Preparation of biodiesel by transesterification of castor oil catalyzed by flaky halloysite supported ZnO/SnO2 heterojunction photocatalyst

Biodiesel belongs to renewable energy and is a potential substitute for petrochemical diesel. In this study, ZnO/SnO2@halloysite heterojunction photocatalyst was prepared by loading semiconductor zinc oxide and tin dioxide on layered kaolin by the improved wet calcination method, which was used to photocatalytic the transesterification of castor oil and ethanol to produce biodiesel. TG-DTG, BET, XRD, UV–vis and SEM-EDS were used to characterize the composition, specific surface area, light absorption characteristics and photocatalytic performance of the samples. The reaction conditions were optimized by the Taguchi method. The optimum reaction conditions were as follows: catalyst dosage 4 wt%, molar ratio of ethanol to oil 12:1, reaction temperature 75 °C. After 2 h, the yield of biodiesel was 96.75 %. In the study of catalyst reusability, the yield of biodiesel is still above 85 % after being reused for 5 times. The kinetics of photocatalytic transesterification was studied, and the results accorded with quasi-first-order kinetics. The activation energy was 75.02 kJ/mol. Lastly, an eight-step reaction mechanism of photocatalytic transesterification was proposed. The prepared castor oil biodiesel can be prepared into B20 biodiesel through additives or mixing to meet ASTM D6751 and EN 14214 standards.