Continuous esterification of oleic acid to ethyl oleate under sub/supercritical conditions over Γ-Al₂O₃

Esterification of oleic acid with ethanol was conducted under subcritical and supercritical conditions in a packed-bed reactor containing γ-Al₂O₃. The presence of γ-Al₂O₃ significantly improved the reaction rate such that the 42% yield achieved at 325 °C, 200 bar, and 1-min residence time without the alumina was increased to 98% at the same conditions when alumina was present. The catalytic capacity was attributed to Lewis acid sites on the surface of alumina, and non Brønsted acid sites were detected. Experiments to study the kinetics were executed at a pressure of 200 bar, elevated temperatures (200, 225, 275, 300, and 325 °C), and residence times of half to 8 minutes. Mass transfer limitations were estimated to be negligible via the Mears and Weisz-Prater criteria. Kinetic analysis based on the one-step model demonstrates that the overall reaction was endothermic, and an Eley-Rideal (ER) reaction mechanism was proposed to describe each elementary reaction step. The stability of γ-Al₂O₃ on product conversion was tested via a 25 h operation under 325 °C, 200 bar, and 1 min residence time, and decrease of the conversion was not observed. However, results of the catalyst analytical characterization shows a decrease of the acid site density and surface area, supporting the occurrence of catalyst degradation. The addition of water slightly decreased the yield, while the pressure change from 200 to 100 bar did not have an obvious effect on the conversion.