Tunable Microporous Bimetallic Carboxylate-Pyrazolate Metal–Organic Frameworks for CO ₂ 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 Cu 2+ or Zn 2+, while carboxylates bind Al 3+, generating chain-based inorganic building units built up from connected M 2+ -pyrazolate polyhedra and μ 2 –OH-corner-shared AlO 6 octahedra, respectively. The resulting structures feature dual ultranarrow tunnel-like pores, one decorated with μ 2 –OH groups. MIP-212(Al/Cu) combines pore confinement with Cu 2+ open metal sites (OMS) to deliver benchmark-level CO 2 uptake at low pressure (2.30 mmol g –1 at 0.15 bar, 298 K) and a CO 2 /N 2 Ideal Adsorbed Solution Theory (IAST) selectivity of ∼30. However, the OMS also imparts marked hydrophilicity, diminishing CO 2 uptake under humid conditions. Markedly, replacing octahedral Cu 2+ with tetrahedral Zn 2+ 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 H 2 O) and superior CO 2 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 CO 2 capture under realistic, moisture-rich environments.