Cis/Trans Mononuclear Copper(II) Nodes with Dual Open-Metal and Lewis-Base Sites: A Metal–Organic Framework Enabling Selective CO₂ Capture from Flue Gas

The development of porous materials for the selective capture of CO2 from flue gas and biogas is crucial for ecological conservation and clean energy advancement. Herein, a novel three-dimensional copper-based metal-organic framework (Cu-MOF) was solvothermally synthesized by using a multifunctional ligand abundant in carboxyl and triazole groups. Inorganic secondary building units (SBUs) feature two types of square-planar mononuclear copper SBUs: a highly polar cis configuration and a symmetric trans configuration. This dual-SBU design maximizes the density of open-metal sites (OMSs) (2.84 per nm3) while simultaneously enhancing both polarity and structural stability. The ligand contains an abundance of uncoordinated N and O atoms, which furnish densely populated Lewis-base sites (LBSs) throughout the pores. The dual-site distribution of OMSs and LBSs present in two distinct polar channels results in a relatively high BET specific surface area, a substantial CO2 uptake capacity, and a CO2/N2 selectivity of 112. Breakthrough experiments reveal a dynamic CO2 uptake of 2.42 mmol g-1 per cycle with well-maintained recyclability. Grand-canonical Monte Carlo and DFT calculations reveal the synergistic binding of CO2 to both OMSs and LBSs, underscoring the practical potential for low-energy CO2 separation from flue gas and biogas.