Synergistic effect of mixed methanol/ethanol on transesterification of waste food oil using p-toluenesulfonic acid as catalyst

In this study, inexpensive waste food oil was used as raw material, the p-toluenesulfonic acid as catalyst, the methanol/ethanol as a transesterification agent to prepare biodiesel was reported. The results indicated that the mixed methanol/ethanol contributed to improving the biodiesel performance. Specifically, when mole percentage of ethanol in the total alcohol was 30%, the yield reached 99.55%, and heating value reached 40.47 MJ/Kg. It was higher than that of single methanol and single ethanol as transesterification agent, respectively. The concentration of saturated fatty acid esters in the biodiesel were also increased by mixed methanol/ethanol as transesterification agent (i.e. increase in the proportion of H in C/H), resulting in higher biodiesel heating value. The mixed methanol/ethanol also showed a good synergistic effect in the biodiesel preparation process. The solubility of ethanol in oil is superior to that in methanol, and ethanol plays a double role of the cosolvent and the transesterification agent. Methanol weakened the drawbacks of the steric effect and emulsification effect of ethanol. This study showed that the methanol can be partially substituted by ethanol. It reduced not only the dependence on methanol, but also the yield and heating value of biodiesel can be increased.