Fabrication of an efficient bifunctional acid catalyst based on MIL-88B(Fe) for solvent-free ring-opening of epoxides with alcohols

Abstract

A new bifunctional heterogeneous acid catalyst with both Lewis and Brønsted acid sites was successfully synthesized via the post-synthetic modification (PSM) method. To this end, a stable metal–organic framework (MOF), namely MIL-88B(Fe), with several coordination unsaturated sites (CUSs) as Lewis acids was modified with sulfamic acid to increase the acidity of the MOF. Both X-ray diffraction (XRD) and N₂ sorption techniques confirmed the introduction of sulfamic acids into the MOF structure through the displacement of the (101) crystal plane to lower angles and increase in the surface area (BET method) and pore volume, respectively, owing to the swelling effect of this MOF. Furthermore, the existence of sulfamic acids in the backbone of the MIL-88B(Fe)/SA catalyst was detected using energy dispersive X-ray (EDX) and FT-IR analyses. The solvent-free ring-opening of styrene oxide with methanol was initially conducted to evaluate the catalytic performance of the catalyst. Compared to MIL-88B(Fe) with Lewis acidity, bifunctional MIL-88B(Fe)/SA demonstrated the highest styrene oxide conversion (95%) owing to the synergy between Lewis and Brønsted acid sites. Besides, some epoxides and alcohols were utilized in this catalytic reaction, and its reusability and stability were found excellent in consecutive catalytic runs.