Orientation Manipulation And Gas Separation Performance Of MOF Membranes

Metal-organic framework(MOF)has become the ideal candidate for membrane-based separation considering the high surface areas,tailorable pore apertures,and rich functionality.It has been well recognized that microstructural optimization is the key to separation performance improvement of MOF membrane.Secondary growth enabled more precise control over the microstructure of MOF membranes by decoupling the nucleation and growth of MOF.In this study,the grain boundary structure,in-plane and out-of-plane orientations of the MOF membranes were optimized by combining the manipulation of the seed layer orientation and controllable epitaxial growth.MOF membranes with excellent gas separation performance were prepared.(1)NH2-MIL-125,which exhibited high thermal stability and preferential adsorption capacity for CO2,have shown great promise for efficient H2/CO2 separation.However,uncontrollable hydrolysis of common metal sources even under transient air exposure severely hindered controllable preparation and microstructural optimization of the relevant MOF membranes.In order to optimize the microstructure of NH2-MIL-125 membranes,we first explored the feasibility of the use of Two-dimensional transition metal chalcogenides(TMDC)and Mxene as metal sources of Ti-MOF,respectively.NH2-MIL-125 particles could be successfully prepared by using TiS2 as titanium source under solvothermal conditions,while Ti3C2Tx was only partially converted.The discrepancy in reactivity inspired us to further manipulate and optimize the microstructure of MOF membranes.(2)Effects of titanium sources and heating methods on the microstructure and H2/CO2 separation performance of NH2-MIL-125 membranes were investigated further.It was found that single-mode microwave heating significantly promoted the in-plane epitaxial growth rate of NH2-MIL-125 crystals.Moreover,with TiS2 as the titanium source,obtained NH2-MIL-125 membranes became well-intergrown,membrane thickness was decreased,and intercrystalline defects were reduced.Prepared NH2-MIL-125 membranes exhibited the H2/CO2 separation factor of 17.6.The structure-performance relationship between the microstructure parameters and the gas separation performance was established.It was found that promoted in-plane growth of cry stals is beneficial to reduce the density of grain boundary defects,thereby improving the permselectivity of NH2-MIL-125 membranes.(3)Based on the above achievements,preferentially c-oriented NH2-MIL-125 membranes were further prepared via oriented epitaxial growth.In brief,a turbulent air-liquid interfaceassisted self-assembly(ALIAS)method was developed and employed for facile self-assembly of disk-shaped NH2-MIL-125 seeds into uniform c-oriented NH2-MIL-125 seed monolayer.The use of TiS2 as the metal source and employment of single-mode microwave heating during epitaxial growth were found indispensable for the preparation of well-intergrown,twin-free coriented NH2-MIL-125 membranes.Prepared NH2-MIL-125 membranes exhibited superior H2/CO2 selectivity in comparison to randomly oriented counterparts,thereby demonstrating the significance of precise preferred orientation control in improving the separation performance of MOF membranes.(4)Three-dimensional orientation control of MOF membrane is expected to further increase the pore distribution uniformity,reduce the intercrystalline defects and therefore,improve the H2/CO2 separation performance.With uniform octahedral-shaped NH2-UiO-66 as seeds,uniform NH2-UiO-66 seed monolayer with both(111)out-of-plane and regional in-plane orientations could be obtained with the turbulent ALIAS method.Among various factors,the use of ZrS2 as metal source played a vital role in warranting the formation of well-intergrown,twin-free NH2-UiO-66 membrane maintaining the preferred orientation inherited from the seed layer.Prepared NH2-UiO-66 membrane exhibited a H2/CO2 separation factor as high as 28.4,which was 3-4 times higher than that of randomly oriented counterparts.This study confirmed the significance of 3D preferred orientation control in separation performance enhancement of MOF membranes.