Preparation Of Metal-Organic Framework ZIF-8/PVDF Composite Membranes And Adsorption Of Iodine In Water

Metal-Organic Frameworks（MOFs）are porous skeleton materials in which metal ions or metal clusters and organic ligands self-assemble into a certain topological structure.At present,the synthesis and application of metal-organic frameworks membrane has become one of research hot spots,including ZIF-8 as a classic metal-organic framework material,which has good application value in the gas phase and liquid phase adsorption separation,with high chemical stability,thermal stability,uniform distribution channel and rules of sodalite structure.In this paper,poly（4-vinylpyridine）（P4VP）brushes were grafted firstly from PVDF membrane via surface-initiated atom transfer radical polymerization with activators generated by electron transfer（SI-AGET ATRT）method which the chelating effect of the pyridine groups on P4VP side chains and Zn²⁺to promote the enrichment of Zn²⁺on the surface of the membrane supports.Then,a dense,continuous,well-intergrown ZIF-8 membranewas prepared on the functionalized surface by counter-diffusion method in situ growth method.Finally,on the capture iodine performance of ZIF-8/PVDF composite membranefor iodine in water was explored to study the effects of pressure,concentration,pH and interference ions on the adsorption performance.The main contents of this paper include:The First Chapter It introduced the concept and features of metal-organic framework materials,the carrier of metal-organic frameworks membrane processing method and the preparation methods,and research progress and application of zeolite imidazole frameworks and ZIF-8.The Second Chapter In order to prepare a dense,continuous,and well-intergrown ZIF-8 layer on a polymer membrane,it was first necessary to functionally modify the surface of the polymer membrane.This chapter attempts to use the"Top-to-Down"surface-coating polydopamine method and"Down-to-Top"surface-initiated polymerization grafted 4-vinylpyridine method to modify the surface of the PVDF microporous membrane supports;and self-assembled ZIF-8polycrystalline layer was prepared on the surface of the two modified membranes by counter-diffusion method.The experimental results showed that the poly4-vinylpyridine modified porous membrane was more conducive to the independent loading of ZIF-8 crystal on the surfaceunder the same reaction conditions;the effect of sodium formate as deprotonation is to 2-methylimidazole solution which affected the degree of densification of ZIF-8 crystals byadding different concentrations of sodium formate.When Zn²⁺:m Im:SF is 1:8:1.340,a dense,continuous,well-intergrown ZIF-8/PVDF-g-P4VP membrane was prepared with a thickness of about 5μm.The Third Chapter Due to its large specific surface area,high porosity and pore volume,ZIF-8 was a highly appropriate capture for iodinein water.This chapter,ZIF-8/PVDF-g-P4VP composite membranecaptured for iodinein water by the static and filtration adsorption experiments to discuss the influence of pressure,concentration,and pH on filtrationand adsorption performance.The ZIF-8/PVDF-g-P4VP composite membrane with a mass of 0.085 g and a membrane area of 8.55 cm² statically adsorbed iodine in 100 mL of 0.2572 mmol/L iodine solution.The removal rate could reach 95%or moreafter 1375 min.Through electron microscopy,it could be known that ZIF-8/PVDF-g-P4VP composite membrane adsorbs iodine,which has a good interaction with the Me IM connector on the ZIF-8frame.At the beginning of the filtration adsorption process,the rapid adsorption process occurs,and the surface adsorption was slow with time.The electron micrographs showed that the ZIF-8 crystals on the surface of the ZIF-8/PVDF-g-P4VP composite membrane were hydrolyzed,blocked the pores of the membrane which the retention of iodine had led to an increase in iodine removal rate.The Fourth Chapter Studies showed that ZIF-8/PVDF-g-P4VP membranewas not stable in aqueous solutions and ZIF-8 crystals were prone to hydrolysis hydrolyze,which was attributed to the elimination of ligand protection.The work showed the improvement of ZIF-8/PVDF-g-P4VP membrane stability in water through utilization of a ligand exchange postmodification method which replaces 2-methylimidazole ligands on the outer surface of ZIF-8/PVDF-g-P4VP membranes by the more hydrophobic 5,6-dimethylbenzimidazol,and named as ZIF-8-DMBIM membrane.The modified ZIF-8/PVDF-g-P4VP membranes retained identical morphologies and crystallinities after static and dynamic water immersion,especially improved the performance of the application of adsorption and membrane separation.ZIF-8-DMBIM membrane was used to remove iodine in water,which studied the effect of coexisting cation and anion on adsorption performance.The experimental results showed that the ZIF-8 cage geometry allowed each iodine atom to form two CT complexes between opposing 2-methylimidazolate linkers,created the ideal binding site to maximize iodine retention.