Due to the endothermic reaction of the esterification process, the activation energy was required to activate the protonation of FFA by a solid GS-9137 catalyst  and . To study the influence of the reaction temperature on the FFA conversion, a number of experiments with temperatures ranging from 65 to 90 °C were carried out using the ICG(15)-SO3H catalyst. As shown in Fig. 6(c), it was found that the optimum reaction temperature was 75 °C. At this point, the FFA conversion was increased up to 95.4% from 75.2% at the temperature of 65 °C. However, the FFA conversion was slightly decreased when the temperature was higher than 75 °C. As reported by Leung and Guo , they produced biodiesel from neat canola oil and used frying oil. They mentioned that higher temperatures above 50 °C had negative impacts to the reaction. Hayyan et al.  had studied the reduction of the FFA content in sludge palm oil via an acid catalyst. They also reported the reduction of FFA was decreased when the reaction was at above 60 °C, and it depended on the viscosity of the feedstock. At this point, at the 75 °C reaction temperature, the activation energy was sufficient to protonate the carbonyl groups from FFA and resulted in the maximum reaction rate.