| The separation of CH4-N2 and SF6-N2 is of great significance to the conservation of resources and the protection of the environment.As a new kind of porous material,metal-organic frameworks(MOFs)have exhibited potential applications in gas separation owing to the good designability as well as structural and chemical adjustability.However,the current common problem is that the separation selectivity or adsorption capacity is relatively low,which can not meet the industrial requirements,and the sorbent selection parameter(SSP)value is a comprehensive index to evaluate the separation performance of adsorbents,so the SSP value of most adsorbents for a specific mixed system is relatively low.Therefore,several MOF materials were synthesized in this work,and the separation properties of CH4-N2 and SF6-N2 were studied.The research results of this paper are summarized as follows:(1)A three-dimensional Cu-MOF with two different types of micropores was synthesized,exhibiting a high selectivity for CH4-N2(10.00-12.67)and the highest sorbent selection parameter value(65.73)among the reported materials.The CH4 molecule interacts with the framework to form multiple van der Waals interactions both in hydrophilic and hydrophobic pores,indicated by density functional theory calculations to gain a deep insight into the adsorption binding sites.In contrast,the weak polarity feature of the hydrophobic pore and the occupied open metal sites in the hydrophilic pore result in a very low adsorption uptake of N2.The excellent separation performance combining the good stability and regenerability guarantees this Cu-MOF to be a promising adsorbent for an efficient separation of the CH4-N2 mixture.(2)A strategy was proposed to construct specific cages using an aliphatic ligand with suitable size to preferentially adsorb CH4 molecules in metal-organic frameworks(MOFs).As a result,a series of MOFs with trans-1,4-cyclohexanedicarboxylicacid(H2CDC)as ligand and different metal centres(M-CDC,M=Al、Cu and In)exhibit excellent separation performances for CH4-N2 mixture,including the higher selectivity(13.1-16.69)and sorbent selection parameter(SSP)than those in all of the reported porous materials,to the best of our knowledge.Especially for the integrated descriptor SSP,the value of Al-CDC is up to 82.0,which is at least 2～3 times higher than those in the reported materials.Breakthrough experiments indicate that CH4-N2 can be completely separated using the packed column of Al-CDC.Theoretical calculations confirmed the preferential trap of CH4 in the cages formed by the aliphatic ligand.In addition,the separation performance can be well maintained even after at least 10 cycles and the treatment of the MOF sample with boiling water,acid and base solution.These results not only reserve promising candidates for CH4-N2 separation,but also provide useful information for the separation of other weak adsorbates with similar properties.(3)A kind of metal-organic framework material,Cu-MOF-OCH3,with different pore structures was synthesized.The material has the double functionalization characteristics of open metal sites and methoxy,and shows good hydrothermal stability and SF6 regeneration performance.The material has excellent comprehensive separation performance.The separation selectivity(361)and the adsorbent selectivity parameter(SSP)value(780)at 298 K and 1 bar are much higher than those reported in the literature.Theoretical calculation shows that the synergistic effect of open metal sites in hydrophilic channel and abundant methoxy group in hydrophobic channel is the main reason for the excellent SF6-N2 separation performance of the MOF material.The results can provide reference for the design and development of SF6-N2 high efficiency separation materials. |
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