Although transesterification catalyst-free in supercritical methanol
can be an alternative method [11], this supercritical process
requires very high temperatures and extremely high pressures conditions
which can incurred high capital cost. Additional methods to
produce biodiesel are the use of ultrasonic reactors and microwaves.
Ultrasonic reactors produce good conversion yields; however, the
use of a catalyst is still necessary apart from the high consumption
of electricity to produce the biodiesel [12]. For the microwaves
method, both heterogeneous and homogeneous catalysts can be
employed to obtain lower reaction times than without the use of
microwaves [13,14]. Such methods are not discussed in this paper.
Now, the heterogeneous catalytic process has potential to overcome
the disadvantages of homogeneous catalytic process, in particular
from the sustainability standpoints, some of which are
quoted and listed as follows: