Preparation Of Metal-organic Framework Membranes And Their Hydrogen Purification Properties

Since the end of the last century,hydrogen was recognized as the most promising energy to overturn the traditional energy market for its unique properties.Today,steam methane reforming(SMR)in combination with water-gas shift(WGS)reaction is still the dominant strategy to produce hydrogen,in which CO₂ and CH₄ are unavoidably involved in the process.To obtain pure hydrogen,the separation of H₂from CO₂ and CH₄ is one of the most significant steps for H₂ production.Comparing with common separation technologies such as pressure swing adsorption or cryogenic separation,porous membranes are expected to contribute in the coming transition from energy-intensive processes to more sustainable ones.Recently,metal-organic frameworks(MOFs)have become the focus of membrane community because of their rationally controllable pore size and easy functionalization.Due to its reasonably controlled pore size and easy functionalization,metal-organic framework materials(MOF)have attracted wide attention in membrane separation.Since gas separation is highly determined by the kinetic diameter of molecules and their interaction with the separation materials,the potential of MOF membranes in gas separation has been well demonstrated due to their highly diversified structures and pore size as well as strong adsorption affinities.In the thesis,studies have been conducted on three MOF membrane materials and their applications in the separation of H₂/CO₂ and H₂/CH₄.The main research results are as follows:1.Separation performances of gases through polymeric membranes are restricted by a trade-off between permeability and selectivity,which is called the Robeson upper limit.Therefore,mixed matrix membranes(MMMs),with hybrid advantages of both inorganic and polymeric membranes,are developed combining polymeric materials with well-selected fillers.So far,a variety of inorganic materials,such as zeolites,silica,carbon nanotubes and carbon molecular sieves can be used as fillers of MMMs.However,owing to the intrinsic differences between the inorganic and polymeric phases,interface voids and rigidified polymer can easily appear in MMMs with inorganic fillers.In particular,zeolitic imidazolate frameworks(ZIFs),a subfamily of MOFs based on bivalent transition metal cations and bridging substituted imidazolate anions,have emerged as a novel type of crystalline porous material for preparation of superior molecular sieve membranes.We synthesized a new type of ZIF with a large number of benzene rings in the side chain as a filler,and synthesized a new type of MMMs by using polyimide(PI)which has a large number of five-and six-membered rings as the matrix.At a filler ratio of 10%,the separation factor of the mixed gas of H₂/CO₂ and H₂/CH₄ of the membrane can reach to 10.85 and 32.02,respectively.But the permeability of the film is still low.2.The permeability of pure MOF membrane is often higher than that of MMMs,so we have synthesized a pure MOF membrane.As one of the most stable MOFs,UiO-66 has attracted intense attention for the preparation of molecular sieving membranes.A new post-synthesis modification(PSM)strategy was developed to enhance the separation selectivity of supported UiO-66-NH₂ membranes on porous ceramic tubes called UiO-66-NH₂@SA by using salicylaldehyde(SA)modification.The mixed gas separation factors of H₂/CO₂ and H₂/CH₄ can be improved due to constrictions of the pore size through reaction of-NH₂ with SA giving iminophenol moieties.Notably,SA-modified UiO-66-NH₂ membranes are rather stable,at a high operation temperature of 200?and 1 bar,mixture separation factors of 11.3 for H₂/CO₂ and 22 for H₂/CH₄ can be obtained.Further,no obvious performance degradation has been observed after 3 months storing in air.This high stability of UiO-66-NH₂@SA membrane could provide possibility for the industrial applications of hydrogen separation and purification.3.In order to further obtain MOF membranes with greater permeability and cheap raw materials,we explored a new way of making membranes.Polymer substrates especially polymer hollow fibers are recognized as promising substrates used for gas separation.Inspired by“like grows like”principle,hydrophobic ZIF-8membranes were expected to be prepared on the polyvinylidene fluoride(PVDF)substrates which also shows high hydrophobicity.However,the swelling of polymer substrates under solvothermal conditions does affect the fabrication of ZIF-8membranes.In this study,we reported one simple method to scrap PVDF solution by using a 200?m thickness membrane fabricator on copper nets(CNs)to solve the swelling problem.Supported by CNs,the swelling of PVDF substrates was prohibited and the nucleation and growth of denser membranes were promoted.Finally,highly hydrogen permselective membranes were fabricated on CNs supported PVDF substrates.At 25?and 1 bar,the equimolar binary mixture separation factors of H₂/CO₂ and H₂/CH₄ were found to be 10.68 and 16.38,respectively,which by far exceed the corresponding Knudsen coefficients.Furthermore,the as-prepared ZIF-8membrane in this study exhibits excellent stability at a relatively broad range of operating temperature and long operating time,which provides possibility for industrial applications.