Structure Design And Control Of Thin-film Composite Membranes For CO₂ Separation

CO₂ membrane separation technology is essential in CO₂ mitigation and energy gas purification.Fabrication of ‘defect-free’ and thin-film composite（TFC）membranes can significantly improve the membrane permselectivity.However,pore penetration appeared in composite membrane preparation by coating method seriously limits the membrane performance;membranes prepared by interfacial polymerization using monomers with long and flexible segments shows high CO₂ permeance,while segments in the membrane are mobile and thus the low diffusion selectivity,which leading to the low selectivity of the membrane.Increasing selectivity generally results in a significant decrease in CO₂ permeance,and thus membrane performance improvement is limited.To solve these problems,three aspects including gutter layer design,structure of coating solution optimizinzation by nanowires and rigid and contorted structure introduction into the selective layer were focused on in this work.Zinc（Ⅱ）Tetrakis（4-carboxy-phenyl）porphyrin)（Zn-TCPP）nanosheets with a pore size larger than 1 nm were selected as the gutter layer material.The Zn-TCPP gutter layer with flat surface and a thickness of only 66 nm was prepared on polyacrylonitrile（PAN）support layer by vacuum filtration.Benefiting from the porous structure of Zn-TCPP nanosheets and the ultrathin thickness of the gutter layer,Zn-TCPP/PAN exhibited the CO₂ permeance higher than 20000 GPU.The polyvinylamine（PVAm）solution was used as the coating solution to prepare selective layer by coating method on the Zn-TCPP/PAN support to obtain the multilayer composite membranes.The gutter layer was superiorly adhered with the support layer and the selective layer,ensuring the structural stability of the multilayer composite membrane.The ultra-high permeance of the gutter layer reduced the gas mass transfer resistance and refrained the pore penetration,and thus the separation performance of the composite membranes was significantly improved.At the feed pressure of 0.15 MPa,the multilayer composite membrane showed the CO₂ permeance of 788 GPU and the CO₂/N2 selectivity of 58.Amine group-functionalized titanium dioxide（TiO₂-NH₂）nanowires were chosen as fillers to incorporated into PVAm solution,and the composite membrane was prepared on the surface of the polysulfone（PSf）support layer by coating method.The cross-linked network structure was formed by the polymer and TiO₂-NH₂ nanowires in the coating solution,and thus the pore penetration of the coating solution was alleviated when composite membranes were prepared.In addition,the enhanced rigidity of the selective layer by introducing TiO₂-NH₂ nanowires mitigated the penetration of the selective layer into the pores of the support layer under high pressure and enhanced the stability of the composite membrane.By adjusting the PVAm concentration in the solution and selecting the optimal loading of TiO₂-NH₂ nanowires,the ultrathin and defect-free composite membranes with the selective layer thickness less than 100 nm were prepared.At the feed pressure of 0.15 MPa,the PVAm（0.25）-TiO₂-NH₂/PSf composite membrane showed the CO₂ permeance of 526 GPU and the CO₂/N2 selectivity of 73.High-performance membranes for CO₂ separation were prepared by trimesoyl chloride（TMC）in organic phase and amino-containing N-methyldiethanolamine（MEDA）and 5,5’,6,6’-tetrahydroxy-3,3,3’3’-tetramethyl-1,1’-spiralbis-indane（TTSBI）with rigid and contorted structure in aqueous phase.The free volume was increased and the pore structure of the selective layer was optimized by introducing monomers with contorted structure.The size-sieving effect of the micropores in the selective layer was enhanced and the pore structure feature was conducive to improving the selectivity of CO₂/N2 and CO₂/CH4.The membrane exhibited state-of-the-art separation performances for both CO₂-N2 and CO₂-CH4 separation.At the feed pressure of 0.11 MPa,the as-prepared membrane with the CO₂ permeance of about1800 GPU and the CO₂/N2 selectivity of 370 were achieved;the CO₂ permeance only reduced by 10% and the CO₂/N2 selectivity was 3 times of the membrane prepared without TTSBI.