Preparation Of Cross-Linked ZIFs-Based Composite Membranes And Gas Separation Performance Study

Membrane separation technology with the advantages of energy saving and consumption reduction,small investment,small footprint has wide applications in natural gas purification and CO₂ capture.Among various separation membranes,composite separation membranes doped with MOFs,also known as mixed matrix membranes（MMMs）,have the advantages of high permeability of inorganic membranes and easy processing of organic membranes.They can overcome the trade-off relationship of permeability and selectivity.It makes both of them improve at the same time,which has attracted the attention of many scholars.The commonly used polymer matrices for MMMs are mainly polyimide,polyamide and self-polymerizing microporous polymer.Metal-organic frameworks（MOFs）have the advantages of good molecular sieving performance,strong structural controllability,and easy modification,making them ideal fillers for the preparation of MMMs.However,with the addition of the second phase MOFs,non-selective pores appear at the two-phase interface between the polymer and filler MOFs,and some MOFs particles agglomerate.Such structural defects lead to a significant decrease in the separation selectivity of the membrane.In order to solve the above problems,the interface modification of MMMs is usually required to achieve the goal of repairing the defects in the membrane and improving the gas separation performance.In this paper,ZIF-8 was selected as the dispersed phase,and 6FDA-type copolyimide 6FDA-BI:DAM（1:1）（6FBD）containing imidazole groups was used as the continuous phase to prepare MMMs.The modification of ZIF-8（ZIF-8-NH₂）by ammonia functionalization to solve the problem of two-phase interface defects achieves a significant improvement in gas separation performance.Through the characterization of the physical and chemical structure of membrane materials and the evaluation of CO₂/CH₄,H₂/CH₄,CO₂/N₂ separation performance,we investigated the influence of-NH₂ functional group on the comprehensive performance of the membrane.Compared with ZIF-8,the amino-modified ZIF-8 can form hydrogen bonds with the polymer,which significantly improves the interfacial force between the two phases and reduces the defect structure in the membrane.The CO₂/CH₄ and H₂/CH₄ selectivities of the 30 wt.%amino-modified ZIFs composite membranes were improved by 20%and 29%compared with6FBD/ZIF-8 MMMs.The 6FBD/30%ZIF-8-NH₂ membrane has excellent separation performance at low temperature,and the ideal selectivity for CO₂/CH₄ increased from16.6 at 60℃to 119 at-20℃,an increase of more than 6 times,while the 6FBD/30%ZIF-8 is only 2.5 times better.This work demonstrates that selecting polymers and MOFs with similar structures and amino-modification of MOFs are effective means to suppress interfacial defects.As the doping amount of the filling phase increases to 40 wt.%,some defect structures still exist at the two-phase interface of the 6FDA-type polyimide/ZIFs MMMs,resulting in a sharp decrease in the gas separation selectivity.In order to further improve the gas separation performance of the membrane,this work further modified the high-loaded MMMs by thermal crosslinking,aiming to build a covalently crosslinked network at the phase interface and repair the defect structure in the membrane.In this work,three different types of polyimides（6FDA-BI（6FB）,6FDA-DAM（6FD）and 6FBD）were selected as continuous phases,and ZIF-8 and ZIF-7 as dispersed phases to prepare cross-linked MMMs.We investigated the effects of ZIFs and polymer structures,oxidative crosslinking conditions on the gas separation performance of the membranes by chemical structure characterization and gas separation performance evaluation of the crosslinked membranes.The results show that after oxidative cross-linking,a complex cross-linked network is formed between the two phases in the MMMs,which effectively repairs the non-selective pores in the membrane and greatly improves the gas selectivity of the cross-linked membrane.Among them,6FB/ZIF-8 has the best gas separation performance after heat treatment at 325°C for 10 h.The selectivities of H₂/CH₄ and CO₂/CH₄ are as high as 584 and 145,which are 3.5 times and 3.3 times that of the untreated membrane.The H₂/CH₄ and CO₂/CH₄ separation performance exceeds the 2008 Robeson upper limit.Therefore,the thermal cross-linking method can effectively repair the defect structure in MMMs and significantly improve the gas separation performance of the composite membrane material.