Water Vapor Sorption in Hybrid Pillared Square Grid Materials

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Summary:
The authors from University of Limerick, Bernal Institute developed a family of hybrid ultramicroporous SIFSIX-pillared square-grid materials (SIFSIX-1-Cu, ‑2-Cu-i, ‑3-Ni, ‑14-Cu-i) with reversible/irreversible water-vapor-induced phase switching, achieving benchmark hydrolytic stability and recyclability for gas-separation applications.

Background:
1. To address moisture instability in porous MOFs/PCPs, previous researchers introduced hydrophobic ligands or post-synthetic modifications, yet most materials still collapse or lose porosity under humid conditions.
2. The authors proposed an innovative comparison of four isoreticular SIFSIX-pillared pcu networks to uncover structure–hydrolytic-stability relationships and identify a robust candidate (SIFSIX-2-Cu-i).

Research Content:
1. Synthesis: Solvothermal reaction of Cu²⁺ or Ni²⁺ hexafluorosilicate with 4,4′-bipyridyl, 1,2-bis(4-pyridyl)acetylene, pyrazine, or 1,2-bis(4-pyridyl)diazene in MeOH/EG at 120 °C.
2. Characterizations:
   1) BET: ultramicropores (≤0.7 nm), Langmuir areas 350–550 m² g⁻¹ (77 K N₂).
   2) PXRD & SCXRD confirm pcu topology; after humidity exposure SIFSIX-1-Cu & ‑14-Cu-i convert to non-porous sql-c*, whereas ‑2-Cu-i remains intact.
   3) Dynamic vapor sorption (DVS, 298 K): reversible negative water adsorption (–1.7 to –2.6 mmol g⁻¹) at 40–50 % RH for SIFSIX-1-Cu, ‑3-Ni, ‑14-Cu-i; type-V isotherm for ‑2-Cu-i (14.6 mmol g⁻¹ at 90 % RH).
3. Application: CO₂/N₂ separation at 298 K; SIFSIX-2-Cu-i retains full capacity after 14 d at 75 % RH & 45 °C; SIFSIX-3-Ni withstands 10 humidity/thermal cycles without loss.
4. Mechanism: Weaker Cu–F(pillar) bonds and linker–linker repulsion (diazene stacking) in SIFSIX-14-Cu-i facilitate pillar displacement by H₂O → sql-c*. Strong CH···π “locks” in SIFSIX-2-Cu-i prevent displacement, imparting moisture resistance.

Outlook:
The study delivers design rules—linker rigidity, weak non-covalent “locks”, and metal–pillar bond strength—to craft next-generation moisture-tolerant porous sorbents for real-world CO₂ capture and noble-gas separations.

Water Vapor Sorption in Hybrid Pillared Square Grid Materials
Authors: Daniel O’Nolan, Amrit Kumar, Michael J. Zaworotko
DOI: 10.1021/jacs.7b01682
Link: https://pubs.acs.org/doi/10.1021/jacs.7b01682

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