Tunable Microporous Bimetallic Carboxylate-Pyrazolate Metal-Organic Frameworks for CO₂ Capture

Herein, we report two heterometallic ultramicroporous metal-organic frameworks, MIP-212(Al/Cu) and MIP-212(Al/Zn) (MIP stands for Materials from Institute of Porous Materials of Paris), synthesized via a hard-soft acid-base design strategy. In these robust pyrazolate-carboxylate architectures, pyrazolates selectively coordinate Cu²⁺ or Zn²⁺, while carboxylates bind Al³⁺, generating chain-based inorganic building units built up from connected M²⁺-pyrazolate polyhedra and μ₂-OH-corner-shared AlO₆ octahedra, respectively. The resulting structures feature dual ultranarrow tunnel-like pores, one decorated with μ₂-OH groups. MIP-212(Al/Cu) combines pore confinement with Cu²⁺ open metal sites (OMS) to deliver benchmark-level CO₂ uptake at low pressure (2.30 mmol g^-1 at 0.15 bar, 298 K) and a CO₂/N₂ Ideal Adsorbed Solution Theory (IAST) selectivity of ∼30. However, the OMS also imparts marked hydrophilicity, diminishing CO₂ uptake under humid conditions. Markedly, replacing octahedral Cu²⁺ with tetrahedral Zn²⁺ centers in MIP-212(Al/Zn) suppresses OMS while preserving framework topology, resulting in significantly lower water affinity (up to ca. 4-fold reduction at 0.2 bar of H₂O) and superior CO₂ breakthrough performance at 50% RH. These findings demonstrate that metal coordination geometry is a powerful lever to modulate hydrophilicity and sorption behavior in MOFs, enabling the rational design of sorbents for efficient CO₂ capture under realistic, moisture-rich environments.