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To expand the structural diversity and optimize the material performance, it is essential but challenging to regulate MOF structures in a predictable and controllable manner. In this work, by manipulating the substituents to engineer the ligand conformations, we designed and synthesized two asymmetric tricarboxylate ligands, and used them to successfully target two copper-tricarboxylate frameworks with diversified topologies depending on the ligand conformations. Besides, the ligand asymmetry induced the formation of two uncommon kinds of copper-carboxylate clusters, thus greatly expanding the library of copper-carboxylate secondary building units. Furthermore, the two compounds also displayed tunable gas adsorption properties pertinent to C2H2 separation and purification. At 298 K and 1 atm, the uptake capacity of C2H2 varies from 79.5 to 104.6 cm3 (STP) g-1, while the adsorption selectivities of C2H2 with respect to CO2 and CH4 are in the range of 2.3-3.8 and 15.3-21.6 for the equimolar components, respectively. Compared to the nitro counterpart, the methoxy MOF features higher C2H2 uptake capacity, larger C2H2/CO2 and C2H2/CH4 adsorption selectivities, and lower regeneration energy. This work demonstrates that simple ligand modification can be used to engineer the structures and tune gas adsorption properties of the resulting MOFs.

The design and construction of porous materials for high-efficient adsorption-based separation of CH

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 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.

Ssa-type MOFs constructed from dicopper paddlewheels and bent diisophthalate ligands exhibit a promising potential for gas adsorption which benefits from their rich open copper sites and polyhedron-based cages with suitable sizes. However, the rational construction of such types of MOFs is exceedingly challenging because the bent diisophthalate ligands employed are inclined to exhibit various conformations and thus are prone to form MOFs with varied topologies. In this work, by pre-organizing the ligand's conformation, we successfully targeted an ssa-type MOF ZJNU-57 from a bent diisophthalate ligand. More significantly, ZJNU-57 exhibits excellent hydrolytic stability and high C2H2 and CO2 uptake capacities as well as impressive C2H2/CH4 and CO2/CH4 adsorption selectivities, indicating its promising potential for C2H2/CH4 and CO2/CH4 separation, which are relevant to acetylene production and natural gas purification. This work not only provides a rare water-stable MOF based on the Cu2(COO)4 cluster for highly selective adsorption of C2H2 and CO2 from CH4, but also demonstrates that the ligand conformation-controlled assembly strategy may be an efficient approach toward the construction of MOF materials with definite topologies for specific applications.