Structure Optimization Of Hybrid Membranes For Small Molecules Gas Separation Using Nanosized Microporous Zeolites

The product gases from different processes and purge gas discharged producing hydrogen or using hydrogen as raw material are typical hydrogen-rich mixtures.The purification,separation and reuse of hydrogen from gas mixtures are of great importance in improving the utilization efficiency of hydrogen resources,optimizing hydrogen production process diversification,and environmental protection.Compared with Pressure Swing Adsorption（PSA）and Cryogenic separation,the membrane separation technology has great advantages such as straightforward operation,energy conservation,environmental friendliness,high efficiency,and small footprint in hydrogen separation.The core part lies in the research and development of membrane materials with high separation performance Therefore,diversified and adjustable hybrid membrane materials have become one of the hot topic in membrane development,due to their ability to overcome the performance limitations of single membrane material.The hybrid materials bring the advantages of multiple components,break through the separation performance upper limit of pure material,and also improve their mechanical and thermal stability.In this study,the cooperative regulation of three phases in hybrid membrane has been achieved by controlling the（i）matrix type,（ii）filler’s morphology and（iii）optimizing the method for preparation of membranes,which strengthen the interactions between the matrix and filler phases limit the interfacial gap.A serious of hybrid membranes with high hydrogen selectivity and permeability have been prepared successfully for the separation of small molecules,including:（1）Employing polyetherimide（PEI）as the matrix phase and nanosized SOD-type zeolite（crystals with a size smaller than 50 nm）as the filler phase,the SOD/PEI mixed matrix membranes were prepared via solvent evaporation method.Herein,the PEI exhibits a good filming property and relative high thermal resistance with a large number of oxygen-containing group as active sites for coupling with the SOD nanocrystals.Nanosized SOD-type zeolite is the inorganic porous material with high hydrothermal stability and high chemical stability,containing rich Si-OHs groups on the surface.Its pore window has perfect hydrogen sieving size(d_pore≈0.28 nm,d_H2=0.289 nm),and the small particle size（<50 nm）reduces the gas transmission resistance significantly.Thus,in the SOD/PEI mixed matrix membranes,the SOD crystals（filler phase）provide high selectivity for hydrogen with fast diffusion.Besides,under the influence of hydrogen bond between the filler and the matrix phases,small-sized gas channels with high diffusion resistance were formed at their interfaces,which not only alleviate the negative effect of the interface on the membrane’s selectivity,but also promote the efficient diffusion of hydrogen.After optimization of the preparation methodology,the ideal H₂/N₂selectivity is 16.9,higher than the Knudsen diffusion separation selectivity.Finally,the SOD/PEI mixed matrix membranes have achieved both high hydrogen permeability of 4762Barrer（110 GPU）and high H₂/N₂selectivity of 33.1.Due to the potential oplasticization of polymer materials,the application of H₂/CO₂separation system has not been studied.（2）The“sandwich type”GO/SOD/GO hybrid membranes have been prepared via layer-by-layer method,where graphene oxide（GO）with higher compatibility and permeability has been adopted to replace the PEI as a matrix phase,and the SOD type zeolite/graphene oxide composite material was synthesized using in-situ solvent-free coupling method.The matrix GO is a two-dimension amphiphilic carbon derivative material,whose typical stacking morphology can form nanoscle channels appropriate for small gas molecules.Additionally,there are a large number of hydroxyl groups,epoxy groups and other oxygen-containing groups on the GO’s surface,providing coupling active sites for the SOD type nanocrystals.In terms of the filler phase,the SOD type nanocrystals/graphene oxide composite material not only utilize to provide the selective diffusion paths for hydrogen,but also to promote the contact and interaction between SOD and GO to further reduce the interfacial gap and defects.The ideal H₂/N₂selectivity of this hybrid membrane is 38 higher than the SOD/PEI hybrid membrane,which means the in-situ coupling method could enhance interface optimization.Furthermore,the GO/SOD/GO membrane avoid the potential plasticization effect observed for the SOD/PEI mixed matrix membranes,thus resulting in highly performing H₂/CO₂ separation membrane.The GO/SOD/GO hybrid membrane has shown a H₂/CO₂selectivity of 45.5 and a H₂permeance of 4005 GPU.（3）The structure features of the SOD/GO hybrid membrane have been optimizaed by applying the in-situ zeolite precursor transformation and cross-linking approach.The GO was used as a matix to confined the crystallization of the amorphous nanoparcitls（an average size of 12 nm）in the precursor and to transform them into crystalline SOD keepling the same size;the high concentration of hydroxyls on the surface of the amorphous and crystalline nanoparticles promote the strong interactions with the GO.The in-situ conversion process induce the formation of stronger Si-O-C chemical bonds and hydrogen bonds between SOD and GO,enhancing the interaction between filler and matrix phases,reducing the interfacial space,promoting the high-density stacking morphology and restricting the expansion of membrane’s structure.The ideal H₂/N₂selectivity of this hybrid membrane is 41,which is the highest in this study,showing that the in-situ cross-linking method could best regulate the interface.The SOD/GO hybrid membrane has reached the optimal performace in terms of selectivity and stability.The SOD/GO membranes exhibited the highest H₂/CO₂ selectivity of105 and relatively high hydrogen permeability 1050 GPU,and they showed high stably for a long period at high temperature of 200°C and in humid environment.