Development of SO42-/ZnAl2O4-ZrO2 composite solid acids for efficient synthesis of green biofuels via the typical esterification reaction of oleic acid with methanol

A novel series of SO42-/ZnAl2O4-ZrO2 composite solid acids for efficient synthesis of green biodiesel via the typical esterification reaction of oleic acid with methanol were prepared by a simple sol-gel-impregnation method. Their structures and acid properties were studied by means of XRD, FE-SEM, TG, NH3-TPD, XPS, FT-IR, NH3 adsorption FT-IR spectra and acid-base titration. The experimental results revealed that the addition of ZnAl2O4 was successfully achieved to stabilize the active tetragonal phase of ZrO2 in SO42-/ZnAl2O4-ZrO2 composite solid acids. Both ZnAl2O4 and ZrO2 acted as active components and participated in the formation of active acid center structure for SO42-/ZnAl2O4-ZrO2 composite solid acids. As a result, the comprehensive acidic properties of SO42-/ZnAl2O4-ZrO2 composite solid acids were effectively regulated by the mass ratio of ZnAl2O4 to ZrO2. Among them, SO42-/ZnAl2O4-ZrO2 (8:2) exhibited the highest catalytic activity and the better reusability in the esterification reaction of oleic acid with methanol, which might be ascribed to its supreme number of acid sites, its excellent structural stability and its better stability of the surface active sites. The kinetic and thermodynamic analysis demonstrated that SO42-/ZnAl2O4-ZrO2 (8:2) composite solid acid could effectively catalyze the synthesis of green biodiesel because of its relatively lower values of activation energy in the esterification reaction of oleic acid with methanol. The inevitable loss of sulfate species on the surface of SO42-/ZnAl2O4-ZrO2 (8:2) might be one of the major reasons for its slight deactivation during acid catalyzed esterification reaction.