On the other hand, acid catalysts can catalyze simultaneous
esterification and transesterification reactions but the rate is about
4000 times slower than that of base catalysts[26]. Slow rate of
reaction often necessitates drastic operating conditions, including the use of high temperatures. Consequently, in a concerted
approach to annul the constraints of high feedstock cost and
stimulate the growth of biodiesel industry, researchers have been
engaged in development of appropriate heterogeneous acid catalysts and biodiesel technologies for transesterification of lower
grade fats and oils. The ideal technology would entail a continuous
flow reaction without deactivation or consumption of catalyst,
minimizes or eliminates the downstream separation and purification protocols[5]and gives complete conversion under mild reaction conditions.
Promising solid acid catalysts have been reported including sulfated zirconia [27,28], sulfated tin oxide [6], amorphous carbon
catalysts [29–31], and supported heteropoly acid catalysts
[32,33]. However, acid-catalyzed transestesrification is usually
energy intensive, requires long reaction times and there is still
need for further improvements before they can replace their
homogeneous Bronsted acid counterparts. Thus, further scientific
and technological advances in heterogeneous catalysis are imperatives for future development of biodiesel[34,35], which may lead
to development of more appropriate technologies for biodiesel
production from waste oils