| Natural gas primarily consists of methane is clean energy with increasing demand.The key step in natural gas purification is the separation of CO2/CH4.However,traditional thermodynamically selective adsorbents exhibit a strong affinity to carbon dioxide,thus requiring high energy cost for the regeneration of adsorbents.In contrast,kinetic separation of CO2/CH4 is preferred for the PSA process,but a long-standing challenge is to precisely control the aperture size in a critical range to achieve a tremendous discrepancy in diffusion rate.Here,we report a guest solvent-directed strategy for fine-tuning pore size within 0.2-0.4 A and to keep the pore,surface chemistry consistent.The thermodynamic and kinetic separation performance of a series of CuMOFs for CO2/CH4 was studied,revealing the relationship between diffusion and pore size,and expecting to promote the development of CO2/CH4 separation technology.Five kinds of one-dimensional ultra-microporous metal-organic frameworks were synthesized using bent ligands and divalent metal copper salts under different hydrothermal conditions.Their crystal structures,pore characteristics,and stability were characterized.The results showed that guest solvent-directed strategy can effectively change the linker at the top of the six-connected binuclear copper,causing the dihedral angle of the bent ligand to change.Therefore guest-solvent directed isomeric micropores that only differ in the aperture size of 0.2-0.4 A.The periodically expanded and contracted aperture of CuFMOF,CuFMOF·H2O,CuFMOF·CH3OH,and CuFMOF·DMF with the narrow bottleneck size of 3.2 × 3.5,3.2 ×3.7,3.5 × 3.7,and 3.3 × 3.9 A2,respectively.Simultaneously,CuFMOFs have apertures similar to the kinetic diameter of CO2(3.3 A)and CH4(3.8 A),and excellent water and thermal stability.The single-component adsorption equilibriums and kinetic curve of CO2 and CH4 and column breakthrough tests for mixture gas on CuFMOFs was determined.The results of Ideal Adsorbed Solution Theory(IAST)and adsorption enthalpies(Qst)revealed that CuFMOFs have barely significant differences in affinity for CO2 and CH4.The kinetic curves demonstrated that the diffusion rate of CO2 and CH4 increases with the increase of pore aperture.CuFMOF with the smallest pore aperture had the slowest diffusion rate,with a kinetic selectivity of 16.1 at 298 K;CuFMOF·DMF with the largest pore aperture had the fastest diffusion rate,with a kinetic selectivity of 36.1 at 298 K.The experimental results displayed that CuFMOF·CH3OH exhibits the best kinetic separation performance thanks to the optimal aperture size.The periodically expanded and contracted aperture with the ideal narrow bottleneck size of 3.5 × 3.7 A2 enables more effective trapping of CO2 and impedes the diffusion of CH4,offering ultrahigh kinetic selectivity(273.5)and equilibrium-kinetic combined selectivity(64.2)at 278 K,comparable to carbon molecular sieves(CMS-3A and BF-CMS comprehensive selectivities are 64.9 and 69.8,respectively),and exceeds most commercial adsorbents.At the same time,excellent cycle performance and easy regeneration of CuFMOFs both confirmed by the column breakthrough tests.The separation mechanism of CO2 and CH4 in the delicate one-dimensional apertures was studied at the molecular level by density functional theory,Monte Carlo,and molecular dynamics simulation.The simulation results illustrated that the diffusion energy barriers of CO2 and CH4 decrease with the increase of the pore aperture.CuFMOF with the smallest pore aperture had the highest CO2 and CH4 diffusion energy barriers(22.4 and 69.8 kJ/mol,respectively).While,the diffusion energy barrier of methane(38.4 kJ/mol)in CuFMOF-DMF with the largest pore aperture was reduced by 45%,indicating that the large pore is not conducive to the kinetic separation of carbon dioxide and methane,CuFMOF-CH3OH with optimal aperture had a high methane diffusion energy barrier and low carbon dioxide diffusion energy barrier(55.6 and 15.4 kJ/mol,respectively),featuring periodically outspread and contracted the cross-section of the channel which allows the entrance of the CO2 but severely hindering CH4 diffusion to achieve efficient kinetic separation. |
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