Microporous organic framework materials such as Porous Organic Cage (POC) and Metal-Organic Framework (MOF) with adjustable pore size exhibits good gas selective transport behavior during the membrane separation application. However, the uncontrollable intercage spaces and lattice flexibility lead to the sacrifice of membrane performance. The vigorous growth of POC membrane, such as rapid precipitation during spin-coating process, will cause disordered arrangement, resulting in abundant non-selective intercage spaces. To address that, an electrostatic-induced crystal-rearrangement strategy was proposed to induced the POC rearrangement on membrane surface to eliminate non-selective intercage spaces, constructing surface-crystalized POC membranes with regular channels (Figure 1a). The crystal-rearranged POC membrane exhibited effective CO2/N2 separation performance. In addition to the uncontrollable intercage spaces, lattice flexibility of porous material also limited their application in separation. For example, the ligand rotation of MOF makes them lack the priority for precise molecular sieving. Electrostatic-anchoring strategy was designed to fixing the ligand rotation in ZIF-8. Electrostatic inducer trifluoroacetate anchored at and blocked the six-membered windows of ZIF-8, and meanwhile induced the positive 2-MIM ligand rotation, thus opening neighbored four-membered windows with a constant size of 3.4 Å (Figure 1b). The obtained ZIF-8 with micro-regulated nanochannels exhibited excellent CO2/N2 selectivity in adsorption and membrane-based separation. In all, specific optimization strategies are proposed based on electrostatic interaction, which provide new inspirations for the nanochannels regularization of microporous organic framework materials membrane for gas separation.

Nanochannels regularization of microporous organic framework material membranes enable efficient CO2 separation

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