Three in one: Rational engineering of multifunctional MIL-101-based ionic catalysts for carbon dioxide-epoxide cycloaddition

Abstract

CO2-epoxide cycloaddition represents one of most attractive approaches for chemical fixation and utilization of CO2, but highly efficient and fully heterogeneous green catalysis is still challenging. Ionic metal-organic frameworks (IMOFs) are nice porous matrices heterogenizing the essential free nucleophilic anions and meanwhile providing Lewis acids, but the catalytic efficiency has been limited for low anion content and lack of auxiliary catalytic sites. In this work, we report stepwise optimization of IMOF catalysts by integrating multiple catalytic sites into MIL-101. Various monocationic IMOFs with quaternary ammonium and bromide anions were prepared by facile alkyne-azide click chemistry. Different numbers of hydroxyl groups were introduced in this ionization step to optimize the activity. To introduce more anions, dicationic IMOFs were created by a second ionization step, N-quaternization of triazolyl to triazolium. By the stepwise ionization and functionalization, we demonstrated the synergic catalytic effects among different sites, the impacts of hydroxyl/bromide content, and the size effect of the cationic group. The optimal IMOFs are superior to previous ionic heterogeneous catalysts in absence of cocatalysts. In particular, the dicationic catalyst can quantitatively convert epichlorohydrin to the carbonate within 1 h at 1 MPa CO2 and 80 °C, the apparent turnover frequency being up to 478 h−1. The highly active and selective, fully heterogeneous and recyclable catalyst has the appeal for practical applications.