Ionic Liquid Modified Polyimide Membranes For NH₃/CO₂ Separation

Melamine production is large in China,and its production process inevitably produces a large amount of tail gas containing ammonia（NH₃）and carbon dioxide（CO₂）.Excessive emissions of NH₃ causes environmental problems such as haze weather and groundwater pollution,which threatens human health.At the same time,NH₃ is an important raw material for industrial and agricultural production,which is used in the synthesis of fertilizers,fibers,and pharmaceuticals.Water scrubbing is the traditional method for NH₃ separation and recovery,but there is some problems such as large solvent consumption,high energy consumption,substandard discharge,and secondary waste water.Therefore,the development of new green and efficient NH₃/CO₂ separation technology is of great significance to environmental protection,resource utilization,and the development of the chemical industry.Membrane separation technology has the advantages of small equipment area,low energy consumption and environmental friendliness,and has become one of prospective technologies for gas separation.However,there are few studies on membrane separation of NH₃/CO₂.which mainly focused on the polymer membranes,and also face to the challenge of the trade-off between permeability and selectivity.In this paper,polyimide（PI）membranes with good thermal stability,stable physical and chemical properties,and high gas permeability were prepared and used for the separation of NH₃/CO₂.Considering that the separation performance of the polymer membrane needs to be improved,we introduced ionic liquids（ILs）with adjustable structures and properties to modify the polyimide membrane,so that the prepared ILs/PI blended membranes possess high NH₃ permeability and NH₃/CO₂ selectivity.We also investigated the influence of different temperature and pressure on membrane separation of NH₃/CO₂,and study the influence of different ionic liquids on membrane separation performance.The main research content and results of this thesis are as follows:（1）In view of the small molecular dynamics diameter of NH₃ and the separation of NH₃/CO₂ under high temperature,three polyimides（6FDA-DAM,6FDA-IM,PMDA-DAM）with high thermal stability were synthesized in this paper,and the chemical structure and solvent solubility of PI were characterized.Three polyimide membranes were prepared by solvent phase transfer method,and their thermal stability molecular chain spacing and FFV were characterized and analyzed.The results show that the three polyimides have higher thermal decomposition temperature（>500℃）.The separation performance of three polyimide membranes for NH₃ was studied.The order of NH₃ permeability is 6FDA-DAM>PMDA-DAM>6FDA-IM,which is consistent with the law of polyimide chain spacing and FFV.Among them,the 6FDA-DAM polyimide membrane has the highest NH₃ permeability of 1279 Barrer.（2）The effects of different temperatures and pressures on the NH₃/CO₂ separation performance of the polyimide membranes were investigated.The results show that as the temperature increases,the NH₃ permeability of the polyimide membranes gradually increases,but the NH₃/CO₂ selectivity decreases.At 120℃,the permeability of 6FDA-DAM to NH₃ can reach 2130.3 Barrer,which is due to the larger FFV of 6FDA-ADM.The influence of pressures on the separation performance of the 6FDA-DAM membrane were investigated.With the increase of pressure,the CO₂ and N₂permeability of 6FDA-DAM membrane gradually decreased,but the NH₃ permeability gradually increased.At 30°C and 5 atm,the NH₃ permeability of 6FDA-DAM membrane to reaches 2787.7 Barrer,and the selectivity of NH₃/CO₂ is 7.3.（3）In order to further improve the separation performance of NH₃/CO₂,ILs with higher NH₃ solubility for were introduced,and a series of ILs（[Bim][NTf₂],[Bmim][NTf₂]）/polyimide blended membranes were prepared.The effect of different ILs’structure and content on the NH₃ separation performance of membranes was studied.The results show that the NH₃/CO₂ separation performance of[Bim][NTf₂]/polyimide blended membrane was improved obviously due to the stronger hydrogen bond between[Bim][NTf₂]and NH₃.With the increase of the content of IL,the NH₃ permeability of[Bim][NTf₂]/polyimide blended membrane first decreases and then significantly increases.When the content of IL is 100 wt%,the NH₃permeability is 1277 Barrer.At the same time,the selectivity of NH₃/CO₂ is significantly improved,reaching 159,which is about 60 times that of pure membrane.（4）The effect of different temperatures on the NH₃/CO₂ separation performance of[Bim][NTf₂]/6FDA-DAM blended membrane was studied.The results show that as the temperature increased,the permeability of the blended membrane to NH₃ and CO₂gradually increased,but the selectivity of NH₃/CO₂ decreased.At 120℃,the NH₃permeability of the blended membrane is 1960 Barrer,and the NH₃/CO₂ selectivity is about 23.However,the NH₃/CO₂ selectivity is still increased by about 7.8 times compared with that of the pure membrane...