Designing And Property Investigation Of Novel Membranes For High Performance Separation

Membranes have received great attention due to their high separation efficiency,ease of operation,and environmental friendly in comparison with conventional technologies.To enable wider application of membrane separation technology,membrane with both high permeability and high selectivity are required.An ideal membrane is expected to have an ultrathin selective layer for low-resistance permeation,and simultaneously well-defined membrane pores matching to the size of the target molecules for precise sieving.However,such structure is difficult to be achieved by conventional porous membranes.Owing to the membrane formation mechanisms,the conventional membranes usually possess broad pore size distribution and micrometer-scale thickness,giving rise to relatively poor molecular size selectivity and low transportation efficiency.In this thesis,high-performance membranes with ultrathin separating layer and uniform porous structure were designed under the direction of Hagen-Poiseuille equation.Two dimensional(2D)nanosheets and metal-organic frameworks(MOFs)were used as building units for highly permeable and selective membranes.In addition,aiming to improve the antifouling property of oil/water separation membrane,super-wetting surface constructed by 2D nanosheets array were designed on mesh membranes.This thesis includes mainly four sections as follows:(1)Ultrathin MOFs membranes for high-performance hydrogen separation.MOFs have recently received considerable interest for gas separation.However,it remains a great challenge to obtain thin and continuous MOFs membranes as molecular sieves due to the difficulties in controlling the growth of MOFs crystals.In this part,an ultrathin and continuous ZIF-8 membrane was fabricated via a spatially confined contra-diffusion process,in which a nanoporous network film was utilized as an interlayer mediator to control the nucleation and growth of ZIF-8 crystals.By such a design,a free-standing ZIF-8 membrane as thin as～550 nm was obtained.The membrane exhibited high H2 permeability as well as high selectivity for separation of gas pairs such as H2/CO2,H2/N2 and H2/CH4.(2)Self-assembled 2D cystine and macrobiomolecule nanosheets.Chiral cystine crystalline nanosheets were synthesized via a nanowire oriented on-surface growth strategy.The formation of cystine nanosheets was induced by a trace amount of Cu2+ions released from Cu(OH)2 nanowires perpendicularly grown on a copper mesh,which acts as catalysts for the transformation from cysteine to cystine.This method can also be used in the self-assembling of macrobiomelecules which possess cysteines on their surfaces.Tobacco mosaic virus(TMV),a cylinder-shaped protein containing an inner pore with diameter of 4 nm,was cross-linked via a Cu2+catalyzed disulfide-bond forming reaction with 2D orientation.By such a design,ultralarge single-layer TMV nanosheets with lateral size of tens of micrometers and well defined nanopores were successfully developed.(3)Designing of homoporous membranes based on 2D assembly of biomacromolecules.As mentioned above,TMV nanosheet with well-ordered and uniformly distributed 4-nm-diameter nanopores is an ideal candidate for the designing of homoporous membranes.Ultrathin ultrafiltration membranes as thin as～40 nm were obtained through simple vacuum filtration.The membrane was capable of performing precise size-selective transportation of substances with 1 nm size difference while maintaining high water permeance up to～7000 L m-2 h-1 bar-1.(4)Designing of inorganic membranes with superhydrophilic and outstanding antifouling property for oil/water separation.Developing of an effective and sustainable method for the purification of oily water is highly desired.In this part,inorganic mesh membrane made up of cupric phosphate(Cu3(PO4)2)with a special intersected nanosheets-constructed structure was designed.Combing the hierarchical structural feature with strong hydration ability of Cu3(PO4)2,the nanosheets-wrapped membrane exhibits superior properties of superhydrophilic and underwater anti-oil-fouling for effective separation of various viscous oils from water,such as heavy diesel oil,light crude oil,and even heavy crude oil.