Cage-Space-Partitioned Metal–Organic Frameworks for Efficient Carbon Dioxide Capture and Conversion

Abstract

Metal–organic frameworks (MOFs) have demonstrated potential for CO2 capture and conversion, which are of great importance to alleviate current global environmental problems. Considering that MOFs with large pores are not conducive to adsorption under atmospheric environments, it is critical to control MOF materials with suitable pore sizes and catalytic sites to facilitate CO2 adsorption and fixation. Based on this, cage space partition (CSP), a new strategy to precisely regulate the pore sizes of MOFs, is proposed herein. The feasibility of the CSP strategy is demonstrated in an extra-large metal–organic cage ([M60(BTC)24], M = Co or Ni, BTC = benzene-1,3,5-tricarboxylate), which connects adjacent small cages ([M12(BTC)12]) to form a parent skeleton. For the first CSP process, four typical pyridine-based triangular ligands (TPT, 2,4,6-tris(4-pyridyl)-1,3,5-triazine) are symmetrically inserted into the M60-cage via open metal sites, which transfer the parent skeleton into a novel CSP-MOF (SNNU-337). Furthermore, two larger tri-pyridine ligands (TPHAP, 2,5,8-tri(40-pyridyl)-1,3,4,6,7,9-hexaazaphenalene) are involved to fulfill the second CSP process through residue open metal sites distributed on the inner cage surface, which lead to another isostructural CSP-MOF material (SNNU-338). Oriented by the continuous π–π interactions, the trapped TPT and TPHAP partitioners are divided into two groups, which finally divided the whole large pore into seven small sections. Benefiting from the two-step CSP process, the low-pressure CO2 adsorption capacity of MOFs is remarkably enhanced. Grand canonical Monte Carlo simulations clearly indicate that the introduction of partition agents successfully regulates the internal aperture of the cage and thus enhances the interactions between the MOF skeleton and CO2 molecules. Moreover, the synergistic effects of CSP in large M60-cages and open metal sites in M12-cages make SNNU-337/338 MOFs excellent catalysts to catalyze CO2 cycloaddition with various epoxides.