One-pot fabrication of a stable POM/MIL-101(Fe) hybrid catalyst supported on a chitosan biopolymer matrix: A study on biodiesel production through RSM optimization, experimental studies, and mechanistic insight

Herein, a heterogeneous polyoxometalate (POM)@Metal organic framework (MOF) hybrid composite was developed by embedding MIL-101(Fe) and Keggin H3PW12O40 on chitosan (CS-HPM) using a simple one-pot solvothermal technique. The highly active CS-HPM catalyst, bearing both Lewis and Brønsted acid active sites (total acidity = 2.992 mmol/g), was employed for microwave-assisted esterification of oleic acid with methanol to produce biodiesel. Following theoretical response surface methodology (RSM) equipped with a central composite design (CCD), a maximum conversion efficiency of 98.9 % was achieved using CS-HPM catalyst at optimal conditions of 21:1 MTOR, 8.8 wt% catalyst loading, and 80 °C temperature within 45 min. The 1H NMR, 13C NMR, and GC-MS studies were performed to confirm and quantify the production yield. A low activation energy (Ea) of 31.39 kJ/mol with a pseudo-first-order rate constant of 0.0783 min−1 at 80 °C revealed the high catalytic performance of CS-HPM catalyst towards oleic acid esterification. The thermodynamic studies demonstrated a non-spontaneous endergonic reaction with ΔH°, ΔS°, and ΔG° values of 34.33 kJ/mol, −169.85 kJ/mol, and 94.26 kJ/mol, respectively. Furthermore, the catalyst could be regenerated and utilized up to four catalytic cycles, exhibiting a retention of over 83 % of its catalytic efficacy. A plausible mechanism for biodiesel production over the CS-HPM catalyst surface was discussed.