Absolute Separation Of Ethylene And Ethane Using Bimetallic Ions Exchanged Industrial Zeolites

As one of the largest chemical products in the world,ethylene plays an important role in the development of national economy and society.And ethylene production is an important indicator of a country’s petrochemical development level.Ethylene mainly comes from petroleum cracking,which produces a large number of mixed gases of ethylene and ethane.At present,cryogenic distillation（0.7-2.8 MPa,183-258 K）is mainly used in industry to separate such mixed gases.However,due to the close boiling points（-103.9°C and-88.6°C,respectively）between ethylene and ethane,cryogenic distillation process has low efficiency and high energy consumption.Adsorption,as a separation technology at room temperature,has the advantages of low energy consumption and simple equipmentwhich is expected to be a substitute or supplementary technology for ethylene/ethane separation,and its core is high-efficiency adsorbent.As a stable traditional industrial adsorbent,molecular sieves have been widely used in the field of adsorption and separation.However,due to the similar molecular physical and chemical properties of ethylene and ethane,the highly selective adsorption separation,especially the highly selective（∞）absolute separation,is facing great challenges.Therefore,it is of great theoretical and practical value to separate ethylene and ethane using ion-exchanged industrial molecular sievesIn this work,zeolite 4A with pore size close to the molecular diameters of ethylene and ethane（4.163 and 4.443?,respectively）was chosen as research object.And we investigated the effects of pore size and surface chemistry of Ag-Ca-4A controlled by bimetallic ion exchange method.XRD and SEM characterization confirmed that the crystal structure and particle size of the sample were basically unchanged during the twice ion exchange process,while the elemental mapping analysis of TEM verified that the elements such as Ag,Na,Ca were evenly distributed on Ag-Ca-4A.ICP elemental analysis found that the silver content in Ag-Ca-4A is only 5.84%,much lower than the content ofπ-complexation metal ions inπ-complexation sorbents reported in the literature.The chemical composition and distribution of different metal ions of ion-exchanged molecular sieves were further calculated,and the influence of the type and quantity of exchangeable metal ions on the pore sizes of molecular sieves was investigated.In this work,we tested the performance of bimetallic ion-exchanged Ag-Ca-4A for absolute separation of ethylene and ethane.At room temperature and atmospheric pressure（298 K,1 bar）,the adsorption capacity of Ag-Ca-4A for ethylene and the IAST-predicted selectivity of ethylene/ethane can reach as high as 3.7 mmol/g and 17568（IAST selectivity）,respectively.At the same time,Ag-Ca-4A hardly absorbs ethane,showing excellent separation performance of ethylene and ethane.Fixed bed experiments further showed that Ag-Ca-4A can achieve nearly absolute separation of ethylene and ethane at room temperature and atmospheric pressure,so as to recycle ethylene efficiently.The isosteric heat of adsorption for C₂H₄ ranges from 45 to 65 k J/mol in Ag-Ca-4A,which is lower than otherπ-complexation adsorbents.Kinetic adsorption experiments confirmed that the existence of Ag⁺will greatly promote the kinetic adsorption rate of ethylene on the materials.In addition,Ag-Ca-4A has stable multiple cycle regeneration performance,simple and controllable preparation method and good industrial application potential.In this work,the influence of ion concentration and initial diameters of zeolites on the absolute adsorption of ethylene/ethane was investigated.It was revealed that the excellent separation performance of Ag-Ca-4A for ethylene/ethane was due to the synergistic effect of C₂H₄ diffusion,a small amount of Ag⁺exchanges to finely tune the pore size of the zeolite so as to exclude C₂H₆,meanwhile strengthen C₂H₄ adsorption affinity for improving the ethylene adsorption capacity;it was also found that the introduction of Ag⁺promoted the kinetic adsorption rate of ethylene in the confined pore.In addition,in this work,the adsorption process of ethylene molecules on Ag-Ca-4A is simulated.The results showed that in the microporous confined pores,Ag⁺induced the stretching of the C-H bond and reduction of H-C-H bond angle of the C₂H₄ molecule,the C=C bond lengthens from 1.32?to 1.4?,while the H-C-H bond angle reduces from 120°to 112°,the length of ethylene along its Y axis also decreased from 4.29?to 4.20?,provideing a good kinetic condition for ethylene molecules to enter the microporous channel quickly.