It has been found that the application of oil with FFA content
more than 2.5% in transesterification reaction leads to the production of a lot of soap in the presence of typical basic catalysts.
So, esterification reaction in the presence of typical acid catalysts is carried out at first in order to reduce the acid content of waste
oils[11–13]. The application of homogeneous catalysts leads to the
requirement for further purification process. Therefore, the application of heterogeneous catalysts in the esterification has received
much attention[14]. The inorganic-oxide solid acids (such as zeolite
and niobic acid) have low concentration of the active acid sites and
as a result low activity in adverse conditions. The activity of ionexchange resins such as amberlyst and nafion is still lower than
sulfuric acid in spite of having strong acid sites (–SO3H). These
catalysts are also expensive and have poor recovery characteristics. Sulfated zirconia has high catalytic activity, but zirconium is
a rare and expensive metal[15,16]. According to the recently conducted research works, carbon-based acid catalysts can be applied
as appropriate catalysts for the esterification reaction. These kinds
of catalysts can be made from renewable resources and have suitable characteristics such as low cost, high surface area, thermal
stability, and high activity[17,18]. The application of these catalysts in biodiesel production has been investigated recently[18–20].
Effective mixing between heterogeneous phases (i.e., oil, alcohol,
and solid catalyst) in the esterification reaction is a challenge. Ultrasonic irradiation is a process of emission of compressed waves
with frequencies above the range of human hearing. Acoustic cavitation is formation, growth, and collapse of bubbles in a liquid under
ultrasound irradiation. Ultrasonic waves provide mechanical energy
for mixing and activation energy required to initiate the reaction.
The collapse of cavitation bubbles destroys phase’s boundaries and
creates the emulsion in the reaction mixture. The main advantages of ultrasound irradiation in biodiesel production may be
reduction in reaction time, molar ratio of alcohol to oil, and energy
consumption. The application of solid catalysts has been recommended for biodiesel production due to their high lifetime and
stability[1,21–24]. Report on the production of biodiesel using ultrasound and solid acid catalysts is hardly available