Are Hemilabile Metal–Organic Frameworks Overlooked as Promising Water-Stable Adsorbents? Elucidating Their Physical and Hydrolytic Properties Using Machine-Learned Potentials

Poor water stability of many metal-organic frameworks (MOFs) is a persistent bottleneck toward their practical applications. Hemilabile STAMs (St. Andrews Materials) demonstrate greater water stability, as well as improved adsorption performance under humid conditions, compared to compositionally similar HKUST-1. Yet, the fundamental properties of STAMs remain largely unexplored. Herein, we leverage machine-learned potentials (MLPs) to simulate physical and hydrolytic properties, water stability and adsorption in STAMs functionalized with various hydrophobic/hydrophilic groups. Encouragingly, STAMs are predicted to exhibit high mechanical strength and low heat capacity, which are desirable attributes in adsorption processes. Moreover, defective STAMs are shown to possess greater mechanical robustness than defective HKUST-1. From MLP-based molecular dynamics simulations, we demonstrate that Cu-paddlewheels of STAMs are relatively stable even at high water loadings. The hydrolysis mechanism of Cu-paddlewheels is examined meticulously, and for the first time, we quantitatively reveal a higher energy barrier is required to hydrolyze Cu-O bonds in STAMs than in HKUST-1, unambiguously elucidating the crucial role of Cu···OH₂ interactions in water stability of hemilabile STAMs. From MLP-based Monte Carlo simulations, water adsorption isotherms predicted in STAMs match well with experimental data. Importantly, it is concretely demonstrated that water adsorption in the hexagonal pore can be tailored by modifying the hydrophobicity of functional groups, while adsorption in the triangular pore is affected marginally. Beyond showing that STAMs are promising adsorbents, we provide new microscopic insights into physical and hydrolytic properties in Cu paddlewheel-based MOFs. More generally, our findings suggest that incorporating hemilability is a promising but underexplored strategy for the development of new water-stable MOFs.