Defect Engineering in the Porous Salt CAU-55: The Relationship between Cluster Defects and Sorption Properties

While defect chemistry is well established for metal–organic frameworks (MOFs), the nature and role of defects in porous salts remain unexplored. Here, we report on the synthesis of the porous salt CAU-55-Cl ([Al₂₄(OH)₅₆(O₂CCH₃)₁₂]Cl₄), which exhibits different sorption properties due to varying defect concentrations that can be achieved by adjusting the molar ratios of the starting materials.

Infrared spectroscopy, elemental analysis and thermogravimetric measurements confirmed that the composition of the framework remained unchanged, suggesting that variations in porosity arise from cluster defects (missing Al₂₄ clusters) rather than ligand defects (missing acetate ligands) in CAU-55-Cl. Ar (87 K), N₂ (77 K), CO₂ (273 K), and H₂O (298 K) sorption experiments revealed a clear transition from exclusively ultramicroporosity for the defect-free CAU-55-Cl, only accessible for H₂O and CO₂, to the presence of additional pores associated with the presence of cluster defects, leading to significantly higher apparent specific surface areas and adsorption capacities with increasing defect concentrations.