Synthesis of biodiesel from soybean oil by coupling catalysis with subcritical methanol

Biodiesel synthesis from soybean oil and methanol was investigated under supercritical and subcritical conditions. Under the supercritical conditions, the maximum methyl ester yield exceeded 98% when the molar ratio of methanol to oil was 42:1 and the reaction temperature ranged from 260 °C to 350 °C. In order to decrease the operational temperature and pressures and to increase the conversion efficiency of methanol, first co-solvent was added to the reaction mixture to improve the reaction process, and then a novel idea was presented in which catalysis and supercritical effect were coupled together. Thus, with 2.5 wt% hexane, temperature of 300 °C, methanol to oil ratio of 42, a 85.5% conversion is observed in 30 min, while a 62.2% conversion is observed without hexane in the same condition; with less carbon dioxide, temperature of 300 °C, methanol to oil ratio of 42, a 91.6% conversion is observed in 20 min, while a 51.4% conversion is observed without carbon dioxide in the same condition; With only a little amount of potassium hydroxide as the catalyst (KOH/oil = 0.1 wt%), a 98% yield of methyl esters was obtained in 10 min at a reaction temperature of 160 °C and the molar ratio (methanol/oil) of 24:1. In contrast, above 1 wt% of catalyst is required in the conventional alkali-catalyzed method; while only 6% yield of methyl ester was obtained at 260 °C (corresponding to subcritical conditions) without the catalyst.This result demonstrated that by coupling the catalysis and subcritical operation, the amount of catalyst could be largely reduced and the methanol utilization could be significantly enhanced. Thus, the present method offers some advantages over both the conventional alkali-catalyst method and the expensive supercritical method.