In Silico Design Of PAFs Membrane Materials For CO₂ Seperation Under Humid Condition

The excessive emission of greenhouse gas carbon dioxide（CO₂）is the unprecedented threats and challenges to the human survival and development.It is urgent to reduce the concentration of CO₂in the atmosphere.Organic porous membranes are promising materials for post-combustion CO₂separation because of their high CO₂permeation selectivity and good stability.However,the direct separation of CO₂from the humid flue gas is one of the most challenging problems.Ionic porous aromatic framework materials（iPAFs）,with high specific surface area,low skeleton density and stable structure,are widely used in the field of gas separation.In this thesis,by using computational simulation methods,we designed and constructed different iPAFs membranes,and investigated systematacially the influences of bromobenzene topology,counteranion and skeleton hydrophobicity on the CO₂separation performance of iPAFs membranes in humid environment.The CO₂separation mechanism in humid environment was revealed from thermodynamics and dynamics.Specific research work is as follows:1.Based on bromophenyl base with different spacing and topology,a series of iPAF-5membranes were built by simulating the in-situ polymerization of dibromobenzene（i-DBB）and 1,3,5-triacetylidene benzene（TEB）modified by N-methylimidazole.It was found that the adsorption curve of CO₂in the low pressure region was in good agreement with the experiment one when bromophenyl spacing was 12（?）.The adsorption capacity of CO₂in the high pressure region is greater when bromophenyl bases are placed in the diamond topology.Based on 12（?）bromophenyl spacing and diamond topology,the iPAF-5 membranes with different counteranions（including BF₄^-,PF₆^-,Tf O^-and NTf₂^-）were constructed,and their structural properties and CO₂adsorption performance were investigated in detail.The results showed that the specific surface areas of the membranes decrease with the increase of the volume of counteranions,while the pore distribution changes only slightly.The CO₂adsorption capacity of four iPAF-5 membranes was calculated at 273 K by GCMC method.The results showed that the CO₂adsorption capacity of BF₄^-,PF₆^-,Tf O^-and NTf₂^-were gradually reduced in dry or humid environment,which was significantly correlated with the specific surface area of the structure.The separation selectivity predicted by IAST theory showed that,compared with iPAF-5（BF₄）and iPAF-5（PF₆）membranes,iPAF-5（Tf O）and iPAF-5（NTf₂）membranes exhibit better CO₂/H₂O adsorption selectivity in thermodynamics.It is a more potential iPAFs membrane material for the separation of CO₂under humid conditions.The study on adsorption of CO₂and H₂O binary gas mixture is consistent with the predicted results of IAST theory.The kinetic study found that H₂O molecules diffused faster than CO₂molecules in several iPAF-5 membranes,which means that the separation of CO₂and H₂O was not conducive to dynamics.Therefore,adsorption selectivity dominates the separation selectivity of CO₂and H₂O in the four iPAF-5 membrane materials.The study of ternary gas mixture（CO₂/H₂O/N₂）showed that iPAF-5（Tf O）membrane had the best CO₂/H₂O adsorption selectivity,while iPAF-5（BF₄）membrane had the best CO₂/N₂adsorption selectivity.2.In situ polymerization of（4-bromo-3-（3-methylimidazomyl）phenyl）tri（4-bromophenyl）methane（i-TBPM）was simulated based on 12（?）spacing and diamond bromophenyl base.A series of iPAF-1 membranes were constructed by skeleton fluoridation and BF₄^-,PF₆^-,Tf O^-and NTf₂^-counteranions replacement to modulate iPAF-1 membranes’chemical function.Structural characterization indicated that both skeleton fluorination and the increase of the volume of counteranions would lead to the decrease of the specific surface area of iPAF-1membrane,but had no obvious effect on the pore size distribution.Under dry conditions,the CO₂adsorption capacity of non-fluorinated iPAF-1 and perfluorinated iPAF-1 membranes with five counteranions was calculated by GCMC method at 273 K.The results showed that the order of CO₂adsorption capacity was HBR>FBF>FBR>FPF>FTA>FTB.IAST theory predicted that in humid environment,FTA membrane in the low pressure region showed the best CO₂/H₂O separation performance,while FBF membrane in the high pressure region exhibited the best CO₂/H₂O separation performance.The study on adsorption of CO₂and H₂O binary gas mixture is consistent with the predicted results of IAST theory.Kinetic studies of six iPAF-1 membranes showed that diffusion selectivity was not beneficial for the CO₂/H₂O permeation selectivity.This implies that thermodynamic factors dominate their CO₂/H₂O permeation selectivity.It was found from studying the adsorption of CO₂/H₂O/N₂ternary gas mixture that FPF membrane is the best for CO₂/H₂O seperation,while the CO₂/N₂adsorption selectivity of FTA membrane is the highest.