Enhanced CO₂ Separation Performance Of Polyelectrolyte-inorganic Hybrid Membrane Surpassing The Trade-off Limit

Membrane technology is one of the most promising CO₂ capture technology, which requires high permeability, high selectivity and low cost for CO₂ separation membrane materials. With the development of gas separation membrane, the instinct trade-off effect between permeability and selectivity for polymeric materials becomes one of the major obstacles for further improvement in separation performance. It is necessary to develop new materials for gas separation membrane. Sulfonated poly ether ether ketone（SPEEK） polymer electrolyte was used to fabricate membranes for efficient CO₂ separation. However, the intrinsic trade-off effect for polymer also restricts the application of SPEEK. This study is mainly focus on how to simultaneously improve permeability and selectivity of SPEEK membrane to surpass the trade-off limit. A series of novel SPEEK-based hybrid membranes for CO₂ separation were designed and fabricated by facile and versatile methods. The effect of hybrid membrane interface compatibility, free volume properties, crystallinity and chain stiffness on the gas separation performance was elaborated by regulating physical and chemical structure of membrane. This study is expected to offer guidance to solve trade-off effect. The details were summarized as follows:Enhanced CO₂ permeability by regulating the fractional free volume of membrane: Based on the interface morphology theory, the novel fillers（e.g. titanium nanotubes（TNTs）, metal organic frameworks（MOFs） and graphene oxide（GO）, etc.） were incorporated into SPEEK matrix to fabricate hybrid membranes. The fractional free volume or crystallinity was regulated to intensify the diffusion mechanism. The fractional free volume, crystallinity, chain stiffness and interchain spacing were precisely regulated by adjusting the type and content of fillers, and the diffusion mechanism was enhanced. The enhanced permeability of the hybrid membrane was attributed to the enlarged size of free volume fraction and cavity. When the content of aminated TNTs was 8wt%, the CO₂ permeability and CO₂/CH4（N2） selectivity of hybrid membrane were 2090 Barrer and 57（62）, respectively, surpassing the trade-off limit.Enhanced CO₂ selectivity by regulating the acid sites of membrane: Based on the interface morphology theory and the specific affinity to CO₂, the sulfonated MOFs with high porosity were incorporated into SPEEK matrix to fabricate hybrid membranes, and the solution mechanism and diffusion mechanism were expected to improve. Good interfacial compatibility between sulfonated MOFs and SPEEK was shown when the loading was as high as 40 wt%. The introduction of sulfonated MOFs with high porosity significantly increased the gas diffusion coefficient and solubility selectivity for CO₂/CH4（N2） systems. When the content of sulfonated MOFs was 40wt%, the CO₂ permeability and CO₂/CH4（N2） selectivity of hybrid membrane were 2064 Barrer and 50（53）, respectively, surpassing the trade-off limit.Enhanced CO₂ selectivity by regulating the base sites of membrane: Based on the interface morphology theory and acid-base reaction, abundant amino decorated MOFs with high porosity were incorporated into SPEEK matrix to fabricate hybrid membranes. The reaction mechanism was introduced into membranes, and the reaction mechanism and diffusion mechanism were expected to collaboratively improve. Under humidified state, The HCO3- produced by the MOFs rich in amine groups facilitated the transport of CO₂. Both the CO₂ permeability and CO₂/CH4（N2） selectivity were significantly enhanced. When the content of amine groups decorated MOFs was 40 wt%, the CO₂ permeability and CO₂/CH4（N2） selectivity of hybrid membrane were 2490 Barrer and 72（80）, respectively, surpassing the trade-off limit.Enhanced CO₂ selectivity by regulating the acid-base sites of membrane: Based on the interface morphology theory, specific affinity to CO₂ and acid-base reaction, sheet-shaped materials GO was incorporated into SPEEK matrix to fabricate hybrid membranes, and the diffusion mechanism was expected to improve. The reactivity mechanism and solubility mechanism were introduced into the membrane by doping the acid-basic site decorated GO into membranes, and the diffusion mechanism, reactivity mechanism and solubility mechanism were expected to intensify. The enlarged fractional free volume and the decreased crystallinity enhanced diffusion mechanism, and the gas permeability increased. The selectivity was significantly enhanced by improving the diffusion selectivity, solubility selectivity and reaction selectivity. When the content of amino acid functionalized GO was 8wt%, the CO₂ permeability and CO₂/CH4（N2） selectivity of hybrid membrane were 1247 Barrer and 82（115）, respectively, surpassing the trade-off limit.