ZIF-67/ZIF-8 Composite Membrane For Efficient Greenhouse Gases Separation

Metal-organic frameworks（MOFs）,a kind of novel crystal hybrid porous materials,have attracted significant interest in membrane separation,owing to the high specific surface area,uniform pores and flexible inorganic and organic compositions.However,MOF based membrane was limited by the tedious fabrication process and scale up feasibility.In this work,a novel phase transformation interfacial growth method（PTIG）,immobilizing zeolitic imidazolate framework（ZIFs）on polymer substrate,was proposed for separating green house gas（carbon dioxide and methane were selected as models）.Specifically,the formation of polymer substrate and MOF selective layer were simultaneously realized by introducing metal ion precursors into the casting solution and immesing nascent membrane into the water solution containing organic ligands.Based on the characterizations of the prepared flat sheet membrane and hollow filber membrane,corresponding H₂/CO₂and H₂/CH₄separation performances were investigated.The main contents were listed as follows:（1）ZIF-8 membrane was simply pre Pared via PTIG method.Zn²⁺,one of the precursors of ZIF-8,was introduced into the PVDF casting solution system,and2-methylimidazole,another precursor of ZIF-8,was dissolved into the gel bath before membrane preparation.In the process of film formation,ZIF-8 crystals rapidly nucleated,grew,formed a continous and defect-free selective layer on the nascent membrane surface.The existence of the ZIF-8 layer on the polymer substrate was confirmed by SEM and XRD.The single gas separation results showed that the H₂permeance reached 7×10^-7mol m^-2s^-1Pa^-1at 80°C and 0.2 MPa.Meanwhile,ideal separation factors corresponding to H₂/CO₂,H₂/N₂and H₂/CH₄were 6.7,10.4 and13.4,respectively.By comparison,the results exceeded the Robeson upper limit.（2）ZIF-67 hollow fiber membrane was prepared with ZIF-67,which possesses structural similarity and identical theoretical aperture to ZIF-8,via PTIG method.Co²⁺,one of the precursors of ZIF-67,was introduced into the PVDF casting solution system,and 2-methylimidazole,another precursor of ZIF-8,was dissolved into the inner core fluid before membrane preparation.During the fabrication process,the two precursors of ZIF-67 were nucleated by contact.And a continous growth led to a defect-free ZIF-67 selective layer on the internal surface of the nascent membrane ultimately.The single gas separation results showed that the H₂permeance reached7.27×10^-7mol m^-2s^-1Pa^-1at 80°C and 0.2 MPa.Meanwhile,ideal separation factors corresponding to H₂/CO₂,H₂/N₂and H₂/CH₄were 6.85,9.61 and 11.29,respectively.The gas separation performances of the ZIF-67 hollow fiber membrane prepared with PTIG method were comparable to that prepared with fabrication methods in Robeson upper limit.