Design And Synthesis Of Porous Aromatic Frameworks For Ethylene Separation

Small hydrocarbons separation exerts an enormous function on petrochemical industry.Utilization of traditional industrial purification technology to separate these hydrocarbons consumes a great deal of energy,which not only aggravates the global energy crisis,but also causes environmental pollution.Ethylene is one of the world’s largest annual consumption of chemical raw materials,and closely related to our social life;however,impurity molecules such as ethane and acetylene can poison catalysts in the process of ethylene polymerization and cause great economic loss.Currently,energy-and cost-intensive methods such as distillation and partial hydrogenation are mainly used to purify ethylene.Pressure swing adsorption（PSA）is a potential gas separation method in which the core is adsorbent.Porous materials are excellent adsorbents because of their high porosity and easily modified framework.Porous organic frameworks（POFs）,an emerging family of porous materials,are conectted by covalent bonds between organic building blocks.They have shown great potential applications in many areas,such as gas separation,pollutant removal,photocatalysis,electrocatalysis,heterogeneous catalysis,proton conduction,and energy storage and conversion.Various POFs with different topologies can be prepared by many polymerization reactions and synthesis strategies.Due to the diversity of building blocks,POFs endowed functional groups have been surely prepared.In addition,functional groups can also be introduced into the POFs skeletons via appropriate post-modification strategies.By properly modifying and functionalizing POFs,the resultant functional POFs further exhibit greater application potential in many aspects.POFs can be classified into amorphous porous materials and crystalline porous materials.Recently,researchers prefer to prepare crystalline POFs,mainly because of its clear topological structure,which enables us to regulate the functionalization of materials at the molecular level and better study the role of POFs in many fields of application.Lately,more and more attention has been paid to the utilization of POFs for selective separation of ethylene.In this paper,we prepared a series of functionalized porous aromatic frameworks（PAFs）,evaluated their properties in selective adsorption of acetylene over ethylene and ethylene over ethane,and further explored the mechanism of those selective adsorption behaviors.A polyimide crystalline POFs,PAF-110,was prepared from 1,4,5,8-naphthalene tetracarboxylic anhydride and tri-（4-aminophenyl）amine via reversible imidization reaction,the acetylene/ethylene separation performance of PAF-110 was studied.Based on TEM observations,the hexagonal crystal structures of PAF-110 were constructed with AA and AB stacking models,and their powder X-ray diffraction patterns were simulated.By comparing the experimental results with the simulated powder diffraction patterns,it is found that AA stacking model is more reasonable.Stability experiments showed that PAF-110 not only possesses excellent thermal stability and chemical stability,but also exhibits outstanding long-term stability.Gas adsorption isotherms revealed that acetylene and ethylene adsorption capacities of PAF-110 were 2.23 and 1.29 mmol/g,respectively.PAF-120 has a higher affinity for acetylene molecules.Calculations predicted the electrostatic interaction between the carbonyl oxygen atom within PAF-110 and acetylene and ethylene hydrogen atoms,Results showed that binding energy between acetylene and PAF-110 is larger and acetylene bind closer to the framework of PAF-110.IAST calculated results showed that at 298K and1bar the equimolar acetylene/ethylene selectivity of PAF-110 is 3.9,which is the highest value among POFs for this separation.Breakthrough experiments exhibited that PAF-110 can effectively separate the two gases and its properties may provide some reference significance into utilization of POFs for selective separation of acetylene and ethylene.Based on the above work,we continued to explore whether this kind of materials can selectively separate acetylene and ethylene.A new stable polyimide crystalline POFs,PAF-120 was fabricated.Structural simulation shows that AA stacking model of PAF-120 is more reasonable.Nitrogen adsorption experiment showed that BET surface area of PAF-120 is 801m²/g,and pore size distributions（PSDs）showed that the pore diameter of PAF-120 is 2.9 nm,which is consistent with that of the simulated crystal structure.Static adsorption experiments of acetylene and ethylene showed that PAF-120 has great promise in separating these two gases.At 298 K and 1 bar,IAST calculation showed that the separation ratio of acetylene and ethylene is 4.1,which exceeds those of the reported organic porous materials for this separation.Meanwhile,the selectivity of carbon dioxide/nitrogen is up to 68.7.Therefore PAF-120 shows selective adsorption of acetylene over ethylene and carbon dioxide over nitrogen and has great potential in petrochemical and environmental protection.First,we synthesized the functionalized PAF-1,PAF-1-PO₃H₂.By utilizing silver tetrafluoroborate and silver nitrate for ion exchange,we further prepared two functionalized materials with different silver ion contents,P1PAg1 and P1PAg2.ICP results showed that the silver content within P1PAg2 is higher than that of P1PAg1.Ethylene and ethane adsorption isotherms show that P1PAg1 can selectively separate ethylene and ethane.IAST calculation results showed that the separation factors of P1PAg1 and P1PAg2 for ethylene and ethane are different.At 298K and 1bar,the ethylene/ethane selectivity of material P1PAg1 is 4.8,indicative of great potential in separating these two gases.π-complexation that occurs between silver ions and ethylene molecules allow P1PAg1 preferentially adsorb ethylene molecules,thus achieving the purpose of separating ethylene and ethane.The functionalized material P1PAg1 has great potential in selective separation of ethylene and ethane.