Among all these heterogeneous bases, calciumoxide (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 rawmaterials 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.
Rice husk (RH) is a major agricultural waste which can be used as a
fuel, and the combustion product, ash, is also considered as a waste
product. Recently, it has been reported that ca. 70 million tons of rice
husk ash was produced yearly worldwide [19]. RH and its ash have
attracted great interest in terms of the extraction of the SiO2 component,
which can act as a cost-effective source for the synthesis of some important
catalysts/supports such as zeolites and mesoporous silica [20–22].
In addition, the physicochemical characteristics of the acquired rice
husk ash (RHA) vary considerably along with the burning temperature
[23]. To the best of our knowledge, few investigations have been made
by using RHA as a catalyst support for biodiesel synthesis till date.