| Separation is the most energy-consuming and investment-intensive process in the chemical industry.Efficient separation and purification of mixtures with extremely similar molecular structures and physical properties is a critical but challenging industrial process.Adsorptive separation,as a supplementary or alternative technology to the conventional heat-driven process,can greatly reduce the energy consumption and lower production cost for these chemical commodities.Metalorganic frameworks(MOFs)containing open metal sites have typical Lewis acid sites,which can induce molecular polarization and enhance the interaction with guest molecules.However,factors such as poor water or thermal stability and hard to scaleup production cost greatly limit their applications in industrial separation.In this work,Hoffmann-type and benzoquinone-based MOFs with planar open metal sites are introduced to construct MOFs with the characteristics of chemical stability and easy scale-up and molding preparation,for the adsorption and separation of noble gases,C5-C8 hydrocarbon isomers with extremely similar properties.Besides,the separation performance and adsorption mechanism of these molecules were systematically studied,which provided certain contribution to the industrial application of MOFs materials with theoretical and technical support.Hoffmann-type MpzNi(M=Co,Ni)materials were successfully prepared by using the metalloligand potassium tetracyanonickelate and pillared ligand pyrazine based on the metal center replacement strategy.Their structures were confirmed by Rietveld refinement of powder X-Ray diffraction,and thermodynamic separation performance of Xe/Kr separation performance on MpzNi samples was systematically evaluated.The frameworks have abundant planar open metal sites and suitable pore size,which can synergistically and efficiently discriminate Xe molecules based on surface polarization and confinement effects.The adsorption capacities of CopzNi and NipzNi materials for Xe were as high as 2.49 and 1.88 mmol/g at low pressure of 20 kPa and 298 K,and the 20/80 Xe/Kr IAST selectivity reached 14.0 and 10.8,which are comparable to the famous Co3(HCOO)6(12)and SBMOF(16).The separation performance achieved a balance between adsorption capacity and separation selectivity.The fixed-bed breakthrough experiments directly proved that the MpzNi material has excellent separation performance for the Xe/Kr mixtures.The dynamic adsorption capacity of CopzNi for Xe reached 2.35 mmol/g,ranking first among the reported adsorbents.Grand canonical Monte Carlo(GCMC)simulations and Firstprinciples density functional theory(DFT)calculations showed that the strong interactions between Xe molecule and MpzNi at the molecular level are the result of the synergistic effect of suitable pore size and adjacent open metal sites.Considering the abundance of planner open metal sites in the MpzNi materials,it is expected to utilize the differences in electrostatic potential among pentadiene isomers for their adsorption separation.The single-component adsorption isotherm curves showed that the trans-piperylene adsorption capacities of CopzNi and NipzNi materials are as high as 2.09 and 1.87 mmol/g at 303 K and extremely low pressure of 5 torr,which are comparable to the benchmark porous material zeolite 5A(2.25 mmol/g,298 K).The Henry selectivity of CopzNi for trans-piperylene/isoprene,cispiperylene/isoprene,and trans-/cis-piperylene are 215.7,76.9,and 2.8,respectively.The separation performances were confirmed by fixed-bed breakthrough tests and were better than zeolite 5A which can only separate isoprene.The dipole-dipole(Niδ+…Cδ-)and multiple hydrogen bonds(C-Hδ+…Nδ-)as well as the π-π stacking interaction between the electric rich pentadiene isomers and the aromatic pyrazine ligands.On this basis,the separation and purification performance of MpzNi for hexane isomers was further explored.The single-component adsorption isotherms showed the MpzNi samples exhibited size sieving effects on 22DMB at all tested temperatures,and the nHex and 2MP guests exhibited different adsorption behaviors at different temperatures due to the affinity differences.Based on the IAST model,the selectivities of CopzNi and NipzNi to the equimolar ratio of 2MP/22DMB at 303 K were 420.0 and 27.3,respectively,and the selectivities of the equimolar ratio of nHex/2MP at 363 K were 171.1 and 73.7,respectively,indicating that CopzNi material has a very desirable ability to capture nHex at 363 K and separate 2MP at 303 K.The fixed-bed breakthrough experiment confirmed the excellent separation performance of the multi-component equimolar hexane isomer.CBMC simulations and DFT calculations revealed that the Niδ+…Cδ-dipole-dipole and C-Hδ+…Nδsupramolecular interactions played important roles in the frameworks.The linear nHex fits better with one-dimensional pores and can interact with more binding sites in the frameworks,which is the main reason for the affinity differences between nHex and 2MP.In view of the excellent separation performance of planar open metal sites,we further designed and constructed Zn-dhbq framework material based on 2,5dihydroxy-1,4-benzoquinone organic ligand,and its crystal structure was solved through rietveld refinement of powder X-Ray diffraction.The activated Zn-dhbq contains abundant planar open metal sites and aromatic benzoquinone ligands,which can provide strong adsorption binding sites for xylene molecules.The adsorption isotherm curves showed the Zn-dhbq was nearly size-exclusive for MX and OX at all temperatures,while having different gate opening pressures for PX at different temperatures,and had the adsorption capacity of 220 mg/g for PX at 303 K.The fixed-bed breakthrough tests further demonstrated Zn-dhbq material can efficiently separate PX from the xylene isomers.The π-π stacking formed by aromatic benzoquinone ligands and xylene molecules,the dipole-dipole interactions between open metal sites and methyl carbon,and the hydrogen bonds built from xylene molecules with the oxygen atoms in the ligands. |
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