Manipulating Micropore Structures In Polymer Of Intrinsic Microporosity Membranes Towards Enhanced Gas Separation Performances

Highly efficient carbon capture and olefin/paraffin separations are major demands in chemical industries.The non-thermal gas separation membrane technology is expected to play an important role in this field and greatly reduce the energy consumption in the separation processes,contributing to achieve carbon neutrality.Membrane materials with high permeability,high selectivity and high stability are the key to membrane technologies.Polymers of intrinsic microporosity（PIMs）integrate the merits of excellent processability and functional tunability for conventional polymers and abundant microporous structures for polymers with framework structures.Therefore,PIMs possess unique advantages,and are regarded as a class of ideal membrane materials for gas separations.However,PIM membranes exhibit low olefin/paraffin selectivity and serious aging behaviors in carbon capture,which inhibit the commercial application of PIM membranes.In this study,focusing on these grand challenges,we explore new pathways and propose the strategies of postsynthetic modification and crosslinking to manipulate the micropore structures of PIM membranes.Based on the raw material of PIM-1,highly efficient C₃H₆/C₃H₈ separation and enhanced aging resistant performance are achieved through constructing hydrogen-bonding,coordination and covalent crosslinking networks between polymer chains,respectively,and optimizing membrane structures.The main contents in this study are summarized as follows.1.AO-PIM-xh polymers are prepared by amidoxime-functionalizing PIM-1 with various reaction time.Owing to amidoxime groups,the interchain interaction is transformed from weak Van der Waals’force in PIM-1 to stronger hydrogen-bonding interaction in AO-PIM-xh.Thus,AO-PIM-xh membranes possess smaller ultramicropore sizes and narrower micropore size distribution,as well as more prominent size sieving effect toward propylene/propane gas pair.The pure-gas C₃H₆permeability of AO-PIM-20h membranes is 50 Barrer and the C₃H₆/C₃H₈ selectivity is23,which is 3.1 times higher than that of PIM-1 membranes.2.Hydrogen-bonding crosslinking networks are constructed by introducing bipyridine into AO-PIM membranes.Hydrogen-bonding will be formed between nitrogen atoms of bipyridine and amidoxime groups,and enlarge the micropore size slightly.Also,the crosslinking structure could induce rearrangement of partial polymer segments.With the same content,utilizing different isomers of bipyridine could further achieve finer manipulation of micropore structures.AO-Bp（22）-16 membranes exhibit optimized pure-gas separation performance with C₃H₆ permeability of 184 Barrer,which is 2.6 times higher than that of AO-PIM membranes.The C₃H₆/C₃H₈ selectivity is improved to 30.3.Coordination crosslinking networks are constructed by introducing metal ions into AO-PIM membranes.Metal ions could convert the original hydrogen-bonding to coordination bonding and change the interchain interactions.Meanwhile,the sizes and complexation capabilities are different among different kinds of metal ions.Thus,the coordination bonding strength varies and the micropore structures could be manipulated within a certain range.AO-K-16 membranes exhibit optimized pure-gas separation performance with C₃H₆ permeability of 147 Barrer and C₃H₆/C₃H₈ selectivity of 50,which is superior to the performance of most polymeric membranes.Besides,the strong coordination bonding suppresses the unfavorable structure evolution of AO-PIM membranes under high feed pressure and enhances the plasticization resistant performance.AO-K-16 membranes exhibit only 2 times the C₃H₆ permeability under 6bars of that under 1.5 bars.For the first time,the metal ion-modified AO-PIM membranes exhibit both high C₃H₆/C₃H₈ selectivity（>35）and high plasticization resistant performance with C₃H₆ permeability higher than 100 Barrer.4.Covalent crosslinking networks are constructed by copolymerizing carbon dots（CDs）with the monomers of PIM-1 to prepare PIM-CD membranes.Owning to the tiny sizes and abundant functional groups,CDs are able to react with the monomers of PIM-1 and to function as nodes to covalently connect PIM-1 segments.With the introduction of stronger covalent bonding between polymer chains,micropore structures in PIM-1 are more stable and the tendency of dense stacking is suppressed.Thus,PIM-CD（8）membranes exhibit excellent aging resistant performance and its CO₂permeability decreased by only 5%during 70 days,whereas the PIM-1 membranes exhibit a CO₂ permeability loss of 25%.