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

Atomic-level understanding of the gate-opening phenomenon inflexible porousmaterials is an important step toward learning how to control, design, and engineer them forapplications such as the separation of gases from complex mixtures. Here, we report suchmechanistic insight through an in-depth study of the pressure-induced gate-opening phenomenon inour earlier reported metal-organic framework (MOF) Zn(dps)2(SiF6) (dps = 4,4 '-dipyridylsulfide),also called UTSA-300, using isotherm and calorimetry measurements,in situinfrared spectroscopy,andab initiosimulations. UTSA-300 is shown to selectively adsorb acetylene (C2H2) over ethylene(C2H4) and ethane (C2H6) and undergoes an abrupt gate-opening phenomenon, making thisframework a highly selective gas separator of this complex mixture. The selective adsorption isconfirmed by pressure-dependentin situinfrared spectroscopy, which, for thefirst time, shows thepresence of multiple C2H2species with varying strengths of bonding. A rare energetic feature at thegate-opening condition of theflexible MOF is observed in our differential heat energies, directlymeasured by calorimetry, showcasing the importance of this tool in adsorption property explorationofflexible frameworks and offering an energetic benchmark for further energy-based fundamentalstudies. Based on the agreement of this feature withab initio-based adsorption energies of C2H2inthe closed-pore structure UTSA-300a ("a"refers to the activated form), this feature is assigned to theweakening of the H-bond C-HmiddotmiddotmiddotF formed between C2H2andfluorine of the MOF. Our analysisidentifies the weakening of this H-bond, the expansion of the closed-pore MOF upon successive C2H2coadsorption until its volumeis close to that of the open-pore MOF, and the spontaneous gate opening to energetically favor C2H2adsorption in the open-porestructure as crucial steps in the gate-opening mechanism in this system