Zeolitic Imidazolate Frameworks (ZIFs)- Based Pervaporation Membranes For Alcohol Dehydration

Zeolitic imidazolate frameworks(ZIFs),as an emerging subclass of metal-organic frameworks(MOFs),consist of imidazolate ligands and metal irons or clusters.ZIFs exhibit excellent thermal and chemical stability,controllable pore aperture,high surface area,and intrinsic porosity.Therefore,they are very attractive for numerous applications,including gas adsorption and storage,chemical sensors,catalysis,drug delivery,and membrane separation.Further,compared with traditional organic materials,ZIF materials have greater potential in fabricating separation membranes because of their tunable pore size and adjustable functionality.The purpose of this thesis is to develop ZIF-based hybrid or continuous membranes and explore their applications in pervaporation dehydration of alcohols.Firstly,novel ZIF-90-APTES-PVA hybrid membranes are fabricated by incorporating ZIF-90 nanoparticles and APTES(3-aminopropyltriethoxysilane)into PVA(poly(vinyl alcohol))matrix simultaneously for pervaporation dehydration of ethanol,where APTES also works as both a modifier of ZIF-90 and a crosslinker of PVA chains.The ligand of ZIF-90,imidazole-2-carboxaldehyde(ICA),contains a free aldehyde group which can react with the amino group of the APTES.The silicon ethoxylate group of the APTES can also react with the hydroxyl group of PVA.Here APTES acts as the bridging agent between ZIF-90 particles and PVA matrix and increases the compatibility between ZIF-90 and PVA.Therefore,ZIF-90-APTES-PVA hybrid membrane shows better separation performance than APTES-crosslinked PVA membrane and ZIF-90-PVA hybrid membrane.Various characterization techniques are employed to investigate the physiochemical properties and the morphologies of ZIF-90-APTES-PVA hybrid membranes.The effects of ZIF-90/APTES mole ratio and ZIF-90 loading on the membrane performance are explored.The results indicate that,compared to pristine PVA membrane,the separation factor of the optimital ZIF-90-APTES-PVA hybrid membrane is improved from 175 to 552 at the operation temperature of 40 °C.In addition,ZIF-8 continuous membrane on the polymeric substrate is also prepared with a novel covalent-assisted seeding method.Well-intergrown ZIF-8 membrane on polyimide(PI)substrate is fabricated with ICA as the covalent agent between ZIF-8 layer and PI substrate.Firstly,the PI substrate is crosslinked with ethylenediamine and the amino groups are grafted onto the PI substrate.Then,the ICA molecules are grafted onto the PI substrate through the imine condensation.The imidazoline groups of ICA act as the nucleation sites to form a ZIF-8 seed layer which is well-attached to the PI substrate through covalent bond.Through secondary growth,thin and defect-free ZIF-8 layer is subsequently fabricated and well-attached on PI substrate by the epitaxial growth of ZIF-8 seed layer on top of it.ZIF-8/PI membrane synthesized with ICA covalent-assisted seeding method shows improved pervaporation performance for isopropanol dehydration.Subsequent surface coating with polydimethylsiloxane or polydopamine is applied to mitigate the grain boundary of ZIF-8/PI membrane,which results in a further performance enhancement.The effect of operation temperature on the separation performance is also carried out.This novel covalent-assisted seeding method provides new perspective on the fabrication of ZIF membranes on polymeric substrates.