The Preparation And Gas Separation Properties Of MFI Zeolite Nanosheet Membranes

Membranes have many functions,such as separation,concentration,purification and refinement etc.,which feature easy operation,high efficiency and facile control etc.,finding more and more implications in the field of separation.Polymeric membranes exhibit merits of low cost and easy manipulation,however,there's a trade-off between permeability and selectivity,i.e.,Robeson upper limitation.Inorganic molecular sieves,possessing regular structure with pore of molecule comparable size,are ideal membrane separation materials.Organic-inorganic mixed matrix membranes prepared by incorporating of zeolite molecular sieves in polymer combine the easiness in the fabrication of membranes of polymers and the high selectivity of inorganic.In this thesis,pore-open MFI zeolite nanosheets were used as building blocks for the fabrication of MFI zeolite nanosheet membranes and MFI zeolite nanosheets-PDMS mixed matrix membranes.Gas separation experiments were also performed to evaluate the membranes separation properties.The main content consists of two parts as follows:(1)Preparation and gas separation properties of MFI nanosheet mixed matrix membranes.Open-pore MFI zeolite nanosheets were prepared by removal of organic structure-directing agents(OSDA)from multilamellar MFI zeolites(ML-MFI)using a mixture of H2SO4 and H2O2(known as piranha solution),followed by ultrasonication and sedimentation for purification.The exfoliated MFI zeolite nanosheets were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),transmittance electron microscope(TEM)and N2 adsorption-desorption analyzer,indicating that after treatment with piranha solution the organic structure-directing agents were removed completely and the micropores were completely opened with micropore specific surface area of 274m2/g.MFI nanosheets/polydimethylsiloxane(PDMS)mixed matrix membranes(MMMs)were prepared by adding the open-pore MFI zeolite nanosheets into PDMS.SEM indicated that the MFI zeolite nanosheets could be uniformly dispersed in the PDMS matrix forming MMMs.The gas separation properties for CO2/N2 and CO2/CH4 of the membranes with different MFI contents were investigated.The effects of temperatures and pressures on the gas separation performance of the MMMs were studied.The results showed that with the increase of MFI contents,both the gas permeability and selectivity gradually increased.The MMM with MFI nanosheets content of 0.12 wt.%exhibited permselectivities of 20.6 and 6.3 for CO2/N2 and CO2/CH4,respectively,while the permeability of CO2 reaches 11616 barrer.MFI nanosheets/PDMS mixed matrix membrane presented better separation performance compared to the previously reported silicalite-1/PDMS and GO nanosheets/PDMS mixed matrix membranes.The MFI nanosheets/PDMS MMM(0.12 wt.%)surpasses the limitation of the Robeson upper bound for CO2/N2 separation.(2)Preparation and gas separation of MFI nanosheet membranes.MFI zeolite membranes were prepared by depositing the open-pore MFI nanosheets dispersed in aqueous solution on home-made alumina disc supports by a simple vacuum-assisted filtration method.Characterizations of SEM,XRD,etc.showed that continuous and defect-free MFI nanosheet membranes with thickness of 400 nm were fabricated from the MFI zeolite nanosheets via the interlayer force.Finally,the gas separation of butane isomers on the MFI zeolite nanosheet membranes was conducted,indicating that an ideal selectivity for n-butane over i-butane and a permeance of n-butane of 5.8 and 2.2×10-7 mol·m-2·s-1·Pa-1 were achieved,repectively.