Elucidating the Sigmoidal Adsorption Behavior of Xenon in Flexible Hofmann‐Type MOFs Through Experiments and Molecular Dynamics with Machine Learning Potentials

ABSTRACT Sigmoidal adsorption in flexible metal‐organic frameworks (MOFs) can be achieved through a dramatic phase transition from narrow to large pores; however, such a unique phenomenon, which is not induced by abrupt phase changes, has rarely been reported. We report a series of flexible Hofmann‐type MOFs, CoNi‐Pipe , NiNi‐Pipe , and CoPd‐Pipe , pillared with a dimensional (3D) piperazine linker. Owing to the rotational flexibility and 3D dimensionality of the piperazine linker, CoNi‐Pipe and CoPd‐Pipe exhibited Xe‐induced gate opening and the corresponding sigmoidal adsorption isotherms. CoNi‐Pipe revealed not only excellent Xe/Kr separation but also Xe recovery performance, attributed to the greatest linker rotation and flexibility, along with moderate Xe binding strength, as validated by the machine learning potential‐based molecular dynamics simulations and van der Waals‐corrected DFT calculations. Thus, this work provides an effective approach for gaining a deeper understanding of the structural dynamics in flexible frameworks and expanding the repertoire of computational studies.