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The design and construction of porous materials for high-efficient adsorption-based separation of CH4 from C2H2 and CO2 is a fundamental goal in the chemical industry. In this work, a lactam-functionalized bent diisophthalate linker, 5,5'-(1-methylpyridin-2(1H)-one-3,5-diyl) diisophthalic acid, was judiciously designed and utilized to construct a dicopper paddlewheel-based MOF under solvothermal conditions. Single-crystal X-ray diffraction revealed that the resulting solid is a three-dimensional mfj-type network incorporating Lewis-acidic copper sites and accessible lactam groups as two different kinds of functional sites. Furthermore, gas adsorption studies indicated that the material exhibited great promise for C2H2/CH4 and CO2/CH4 separations. In particular, compared to the unmodified parent compound, the resultant MOF exhibits 9.4% and 12.9% higher C2H2 and CO2 uptake capacities and 48.0% and 28.3% higher C2H2/CH4 and CO2/CH4 adsorption selectivities for the equimolar gas mixtures at 298 K and 1 atm, despite the lower specific surface area and pore volume. This work reported the first lactam-functionalized MOF displaying significantly enhanced adsorption of CO2 and C2H2 over CH4.

Evaluating the effect of the substituents on structural stabilities and gas adsorption properties of MOFs is fundamentally important for rational design and synthesis of new MOFs with better performance. For this purpose, three isoreticular copper-based MOFs (ZJNU-87, ZJNU-88 and ZJNU-89) with ssa-type topology were successfully constructed from bent diisophthalate ligands bearing different substituents. Permanent porosity studies reveal that the substituent has a significant effect on framework stabilities against desolvation. Utilizing nonpolar n-hexane as an activation solvent can give more optimized permanent porosity for nonpolar or less-polar substituent-modified ZJNU-88 and ZJNU-89. Furthermore, their gas adsorption properties with respect to C2H2, CO2 and CH4 were systematically investigated, revealing their promising potential for selective C2H2/CH4 and CO2/CH4 separations. In particular, methoxy-modified ZJNU-89 performs better than the methyl-modified ZJNU-88 in terms of uptake capacity and adsorption selectivity, which might be attributed to more suitable pore space of ZJNU-89.

Three copper-based ligand-originated MOF isomers (ZJNU-81, ZJNU-82 and ZJNU-83) derived from methyl-functionalized V-shaped diisophthalate ligands were solvothermally synthesized and structurally characterized. Single-crystal X-ray diffraction studies revealed that the position of the methyl group in the central phenyl spacer of diisophthalate ligands exerted a significant effect on determining the final structures of the resulting MOFs, which was rationalized to result from the steric effect imposed by the methyl group controlling the conformational structures of the ligands during the self-assembly process. Furthermore, their gas adsorption properties with respect to C2H2, CO2, and CH4 were systematically investigated and comparatively analyzed. Gratifyingly, the three MOFs exhibited respectable C2H2 and CO2 uptake capacities as well as impressive C2H2/CH4 and CO2/CH4 adsorption selectivities. Although the disparity is small, the different gas uptake capacities and adsorption selectivities exhibited by the three MOFs demonstrate that the position of the methyl group has a certain impact on gas adsorption properties. This work not only reported three MOFs with promising potential for C2H2/CH4 and CO2/CH4 separations, but also more importantly afforded a fundamental understanding of the positional effect of functional groups on the structures and gas adsorption properties of the resulting MOFs, which provides valuable guidance for future design and synthesis of porous MOFs displaying better performance.

The combination of an angular diisophthalate ligand, 5,5′-(naphthyl-2,7-yl)diisophthalate (H4L), and copper ions under different solvothermal conditions afforded two polymorphous metal–organic frameworks (ZJNU-77 and ZJNU-78) with the same framework composition of [Cu2(L)(H2O)2], providing a platform to investigate the relationship between MOF polymorphism and gas adsorption properties. As determined by single-crystal X-ray diffraction, ZJNU-77 and ZJNU-78 exhibited three-dimensional networks crystallizing in different space groups. Their structural differences were mainly manifested by the ligand's conformation, the level of framework interpenetration and the network's topology. Interestingly, gas adsorption studies showed that the two compounds after desolvation displayed comparable gas adsorption properties with respect to C2H2, CO2 and CH4, despite their different surface areas and pore volumes. The C2H2, CO2, and CH4 uptake capacities at 298 K and 1 atm are 120.2, 78.1, and 18.4 cm3 (STP) g−1 for ZJNU-77, and 122.0, 82.0, and 18.9 cm3 (STP) g−1 for ZJNU-78, respectively. The IAST adsorption selectivities for the equimolar C2H2/CH4 and CO2/CH4 mixtures are 28.6 and 5.7 for ZJNU-77, and 28.4 and 5.9 for ZJNU-78 at 298 K and 1 atm. These results indicate that besides the surface area, the pore size also plays a crucial role in gas adsorption. This work not only represents an intriguing example of MOF polymorphism achieved by controlling solvothermal conditions, but also provides an insight into the correlation between MOF polymorphism and gas adsorption properties.

The ligand truncation strategy provides facile access to a wide variety of linkers for the construction of MOFs bearing diverse structures and intriguing properties. In this work, we employed this strategy to design and prepare a novel bent diisophthalate ligand, and used it to successfully construct a copper-based MOF ZJNU-51 with the formula of [Cu2L(H2O)2]·5DMF (H4L = 5,5'-(triphenylamine-4,4'-diyl) diisophthalic acid), which was thoroughly characterized by various techniques including FTIR, TGA, PXRD and single-crystal X-ray diffraction. ZJNU-51 is a two-fold interpenetrated network in which the single network consists of dicopper paddlewheel units connected by the organic ligands and contains open channels as well as six distinct types of metal-organic cages. Furthermore, gas adsorption properties with respect to C2H2, CO2, and CH4 were systematically investigated, demonstrating that ZJNU-51 is a highly promising material for C2H2/CH4 and CO2/CH4 separations. Specifically, the IAST adsorption selectivity at 298 K and 1 atm reaches 35.6 and 5.4 for the equimolar C2H2/CH4 and CO2/CH4 gas mixtures, respectively. More significantly, as revealed by PXRD and N2 adsorption measurements, ZJNU-51 exhibits excellent chemical stability, which lays a good foundation for its practical application.