Structural Design And Optimization Of Polyaryl Piperidine Anion Exchange Membranes For Acid Recovery

With the rapid development of industry,natural resources are decreasing,and the consequent environmental problems are attracting widespread attention.And how to recycle and harmlessly treat the acidic wastewater among them has become a hot research topic.Usually,acidic wastewater can be treated by conventional techniques such as chemical precipitation,coagulation,flocculation,adsorption,etc.,but these routes involve heating and adding alkaline solutions,thus causing more resource consumption.Among the many membrane treatment technologies,diffusion dialysis（DD）has the advantages of simple operation,low energy consumption and easy operation,showing a broad application prospect.Anion exchange membrane（AEM）,as the core component of diffusion dialysis,has the"trade-off"effect between acid flux and ion selectivity,which is the core problem to be solved in DD application.Therefore,the development of AEMs with high DD performance and excellent dimensional and chemical stability has been the goal of many researchers.It has been shown that by adjusting the morphology between hydrophilic charged groups and hydrophobic main chains in the membrane to induce the formation of phase separation structures,or by using cross-linking agents to adjust the dense network of the membrane,and by introducing hydrogen-rich auxiliary groups（e.g.carboxyl,hydroxyl）in the membrane,the ion transport resistance can be reduced,thus ensuring excellent separation and maintaining high acid fluxes.Therefore,based on the above strategies,we prepared a series of poly（aryl piperidine）based polyelectrolytes with the goal of preparing high performance AEMs by rational molecular design to balance the problem between acid flux and ion selectivity and to further improve the comprehensive performance of AEMs.The specific studies are described as follows:（1）Based on improving the dimensional stability and diffusion-dialysis performance of AEM,a series of poly（aryl piperidine）anion exchange membranes with different dual cation string contents were prepared by first copolymerizingα,α,α-trifluoroacetone with N-methyl-4-piperidone and biphenyl,and then introducing flexible long side chains containing piperidine cation groups into the polymer by Menshutkin reaction.The TGA results showed that the introduction of the flexible long side chains reduced the thermal stability of the membranes,and the higher the proportion of the double cation string in the polymer,the worse the thermal stability of the polymer,but the mass loss of all membranes was less than 5%below 200°C,and tensile tests showed that the tensile strength of the prepared AEMs ranged from 6.10MPa to 44.10 MPa,with good thermal and mechanical properties.The tensile strength of the prepared AEMs ranged from 6.10 MPa to 44.10 MPa,which had good thermal and mechanical properties.As the concentration of ionic groups in the membranes increased,the water absorption and swelling rate increased and the microphase separation structure became more obvious,which helped the transfer of protons in the membranes and increased the acid flux,but the corresponding ion selectivity decreased.All three membranes prepared were immersed in the simulated acidic waste solution for 7 days,with mass loss rates below 5%,showing good chemical stability.Although the QPBPip33Ac membrane had the highest IEC value,the mechanical properties of this membrane were poor and the excessive water absorption and swelling destroyed the dimensional stability of the membrane,which was not favorable for the selective separation of ions.The above results show that by changing the concentration of ionic groups within the membrane,the dimensional stability and mechanical properties of the membrane can be effectively improved,and the acid flux can be increased,which provides a new idea for the design of the molecular structure of the polymer.（2）To improve the problem of low acid flux due to cross-linking,we first quaternized the poly（biphenyl piperidine）polymer directly and introduced unsaturated olefins,and then performed the"Click"reaction with the cross-linking agent containing"dithiol".The cross-linked anion exchange membrane QPBP-HT/OT-X%with different cross-linking degree and different cross-linking agents was prepared by adjusting the content of cross-linking agents 1,6-hexanedithiol or 3,6-dioxa-1,8-octanedithiol.The test results showed that the water absorption and swelling rate of the cross-linked membranes decreased,and the tensile strength was significantly higher than that of the uncross-linked membranes at 30.78 MPa.In terms of microscopic morphology,the QPBP-OT-X%AEM containing EO groups show relatively larger ionic domains and more pronounced microphase separation structures than the less self-assembled hydrophobic QPBP-HT-X%AEM.Diffusion dialysis tests showed that the acid flux of QPBP-OT-X%AEMs with hydrophilic EO groups in the membrane was significantly higher than that of QPBP-HT-X%,and even QPBP-OT-5%AEM achieved an acid flux of 41.72×10^-3 m/h,and the separation factor S also reached 97.41,which were better than the initial membrane(U_H⁺=33.24×10^-3 m/h,S=59.56);for the chemical stability test,it could be found that with the extension of soaking time,the membranes showed a higher quality than the initial membrane.In the chemical stability test,it can be found that the membranes showed mass loss with the extension of soaking time,but at most not more than 8%,showing a strong stability in acidic environment.The above results indicate that by introducing hydrophilic EO groups into the cross-linked structure,the ion selectivity can be effectively improved and the acid flux can be maintained high,which effectively balances the"trade-off"effect between the two and is a very promising modification method.