Unprecedented NH2-MIL-101(Al)/n-Bu4NBr system as solvent-free heterogeneous catalyst for efficient synthesis of cyclic carbonates via CO2 cycloaddition.

Abstract

Amine-functionalised framework NH2-MIL-101(Al) was synthesized using a solvothermal and microwave method and characterized by PXRD, FT-IR, TGA, SEM-EDX, and BET surface area analysis. The desolvated framework, in the presence of co-catalyst tetrabutylammonium bromide (TBAB), acted as an excellent heterogeneous catalyst for the solvent-free cycloaddition of carbon dioxide (CO2) with epoxides, affording five-membered cyclic carbonates. Using styrene oxide, the NH2-MIL-101(Al)/TBAB system showed more than 99% conversion, affording 96% yield and 99% selectivity with a turn over frequency of 23.5 h-1. This validated the synergistic effect of the quaternary ammonium salt during CO2 cycloaddition. The catalyst could be recycled at least five times without a noticeable loss in activity, while leaching test showed no leached Al3+ ions throughout the reaction. Thorough analysis of the reaction parameters showed that the optimum conditions for obtaining the maximum yield and highest selectivity were 120 °C and 18 bar of CO2 for 6 h. The outstanding conversion and selectivity were maintained for a range of aliphatic and aromatic epoxides, corroborating the duel benefit of the micro-mesoporous system with amine functionality, which offered easy access for reactant molecules with diverse sizes, and provided inspiration for future CO2 cycloaddition catalytic systems. We also propose a rationalized mechanism for the cycloaddition reaction mediated by NH2-MIL-101(Al) and TBAB based on literature precedent and experimental outcome.