Electrostatic immobilization of ionic liquids onto SBA-15 as heterogenized catalysts for esterification of oleic acid with trimethylolpropane

To overcome the drawbacks of homogeneous catalysts in the synthesis of trimethylolpropane trioleate (TMPTO), ionic liquids with different acidity were electrostatically immobilized onto SBA-15 to obtain heterogenized catalysts. The catalytic performance of the as-prepared two catalysts (IL1/SBA-15 and IL2/SBA-15) was investigated at different temperatures (100–140 °C) and molar ratio of oleic acid (OA) to trimethylolpropane (TMP) (3:1 to 5:1) at fixed catalyst dosage of 3 wt% and reaction time of 20 h; where the catalyst IL1/SBA-15 with higher acidity exhibits higher activity. The optimum temperature (at OA:TMP = 3:1) was obtained at 130 °C, where the total conversion was 86.1% and 70% with selectivity towards TMPTO of 95.6% and 92.96% for IL1/SBA-15 and IL2/SBA-15, respectively. Above this temperature, the selectivity began to decrease. The effect of OA: TMP molar ratio showed the increase of conversion (92.7% and 79.1%) with the increase of OA: TMP molar ratio up to 4.5. Further increase in OA concentration results in a damping of the catalytic efficiency of the two catalysts. Through six consecutive runs, the periodicity test refers to the relative stability of the employed catalysts with a slight decrease in activity. All results were interpreted and discussed in detail.