aqueous media), thus reducing the biodiesel production efficiency [183]. To address this obstacle, several adsorbents such as superabsorbent polymer, silica gel, alumina, and molecular sieve have been used to remove the water from the reaction mixture, enhancing the reaction efficiency [183,212,213].
Similarly, the type of acyl acceptor used also affects the lipase-catalyzed reaction. Studies have reported that lipase is deactivated using a high amount of methanol or ethanol, lowering the biodiesel yield [181,185]. In addition, the use of methanol or ethanol as an acyl acceptor for biodiesel production resulted in the formation of by-product glycerol [181]. This by-product also inhibits the activity of lipases, especially immobilized lipases because it can easily accumulate on the surface of immobilized lipases [181]. To address this obstacle, methyl acetate is proposed as another alternative acyl acceptor for biodiesel production [185]. The use of methyl acetate prevents the inhibition of lipase caused by methanol/ethanol and by-product glycerol (no glycerol produced in the reaction), thus enhancing the reaction rate [185]. Besides this method, ultrasounds [214,215] or very hydrophobic supports [216,217] can be used as another approach to lower the negative effect of glycerol on the enzyme. Studies have reported that ultrasounds can stir the enzyme particles from the inside and avoid the formation of the glycerin/water phase [215,218].
Another concern for each specific lipase in biodiesel production is associated with the oil source [218]. Fats/oils are a very heterogenous substrate, which are mainly comprised of triglycerides, low levels of mono and diglycerides, and some FFA [218]. Therefore, enzyme specificity affects the enzyme activity over each substrate [218]. To address this issue, the combination of different lipases (combi-lipase) has been proposed for biodiesel production [190,219]. There are several types of combi-lipase, which include co-immobilized lipases (different lipases immobilized on the same support), a mixture of individually immobilized lipases, and a mixture of free lipases [218]. Guan et al. [219] firstly reported the use of R. miehei lipase and P. cyclopium lipase mixture (in a liquid form) for biodiesel production from soybean oil. The result showed that the R. miehei lipase (individual enzyme) resulted in 68.5% biodiesel yield, but the yield increased to 95% when using the mixture of R. miehei and P. cyclopium lipases [219]. This was due to the use of lipases with different specificities [218,219]. In another study, the individual use of R. oryzae lipase and C. rugosa lipase resulted in 94.36% biodiesel yield at a reaction time of 9 h and 92.63% biodiesel yield at a reaction time of 30 h, respectively [220]. However, the biodiesel yield reached 98.16% (at a reaction time of 6 h) by using the mixture of both enzymes [220]. Similarly, various combilipases such as lipase cocktail (67% C. antarctica lipase B and 33% R. miehei lipase) [190]; immobilized C. rugosa and R. miehei lipases [175,202], co-immobilized R. miehei lipase and C. antarctica lipase B [193]; a mixture of 10% T. lanuginosus lipase, 75% C. antarctica lipase B, and 15% R. miehei lipase [221]; a mixture of lipases from porcine and T. lanuginosus (in both liquid and immobilized forms) [222]; a mixture of immobilized C. rugosa and R. oryzae lipases [223], and co-immobilized C. rugosa and R. oryzae lipases [224,225] were also tested for biodiesel production. These combi-lipases showed a higher biodiesel yield than the individual enzymes [175,190,193]. Mixtures of the same enzyme immobilized using different protocols/support materials also affect the biodiesel yield. Toro et al. [226] immobilized the same lipase (T. lanuginosus lipase) on two different supports (Purolite ® ECR1604 and Lewatit ® VPOC1600) and used them for biodiesel production from palm olein. The biodiesel yield reached 70.3% (for lipase immobilized on Purolite ® ECR1604) and 78.2% (for lipase immobilized on Lewatit ® VPOC1600). Notably, the biodiesel yielded increased to 86.1% when the mixture of the two individually immobilized lipases was used [226]. This could be explained by the fact that the enzyme features (flexibility of their active site and their mechanism of action) can be modulated by changes in the immobilization protocol [172]. Consequently, the changes in the support feature influence the stability, activity, and specificity of the lipase [172,218].
Generally, although both immobilized and liquid lipases (individual lipases or combilipases) show effectiveness for converting oil into biodiesel, their industrial application is still limited due to the high cost of the enzyme as compared to chemical catalyst [227,228]. Therefore, further studies on lipase-catalyzed biodiesel production are still required to improve the efficiency and economic feasibility of the process.

Catalyst Reusability
For biodiesel conversion, the catalyst's effectiveness is not only determined by its catalytic activity but also its recoverability and reusability. Since homogenous catalysts cannot be reused for the next batch of production, heterogeneous catalysts play an important role in reducing production costs. Their recyclability not only lowers production costs but also maximizes environmental protection [229]. As compared to homogenous catalysts, one of the benefits of heterogeneous catalysts is that they can be reused several times. Furthermore, these catalysts may be regenerated or used for other purposes after losing their catalytic activity, such as construction materials, soil stabilizers, cement industries, and phosphate adsorbents [230].
Most of the bio-derived acid and alkali catalysts can be reused 4-7 times to yield biodiesel of 65-85% (Tables 1 and 2). da Luz Corrêa et al. [122] prepared sulfonated carbonbased catalysts from murumuru kernel shell and used them for FFA conversion. The first use of the catalyst resulted in 95.1%, but the FFA conversion was reduced to 84.5% and 66.3% after the second and third catalyst reuses, respectively. The reusability of a solid base oxide catalyst derived from chicken eggshell was investigated by performing transesterification using the same catalyst for 10 cycles, and the yield was found to be marginally reduced after the seventh cycle, which may be due to catalyst pores being blocked, reducing reactant adsorption and desorption [49]. Kirubakaran and Arul [27] also investigated the reusability of a heterogeneous catalyst derived from eggshell. The catalyst could be reused five times to yield 85% biodiesel. After that, the biodiesel yield reduced significantly, suggesting that the catalyst's stability had deteriorated. This is due to the presence of active Ca(OH) 2 phases which reacted partially with the homogenous mixture in the transesterification reaction. In comparison with solid bio-based acid and alkali catalysts, several immobilized lipases show better reusability. Several immobilized lipases can be reused for up to 20 cycles without loss of enzyme activity [181,185]. However, the use of immobilized lipase for biodiesel production is still under lab-scale investigation because of the high cost of the enzyme. Therefore, to be used for industrial biodiesel production, further studies are still required to improve the catalytic activity, stability, and reusability of bio-based catalysts. In addition, a pilot-scale investigation is also needed to evaluate the potential use of these catalysts for biodiesel production before being used for industrial applications.

Environmental and Economic Evaluation
Catalyst selection is one of the crucial issues in biodiesel production with the aim to minimize energy consumption, waste generation and treatment, and reduce production costs [185]. The use of bio-based catalysts (alkali catalysts, acid catalysts, and enzymes) lowers the environmental effect since these catalysts are derived from natural sources (plants, animals, or microorganisms). These catalysts are also easy to separate from the reaction mixture and reuse, reducing the generation of wastewater and chemical residues in the downstream process, especially the purification step. Consequently, the fee for the purification step and waste treatment can be reduced, lowering the production cost.
Several studies have also evaluated the economic feasibility of different biodiesel production processes [231,232]. The bio-derived alkali-and bio-derived acid-catalyzed processes are more economically feasible than the conventional process (H 2 SO 4 -or KOHcatalyzed process) for biodiesel production since the cost of those catalysts (and total biodiesel production cost) is considered lower than that of conventional chemical catalysts (KOH or H 2 SO 4 ) [232][233][234]. Among these two processes, the alkali-catalyzed transesterification seems to be superior to the acid-catalyzed process because the former proceeds at a lower temperature, has a shorter reaction time, and requires a lower molar ratio of alcohol to oil as compared to the latter, as shown in Tables 1 and 2 [232,235]. In addition, the bio-based alkali and bio-based acid catalysts can be synthesized from the same natural source, but the synthesis of bio-based acid catalyst commonly requires one more step (sulfonation) [232]. Consequently, in some cases, the cost of bio-derived acid catalysts can be higher than that of bio-derived alkali catalysts. However, the cost of each specific catalyst depends on various factors including the source, synthesis method, and its reusability [232]. Therefore, it is difficult to compare the cost of all different types of catalysts. Different from bio-derived acid and alkali catalysts, the enzyme is expensive, especially the immobilized enzyme, making the lipase-catalyzed biodiesel production less competitive [236,237]. To reduce the enzyme cost, free lipase (or liquid lipase formulation) has been proposed for biodiesel production [183]. However, the reusability of liquid lipases is limited [183]. Therefore, the enzymatic process is still under investigation to improve its industrial application. Generally, among the three processes, the bio-derived alkali-and bio-derived acid-catalyzed processes are more economically feasible than the enzymatic process [236].
However, no individual studies have been conducted to compare the economic feasibility of the bio-derived acid-, bio-derived alkali-, and enzyme-catalyzed biodiesel production processes. Therefore, more studies are still required to evaluate and compare the economic feasibility of these processes.

Future Prospects and Conclusions
The use of biomass-derived catalysts has become a recent interest to make biodiesel production more sustainable. In addition, the use of these catalysts is promising to reduce the current high cost of biodiesel production, making biodiesel competitive with petrodiesel fuels. Research is therefore aimed to develop environmentally friendly, cost-effective, and efficient biomass-derived catalysts for biodiesel production. Consequently, different natural sources (animals, plants, microorganisms) have been used for synthesizing biobased catalysts including acid catalysts, alkali catalysts, and enzymes. The catalytic activity of these catalysts varies among them. The use of acid or alkali catalysts depends on the quality of the feedstock. Besides, enzymes can be used as an alternative to both acid and alkali catalysts for biodiesel production. These catalysts show their advantages and disadvantages when they are used for biodiesel production. These catalysts show promise for biodiesel production, but these investigations have been stopped at lab-scale investigations. More investigations on these catalysts are therefore needed, especially largescale investigations to prove the potential use of these catalysts for industrial biodiesel production.