Transesterification of palm oil and esterification of palm fatty acid in near- and super-critical methanol with SO4–ZrO2 catalysts

Sulfated zirconia (SO4–ZrO2) catalysts, prepared with three different sulfur loading contents (0.75%, 1.8% and 2.5%) at two calcination temperatures (500 °C and 700 °C), were tested for use in the transesterification of purified palm oil (PPO) and the esterification of palm fatty acid (PFA) in near-critical and super-critical methanol. Techniques including BET, XRD, NH3- and CO2-TPD revealed that the sulfur content and calcination temperature strongly affects the catalyst base–acid site, specific surface area, average pore size, phase structure, and thus the catalytic reactivity. The most suitable sulfur loading content was found to be 1.8% and the optimum calcination temperature 500 °C. The results show that the use of SO4–ZrO2 reduces esterification reaction times, the amount of methanol necessary and the required reaction temperature. The reactions at 250 °C in the presence of the SO4–ZrO2 catalyst at 0.5 w/w% catalyst to PPO or PFA were found to give the highest FAMEs conversions. Under these conditions, 90% and 75% conversions were achieved within 10 and 1 min from PPO (at 25:1 MeOH:PPO molar ratio) and PFA (at 6:1 MeOH:PFA molar ratio), respectively.