Recently, alternative renewable fuels have gained great attentions
worldwide since traditional fossil energy resources are limited and the
use of them usually leads to environmental pollution. Biodiesel as one
of the renewable and clean biofuels has been chosen as a promising
fuel, which was conventionally produced through transesterification
of refined vegetable oils with methanol using homogeneous base
catalysts, such as sodium hydroxide, potassium hydroxide, and sodium
ethoxide [1]. Although such homogeneous base catalysts usually
exhibited high catalytic activity, these catalysts have shown some
disadvantages such as difficulty in recycling after the reaction and an
inevitable production of toxic water. Fortunately, the heterogeneous
catalysts may address some of these problems since the solid catalysts
can be reusable and easy to separate. Besides, a neutralization step of
producing wastewater would be omitted. In general, the heterogeneous
catalysts can be categorized into two types: acids and bases [2]. Since
the acid catalyzed transesterification processes usually required long reaction
time and achieved at high reaction temperature, base catalyzed
transesterification process can be considered as the most attractive
process to date