Preparation And Properties For Gas Separation Of Polyimide With Ladder Dioxinyl-bipyrimidine Structure

With the development of the economy,the overuse of fossil fuels(coal,oil,and natural gas)have caused many environmental hazards such as global warming and worsening climate conditions.The people have been exploring an environmentally and friendly gas separation technology.Membrane gas separation technology not only possess the characteristics of no phase transition and low energy efficiency,but also has the advantages of low investment cost,easy operation and simple equipment.Besides,the technology can also realize H2 and N2 separation,CO2 removal from flue gas,deacidification of natural gas and isomers separation,etc.The gas separation membrane of polymer materials has the characteristics of strong mechanical properties,easy processing and low cost,so it has become one of the materials of choice for large-scale gas separation membranes.Polyimide is a preferred material with stable chemical structure and mechanical properties for gas separation.In recent years,the introduction of trapezoidal bridge rigid structure into polyimide can significantly increase the free volume and obtain a membrane with higher gas permeability,which has become a research highlight of polyimide gas separation membranes.Research on gas separation membranes based on polyimide molecular design and composite materials has been continuously developed.At present,6FDA-based polyimide gas separation membranes have been commercialized due to their excellent stability and gas separation performance.In this paper,from the perspective of molecular structure design,a polyimide with a trapezoidal high nitrogen-rich structure is designed,and three new polyimides containing a ladder dioxinyl-bipyrimidine structure are prepared.Besides,the complexation of cobalt ion and bipyrimidine forms a "cross-linked" network inside the polyimide,which can restrict the movement of the molecular chain and improve the gas separation performance of the polyimide membrane.In the first part,a novel dioxinyl-dipyrimidine diamine was synthesized through a six-step reaction.Three polyimides were synthesized by polymerization of 4,4’-(hexafluoroisopropyl)phthalic anhydride(6FDA)with dioxinyl-bipyrimidine diamine(DMD)and 2,4,6-trimethyl-m-phenylenediamine(DAM)(DMD10-PI-DAM,DMD30-PI-DAM and DMD-PI-0.0).As the content of dioxinyl-bipyrimidine in polyimide increases,the polymer molecular chain spacing(d-spacing)decreases and the glass transition temperature(Tg)decreases,but the thermal decomposition temperature of polyimide increases to 520.6 oC.The gas performance test indicate that DMD10-PIDAM(The polymerization molar ratio of DMD to DAM is 1:9)has excellent gas permeability and DMD-PI-0.0(homopolymer of 6FDA and DMD)shows excellent gas selection performance.The CO2/CH4 gas separation performance of DMD10-PI-DAM and DMD30-PI-DAM exceeds the Robeson upper bound in 1991 and was close to the Robeson upper bound in 2008.In the second part,we selected dioxinyl-dipyrimidine polyimide(DMD-PI-0.0)with multiple complex sites and the best gas separation selectivity as the matrix.Four kinds of cobalt complex polyimide films(DMD-PI-3.0,DMD-PI-10,DMD-PI-15,DMD-PI-20)were prepared by mixing different quality cobalt acetate into the polyimide casting solution.As the content of cobalt acetate increases,d-spacing of polyimide decreases,while Tg gradually increases,indicating that the complexation leads to a tighter molecular chain arrangement.In order to further improve the gas separation efficiency,we prepared the complex polyimide into four thin film composite membranes(TFCMs)(DMD-CPI-3.0,DMD-CPI-10,DMD-CPI-15,DMD-CPI-20).Scanning electron microscope test found that the average thickness of the selective layer within the composite membranes is 500~650 nm,and there is no obvious defect on the surface of the selective layer.The gas separation test found that the DMD-CPI-3.0(mixing 3.0 wt.% cobalt acetate TFCMs)composite membrane obtained a CO2 transmission of 69.9 GPU at 25℃ and 1.5 bar,and the CO2/CH4 selectivity of the DMD-CPI-15 composite membrane was 44.79.In summary,the bipyrimidine polyimide membrane designed in this paper possess excellent gas separation performance.The introduction of cobalt ions into polyimide significantly reduces the molecular chain spacing,because the complexation of cobalt ions restricts the movement of molecular chains.Besides,cobalt acetate was mixed into the synthesized polyimide to prepare thin film composite membranes by spin coating.The composite membranes possess excellent CO2/CH4 gas separation ability.Therefore,this paper provides a highly efficient method to improve the gas separation performance of polyimide by complexing with cobalt,which provides new ideas for future researchers.