Study On The Separation Performance Of Precisely Constructed Two-dimensional Laminar Membranes

Due to the rapid economic and industria development,water resources shortage issues have attracted much attention.It is an important route to gain clean water by treating wastewater efficiently and environmentally.The two-dimensional?2D?membranes composed of nanosheets allow for high-precision separation at the molecular and ion level.To data,2D membranes have been successfully used in nanofiltration,forward osmosis,reverse osmosis processes,but it still faces huge challenges in the fields of wastewater treatment and seawater desalination.On the one hand,it is difficult to precisely regulate the interlayer distance of 2D membranes depending on the current technology.On the other hand,it is hard to achieve efficient sieving of molecules or ions with sub-nanometer sizes.Based on the above descriptions,researches on controlling the interlayer distance of 2D membranes precisely,revealing the transmission mechanism of fluids in nanochannels,improving the separation performances,and reducing energy consumption in the chemical separation process are urgently needed in the field of separation membrane.At the same time,it is also an important way to drive energy conservation and emission reduction in the entire chemical industry.This dissertation focuses on 2D membranes regulation.2D membranes constructed by graphene oxide?GO?nanosheets and molybdenum disulfide?Mo S₂?nanosheets were designed for separation processes such as nanofiltration and forward osmosis.Afterwards,the assembly design of the 2D membrane based on tantalum disulfide?Ta S₂?nanosheets was expanded to improve membrane separation performance and stability performance.Detailed contents are included the following points:?1?Aiming to solve the problem of narrow interlayer channels of graphene oxide membranes and easy swelling and decomposition in water,the authors used the cationic polymer rich with imidazole groups and the anionic polymer rich with sulfonic acid groups to intercalate into the 2D channels of graphene oxide membranes.The graphene oxide-based composite membranes not only greatly improve the stability in extreme water environments such as strong acids and alkalis,but also do not sacrifice dye retention in the nanofiltration mode with the water flux up to 1111 L m^-2 h^-1.In contrast,the original pure graphene oxide membranes only have a water flux of 65 L m^-2 h^-1.For forward osmosis applications for desalination,graphene oxide-based composite membranes also exhibit superior water permeability(2.49 L m^-2 h^-1)and Na Cl rejection?95%?.This is mainly due to the non-covalent bond between the intercalators and the oxygen-containing functional groups on the surface of the graphene oxide nanosheets.The first-principles calculations show that the interaction forces are as high as-43.6 k J mol^-1 and-31.0 k J mol^-1.This work not only provides a non-covalent bonding method for regulating graphene-based membranes,but also can be extended to other 2D membranes composed of transition metal halides,covalent organic frameworks etc.?2?In order to solve the problem that the molybdenum disulfide membranes are stable in water but the water flux is low,the author uses a series of metal cations to intercalate into the interlayer channels of the molybdenum disulfide membranes.Hydrated metal cations have different hydration diameters?6.6-8.6??.After dehydration treatment,channels with sizes of 3.0-8.5?can be assembled to achieve sub-nanometer precision control.The water flux of the molybdenum disulfide membrane assembled by this simple strategy can vary with the size varations of the interlayer channels.Because the channel size is smaller than the size of the hydrated ions in the solution,the adjusted molybdenum disulfide membranes still have excellent rejection performance for metal ions,which can be applied in the field of seawater desalination.Due to the strong inertia of the sulfur atoms on the surface of the molybdenum disulfide nanosheets,in this work,the channel size of the molybdenum disulfide film is controlled at the sub-nanometer scale,and this type of precisely regulated two-dimensional channels is expected to achieve efficient ion screening.?3?In order to avoid the problem of 2D membranes structure defect without relying on increasing the thickness of the film,the author has developed an ultra-thin,high-quality 2D film from large-sized two-dimensional tantalum disulfide sheets.We explored the general rules of crystal preparation and lithium intercalation peeling of two-dimensional sheets,prepared large-sized tantalum disulfide crystals?>6 mm?,high-quality two-dimensional sheets?average lateral size>30?m?,and tried to use large-sized two-dimensional sheet to assemble ultra-thin?<100 nm thickness?defect-free membrane for desalination.This work starts with the preparation of large-layer nanosheets,explores the characteristics and properties of two-dimensional membranes,and improves the permeation rate of solvents,the selectivity of separation,and the stability of membranes.In addition,the results have great significance for other transition metal sulfide materials.