Study On ZIF-8 And Its Sulfide Modified Graphene Oxide Composite Membrane And Its Separation Performance

Nanofiltration has the advantages of high selectivity,energy saving,simple equipment,convenient operation,etc.It is considered to be one of the most promising new technologies for processing organic solvents.The main content of this technology is to find a suitable membrane material and regulate the membrane structure.Therefore,a separation membrane with high flux,excellent selectivity and good stability is obtained.Graphene oxide（GO）is a honeycomb-like two-dimensional nanomaterial composed of single-layered carbon atoms.At the edge of the sheet and its defects,there are many oxygen-containing functional groups.The presence of these functional groups makes it possible to be functionalized,so as to control the distance between GO sheets and optimize the separation performance of membranes.In this study,porous ZIF-8 and its derivative ZnS were used to functionalize the GO sheet.Through the synergy between GO nanosheets and the inorganic porous nanoparticles,the optimizing of the mass transfer channels in membranes was achieved,and the OSN performance of membranes was improved.Firstly,oxygen-containing functional groups on the GO sheet as binding sites were used to coordinate with Zn²⁺in ZIF-8.ZIF-8 crystals were thus in-situ grown on the surface of GO sheet to form ZIF-8@GO composite with a two-dimensional layered structure.The micromorphology,crystal structure,and thermal stability of ZIF-8@GO were characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,and thermogravimetric analysis（TGA）.Then it was blended with polyethylenimine（PEI）and co-deposited on the surface of tubular Al₂O₃ ceramic substrate by vacuum assisted method.To fabricate a stable ZIF-8@GO/PEI composite membrane,chemically cross-linked with glutaraldehyde（GA）molecules was performed.The micro-morphology and surface properties were characterized by scanning electron microscopy（SEM）,atomic force microscopy（AFM）,fourier transform infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）.The membranes were used for the removal of dye molecules in organic solvents.By optimizing the fabrication conditions of membranes,the best performance of removing dyes from methanol was obtained with the flux of 6.1 L m^-2 h^-1 bar^-1,and the rejection of 99%.This performance was better than that of a composite membrane with only a single particle added.This can be attributed to the fact that GO improved the dispersion of ZIF-8 crystals in membrane,while ZIF-8 crystals also increased the inter-layer channels of GO sheets,which together promoted the separation performance of membrane.Secondly,on the basis of previous studies,with thioacetamide（TAA）as the vulcanizing agent and Zn²⁺in ZIF-8 as the zinc source,ZIF-8@GO particles were in situ vulcanized on ceramic tube to prepare ZnS@GO composite membrane.To further increase the stability,a layer of GA-crosslinked PEI solution was further impregnated to prepare a ZnS@GO/PEI composite membrane.The structure and other properties of composite membranes and ZnS@GO composite particles were characterized by SEM,TEM,XRD and FTIR.A ZnS@GO/PEI composite membrane with hollow structure was obtained by optimizing the vulcanization conditions such as vulcanization time,vulcanizing agent concentration,decomposition rate of vulcanizing agent,and particle size of ZIF-8 templates.Then the membranes were applied to the removal of dye molecules in methanol solution.The best separation performance with a flux of 15.8 L m^-11 h^-1 bar^-11 and retention of 96.8%was obtained which reduced the mass transfer resistance of organic molecules in the membrane and realized the optimization of the mass transfer channel and the separation performance.