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.