Self-assembly Of Hydrophilic/Hydrophobic Separation Membranes And Scale Up Of Alcohol Permselective Pervaporation Membranes

Self-assembly as an important method was applied to fabricate ultra-thin dense films, because of its advantage of versatility, simplicity and controllability. In this thesis, dense hydrophilic/hydrophobic separation membranes were successfully built upon flat sheet, hollow fiber and tube substrate, respectively by layer-by-layer(Lb L) assembly or self-assembly of monolayers(SAMs). The relationship between the microstructure, morphology and separation performance of the membranes was investigated by advanced analytical tools. The scale up of the composite membranes was also studied.First, Polycation- and polyanion-coated nanoparticles(NPs) were obtained by tuning the electrical properties of the amphoteric oxide NPs(Zr O2/Al2O3) in acid and basic environments. Zeta potential of polyelectrolytes-coated NPs demonstrated that both polyanion- NPs and polycation- NPs remained stable. The diameters of Zr O2/Al2O3 NPs and polyelectrolyte(PE)-coated Zr O2/Al2O3 NPs were measured by a laser scattering size analyzer and TEM. The results suggest that the NPs were well dispersed, due to the electrostatic repulsion originating from the outer PE layers. This nanohybrid membranes were formed by dynamic layer-by-layer(Lb L) assembly of polycation- and polyanion-coated NPs on platform or hollow fiber substrates. SEM-EDX and AFM analyses suggest that the Zr O2 NPs are mostly distributed on the substrate surface, while the Al2O3 NPs are able to migrate into the substrate membrane pores. This is due to the differences between the sizes of the two NPs and results in an increase of the surface roughness of the Zr O2 nanohybrid multilayer membrane while that of the Al2O3 nanohybrid multilayer membrane decreased. The nanohybrid membranes were used for pervaporation separation of 95 wt% acetone-water mixture. We show that the matching between the size of the NPs and the supporting membrane pore size strongly influence the morphologies and the performance of the multilayer membranes. On the basis of this strategy, organic/inorganic composite membranes were explored and studied in order to overcome the difficulty of fabricating defects-free selective layer on industrial-grade inorganic substrate. Zr O2 nanohybrid membranes were built up on the surface and pores of ceramic tubular substrate. The nanohybrid membranes show excellent nanofiltration performances in the dye removal. The retention of nanohybrid membrane for methyl blue could reach 99.1 %, and the flux was 125 kg/(m2·h·Mpa). The results suggested that the nanohybrid multilayer can modify the defects in the substrate and be used as dense selective layer.Second, surface-modified poly(dimethylsiloxane)(PDMS) membranes were prepared using self-assembled monolayers(SAMs) to fabricate a membrane for use in pervaporation separation of ethanol/water mixtures. A cross-linked PDMS/polysulfone(PSf) composite membrane was transformed by introducing hydroxyl functionalities on the PDMS surface through a UV/ozone conversion process.(Tridecafluoroctyl)triethoxysilane was allowed to be adsorbed on the resulting Si–OH substrate to increase the hydrophobicity of the membranes. Results from FTIR, SEM, XPS, AFM, and contact angle analyses suggest that the fluoroalkylsilane monolayer was successfully formed on the modified PDMS/PSf membranes treated by 60 min UV/ozone exposure. The newly SAMs-modified membrane exhibited a separation factor of 13.1 and a permeate flux of 412.9 g/(m2 h), with a contact angle of 110°. Inspired by the complementary roles of surface energy and roughness on natural nonwetting surfaces, a superhydrophobic surface was successfully designed and prepared by SAMs modification on hierarchical ZIF-8/polymer hybrid membrane. SEM and AFM showed that the film surface became much rougher after ZIF-8 NPs were embedded within it. The enhanced surface roughness could amplify the wettability of membranes. Moreover, the rougher surface of the ZIF-8/PDMS membrane provided more sites for the deposition of SF chains, so that the hydrophobicity could be improved. The as-prepared membrane was superhydrophobic(CA = 152.4°) and exhibited the best overall performance for n-butanol pervaporation with a separation factor of 84.8 and flux of 1339 g/(m2·h).Third, PDMS/PSf composite membranes were scaled up using a roll-coating method. The roll-coating system for the construction of composite membranes was built. The thickness of the PDMS separation layer was successfully controlled on PSf substrate by roll-coating different layers with guidance provided by the results of scanning electron microscopy and X-ray diffraction pattern. The membranes are used for pervaporation separation of ethanol/water mixtures. The composite membranes with 30 layers(the thickness of selective layer is about 4 μm) had a flux of 1493 g/(m2·h) and a separation factor of 11.6 in laboratory. Then the pervaporation experiments were performed in a large cell using a plate-and-frame membrane module with a total membrane area of 0.36 m2. A total flux of 900~2200 g/(m2·h) with 46~57 wt% ethanol in the permeate was achieved. A pilot plant located at an industrial site, with a plate-and-frame membrane module and a total membrane area of 2.16 m2 was further studied. A total flux of 1000 g/(m2·h) with 60 wt% ethanol in the permeate was achieved by the 2.16 m2 pilot-scale pervaporation facility, which showed technical feasibility for industrial application.