Identifying the Gate-Opening Mechanism in the Flexible Metal-Organic Framework UTSA-300

Atomic-level understanding of the gate-opening phenomenon in flexible porous materials is an important step toward learning how to control, design, and engineer them for applications such as the separation of gases from complex mixtures. Here, we report such mechanistic insight through an in-depth study of the pressure-induced gate-opening phenomenon in our earlier reported metal-organic framework (MOF) Zn(dps)₂(SiF₆) (dps = 4,4'-dipyridylsulfide), also called UTSA-300, using isotherm and calorimetry measurements, in situ infrared spectroscopy, and ab initio simulations. UTSA-300 is shown to selectively adsorb acetylene (C₂H₂) over ethylene (C₂H₄) and ethane (C₂H₆) and undergoes an abrupt gate-opening phenomenon, making this framework a highly selective gas separator of this complex mixture. The selective adsorption is confirmed by pressure-dependent in situ infrared spectroscopy, which, for the first time, shows the presence of multiple C₂H₂ species with varying strengths of bonding. A rare energetic feature at the gate-opening condition of the flexible MOF is observed in our differential heat energies, directly measured by calorimetry, showcasing the importance of this tool in adsorption property exploration of flexible frameworks and offering an energetic benchmark for further energy-based fundamental studies. Based on the agreement of this feature with ab initio-based adsorption energies of C₂H₂ in the closed-pore structure UTSA-300a ("a" refers to the activated form), this feature is assigned to the weakening of the H-bond C-H···F formed between C₂H₂ and fluorine of the MOF. Our analysis identifies the weakening of this H-bond, the expansion of the closed-pore MOF upon successive C₂H₂ coadsorption until its volume is close to that of the open-pore MOF, and the spontaneous gate opening to energetically favor C₂H₂ adsorption in the open-pore structure as crucial steps in the gate-opening mechanism in this system.