Constructing A Two-dimensional Mixed Matrix Membrane Based On G-C₃N₄

Selective transport of water and ionic molecules is essential in water-related environments,resources,energy,and other fields.The two-dimensional?2D?membrane constructed from 2D materials,due to its advantages of high separation performance,good chemical and thermal stability,and excellent mechanical properties,is expected to achieve mass transmission controlled at the nano-and sub-nanometer level.Orderly stacking of these 2D ultra-thin nanosheets can build a2D layered membrane with an well-organized layered structure.As a result,many channels are created between layers,and porous nanosheets will naturally provide more channels for fast and selective large-scale transport of water,ions,and molecules,which has attracted researchers'great research and application interests.At present,there are not many two-dimensional materials that have been developed and assembled into layered membrane,so many types of two-dimensional membranes will be developed and researched.In this dissertation,2D g-C₃N₄nanosheets are taken as the main research subject,and the improvement of the separation performance and membrane stability of two-dimensional membranes are taken as the research goal,further explore the mass transfer mechanism and expand other applications of g-C₃N₄nanosheets.The specific research content of the paper is as follows:?1?Inspired by the layered structure of graphene oxide,we inserts sulfonated polyphenylene oxide?SPPO?/methyl blue?MB?/evens blue?EB?molecules with SO₃H between adjacent g-C₃N₄nanosheets in this paper.The g-C₃N₄based membranes of SPPO/g-C₃N₄,MB/g-C₃N₄,and EB/g-C₃N₄were prepared by vacuum filtration.The insertion of sulfonic acid group successfully opened the tightly packed g-C₃N₄nanosheet membrane.The modified g-C₃N₄based membranes can increase the water flux by two orders of magnitude compared to the pure g-C₃N₄membrane while maintaining excellent separation performance.The SPPO/g-C₃N₄membrane showed extremely high water permeability with the water flux of 8867 L h^-1m^-2bar^-1,and the MB dye molecules were retained by 100%,while the original g-C₃N₄membrane showed only flux of 60 L h^-1m^-2bar^-1.The g-C₃N₄based membrane's retention for dye molecules with different sizes and charges is based on electrostatic repulsion and size screening.At the same time,since the SO₃H site on the SPPO/MB/EB molecule firmly binds the N atoms on g-C₃N₄,they are strongly bound and distributed on the g-C₃N₄nanosheets through acid-base interaction.This enables the nanochannels of g-C₃N₄based membranes to be stable for a long time in acidic,alkaline,multiple cycles,and high pressure environments.?2?Inspired by the structure of aquaporins,a binary cooperative interface heterojunction composite membrane was constructed by using grapheme oxide?GO?and g-C₃N₄two-dimensional materials in this paper.This type of GO/g-C₃N₄composite membrane has multiple inter-layer transportation channels,and heterojunction channel structures with alternating different sizes and hydrophilic end and hydrophobic end alternate arrangement,hydrophilic channels with ultra-low friction and excellent membrane stability.The water flux of 37.5%GO/g-C₃N₄composite membrane is 110 L h^-1m^-2bar^-1,which is 3 times higher than that of pure GO membrane,and can achieve 100%retention of dye molecules with sizes above1.1 nm.The retention of Na₂SO₄solution can reach 92.5%,and its desalination performance far exceeds other reported two-dimensional separation membranes.This is due to the alternating arrangement of wide and narrow hydrophilic and hydrophobic channels in the composite membrane.The larger GO hydrophilic channels provide high water permeability,while narrow hydrophobic g-C₃N₄channels provide higher ion retention.The two-dimensional nano-coordinate interface composite channel constructed by two kinds of nanosheets can efficiently promote water transport and block salt ions.At the same time,the strong interaction between the two kinds of 2D nanosheets makes the composite membrane have a fairly stable 2D channel.The stability of the membrane was investigated by testing the separation performance of the composite membrane for Na₂SO₄under different pH solutions,NaClO solution treatment and high pressure operation.The results show that the water flux and rejection have not changed much,so the composite membrane has the characteristics of resistance to acid,alkali,chlorine and high pressure.