A pair of polymorphous metal–organic frameworks based on an angular diisophthalate linker: synthesis, characterization and gas adsorption properties

Abstract

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.