Mild synthesis of biofuel over a microcrystalline S2O82 −/ZrO2 catalyst

•The S2O82 −/ZrO2 material is synthesized by one-pot method using ammonium persulfate.•The material with stabile S2O82 − component has extraordinarily abundant superacid.•The material has high activity and selectivity for producing biofuel even at 100 °C.•The favorable catalysis results from t-ZrO2 microcrystallites anchored more S2O82 −.•No sulfur leaching in cyclic utilization of the catalyst for the reaction.

A solid super acid catalyst, S2O82 −/ZrO2, was prepared by a new synthesis route of one-pot method with ammonium persulfate via vapor phase hydrolysis to improve biodiesel production using expired soybean oil as reactant. Catalysts were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), and NH3-temperature programmed desorption (NH3-TPD) to determine the optimum pretreatment conditions and understand the role of the active sites for transesterification. S2O82 −/ZrO2 calcined at 500 °C, which showed an amorphous phase by XRD characterization, was identified as composed of tetragonal phase ZrO2 microcrystallites by TEM examination. The microcrystallites which anchored more S2O82 − had abundant super acid and displayed high catalytic activity in the transesterification under mild reaction conditions. The reaction parameters over the catalyst were optimized and both the conversion of soybean oil and selectivity of biofuel reached 100% under the conditions of 110 °C of reaction temperature, 4 h of reaction time and 20:1 of methanol to oil molar ratio. Moreover, the catalyst showed high stability in the reactions and no sulfur leaching was found in the biodiesel products in cyclic utilization.