Study On Fine Porous Structures Of MOFs And Separation Performance Of C2 Hydrocarbons

C2 hydrocarbons（such as ethylene,acetylene and ethane）are very important basic chemical raw materials.The purity of these gases has a crucial impact on the production of target products（such as polyethylene）.It is necessary to obtain high purity ethylene,acetylene or ethane.At present,cryogenic distillation is the main technology used to separate light hydrocarbons in industry,which is expensive and high energy consumption.The physical adsorption separation technology based on solid porous materials has the advantages of simple process,high product purity and low cost,which is expected to gradually replace the traditional cryogenic distillation.As a new type of solid porous materials,metal-organic frameworks（MOFs）have many advantages,such as specific surface area,high porosity,uniform pore size and easy to adjust pore structure,which have great potential in the field of gas adsorption storage and separation.Therefore,it is of great significance to develop MOFs for the separation of C2 hydrocarbons（such as C₂H₂/CO₂,C₂H₂/C₂H₄,C₂H₄/C₂H₆,etc.）.In view of this,starting from the design and synthesis of MOF materials,a series of MOF materials with special pore structure were designed and prepared.Their separation and purification performance for C2 light hydrocarbons were studied and investigated.The structure-activity relationship between the porous fine structure of these MOFs materials and the separation performance for C2hydrocarbons was clarified,which provided a theoretical basis for the industrial application of MOFs materials in the separation and purification of C2 light hydrocarbons.1.an amine-functionalized metal-organic framework,CPL-1-NH₂,was designed and synthesized.Based on the occupation of amino groups in the pore space,the adsorption site selective occupation strategy was proposed and verified for screening C₂H₂/CO₂ mixtures.The results show that CPL-1-NH₂ has a high C₂H₂/CO₂ uptake ratio（8.6）and excellent IAST selectivity（119）at 298 K and 1 bar,which is only lower than UTSA-300a（743）.Comparing the single component adsorption of acetylene and carbon dioxide on CPL-1 and CPL-1-NH₂,it can be found that the modified amino group enhances the adsorption capacity of C₂H₂ on CPL-1-NH₂,and reduces the adsorption capacity and capacity of CO₂.DFT-D calculation results show that the synergistic effect of amino and uncoordinated oxygen atoms enhances the adsorption of acetylene by CPL-1-NH₂.At the same time,the modified amino groups occupy the adsorption sites of carbon dioxide and shield the adsorption of uncoordinated oxygen atoms near the amino groups,which reduces the adsorption capacity of carbon dioxide and the interaction with carbon dioxide.The breakthrough experiments of C₂H₂/CO₂ mixtures in fixed bed shows that CPL-1-NH₂ is an ideal material for C₂H₂/CO₂ separation.The separation performance of CPL-1-NH₂ for C₂H₄/C₂H₆ system was studied.The results showed that the amino group enhanced the interaction between the framework and ethylene,resulting in the"gate-opening"phenomenon,and reduced the adsorption capacity of ethane,which promoted the separation of C₂H₄/C₂H₆.At 273 K,the adsorption capacity of CPL-1-NH₂ for C₂H₄ is 1.65mmol g^-1,which is about 13.8 times that of C₂H₆(0.12 mmol g^-1)at 1000 mbar.The IAST selectivity of CPL-1-NH₂ to C₂H₄/C₂H₆ mixtures was 12.7.breakthrough experiments show that CPL-1-NH₂ can effectively purify C₂H₄/C₂H₆ mixtures.2.The adsorption properties of fluorine functionalized MOF material Zn-FBA for acetylene,ethylene and ethane and the separation properties of C₂H₆/C₂H₄ and C₂H₂/CO₂ were studied.The adsorption capacity and the isosteric enthalpy of adsorption of Zn-FBA for acetylene,ethylene and ethane showed the opposite order,C₂H₆>C₂H₄>C₂H₂.DFT-D calculations show that due to the larger molecular size and more hydrogen atoms of ethane,there are many C-H…F hydrogen bonding with shorter distance between Zn-FBA and C₂H₆ molecules.Such reverse order adsorption will facilitate the separation of C₂H₄ from C₂H₆/C₂H₄ mixtures.The calculated IAST selectivity of Zn-FBA for C₂H₆/C₂H₄ mixtures with ratios of 50/50,10/90 and 1/15 are2.9,3.0 and 3.0 respectively,indicating that Zn-FBA has high ethane selectivity.Further kinetic analysis shows that the diffusion rate of ethane in Zn FBA channel is lower than that of ethylene,which affects the efficiency of C₂H₆/C₂H₄ breakthrough experiments and leads to obvious tailing phenomenon.Polymerization grade ethylene gas can still be obtained through Zn-FBA bed.Zn-FBA exhibited good C₂H₂/CO₂ separation performance.The IAST selectivity of C₂H₂/CO₂ was 9.1 at 298 K and 1 bar,which was higher than that of most MOFs reported so far.The breakthrough experiments further verified its selective separation performance for C₂H₂/CO₂.3.The separation properties of C₂H₂/C₂H₄ and C₂H₂/CO₂ of Cd-TZ were studied.Cd-TZ showed good acetylene adsorption capacity.The adsorption capacity of acetylene at 298 K was3.10 mmol g^-1 at ambient pressure.At 298 K and 1 bar,the IAST selectivity of Cd-TZ for C₂H₂/C₂H₄（50/50 and 1/99）is 13.3 and 7.3 respectively,which is higher than that of most MOFs reported in the literature.The optimal adsorption sites of acetylene and ethylene in the framework of Cd-TZ were obtained by DFT-D calculations.It was found that the cage of Cd-TZ and the polar sites of nitrogen atom on the surface of channel were very suitable for the adsorption of acetylene.The IAST selectivity of C₂H₂/CO₂（50/50）is 8.3 at 298 K and 1 bar.DFT-D calculation results show that the optimal adsorption site of CO₂ moelcules is near the center of the channel.Therefore,the interaction between carbon dioxide and pore surface is very weak.These results indicate that Cd-TZ has a good application prospect in the separation of C₂H₂/C₂H₄ and C₂H₂/CO₂.