Highly stable and reusable MIL-101(Fe)‑supported acidic ionic liquid catalyst for efficient green biodiesel synthesis

In this study, functionalized ionic liquids were precisely anchored onto the pores and surfaces of the high-specific-surface-area MIL-101(Fe) metal-organic framework (MOF) carrier through covalent bonding, achieving the ordered arrangement of active sites and the modulation of the local microenvironment. Specifically, carboxyl-functionalized imidazolium ionic liquids bridge Fe³⁺ ions and terephthalic acid ligands to construct the supported acidic ionic liquid MIL-101(Fe)-[(CH₂COOH)₂IM][HSO₄] via solvothermal co-assembly. The obtained catalyst efficiently facilitates the esterification of various free fatty acids with methanol for biodiesel production. The catalyst was characterized by XRD, FT-IR, XPS, SEM, BET-BJH, and TGA. The catalyst demonstrated high acidity (6.31 mmol/g), excellent thermal stability, and efficient catalytic performance. Under the operating conditions optimized by response surface methodology (catalyst dosage of 8.64%, methanol-to-oleic acid molar ratio of 16.9:1, reaction temperature of 67.0°C, and reaction time of 4.0 h), the oleic acid conversion over MIL-101(Fe)-[(CH₂COOH)₂IM][HSO₄] reached 97.66%, and it still maintained favorable catalytic performance after 7 cycles. In addition, MIL-101(Fe)-[(CH₂COOH)₂IM][HSO₄] also exhibited broad substrate applicability, and could serve as a promising catalyst for biodiesel preparation.