1. Introduction
Because of the energy and global warming crisis, development of renewable energy has been focused worldwide [1]. Biodiesel is one of the most potential alternative energy since it is renewable and environmental friendly. Biodiesel is produced by transesterification of vegetable oils or animal fats with methanol to produce fatty acid methyl ester (FAME) and glycerol as a by- product [2]. Transesterification can be catalyzed by an acid, base, or enzymes [3]. Heterogeneous catalysts have the advantage that separation and regeneration of the catalyst is easy and cheap [4]. Heterogeneous basic catalysts include alkaline earth metal oxides such as calcium oxide (CaO), magnesium oxide (MgO) and hydrotalcites [5,6].