CAU-11-COOH with a V-Shaped Linker as a Catalyst for the Solvent-Free Synthesis of Cyclic Carbonates from CO₂ and Epoxides

An Al₃₊-based metal-organic framework (MOF), CAU-11-COOH, with a V-shaped ligand, DPSDA (3,3′-4,4′-diphenylsulfonetetracarboxylic dianhydride), was prepared using the solvothermal method, and was characterized using powder X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, elemental analysis, thermogravimetric analysis, Brunauer-Emmett-Teller analysis, and CO₂ adsorption. The catalytic efficiency of CAU-11-COOH was investigated in the solvent-free cycloaddition of carbon dioxide with epoxides, which yielded five-membered cyclic carbonates under mild reaction conditions. CAU-11-COOH with a co-catalyst, tetrabutylammonium bromide (TBAB), gave higher than 98% yield of epichlorohydrin carbonate at 80 °C without a solvent. A plausible reaction mechanism in which the Lewis acidic metal center, an uncoordinated carboxyl group, and a nucleophilic bromide anion operate synergistically is proposed. The CAU-11-COOH catalysts were found to exhibit high thermal stability and could be reused more than four times without any significant reduction in activity.