## 1 Introduction  
The depletion of fossil fuels and increasing ecological awareness has led to a search for alternative fuels make from renewable sources such as plant biomass. In 1990 during an exhibition in Paris, a diesel engine was run for the first time for several hours using peanut oil as fuel; however the high viscosity (10-20 times higher than diesel fuel) and low volatility of vegetable oils hindered the generalised use of such fuels in combustion engines. Furthermore, the decomposition of glycerol leads to the formation of the toxic compound acrolein when vegetable oil is combusted in the engine, which further hampers the direct use of vegetable oils as fuels. Replacing the glycerol by short alkyl chain alcohols (e.g., methanol or ethanol) has since provided a low viscous and useful fuel for diesel engine [1]. Mixtures of methyl and/or ethyl esters of fatty acids obtained through transesterification of the triacylglycerlycerides contained in plant oils are named biodiesel. This biofuel is so far considered as a good substitute for diesel fuel as it can be used in any compression ignition engine without the need for modifications. Biodiesel is safe, renewable, non-toxic, biodegradable in water, free in sulfur compounds, has a high flash point (>130 \({}^{\circ}\)C) and better lubricant properties than diesel. Its use fully complies with the aims and purposes of environmental protection [2]. Biodiesel is usually synthesized through transesterification (also named alcoholysis) of triacylglycerlycerides with methanol, yielding Fatty Acid Methyl Esters (FAMEs) and glycerol as by-product. Because of the reversibility of the reaction, an excess of alcohol is used to shift the equilibrium towards the products side. Three consecutive transesterification reactions are involved in the full conversion of triacylglycerlyceride molecules to biodiesel (see Figure 1). Figure 1: Scheme of the catalyzed transesterification of triglycerides to synthesize biodiesel. Catalysis of the transesterification reaction can be roughly divided into two categories, chemical and enzymatic. Chemical catalyzed transesterification reactions can be carried out by either acid or base, in homogeneous or heterogeneous phase, and selection of the conditions depends on the characteristic of the starting triacylglyceride. Basic catalysts are usually preferred to acid catalysts because they provide better reaction rates, lower temperature requirements, and higher conversion efficiencies. NaOH and KOH are classical catalysts, although sodium methoxide derived from the added NaOH is actually the most applied at industrial scale. The protocol involves the dissolution of the catalyst in methanol by vigorous stirring, and mixing the resulting alcohol/cataly solution with the vegetable oil to give two liquid phases (biodiesel and glycerol) with high yields (\(>\)90%) after several hours at 65-90 \({}^{\circ}\)C. However, when the free fatty acids (FFAs) content of the triglycerides is higher than 1-2% w/w, basic catalysts can also produce saponification as a side reaction. In such cases, acid catalysts (e.g., H\({}_{2}\)SO\({}_{4}\)) are used, but of large amounts of alcohol (e.g., a molar excess of alcohol over the vegetable oil higher up to 10) and the complete removal of free water are required. Homogeneous catalysts usually give better results than heterogeneous catalysts for biodiesel production. However, many problems are associated with homogeneous catalysis, such as high energy consumption, the formation of unwanted soap by-products because of the presence of FFAs, expensive catalyst separation from the reaction mixture, and specially the generation of large amounts of wastewater during products separation and cleaning. These are clear breakdown points for the sustainability of homogeneous basic catalysis, the usual approach applied at industrial scale for producing biofuel.