Defect-engineered and sulfonated MIL-101(Cr) featuring synergistic acid sites for efficient esterification of oleic acid

Biodiesel has emerged as a promising alternative to conventional diesel due to its renewability and environmental friendliness. In this study, a MIL-101(Cr) featuring synergistic acid sites was developed through a combined defect engineering and post-sulfonation strategy, enabling efficient esterification of oleic acid under mild conditions. Defects were introduced in the MIL-101(Cr) through a mixed-ligand approach. The existence of abundant oxygen vacancies in the defect-engineered MIL-101(Cr) was confirmed by electron paramagnetic resonance (EPR), which could provide adequate grafting sites for the subsequent sulfonation. The sulfonic acid-functionalized MIL-101(Cr) catalyst (MRBSX) was then synthesized by post-sulfonation with chlorosulfonic acid as the sulfonation agent. The chemical structure and sulfonic acid density of MRBSX were systematically investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), brunauer-emmett-teller (BET), and Fourier-transform infrared spectra (FT-IR), and other characterization techniques. Acid-base titration combined with elemental analysis (EA) demonstrated the successful loading of sulfonic acid groups in MRBSX, highlighting the promotional role of oxygen vacancies in enhancing acid site density and catalytic efficiency. Under optimized reaction conditions (100 °C, 6 wt%, 3 h, and M/O = 8:1), the MRBSX with 10 mol% benzoic acid achieved a high oleic acid conversion of 96.7 % and a corresponding biodiesel yield of 91.5 %. Therefore, the proposed synergistic strategy of defect engineering and post-sulfonation is effective in constructing high-performance solid acid catalysts, demonstrating promising potential in green energy conversion applications.