Hydrothermal carbonization carbon-based acid catalyst for biodiesel production

Abstract

This study aims to synthesize the production of biodiesel (oleic acid methyl ester) by esterification in the presence of sulfonated hydrothermal carbon based catalyst under microwave irradiation. The catalyst was prepared by hydrothermal carbonization, followed by sulfonation presenting sulfonic acid group as an active site. The catalyst characterization results confirmed good microwave-absorptivity of the catalyst which can be heated directly and accelerate the reaction immediately at the surface of the catalyst. Esterification of oleic acid (OA) with methanol experiments were optimized by using response surface methodology based on central composite design. Three variables: molar ratio of methanol to OA (2.5:1–7.5:1), reaction time (50-70 min) and catalyst loading (2-5 wt%) were studied. The results indicated that molar ratio of methanol to OA was the most influential factor on OA conversion and followed by catalyst loading. While, the reaction time showed negligible effect on OA conversion within the range studied. Based on the model, the optimum OA conversion of 95.55% was predicted at 5.8:1 methanol to OA molar ratio, 60 min and 3.05 wt% catalyst loading. The experimental validation carried out at this condition indicated that the model gave good prediction of OA conversion, with only 2.79% error and the coefficient of regression (R2) of 0.9407. Furthermore, the reaction was found to be reasonably described by the pseudo-first order kinetics. The dependency of the reaction rate constant on temperatures gave the value of the activation energy of 64 kJ/mol. Moreover, the effect of catalyst recycling was studied. The decrease in OA conversion is significant according to the catalyst is not active after first run. As a result, the deactivation can be solved by improving the stability of the catalyst.