Heterogeneous catalytic esterification of oleic acid under sub/supercritical methanol over γ-Al₂O₃

A continuous pack-bed reactor experimental system was employed to investigate the heterogeneous catalytic esterification of oleic acid and sub/supercritical methanol with γ-Al₂O₃ as the catalyst. The experiment conditions were pressure of 10–20 MPa, temperature of 175–325 °C, residence time of 0.2–7 min and methanol/oil molar ratio of 20:1. A global one-step second order reversible kinetic model and a three-step Eley-Rideal mechanism model were proposed. The influence of impurity water and reaction pressure on the supercritical heterogeneous catalytic reaction were examined, and the stability of γ-Al₂O₃ under supercritical conditions was also evaluated. It was found that γ-Al₂O₃ showed excellent catalytic performance in both subcritical and supercritical processes, promoting theoretical maximum biodiesel yield at 20 MPa, 275 °C in 1 min. The impurity water hindered the conversion of biodiesel due to its negative influence on both the esterification reaction and the catalyst. Finally, the stability of γ-Al₂O₃ was tested under supercritical condition for 30 h.