Immobilization of N-oxide functionality into NbO-type MOFs for significantly enhanced C2H2/CH4 and CO2/CH4 separations

Abstract

NbO-type MOFs built up from linear diisophthalate ligands and dicopper paddlewheel-based secondary building units offer an excellent platform to perform pore surface chemistry engineering and understand the structure–property relationship. In this work, we designed and synthesized two N-oxide functionalized linear diisophthalate ligands, and employed them to construct under suitable solvothermal conditions their corresponding NbO-type MOFs termed ZJNU-19 and ZJNU-20. Their gas adsorption properties with respect to C₂H₂, CO₂, and CH₄ were systematically measured, and adsorption selectivities of C₂H₂ and CO₂ over CH₄ were assayed using a well-known ideal adsorbed solution theory, establishing their promising potential for C₂H₂/CH₄ and CO₂/CH₄ separations in connection with acetylene and natural gas separation and purification, which were found to be less dependent on the methyl position. In particular, at atmospheric pressure, the C₂H₂ and CO₂ uptake capacities reach as high as 214.5 and 114.7 cm³ (STP) g⁻¹ for ZJNU-19, and 210.0 and 111.9 cm³ (STP) g⁻¹ for ZJNU-20 at 295 K, while the IAST-predicted C₂H₂/CH₄ (v/v = 1/1) and CO₂/CH₄ (v/v = 1/1) adsorption selectivities are up to 42.2 and 6.4 for ZJNU-19, and 42.1 and 6.2 for ZJNU-20 at 298 K. Furthermore, a comparative investigation demonstrated that N-oxide is a powerful chemical functionality that can be utilized to design and construct porous framework compounds for boosting C₂H₂ and CO₂ adsorptions.