Nowadays, the efforts on finding new catalysts for biodiesel production are pro­gressing intensively. Different types of catalysts are produced and studied in order to maximize biodiesel yield and at the same time minimize catalyst use without sacrificing their catalytic performance.

The research and development on catalysis are expanding at an extensive rate, mainly to ensure that the process can achieve high productivity in shorter time. Traditional raw materials that are commonly used at industrial scale consist of edi­ble sources that are valuable as food supply, such as soybean and canola.

Further implementation of these sources for biodiesel industry can lead to competi­tion in food sector, which may result in deficiencies of food supply for human con­sumption. Nonedible oils from alternative feedstocks are being recognized as a feasible solution to this matter. However, the process involving nonedible sources requires additional steps for refining purpose. Furthermore, the feedstock can account for up to 85% of the cost for biodiesel production (Canakci and Sanli 2008). It is essential to choose feedstock with lower price when considering the competitiveness of biodiesel with petroleum-based diesel fuel. Waste cooking oil is an attractive feed­stock as it can be acquired at low cost, but it contains FFA and water that are undesir­able for the reaction, especially for the process involving base catalyst.

Homogeneous catalyst can drive the reaction forward in a short period as the cata­lyst is in homogeneity with other reactants. The cheaper price also contributes to the broad use of liquid catalyst for biodiesel synthesis. However, the nature of the catalyst makes the separation of biodiesel from reactants harder. Separation of catalyst proceeds by washing with water, which generated wastewater during the purification step. Also, the catalyst is seldom recovered and recycled. Heterogeneous catalyst requires harsher operating conditions, such as elevated temperature, high alcohol to oil molar ratio, and extended reaction time to increase the biodiesel output. This type of catalyst faces mass transfer limitation of oil-alcohol-heterogeneous catalyst (three-phase system) in the initial stage of the reaction. Another problem related to heterogeneous catalyst is leaching of catalyst into reactants, which will then affect its catalytic activity. Regardless of that, solid catalyst is recovered easier at the end of the process and more suitable for continuous process as it can be placed in fixed bed reactor.

Alkaline catalyst such as NaOH and KOH is excellent for catalyzing conversion of triglycerides into biodiesel. The reaction proceeds at faster rates with high FAME yield compared to acid catalyst. A drawback associated with alkaline catalyst is its intolerability towards high FFA and water content, especially involving nonedible feedstocks. The contact between the catalyst and FFA leads to formation of soap, while hydrolysis of FAME generates more FFA during the process. Acid catalyst can put up with feedstock of high FFA content, as esterification takes place and produced alkyl esters. This benefits the production of biodiesel from low-cost lipid sources, and acid catalysts can simultaneously catalyze esterification and transesterification.

Despite its resistance towards high amount of FFA, the acid catalyst is less pref­erable to alkaline catalyst because of its inferiority in terms of reaction times due to the difference in the chemical pathway of the reaction, where alkaline catalyst fol­lows a more direct route for nucleophilic substitution (Lotero et al. 2005). Furthermore, its corrosiveness behavior towards equipments and pipelines at ele­vated temperature is also problematic.

Ionic liquids combine the benefits of homogeneous catalyst (i. e., same phase with the reactants) and heterogeneous catalyst (i. e., easier recyclability) for bio­diesel synthesis. High yield and conversion can be achieved by manipulating the combinations of cations and anions to produce suitable catalysts. They are excellent in catalyzing the reaction, especially Brqrnsted acidic ionic liquids, and the perfor­mance is also up to par when compared with conventional homogeneous catalysts. A gap that must be addressed to enable the deployment of ionic liquids for industrial scale biodiesel production is its cost, which is too expensive and several times higher than conventional homogeneous catalyst. Efficient methods for reclaiming ionic liquids have to be identified in order to overcome the barrier related to its cost. Separation involving thermal application is quite unsustainable when the amount of energy required to generate heat is taken into consideration, while supercritical extraction using carbon dioxide needs to be operated at high pressure to enhance the separation efficiency. There is lack of study on alkaline ionic liquids for the trans­esterification process at the moment, and also the mechanism on how cations and anions react with triglyceride source to catalyze the conversion is still deficient.

The ability of ionic solids in catalyzing chemical synthesis is promising, particu­larly for biodiesel-related process. The hybridization between cations of ionic liq­uids and anions of Keggin-POM heteropolyacids produced solid catalysts that are able to enhance the transesterification reaction, owing to functional ions that can be modified according to the specific processes. They also have high thermal resistance due to the strong ionic bond between the ions. In-depth study on ionic solids as cata­lyst for transesterification or esterification reactions is required to verify its viability in the process. The optimum operating conditions are yet to be determined, and information on the leaching of the catalyst into the reactants is also missing.

To overcome the drawbacks on conventional method in producing biodiesel, the advancement in novel process is progressing rapidly. The introduction of technolo­gies such as ultrasonic irradiation, microwave irradiation, and reactive distillation allows the reaction to reach completion in shorter time. The application of microwave — assisted transesterification is faster than conventional transesterifica­tion, but the scale-up of this technology is difficult.

Reactive distillation allows the reaction and separation of reactants in the same distillation column and can potentially reduce the capital and investment cost if implemented at a larger scale. Further studies need to be conducted to identify new novel processes that have the capability to produce biodiesel effectively at moderate operating conditions without sacrificing the yield and conversion.