Solvent-free mechanochemical route for the construction of ionic liquid and mixed-metal MOF composites for synergistic CO2 fixation

Abstract

The synergistic effect between multiple active sites is a powerful approach to wield control in catalysis. However, it remains a challenge to integrate various active species simultaneously into a porous material in an organized and cooperative manner. Herein, by virtue of the tunable secondary building units (SUBs) and pore structures of metal–organic frameworks (MOFs), we demonstrate a straightforward way to integrate multiple catalytic components into MOFs via mechanochemical synthesis, whereby the phase separation of various components can be avoided. The multi-functionality in the resulting MOFs came from doping the metal nodes with another metal species and encapsulating molecular catalysts in the cavities, as demonstrated by partially replacing the zinc ions with cobalt ions and enveloping ionic liquids (IL) in the ZIF-8 (Zn) framework to yield IL@ZIF-8(Zn/Co). It is shown that compared to the homometallic MOF, the bimetallic one together with the introduced ionic liquid brings about the opportunity to take advantage of the synergism. This is reflected in the superior performance of IL@ZIF-8(Zn/Co) with respect to the related individual components in the industrially relevant cycloaddition of epoxides and CO₂. Given that the strategy presented is applicable to other MOF materials and molecular catalysts, our approach, therefore, opens a new avenue for the introduction of multi-functionality into MOFs for various potential applications.