Among all these heterogeneous bases, calcium oxide (CaO) catalysts
have drawn great interest because of not only their high activity but also
the low cost during the catalyst preparation process [4]. Moreover,
some of the sources of this compound are abundant and renewable
[5]. In particular, many successful utilizations of waste sources, such as
eggshell [6,7], mollusk shell [8–11] and bones [12,13], were reported
for the production of biodiesel. Nonetheless, few studies were focused
on the utilization of the waste sources derived CaO impregnated onto
other wastes sources (such as ash). One such effort in this area was
the investigation conducted by Chakraborty et al. [14], who developed
a heterogeneous coal fly ash-based CaO catalyst using waste eggshell
as raw materials for the transesterification of soybean oil to yield biodiesel.
In addition, several research studies related to the application of
ash-based catalyst for synthesis of biodiesel have been published
(such as palm oil boiler ash [15], cocoa pod husks ash [16], palm
empty fruit bunch ash [17] and wood ash [18]). Those ash-based
catalysts not only showed good catalytic performance but also were
environmentally friendly and renewable for biodiesel production.