Biodiesel production using homogeneous and heterogeneous catalysts: A review

Biodiesel, which is an alternative renewable fuel, is defined as mono alkyl ester of long-chain fatty acids and has properties comparable to those of fossil-based diesel. Biodiesel can be produced from vegetable oils or animal fats. The most common method used to produce biodiesel is a reversible chemical reaction called transesterification. This reaction takes place either in the presence of catalysts at lower temperature and pressure or in the absence of catalysts at higher temperature and pressure in supercritical state. Catalyzed transesterification reaction is preferred in biodiesel production because of the moderate reaction conditions. Homogeneous base catalysis can be used in transesterification when fresh vegetable oil is used as a feedstock due to its low cost, high catalytic activity, and feasibility to operate at low temperatures. Homogeneous acid catalysis is a better choice when the feedstock contains higher amounts of free fatty acids (FFAs). Heterogeneous base and acid catalysis are preferred due to their easy separation from biodiesel, hence reducing number of product purification steps. However, heterogeneous catalysis is still under development and has a promising future in biodiesel industries. In this chapter, various acid- and base-catalyzed esterification and transesterification reactions are discussed, and recent trend in catalyst development is highlighted. It is recommended that a proper selection of catalyst is made in a transesterification reaction, depending largely on the type of feedstock.