Design And Preparation Of Block Conolymer/Ionic Liquid Blend Membrane And Studies On Ammonia Separation Performance

Separation of ammonia(NH3)-containing gases is significant for NH3 resource recycle and utilization and the reduction of air pollution.In industry the common treatment methods for NH3-containiing gases are water-scrubbing and acid scrubbing,and there are problems such as secondary pollution and high energy consumption in separation.Membrane technology shows advantages such as environmentally-friendliness,low energy consumption,small area,as well as easy operation,which indicates a promising prospect in NH3 separation.Up to now,research of NH3 separation membranes are mainly focused on polymeric membranes,of which block copolymers exhibit better NH3 separation properties.Nevertheless,simultaneously achieving high NH3 permeability and selectivity is difficult.Ionic liquids(ILs)used for gas separation membranes can enhance affinity for NH3 of the membranes,increasing NH3 solubility.Meanwhile,microstructures of the membranes are tuned,thus achieving more efficient NH3 separation.In this work,various series of block copolymer/ILs blend membranes with high NH3 permeability and selectivity were designed and fabricated for NH3 separation by using ILs to tune membrane structures and properties.The effects of ILs on membrane microstructures and NH3 separation performance are systematically investigated,which optimizes NH3 separation performance of the membranes.The main research contents and results in this dissertation are as follows:(1)Poly(ether-block-amide)copolymer(Pebax 1657)/ILs membranes were designed and prepared for NH3 separation.The effect of different properties of ILs on NH3 transport was studied.ILs including protic IL 1-ethylimidazolium bis(trifluoromethylsulfonyl)imide([Eim][NTf2])and conventional ILs 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([Emim][NTf2])were used,and[Eim][NTf2]has better hydrogen bond donating ability and higher viscosity compared with[Emim][NTf2].The effects of the ILs on membrane microstructures and NH3 separation performance were systematically investigated.According to characterizations of membrane structures,it was found that[Eim][NTf2]and[Emim][NTf2]can decrease crystallinity of the membranes,which was favorable for gas permeation.Pebax/[Eim][NTf2]membranes exhibited better NH3 separation performance compared with Pebax/[Emim][NTf2]membranes.With the increment of IL content in Pebax/[Eim][NTf2]membranes,NH3 solubility coefficients greatly increased because of the stronger hydrogen bond interaction between[Eim][NTf2]and NH3,while NH3 diffusivity coefficients gradually decreased.NH3 diffusivity coefficients greatly increased and NH3 solubility coefficients slightly increased with the increment of IL content in Pebax/[Emim][NTf2]membranes.Therefore,increasing NH3 solubility in Pebax/IL membranes is a key factor to promote NH3 permeability.(2)By combining the advantages of protic ILs(PILs)for NH3 absorption and the advantages of sulfonated block copolymers with self-assemble properties,series of sulfonated pentablock copolymer(Nexar)/IL membranes were developed for NH3 separation.According to the results of characterizations and calculations,Nexar/PILs membranes exhibited self-assembled and micro-phase separated structures.PILs were located at both hydrophilic blocks and hydrophobic blocks,enhancing connectivity of the ionic domains.Besides,PILs exhibited stronger interaction with NH3,thus largely enhancing NH3 separation performance of the membranes.The effects of ILs categories and IL content on microstructures and NH3 separation performance were systematically explored.The results indicated that NH3 separation performance of Nexar/PILs membranes was better than that of Nexar/conventional ILs and the neat Nexar membrane.PILs can improve NH3 diffusion and solubility in the membranes simultaneously,and the influence on diffusion was more significant.Nexar/[Eim][NTf2]-25 wt%membranes exhibited the highest NH3 permeability.reaching 2711 Barrer,and the corresponding NH3/N2 and NH3/H2 selectivities were 1407 and 262,which were 446.6%,148.6%and 193.1%increasements compared with those of the neat Nexar membrane.(3)2-methylimidazolium bis(trifluoromethylsulfonyl)imide([2-Mim][NTf2])and imidazolium bis(trifluoromethylsulfonyl)imide([Im][NTf2])were chosen to prepare IL-based membranes with multiple sites for improved NH3 separation performance.By characterizations and calculations,it was found that there are hydrogen bond interactions and van der waal force between[2-Mim][NTf2]and Nexar.Microstructures of Nexar/[2-Mim][NTf2]membranes and Nexar/[Im][NTf2]membranes primarily consisted of lamellae.The homogenous distribution of ILs in the membranes made gas transport channels more continuous,which dramatically enhanced NH3 diffusion.Meanwhile,NH3 solubility in the membranes was promoted because of the multiple hydrogen sites of ILs.The effects of ILs content on NH3 separation performance of the membranes were systematically investigated.Membranes had best NH3 separation performance at IL content of 25 wt%,and the separation performance was superior to that of Nexar/PIL membranes.NH3 permeability,NH3/N2 and NH3/H2 selectivities of Nexar/[2-Mim][NTf2]-25 membrane were 3248 Barrer,1662 and 334,respectively.Moreover,the increment of pressure further promoted NH3 separation performance of the membranes.(4)Several Nexar/metal ILs membranes were further designed and fabricated based on the high NH3 solubility in metal ILs.The ILs have different structures,effecting the strengths of interactions with NH3.Therefore,it was possible to tune NH3 separation performance of the membranes.Results of gas permeation measurements indicated that NH3 separation of the membranes with Lithium(Li)was better than that of the neat Nexar membrane,while NH3 separation performance of the membranes with cobalt(Co)was worse.NH3 separation performance of Nexar/2-methylimidazolium lithium bi(bis(trifluoromethylsulfonyl)imide)([2-Mim][Li(NTf2)2])membranes at different ILs content was further studied.The results showed that with the increment of IL content,NH3 diffusivity coefficients and NH3 permeability increased.However,the improvements of NH3/N2 and NH3/H2 selectivities were not obvious.The effect of pressure on NH3 separation performance of Nexar/metal IL membranes was investigated,and it was found that NH3 separation performance of the membranes was gradually promoted by increasing the pressure from 1 bar to 5 bar.Besides,characterizations were used to reveal the micro-phase separated structures of Nexar/metal IL membranes.The increment of IL content contributed to richer ionic domains,thus enhancing NH3 permeability of the membranes.