However, such catalysts suffer the drawbacks of sensitivity to free fatty acids (FFAs) and water in different oil feedstock. For example, when the FFA and water content are above 0.5% and 0.06%, respectively, the basic catalysts (KOH and NaOH) react with the FFA to produce soaps [5e7] Therefore, oils contain high levels of FFA and water, such as waste cooking oil (WCO), which cannot be directly used to produce biodiesels by homogeneous catalysts