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

Abstract Defect engineering has emerged as a powerful strategy to tailor the properties of porous materials. 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. These experimental findings were further corroborated by molecular simulations, which revealed the geometric characteristics of cluster defects and their influence on adsorption behavior. Grand Canonical Monte Carlo (GCMC) and Widom insertion simulations of Ar as the probe confirmed that missing clusters introduce larger, energetically distinct pore environments, aligning with the observed sorption trends.