The performance of the two-step biodiesel production of the SrFe2O4/SiO2-SO3H-NaOH and H2SO4-NaOH systems operated under the best operation condition were shown in Fig. 5. The results show that both SrFe2O4/SiO2-SO3H and H2SO4 could completely consume FFAs in the mixed oil and transform them to biodiesel. However, in the following transesterification step, the conversion of triglycerides was different depending on the catalyst used in the first step. The transesterification conversion after esterification by SrFe2O4/SiO2-SO3H was better than that by H2SO4, and the total conversion of the SrFe2O4/SiO2-SO3H-NaOH system was almost 100% but that of H2SO4-NaOH system was just about 90% (Fig. 5). The reason for this could due to the fact that H2SO4 was not removed after esterification, thereby partially consuming NaOH used to catalyze transesterification. This results in an incomplete
transesterification because the amount of catalyst (NaOH) was insufficient. On the other hand, SrFe2O4/SiO2-SO3H could be easily removed, so there was no negative effect on the next step.