Synthesis and Gas Separation Performance of Highly Permeable Membranes

Author:Jiang Xu

Supervisor:shao lu


Degree Year:2019





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).