Three ligand-originated MOF isomers: the positional effect of the methyl group on structures and selective C2H2/CH4 and CO2/CH4 adsorption properties

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.