Research On Solute Separation And Ion Separation By Triazine Frame Polymer Membranes

Traditional energy-based chemical separation processes,especially in liquid separation,mainly use energy-intensive processes such as evaporation and distillation,which is inconsistent with the current sustainable development strategy.Membrane separation technology is widely used in the separation and purification process because of its advantages of precise separation,low energy consumption,green safety,et al.The design of separation membrane is the key to solve the problem of solute separation and ion separation in liquid by membrane technology.Polymer is the first choice for the construction of separation membrane because of its rich raw materials,easy processing and various functions.Rigid polymers are used as raw materials to construct stable microporous separation channels,which have good chemical stability and solvent resistance.At the same time,selective separation of solutes or ions in solution based on size sieving and electrostatic repulsion of pore structure is expected to replace traditional chemical separation methods to achieve the purpose of energy saving,efficiency improvement and energy structure optimization.Herein,we report a covalent triazine framework membrane(CTF-BP)synthesized by a superacid-catalyzed organosol-gel reaction of commercially available aromatic nitrile monomer.It is found that CTF-BP membrane has the ability to separate solutes and ions in liquid.Based on this separation characteristic,CTF-BP membrane was applied to organic solvent nanofiltration and vanadium flow batteries respectively.The main research is as follows:(1)The three-dimensional network composed of chemically inert C-C and C-H bonds formed during the superacid catalytic crosslinking process endows CTF-BP membranes with good solvent resistance and rich microporous structure.According to the pore size characterization by CO2 adsorption and desorption,the membrane contains a large number of micropores with pore sizes in the range of 0.5-0.85 nm.To improve solvent permeation,a CTF-BP membrane with a thickness of 200 nm was successfully obtained by diluting the polymer membrane solution.This membrane exhibits well permeability to both polar and nonpolar solvents and is dimensionally stable in organic solvents,such as methanol,n-hexane,etc.Size sieving by the robust micropores(<1.0 nm)confers the membrane with a cut-off molecular weight as low as 550 g mol-1,which can effectively separate mixed dyes and is expected solve the environmental pollution caused by printing and dyeing wastewater in the textile industry.In addition,we also found that hydrogen bonds were formed between methanol and six-membered heterocycles in the CTF-BP membrane,which is beneficial to improve the membrane’s affinity for methanol.Under the synergistic effect of size sieving,even if the methanol content is low,methanol still have priority over n-hexane to permeate the membrane.To demonstrate the practical applicability,this membrane is utilized to separate n-hexane containing low concentration of methanol solution,and a maximum separation factor of 1485 and a methanol flux of 3.21 kg m-2 h-1 are obtained.Compared to azeotropic distillation and permeation vaporization separation techniques,CTF-BP membranes have remarkable features of low energy consumption and high fluxes for separating methanol and hexane mixture.(2)Based on the robust micropores,CTF-BP membrane was tried to be used for ion separation with smaller size.To improve the hydrophilicity of CTF-BP membrane,the triazine ring is protonated to obtain CTF-BP+ membrane.Static ion diffusion experiments indicate that the ion transmembrane permeation rate is negatively correlated with the diameter of hydrated ions,which preliminarily proves the ions selectivity of CTF-BP membranes.Protonated CTF-BP membrane was used to construct a fast proton transfer channel to achieve high permeability rate of H+.At the same time,VO2+ was blocked based on size sieving and coulomb repulsion,and the ion selectivity of H/V was as high as 1383.When CTF-BP+membrane is used in vanadium flow battery,it was found that the areal membrane resistance is only 0.7 Ω cm2,and the peak power density can reach 528 mW cm-2.At the same time,it has a good capacity retention rate,which can meet the long-term operation of vanadium flow battery,contributing to the storage and conversion of renewable energy.