Quaternary phosphonium salt-functionalized Cr-MIL-101: A bifunctional and efficient catalyst for CO2 cycloaddition with epoxides

Integration of synergic Lewis acid sites and nucleophilic anions that can facilitate ring-opening of epoxide is a good way to develop efficient catalyst for cycloaddition of CO2 with epoxides. Herein, we prepared a novel metal-organic framework (MOF) catalyst (Cr-MIL-101-[BuPh3P]Br) through grafting quaternary phosphonium salt ionic liquid (IL) on Cr-MIL-101 by a facile post-synthetic approach. The introduction of phosphonium salt IL highly improves the catalytic activity and stability compared with the parent Cr-MIL-101-NH2. In the absence of any solvent and co-catalyst, Cr-MIL-101-[BuPh3P]Br showed excellent catalytic activity for the cycloaddition reaction under moderate reaction conditions. Under optimized conditions, the yield and TOF of propylene carbonate can be achieved 97.8% and 1086.7 h−1, respectively. This is because the synergetic interaction of dual functional sites including Cr3+ as Lewis acid sites in MOF and Br- as nucleophile in IL could promote the ring-opening of epoxide through the coordination of Cr3+ sites with O atom and the nucleophilic attack of Br- on the less sterically hindered β-carbon atom of epoxide, respectively. Comparison with other reported ILs-functionalized Cr-MIL-101 or other kinds of MOFs catalysts reveals that Cr-MIL-101-[BuPh3P]Br has superior catalytic performance and potential. Moreover, Cr-MIL-101-[BuPh3P]Br as a heterogeneous catalyst also showed good chemical stability and reusability.