Catalysis for esterification reactions: a key step in the biodiesel production from waste oils

One possible method to produce biodiesel from waste oils (characterized by high concentrations of Free Fatty Acids, FFA), is the use of a two step process: an esterification reaction of FFA and a subsequent transesterification reaction with methanol of the whole mixture using a basic catalyst. In this review, we report the most important results and the still open challenges in relation to esterification reactions in the presence of homogeneous or heterogeneous catalysts.