The acid catalysts used in homogeneous phases include sulphuric
acid (H2SO4), hydrochloric acid (HCl) and sulphonic acid
[RS(@O)2AOH] where, R is an aryl group and S(@O)2AOH is sulfonyl
hydroxide. Recently, Zhang et al. [7] reported that bagasse
can be used as raw material to prepare carbon-based solid acid
catalysts for the esterification of free fatty acid in waste cooking
oil. The main advantage of the acid catalyst is that the free fatty
acids content in the oil will not affect the reaction. However, the
acid catalysts have not been used as widely as alkali catalysts
due to it has several limitations, such as the demand of costly
separation, slower reaction rate, high reaction temperature
required, high molar ratio of alcohol to oil and purification steps,
high corrosion and non-reusability [