Synthesis And Gas Separation Performance Of Highly Permeable Membranes

Plenty of gaseous waste and by-products will be produced during the utilization of fossil fuels including the CO₂ from combustion,H₂ and hydrocarbons from decomposition.Especially CO₂,the main cause of greenhouse effect,seriously affects the living environment of human beings.Conventional methods for H₂ and hydrocarbons separation are confronted with predicament of high energy costs and secondary pollution.Hence,there is an urgent need to develop advanced eco-friendly gas separation technology for CO₂ capture and H₂ purification.Membrane separation is considered as the most promising separation technology in the future due to its advantages of high energy efficiency and operation simplicity.In recent years,although the performance of conventional polymer membranes has been continuously improved,the permeability of most reported polymer membranes are still too low to meet the needs of industrial applications.Herein,in this study,polyethylene glycol?PEG?and porous metal-organic frameworks?MOFs?are chosen to design three types of high-permeable membranes including MOF-based molecular sieve membranes,MOF/PEG mixed matrix membrane,and PEG-based polymeric membrane,for CO₂capture and H₂ separation based on gas separation theories.The physicochemical and gas permeation properties of these membranes are thoroughly studied.The feature and application direction of each type of membrane are analyzed.The relations of membrane structures and their separation performance are mainly researched.The permeation mechanisms of gases in membranes with different structures are explored.In order to expand the membrane materials and break through the limitation of polymers,extremely simple one-step method is discovered to synthesize defect-free zeolitic imidazolate framework-8?ZIF-8?films in aqueous solution by utilizing dopamine as a modulator.In-situ dopamine polymerization is considered to inhibit homogenous growth of ZIF-8 and slow down ZIF-8's growth rate on substrate,whilst the formed polydopamine facilitates the counter-diffusion growth of ZIF-8 layer on substrate and patch the possible defects simultaneously.The influence of reaction period on membrane morphology and separation performance is investigated.The designed ZIF-8 composite membrane exhibits exceptional high separation performance with H₂/C₃H₈ permselectivity of 6688 and C₃H₆/C₃H₈ permselectivity of99.Though the ZIF-8 membranes have excellent gas separation performance,the poor workability seriously restricts their further application in large scale.Accordingly,PEG-acrylate monomers and UiO-66 type MOFs are adopted to synthesis UV-crosslinked PEG/UiO-66 mixed matrix membranes for further study.Isopropenyl functionalized UiO-66-MA,which is synthesized from UiO-66-NH₂,is used to copolymerize with PEG monomers to create covalent bonds between PEG matrix and the dispersed UiO-66-MA particles.Thus,the interfacial adhesion of UiO-66-MA and PEG is enhanced.Highly dispersed UiO-66-MA particles also can generate highly efficient gas transport passages in the UV-crosslinked PEG membrane.The interfacial covalent connections are clarified via atomic force microscope and¹³C solid state NMR.The permeation results show that the interfacial reinforced PEG/UiO-66-MA membranes exhibit much higher gas permeability than cross-linked PEG and PEG/UiO-66-NH₂ membranes.The highest CO₂ permeability reaches 1450Barrer,with CO₂/H₂ and CO₂/N₂ selectivities of 11.6 and 45.8,respectively.The CO₂plasticization behaviors of these mixed matrix membranes with different interfacial status are further studied to disclose a novel interface assessment method for nanocomposites.To further exploit the potential of PEG-based membranes and overcome the permeability loss of mixed matrix membrane with high MOF loadings,low molecular weight?500 g/mol?polyethylene glycol dimethyl ether?PEGDME?is in situ added into UV-crosslinked PEG to replace MOF fillers so as to fabricate novel semi-interpenetrating PEG membranes?SIPN?in one step.The liquid linear PEGDME lowers the glass transition temperature,increases the chain mobility and expands the free volume size of the cross-linked PEG membrane.Thereby,the gas transport within membrane is greatly improved.Meanwhile,the abundant ethylene oxide units in PEGDME render the membrane more CO₂-philic,further enhancing CO₂ permeability.The obtained SIPN membranes show the highest CO₂ permeability of 2980 Barrer among the reported PEG-based CO₂-philic rubbery membranes with excellent CO₂/H₂?14.7?and CO₂/N₂?45.7?selectivities.Finally,aiming to improve the mechanical strength of SIPN membranes,branched semi-interpenetrating?BSI?PEG membrane is developed by initiating the secondary UV-polymerization of impregnated polyethylene glycol methyl ether acrylate?PEGMEA?to generate root-mimicking PPEGMEA?polyPEGMEA?within thermal cross-linked PEG network that synthesized with PEG-NH₂ and PEG-epoxy.The highly flexible side chains of PPEGMEA can dynamically entangle with cross-linked PEG network to form a semi-interpenetrating structure whilst enhance the softness of the whole membrane.These penetrating-chain-branched CO₂-philic membranes exhibit improved tensile strength,great elasticity,excellent long-term stability and ultrahigh gas permeability.The highest CO₂ permeability reaches 1952 Barrer at 20 atm and 35°C with the excellent CO₂/light gas selectivity?16.0 for CO₂/H₂ and 70.6 for CO₂/N₂?.