Heterogeneous catalyzed transesterification is a merit process to produce superior grade of biodiesel and by-product glycerol in a simpler pathway. Transesterification activity of solid catalyst is as high as homogeneous catalyst with the presence of the characteristics, which were discussed in Fig. 10.2. However, crude biodiesel from heterogenized catalyzed reaction cannot be use directly in vehicle engine as it does not meet the EN 14214 or ASTM D6751 standard. Biodiesel (ASTM or EN) standard is needed to ensure the following important factors in the fuel production process are satisfied: (1) complete transesterification reaction (conversion of oil and selec­tivity of methyl ester), (2) removal of glycerin, (3) removal of catalyst, (4) removal of alcohol, (5) removal of soap product, and (6) complete esterification of FFAs (Balat and Balat 2010). The products are sometime contaminated with alcohol, and un-reacted oil and metal leaching from the solid catalyst during transesterification step. Besides, soap may be generated during the process also contaminates both biodiesel and glycerol phases.

Generally, there are three main approaches for biodiesel purification (Table 10.7) (Leung et al. 2010). The main purpose to perform this step is to wash out the residue catalyst, alcohol, free glycerol from crude biodiesel, and un-reacted raw oil, to make the product competent to ASTM or EN standards. In the case of biodiesel from homogeneous catalyzed reaction, extra chemical treatment is required to remove liquid catalyst. Chemicals (hydrochloric/sulfuric acids or CaO) are used for neur — tralization reaction of alkali or acid catalyst in order to remove the catalyst in the form of metal salts. Unlike homogenized purification step, solid catalyst can be easily removed via gravity separation without extra chemical treatment; the purified biodiesel was obtained from one of these alternative ways (wet wash, dry wash, or membrane extraction).

10.2 Conclusion

Nowadays, the price of biodiesel is still rather high, compared to petrodiesel fuel. The conventional technology used in biodiesel production for industrial scale is based on the transesterification of refined feedstock with methanol using basic homogeneous catalysts. This technology may cause problems in extra purification steps to remove liquid catalyst from the product. Hence, this issue has stimulated the transformation of homogeneous catalyzed reaction to heterogeneous system for economy feasible production cost and better biodiesel quality. From the commercial point of view, solid base catalysts are more effective than acid catalysts and enzymes. The transesterification activity of heterogeneous base-catalyzed reaction is more reactive than the solid acid catalyst in terms of lower reaction temperature, shorted reaction time, and smaller catalyst amount. Besides, some of the solid base catalysts possess similar or better reactivity than homogeneous base catalyst. To develop an

ideal heterogeneous base catalyst with high transesterification activity, the catalyst system should have high surface area with large porous system together with high basicity and strong basic strength on the active sites. The large porous system of catalyst is essential for reaction between large triglyceride molecules with long alkyl chain to avoid diffusion problems. The presence of strong basic active sites enables the reaction to begin and to proceed at an acceptable rate by forming the methoxide anion from the reaction between methanol and basic site of the catalyst. Furthermore, a catalyst with hydrophobic surface is important to improve the adsorption of feedstock with hydrophobic characteristic on the catalyst surface and to prevent the deactivation of catalytic sites by the strong adsorption of polar by­products such as water and glycerol. Although the catalysts presented above seem to be good candidates for biodiesel production, some of these catalysts are not pass­ing through a truly heterogeneous pathway. Great research efforts are still under way to develop the right catalyst with several requirements, i. e., tolerate to high FFA oil, catalyzed esterification and transesterification simultaneously, resistant to cata­lyst poisoning and leach resistant of catalyst active components, active in mild reac­tion condition, and high conversion and selectivity of product. Other than catalyst, selecting of suitable biodiesel feedstock is crucial to achieve the objectives: (1) lower price feedstock to reduce the production costs and (2) to avoid the pressure on the demand of food purpose.