Preparation Of Ion-Selective Separation Membranes Based On Superacid-Catalyzed Polymerization

Anion selective separation membranes,as an advanced separation membrane,have become an indispensable functional components in membrane separation processes such as electrodialysis(ED)and diffusion dialysis(DD),contributing to clean and efficient resource production and recovery.However,most of the anion selective separation membranes used in water treatment are based on the post-modification of commercial polyphenylene ether(PPO),polyether ketone(PEK),polyethersulfone(PES)and other polymers containing ether bond linkages.The instability of such anion selective separation membranes in extreme pH environments(especially in alkaline environments)has greatly limited the application of monovalent/divalent ions separation processes.Therefore,it is very important to explore a facile preparation method of anion exchange membrane with highly acid and alkali resistance,good mechanical properties and ion separation ability.According to the above situation,this thesis has designed the polymer backbone structure via superacid-catalyzed polymerization from the bottom polymerization stage,and then prepared a variety of anion exchange membranes with good mechanical properties,high thermal and chemical stability through subsequent modification for ion separation processes.The main contents of this paper are as follows:(1)First,an anion exchange membrane,containing the carbazole structure,was designed and prepared by superacid-catalyzed polymerization in this article.By adjusting the amount of N-methylpiperidine in the subsequent Menshutkin reaction,polymers with different amount piperidinium ions were obtained.The success of the polymerization and quaternization reaction was proved by 1H NMR result.TG and DMA test results have showed that the series of membranes have excellent thermal stability and mechanical properties.ED results show that this series of membranes have higher ions flux and separation efficiency than commercial membrane Neosepta ACS,In NaCl/Na2SO4 system,the Cl-flux of QPC-Pip-60 reaches 3.32 mol m-2 h-1,and the permselectivity reaches 11.6.In the NaOH/Na2WO4 system,the microphase separation structure makes the OH-flux up to 3.59 mol m-2 h-1 and the permselectivity up to 70.Long-term stability test in seawater simulated solution and alkali stability test in NaOH solution indicate this ionic membrane has excellent cycle test stability and alkali resistance.(2)Then,in order to obtain a higher performance and easy-to-prepare anion selective separation membrane,the electrophilic carbonyl-containing monomers catalyzed by superacid catalyzed polymerization were explored and selected,and the polyarylene backbone containing pyridine structure was obtained based on the polyhydroxyalkylation reaction.Specifically,a series of poly(alkyl-biphenyl pyridinium)(PAB)membrane polymers were synthesized from 4-acetylpyridine and biphenyl via super-electrophilic activation by triflic acid,and then quaternization was conducted using Menshutkin reactions with 1-bromopentane.A simple two-step reaction procedure can obtain QPAB-x series anion exchange membranes with excellent solubility in organic solvents.The desirable mechanical and thermal stability properties have been also confirmed by TG and DMA test.As the previous work,the synthesized membranes were used as anion selective separation membranes in the ED process for anion sieving.In NaCl/Na2SO4 system,the Cl-flux of QPAB-2 membrane is as high as 3.3 mol m-2 h-1 and the permselectivity has reached as high as 11.9.In the alkali recovery ED system,the OH-flux of the QPAB-2 membrane is as high as 3.5 mol m-2 h-1,and the permselectivity is as high as 361.2,which is much higher than the previous work(70).Besides,long-term ED tests with a single QPAB-2 membrane have confirmed this well-designed structure’s good cycle stability,especially alkaline stability.The current study manifest that QPAB-x membranes are prepared efficiently from readily obtainable monomers and confer excellent ion separation performance and alkaline stability.(3)At last,the second work was further expanded in DD,and the pyridine ring in PAB backbone was reacted with the 1-bromo-6-(trimethylammonium)hexyl bromide ionic liquid to obtain a ether-bond-free poly(alkyl-biphenyl pyridinium)polymer containing dicationic side chains,which was applied to the diffusion dialysis in acid recovery process.According to the different amounts of reactants in the Menshutkin reaction,a series of QPAB-C6QA-x with different amount of pyridinium and quaternary ammonium cations were obtained.The IEC of the QPAB-C6QA-x series membranes was within 0.93-1.58 mmol g-1,having a good dimensional and mechanical stability.The existence of the "island-like" micro-phase separation can improve acid flux in the DD process,and the dense membrane structure is beneficial to higher selectivity between acid and metal ions.The AEM diffusion dialysis coefficient is between 0.023-0.030 m h-1,indicating a good acid recovery performance,and the separation factor was in a range of 53-78.In addition,the membrane performance did not show a downward trend in long-term acid recovery test.Therefore,the novel QPABC6QA-x series AEMs have showed high application potential for acid recovery via DD.