An alternative catalyst for conventional transesterification is homogeneous acid catalyst. Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are usually preferred as acid catalysts. They are preferred to base catalyst when the amount of FFA in feed­stock is high due to its high tolerance and less sensitivity towards high FFA concen­tration. The application of acid catalyst can avoid formation of soap.

This is because FFA is esterified to produce fatty acid alkyl esters, and potassium or sodium ions, which can lead to the unwanted saponification reaction, are absent from the process. Moreover, esterification and transesterification can be done simul­taneously using acid catalyst, thus eliminating the need to use basic catalyst and preventing saponification problem.

Marchetti and Errazu (2008) conducted the esterification of oleic acid using acid catalyst (H2SO4) and refined sunflower oil as feedstock. The conversion reached
almost 100% even when the FFA content in the feedstock was 27.22% w/w. Increasing the FFA content in oil feed allows a higher final conversion and also increased the reaction rate. The formation of soap was minimal for this catalyst as the FFA was reduced to 0.54% w/w. Also, the results showed that the process was highly depen­dent on the amount of catalyst. The initial reaction rate was increased as more amount of catalyst is used, but it did not affect much of the final conversion.

Even though the catalyst can tolerate high amount of FFA in feedstocks, using H2SO4 for biodiesel synthesis caused the drawback related to its strong acidic char­acter. Equipments involved in the process are susceptible to corrosion problem, while wastewater used for removing acid catalyst from the resultant biodiesel is hazardous to the environment. Furthermore, acid catalyst is less preferable than basic catalyst because of the slower reaction and requires severe operating condi­tions for high biodiesel yield. Although the productivity can be increased with high temperature and high concentration of H2 SO4, in return it would affect the yield because catalyst could burn some of the oil (Sharma et al. 2008).

Another alternative for usually employed Brqrnsted acid (i. e., H2SO4 and HCl) is Lewis acid, which also can catalyze transesterification of oil into biodiesel. Cardoso et al. (2008b) employed two different acid catalysts for converting oleic acid into ethyl oleate: H2SO4 as a Brqrnsted acid catalyst and tin (II) chloride dehydrate (SnCl22H2O) as a Lewis acid catalyst.

Among the advantages of Lewis acid are less corrosion of the reactors and also being inexpensive. They concluded that there was virtually no difference between the conversion and selectivity of ethanolysis of oleic acid catalyzed by SnCl2 and H2SO4. Also, the activation energy of esterification using SnCl2 was found approxi­mately similar to previous work of H2SO4-catalyzed esterification of FFAs in sun­flower oil.