Efficient and reusable polyoxometalate-based sulfonated ionic liquid catalysts for palmitic acid esterification to biodiesel

Fabrication of polyoxometalate-based sulfonated ionic liquid catalysts was achieved by combining phosphotungstic acid (HPW) and SO3H-functionalized zwitterion ionic complex. The obtained composite materials were well characterized by FT-IR, TG, 1H NMR, 13C NMR and 31P MAS-NMR and investigated their performance as catalyst for esterification reactions of palmitic acid to biodiesel. Response surface methodology (RSM) was applied to optimize esterification of palmitic acid with ethanol by [MIM-PSH]2.0HPW12O40. The effects of various reaction conditions, including ethanol/acid molar ratio, the amount of ionic liquid and reaction time were addressed by Box–Behnken experimental design (BBD). The variance (ANOVA) analysis showed that the reaction time and amount of catalyst were the most significant parameters. And the effect of interaction between the ethanol/acid molar ratio and reaction time was also the most significant parameter affecting the conversion of palmitic acid. The coefficient of determination of this model was 0.9754. The optimum reaction conditions were established as follows: ethanol/acid molar ratio of 13, amount of catalyst of 7%, reaction time of 5 h at 80 °C and got a maximum yield of biodiesel of 91.8%. The [MIM-PSH]2.0HPW12O40 ionic liquid could be reused six times without noticeable drop in activity. Under optimum conditions, catalysts showed a superior catalytic efficiency and reusability due to better superacidity and lower molecular transport resistance.