Calcium oxide is one of the best researched heterogeneous
catalysts as it has higher basicity, lower price, lower solubility, is
non-corrosive, economically benign, lower solubility and easier
to handle than homogeneous base catalysts [14]. Ca(NO3)2, CaCO3,
CaPO4 and Ca(OH)2 are raw materials to produce CaO. Some
researchers believe that calcium methoxide functions as a substantial
solid base catalyst under the reacting conditions [1]. The basic
sites over heterogeneous catalysts are active centers for transesterification
[15]. Natural sources such as egg and sea shells have also
been utilized. Animal bone too can be used as a raw material as it
contains a good amount of CaO. Obadiah et al. (2012) proved that
irrespective of the origin of CaO, the basic CaO acts as a catalyst in
the transesterification [12].
The CaO mediated catalysis in transesterification reaction leads
to the formation of a methoxide anion due to the reaction between
methanol and hydroxide ion. Then, this anion reacts with the triglyceride
to result in a total of three moles of methyl ester and a
mole of glycerol [12]. Solid base catalysts have the tendency to
absorb acid molecules from the ambient environment and lose
activity. CO2 and H2O in air can easily poison the basic sites on a
catalyst surface and change the bulk crystal structure of the
catalyst [15].