Facile Synthesis of Silver-Exchanged Phosphotungstic Acid Immobilized on Sn-Bi Bimetallic Metal–Organic Frameworks for Enhanced Esterification

Metal–organic frameworks (MOFs) are ideal supports for the synthesis of porous composite catalysts. In the present study, Sn-Bi bimetallic metal–organic frameworks (Sn-Bi-MOFs) supported silver-doped phosphotungstic acid (AgPW) catalysts (AgPW@Sn-Bi-BDC and AgPW@Sn-Bi-BDC (NH 2 )) were successfully synthesized via a simple in situ impregnation method, which was subsequently applied to catalyze esterification for the production of biodiesel from oleic acid (OA). The physico-chemical properties of the prepared composite catalysts underwent comprehensive analysis through XRD, FTIR, N 2 physisorption, SEM, EDX, NH 3 -TPD, Py-FTIR, TG, and XPS techniques, confirming the successful impregnation of AgPW on the Sn-Bi-MOFs framework. Among the catalysts tested, AgPW@Sn-Bi-BDC (NH 2 ) exhibited the better catalytic activity than that of Sn-Bi-BDC, Sn-Bi-BDC (NH 2 ), and AgPW@Sn-Bi-BDC, reaching 91.6% of OA conversion with the methanol:OA molar ratio of 20:1 and the catalyst quantity of 0.2 g at 130°C for 4 h. The high activity of AgPW@Sn-Bi-BDC (NH 2 ) is attributed to the available multiscale pore structure, high acidity, and the synergistic action of the Brønsted and Lewis acidic sites. Additionally, the esterification with AgPW@Sn-Bi-BDC (NH 2 ) followed the first-order reaction kinetic model, with an E a of 34.5 kJ/mol. Moreover, the recyclability of the composites was also assessed, demonstrating sustained catalytic activity after four reuses. This approach showed a potential for sustainable and efficient energy production through bimetallic MOFs-based composite catalysts.