| Metal-organic frameworks(MOFs),as a new type of porous material with the tailored-to-application structural tunability,have aroused increasing interests as potential adsorbents in separation applications.One of the most common challenges in various MOF-based separation processes is that an increasing structural similarity of different adsorbates requires increasing elaboration of the pore structure to improve the separation selectivity.Currently,the vast majority of MOFs are composed of aromatic ligands.Although the easy derivatization and rigidity of aromatic rings contribute to easily tunable pores and good framework stability,their structural planarity and inherent chemical features represent a possible limitation and reduced variation in the pore structure of the materials.Herein,this work innovatively constructed several new MOFs based on poly cycloalkyl ligands,and realized precise refinements of the pore size,arrangement of adsorption sites,pore shape,and pore environment by utilizing the three-dimensional structure and the alkyl-rich chemical property of polycycloalkyl ligands,providing more variety in pore constructions.Targeting at the representative and challenging separation of monoatomic gases,alkane homologues,hexane isomers,C8 aromatic isomers,a variety of strategies have been proposed to enhance the adsorption separation processes,including the quasi-three-dimensional pore size refinement,alkyl-rich adsorption site arrangements,thermodynamic-kinetic synergetic effect,and molecular recognition by magnifying the difference between adsorbates.Moreover,as the separation difficulties increasing,these strategies are further realized by extending the single polycycloalkyl ligand to double polycycloalkyl ligands with more structure variety and precision,improving the efficiency of identifying mixtures with similar structures and properties,which provides certain guidance for the design of adsorbents.Utilizing the unique structural rigidity and three-dimensional characteristics of polycycloalkyl ligands,a quasi-three-dimensional pore refinement based on the tunability of both the size and number of aliphatic rings of polycycloalkanedicarboxylate ligands has been realized,which can simultaneously change the onedimensional length of the ligand and the accessible pore space in specific directions.Compared to the aromatic ligands with planar structures,polycycloalkyl ligands are expected to construct pores with higher dimensional accuracy.Based on this strategy,two hydrolytically stable isoreticular new aluminum-based MOFs,ZUL-C1 and ZULC2(ZUL=Zhejiang University Lab),were successfully synthesized and applied in xenon/krypton separation.Validated by X-ray diffraction,simulation,singlecomponent adsorption isotherms tests,and multi-component breakthrough experiments,an expansion of polycycloalkane moiety enables sub-angstrom contraction in specific directions and shortens the distance between the polar hydroxyl sites and xenon atoms.The resultant MOF,ZUL-C2,exhibits strong xenon(400 ppm)capture ability with the capture capacity of 27.4 mmol/kg from the breakthrough tests of used nuclear fuel(UNF)off-gas.Meanwhile,the good balance between xenon/krypton selectivity(19.1)and xenon capacity allows ZUL-C2 to also have an exciting separation performance for the air-separation byproduct mixture with the Kr productivity of 5.54 mmol/g.In addition,the ZUL-C2@HPC pellets prepared with hydroxypropyl cellulose maintain the excellent xenon/krypton separation performance,reflecting a promising application prospect.In addition to realizing the quasi-three-dimensional pore size refinement based on the three-dimensional characteristics of the polycycloalkyl ligand,its polyalkyl chemical properties also bring abundant alkyl adsorption sites to the pores.By replacing the polycycloalkyl ligand,the number of alkyl groups facing the channel can be increased and properly arranged,which enhances the van der Waals interactions with alkane molecules.This strategy significantly improves the affinity of ZUL-C2 to ethane and propane with low-concentration,and its ethane adsorption capacity at 1 kPa reaches 1.16 mmol/g,which is 3.2 times as large as that of ZUL-C1(0.36 mmol/g),surpassing all literature MOFs.The selectivities of ethane/methane and propane/methane at 298 K are 91 and 632,respectively,which are significantly higher than ZUL-C1(22 and 73)and also outperforms all existing benchmark porous materials.Both ZUL-C1 and ZULC2 exhibit dynamic exclusion of methane in the breakthrough experiments with the high-purity methane yields of 5.42 and 11.4 mmol/g,respectively.Besides,both ZULC1 and ZUL-C2 have excellent water stability,hydrogen sulfide stability and recyclability.The ZUL-C2@HPC pellets maintain the outstanding capture ability of ethane and propane,which is positive for practical application.In order to realize the efficient separation for hexane isomers with increasing structural similarity,double polycycloalkyl ligands are applied in MOF construction to break through the framework construction mode based on a single polycycloalkyl ligand and increase the tunability of the pore.Two isoreticular new pillared-layered MOFs,ZUL-C5 and ZUL-C6 were successfully synthesized.Through altering polycycloalkyl ligands,the pore size can be finely refined to be close to the molecular size of the 3-methylpentane,which significantly diminishes the diffusion rate of 2,2dimethylbutane with larger molecular size,thus enhancing the kinetic selectivities.Besides,the linear n-hexane and mono-branched 3-methylpentane could fit well with the cross-type pores at the straight section and the intersection,respectively,generating multiple van der Waals interactions with the abundant alkyl sites on the surface and enhancing the thermodynamic separation.Thus,the efficient separation of hexane isomers was realized based on the thermodynamic-kinetic synergetic effect.Both ZULC5 and ZUL-C6 could realize the approximate dynamic exclusion of 2,2dimethylbutane in breakthrough experiments,and the yields of high-purity 2,2dimethylbutane reach 0.66 and 0.77 mmol/g respectively,exceeding all materials reported so far.For the separation of C8 aromatic compounds,a molecular recognition strategy has been demonstrated to realize the precise discrimination of C8 aromatics by constructing non-aromatic confined pore environment with double polycycloalkyl ligands.The non-aromatic low-polar pore environment avoids strong convergent π-πinteractions between the framework and the common phenyl rings while creating possibilities to amplify the difference between host-guest/guest-guest interactions regarding the different methyl(ethyl)group positions of isomers(i.e.,the difference in molecular shape).Meanwhile,double poly cycloalkyl ligands create more possibilities to construct pores that suitable for C8 aromatics.The resultant pillar-layered MOF,ZUL-C3,undergoes structural transformations with the increase of C8 aromatic uptakes to be adapted to the loaded single/paired ortho-xylene molecules,realizing outstanding C8 separation performance with different configurations.As validated by guest-loaded single crystal structures analysis and simulation calculations,the difference in the methyl(ethyl)groups positions of isomers contribute to different host-guest interactions between isomers and the framework or different guest-guest interactions caused by different stacking mode of guest molecules,verifying the effectiveness of this separation strategy and encouraging researchers to apply it in other aromatic isomers separations. |
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