Fabrication of metal-organic framework-mixed matrix membranes with abundant open metal sites through dual-induction mechanism

Abstract

Metal-organic framework (MOF)-mixed matrix membranes (MMMs) have the potential to overcome the trade-off limitations between gas separation performance and large-scale fabrication. However, most conventional MOF MMMs are used for CH4/N2 separation systems, the preparation of MOF-MMMs with high permeance and good selectivity for N2/CH4 separation has rarely been reported. Here in, we propose supported thin MOF-MMMs with abundant open metal sites to preferentially adsorb N2 and improve the N2/CH4 separation performance. The formation of the selectivity layer with abundant open metal sites was realized through dual-induction mechanism of in-situ Co2+ ion doping and polydimethylsiloxane (PDMS) solution post-modification. The introduction of Co2+ ions restricted the coordination of Zn-N and increased the number of open metal sites in the ZIF layer during its formation. The post-treated PDMS solution with weak acid could further induce the formation of open metal sites. Meanwhile, the electron paramagnetic resonance and synchrotron radiation-based XAFS results also confirmed that the number of open metal sites in the separation layer increased gradually with the in-situ Co2+ ion decoration and PDMS solution post-modification. In addition, the PDMS also could repair the intercrystalline defects of the MOF membranes and reduce their non-selectivity area. Therefore, the synthesized ZIF-8 MMMs showed a high N2 permeance of 1367 GPU and a moderate N2/CH4 selectivity of up to 6.