Solvothermal Metal Metathesis on a Metal–Organic Framework with Constricted Pores and the Study of Gas Separation

Metal–organic frameworks (MOFs) with constricted pores can increase the adsorbate density of gas and facilitate effective CO₂ separation from flue gas or natural gas due to their enhanced overlapping of potential fields of the pores. Herein, an MOF with constricted pores, which was formed by narrow channels and blocks of functional groups, was fabricated from the assembly of a methyl-functionalized ligand and Zn­(II) centers (termed NPC-7-Zn). Structural analysis of the as-synthesized NPC-7-Zn reveals a series of zigzag pores with pore diameters of ∼0.7 nm, which could be favorable for CO₂ traps. For reinforcing the framework stability, a solvothermal metal metathesis on the pristine MOF NPC-7-Zn was performed, and a new Cu­(II) MOF (termed NPC-7-Cu) with an identical framework was produced. The influence of the reaction temperatures on the metal metathesis process was investigated. The results show that the constricted pores in NPC-7-Zn can induce kinetic issues that largely slow the metal metathesis process at room temperature. However, this kinetic issue can be solved by applying higher reaction temperatures. The modified MOF NPC-7-Cu exhibits significant improvements in framework stability and thus leads to a permanent porosity for this framework. The constricted pore structure enables enhanced potential fields for these pores, rendering this MOF with high adsorbate densities for CO₂ and high adsorption selectivity for a CO₂/N₂ gas mixture. The adsorption kinetic studies reveal that CH₄ has a faster diffusion rate constant than CO₂, showing a surface diffusion controlled mechanism for CO₂ and CH₄ adsorption