Quaternary ammonium salt functionalized MIL-101-NH2(Cr) as a bifunctional catalyst for the cycloaddition of CO2 with epoxides to produce cyclic carbonates

The reaction of CO2 with epoxides to produce cyclic carbonates requires high-performance, low-cost, and facilely recycled catalysts. Here, we report a heterogeneous catalyst (MIL-101-N(Bnme2)Br) based on metal-organic frameworks (MOFs) for catalytic CO2 conversion into cyclic carbonates. The MIL-101-N(Bnme2)Br catalyst was prepared as follows: A MOFs precursor (MIL-101-NH2) was firstly synthesized using a method of hydrothermal synthesis, then coupled with the quaternary ammonium salt ionic liquid (N(BzBnme2)Br) via aldehyde-amide condensation. A series of comparative experiments revealed that MIL-101-N(Bnme2)Br catalyst has high activity for catalyzing the cycloaddition reaction to produce cyclic carbonates in the absence of solvents and co-catalysts. As was exampled by the synthesis of propylene carbonate, a high yield (93 %) and selectivity (99 %) was achieved under the optimized conditions (0.93 mol% MIL-101-N(Bnme2)Br catalyst, 1.4 MPa CO2 pressure, 17 mmol propylene oxide, 100 °C, 5 h). More impressive, the MIL-101-N(Bnme2)Br catalyst could be recycled easily by filtration and was reusable up to several times without a significant activity decay. Furthermore, our density functional theory (DFT) calculations suggested that such high catalytic activity mainly resulted from the cooperative catalysis between the coordinatively unsaturated Cr3+ sites and Br anions in MIL-101-N(Bnme2)Br catalyst. Meanwhile, a possible mechanism for the cycloaddition reaction catalyzed by MIL-101-N(Bnme2)Br catalyst was also proposed.