Ligand Asymmetry and Low-Cost Strategy of Cr-Dicarboxylate Frameworks for Highly Efficient Esterification of Levulinic Acid

A ligand asymmetry and low-cost strategy was proposed based on structural correlation analysis and cost consideration for constructing novel metal-organic frameworks (MOFs) adapted to catalyze esterification of biomass-derived levulinic acid to produce levulinate ester biofuels. The symmetry of the two carboxyl groups in monosodium 2-sulfoterephthalate (STPA), the ligand of MIL-101-SO₃H, was appropriately reduced, and monosodium 5-sulfoisophthalate (SIPA) with a cost of 1% of STPA was selected as the targeted asymmetric ligand. A novel Cr-SIPA MOF synthesized using SIPA ligands and chromium ions, exhibited a 65% higher turnover frequency value than MIL-101-SO₃H while maintaining the same 100% ethyl levulinate selectivity as MIL-101-SO₃H. Both Lewis and Bro̷nsted acids contributed to the catalytic activity in the esterification of levulinic acid and ethanol, with Bro̷nsted acid contributing significantly greater activity than Lewis acid. The superior catalytic performance of Cr-SIPA compared to MIL-101-SO₃H is attributed to its high Bro̷nsted acid density and its bundle-assembled ordered array structure. This research provides a new approach to constructing efficient MOFs for heterogeneous catalysis.