Ligand engineering in MIL-101(Cr) for enhanced CO2 cycloaddition: the dominant role of carboxyl over amino functionalization

The functionalization of metal organic frameworks (MOFs) with tailored ligands offers a promising pathway for enhancing their catalytic performance in CO 2 fixation. This work systematically explores the impact of ligand engineering in MIL-101(Cr) through carboxyl (–COOH) and amino (–NH 2 ) functionalization for the CO 2 cycloaddition reaction. Comprehensive characterizations of pristine MIL-101 and functionalized MILs involving MIL-101-COOH, MIL-101-NH 2 , and MIL-101-COOH/NH 2 reveal that –COOH functionalization significantly enhances the surface area (2717 m 2 /g) and reduces the activation energy (17.1 kJ/mol vs. 23.1 kJ/mol for pristine MIL-101), whereas –NH 2 groups improve the CO 2 binding strength but compromise the crystallinity and surface area (532 m 2 /g). Under mild conditions (80 °C and 0.1 MPa CO 2 ), MIL-101-COOH achieves >99 % yield for CO 2 cycloaddition with epichlorohydrin at 8 h and maintains stability over ten cycles, outperforming other MILs (72–82.5 % yields). This work establishes ligand engineering, especially –COOH functionalization, as a critical strategy to optimize MILs for CO 2 fixation to produce cyclic carbonates.