Preparation Of Crosslinked Microporous Polyimide Materials And Their Gas Adsorption Properties

As the most important source of energy in modern society,fossil fuels are vital to human industrial production and social life.But the burning process of fossil fuels has caused serious greenhouse effects such as sea-level rise and global warming.So far,scientists have paid great attention to reducing CO₂ emissions and slowing down the greenhouse effect,and have carried out a lot of targeted research work.CO₂ capture and storage（CCS）is an effective CO₂ emission reduction technology,which is conducive to promoting the realization of"carbon neutrality"and"carbon peaking".Microporous organic polymers（MOPs）are an excellent class of CCS materials and are therefore favored by researchers.As a member of MOPs,microporous polyimide has high thermal stability,excellent pore structure parameters,abundant nitrogen and oxygen heteroatoms and good chemical stability.At present,researchers’research on microporous polyimide mainly focuses on three types:（1）Polyimides of intrinsic microporosity:The microporous structure is relatively uniform,but the building units often contain rigid or twisted structures.The synthesis of such structure monomers is more complex and difficult;（2）Hyperbranched microporous polyimide:The synthesis of hyperbranched rigid precursors is more complicated,and the polymerization process is prone to gel phenomenon;（3）Crosslinked microporous polyimide:The building block needs to have the cross-linkable group has diverse structures.The introduction of the cross-linkable group into the flexible polyimide can limit the configuration transition of the molecular chain and prevent the collapse of the molecular chain,so that the construction of the pore structure can be realized.In this dissertation,the crosslinked microporous polyimide material was the focus of discussion,and various crosslinked microporous polyimides were prepared by designing cross-linkable precursors with different structures.The effects of building block,main chain structure,substituent and crosslinking density on the pore performance parameters and CO₂ adsorption separation performance of crosslinked microporous polyimide were studied.The exploration is as follows:1.We designed and synthesized a 3,3’-bis（2,3,5,6-tetrafluoro-4-vinylphenoxy）-4,4’-biphenyldicarbonate diamine containing a crosslinkable ethenyl group（TFDA）,which was reacted with phthalic anhydride（PA）and 4-phenylethynyl phthalic anhydride（PEPA）containing cross-linkable phenylethynyl group to prepare small-molecule cross-linkable precursors with cross-linkable groups,and then two crosslinked polyimide materials TFPA-CL and TFPEPA-CL with different building blocks were prepared by the thermal crosslinking reaction.Compared with the crosslinkable precursors,the pore parameters of the crosslinked polyimide are significantly improved,the BET specific surface area of TFPA-CL and TFPEPA-CL are90 m² g^-1and 433 m² g^-1 respectively,and the total pore volume is 0.404 cm³ g^-1 and0.520 cm³ g^-1.Between them,TFPEPA has cross-linkable ethenyl groups and phenylethynyl groups.More crosslinking sites improve molecular rigidity and more crosslinked structures,which further improved molecular support and facilitate the maintenance of pore structure.Due to the better superior BET surface area and total pore volume,TFPEPA-CL has better CO₂ adsorption capacity and CO₂/N₂ selectivity,which are 8.23 wt%and 39.6,respectively.The results show that the ethenyl crosslinked structure can provide effective molecular support to construct porous polyimide materials,but the effect of using cross-linkable small molecules as precursors is limited.2.To study the effect of different backbone structures on the pore structure parameters and CO₂ gas selective separation performance of crosslinked microporous polyimide:four kinds of crosslinked microporous polyimides with different main chain structures were prepared by the polymerization reaction and subsequent thermal crosslinking reaction of vinyl diamine monomer TFDA and commercial anhydride monomers（ODPA,BPDA,PMDA and 6FDA）with different structures.All four materials have typical characteristics of microporous materials.The BET specific surface area of the crosslinked microporous polyimide reaches 337 m² g^-1-618 m² g^-1,the pore size is concentrated below 2.0 nm,and the microporous structure is less than0.7 nm.The microporous structure in the crosslinked polyimide network is formed during the thermal crosslinking reaction of crosslinkable ethenyl groups.The synergistic effect of the rigid polymer backbone and the crosslinked structure effectively prevents molecular chains from packing dense and promotes the formation of the microporous structure.These four crosslinked microporous polyimide networks have better CO₂ gas adsorption capacity（5.65 wt%-9.31 wt%）and CO₂/N₂ selective separation ratio（34.4-58.5）.The formation of the microporous structure in the crosslinked polyimide network originates from the thermal crosslinking reaction process of the crosslinkable enethyl group.The synergistic effect of the rigidity of the crosslinked polymer backbone and the crosslinked structure can effectively limit the molecular chain.Compared with the crosslinking system of small molecular precursors,the polymer has an intrinsic free volume,which leads to loose molecular packing and promotes the formation of microporous structure,thereby obtaining a microporous adsorption material with better adsorption performance.3.The effect of substituent polarity on the BET specific surface area,pore size and CO₂ adsorption performance of cross-linked microporous polyimide was studied:two dianhydride monomers with different substituents were designed and synthesized,and two dianhydride monomers with different substituents were designed and synthesized.Based on the diamine monomer TFDA,the cross-linked microporous polyimide materials CH₃-TF-CL and CF₃-TF-CL with methyl and trifluoromethyl substituents were prepared by polycondensation and subsequent thermal cross-linking reaction,respectively.The uncrosslinked polyimide has a lower BET specific surface area(48m² g^-1-84 m² g^-1),and the BET specific surface area of the crosslinked polyimide is significantly improved after the thermal crosslinking reaction(675 m² g^-1-699 m² g^-1),with a large micropore volume(0.142 cm³ g^-1-0.162 cm³ g^-1),good CO₂ gas adsorption capacity（10.80 wt%-10.94 wt%）and CO₂/N₂ selective separation ratio（44.0-51.8）.Due to the higher micropore volume and micropore ratio in the CF₃-TF-CL network,and the polar C-F bond increases the interaction force with CO₂ gas molecules through dipole-quadrupole interaction.The CF₃-TF-CL exhibits better pore parameters and higher selective separation performance for gas adsorption.Experimental results show that the introduction of polar substituents can help to improve the gas adsorption properties of porous organic polymers.4.The carbon dioxide adsorption selectivity of crosslinked microporous polyimide was improved by increasing the degree of crosslinking.We choose the diamine monomer TFDA containing a cross-linkable ethenyl group and the synthesized two dianhydride monomers PEDA and PEQDA containing phenyl side groups and phenylethynyl side groups,respectively,through polycondensation and thermal crosslinking reaction,the obtained two crosslinked microporous polyimides with different crosslinking densities.The crosslinked microporous polyimide network has a higher BET specific surface area(399 m² g^-1-607 m² g^-1).The PEQDA-PI has cross-linkable ethenyl and ethynyl side groups in the polymer main chain,resulting in higher crosslink density and shorter distance between crosslinkable sides and polymer segments,so the PEQDA-PI-CL exhibits smaller pore size（<0.7 nm）,better pore parameters and CO₂ adsorption performance of microporous materials.The crosslinked microporous polyimide network of PEQDA had better CO₂ gas adsorption（9.88 wt%）,CO₂/N₂ selective separation ratio（76.6）and CO₂/CH₄ selective separation ratio（12.4）.The results exhibit that growing up the crosslinking sites of crosslinked microporous polyimide materials is an effective way to construct excellent gas adsorption and separation performance.