At present, the homogeneous catalytic trans-esterification
method is mainly used for the industrial synthesis of biodiesel products with acid or alkali catalysts. However, there exists a critical
deficiency of the soap formation in catalytic process for the reason
of its higher level of free fatty acid (FFA) using WCOs as feedstocks
which can consume the oils and catalysts, resulting in the difficulty for separation of products [10,11]. Therefore, it would become
rather important to eliminate the FFA before biodiesel synthesis.
For instance, Wang et al. employed ferric sulphate as heterogeneous
acid catalyst to achieve the methanolysis of FFA in a new twostep catalysis process for the synthesis of biodiesel, exhibiting the
higher conversion efficiency and several advantages, including no
acidic wastewater, high efficiency, low equipment cost, easy recovery of catalysts, and so on [12]. Zafiropoulos et al. applied a novel
diarylammonium catalyst to esterify the FFA in greases to fatty
acid methyl ester (FAME) and successfully reached the FFA content
of