Green Preparation And Gas Separation Prosperities Of Zeolitic Imidazolate Frameworks Membranes

Separation of mixtures is one of the most important industrial processes,but it consumes as much as half of the total energy of many industrial processes.Among all the mixtures that demand to be separated,the separation of gas mixtures is particularly important for industrial processes,such as the purification of hydrogen from the mixtures,a clean and new energy source.Membrane-based gas separation has attracted more and more attention owing to its unprecedented features,namely,lower cost,higher energy efficiency and smaller carbon footprint compared to conventional energy-intensive methods such as condensation,distillation and PSA.The current gas separation membrane market is dominated by organic polymer membranes,which is contributed to their unique advantages of low cost,high processing and easy scalability.However,both permeability/selectivity trade-off and short lifetimes strongly restrict the further improvement of their performance.Zeolite molecular sieve membranes are substitutes for polymeric membranes as a result of their distinguished thermal and chemical stability.In addition to inherent stability,zeolites have attracted extensive attention due to their well-defined,regular pore structure which endows zeolite membranes with better gas separation property compared to polymeric membranes.However,there are some bottleneck problems in zeolite membranes,such as harsh synthesis conditions,limited topological structure,less post-modification possibility of pore channels and functions.As a new class of crystalline porous materials,Metal–organic frameworks?MOFs?,have been extensively investigated in recent years due to their various structure,high stability,the channel size easy to adjust and the channel easy to function.The most important and outstanding property of MOFs compared to zeolites is that the pores of framework are more designable and functionable,which makes MOFs have great potential in meeting the requirements of different separation conditions.Moreover,most MOFs membranes can be crystallized in mild reaction conditions.Following these merits of MOFs materials,it has a good chance of being favorable candidates for advanced membranes.Zeolitic imidazolate frameworks?ZIFs?,a sub-class of MOFs,which combine the unparalleled properties of both MOFs and zeolites,for instance,the channel structure superiority of MOFs and high stability of Zeolitics,quickly develope into the most concerned MOFs materials.To prepare MOFs membranes with excellent gas separation performance,it is vital to develop the robust membrane synthesis methods.Thus far,solvothermal or hydrothermal method synthesis method was the most widely applicable way to prepare MOFs membranes,which normally requires the dissolution of metal salts and linkers in an organic solvent under high temperature and pressure.Solvent used in the synthesis process drastically reduced the yield of MOFs products as the result of occupying a large space in containers.Moreover,the addition of solvents leaded to much tricky chemical pollution and corrosion.In addition,solvents used in synthesis route naturally resulted in the generation of high pressure,which was not conducive to large-scale formation of molecular sieving membranes.Therefore,it is urgent to develop environment-friendly synthetic methods for MOFs membranes.In this contribution,the well-intergrown and defect-free ZIFs membranes were successfully prepared by solvent-free preparation method.The use of large number of solvents and high pressure was avoided in this fabrication process thus simplified the synthesis process,reduced the energy consumption and emission of pollutants.Hence,this route was certainly suitable for the construction of membrane with large processing area.In addition,the crystallization of MOFs membranes under solvent-free condition showed more outstanding advantages to fill the common intercrystalline cracks,pinhole defects and grain boundary defects on the membrane,which is of great significance for the synthesis of continuous membranes with excellent separation performance.The fabricated membranes showed excellent properties in separation of H₂/CO₂,H₂/N₂ and H₂/CH₄ mixtures.Particularly,it exhibited high permeability and selectivity simultaneously in purification of H₂ from H₂/CO₂ mixture,which gave MOF membranes great potential for applications in the purification of hydrogen produced from steam reforming natural gas.