Transesterification of used vegetable oils with a Cs-doped heteropolyacid catalyst in supercritical methanol

In this study, used vegetable oils (VO) were transesterified in supercritical methanol with a Cs-doped heteropolyacid (HPA) catalyst so as to produce biodiesel. The stability of the prepared catalysts was determined by measuring the amount of dissolved heteropoly anions. Cs2.5PW12O40 exhibited the highest stability in supercritical methanol. Kinetic parameters were determined at the initial stage of the reaction to examine the reaction rate of the process. The effects of the reaction parameters, including the molar ratio of methanol to oil, reaction pressure, temperature, and time, on the content of fatty acid methyl esters (FAMEs) were also investigated in order to obtain optimum reaction conditions. The content of FAME reached 92% under the optimum reaction conditions of 260 °C, 20 MPa, a molar ratio of methanol to oil of 40, and a reaction time of 40 min in the presence of 3% catalyst. The results showed that supercritical methanol compensated for the low reaction rate of solid acid catalytic transesterification, while the Cs-doped HPA catalyst mitigated the harsh operation conditions of the supercritical methanol process.