RSM optimization and kinetics study of α-Fe2O3/g-C3N4@H photocatalyst with S-type heterojunction for esterifying crude castor oil to reduce acidity and synthesize biodiesel

[Display omitted] •S-type heterojunction α-Fe2O3/g-C3N4@halloysite photocatalyst was prepared.•Optimization of reaction conditions of photocatalytic esterification by RSM.•g-C3N4 prepared by doping has better physical and chemical properties.•Photocatalytic esterification follows a reversible second-order kinetic model. The preparation of biodiesel from low-quality oil conforms to the concept of sustainable development. In this study, low-quality castor oil was used as raw material, and α-Fe2O3/g-C3N4@halloysite photocatalyst prepared by two-step calcination method was used to pre-esterify the raw material to reduce the acid value, so that the quality of the raw material met the requirements of a homogeneous alkali catalyst, and then biodiesel was prepared by transesterification catalyzed by NaOH. The results show that the doped g-C3N4 has a larger specific surface area and better photoelectric properties, and the photocatalyst loaded with 10 wt% α-Fe2O3 has stronger light absorption characteristics and electron-hole separation ability. Response surface method was used to optimize the experiment: the molar ratio of ethanol to oil was 11.74:1, the amount of catalyst was 3.44 wt%, and the reaction time was 2.72 h. The reaction temperature was 74.57 ℃, and the conversion rate of FFAs (free fatty acids) was 97.62 %. The catalyst was recycled 5 times, and the conversion rate of FFAs still exceeded 80 %. The photocatalytic esterification reaction in this study conformed to the reversible second-order kinetic model. The physicochemical properties of castor oil-based biodiesel prepared by adding additives or preparing B20 biodiesel meet ASTM D6751 and EN 14214 standards.