Locating Gases in Porous Materials: Cryogenic Loading of Fuel‐Related Gases Into a Sc‐based Metal–Organic Framework under Extreme Pressures

Abstract

An alternative approach to loading metal organic frameworks with gas molecules at high (kbar) pressures is reported. The technique, which uses liquefied gases as pressure transmitting media within a diamond anvil cell along with a single-crystal of a porous metal organic framework, is demonstrated to have considerable advantages over other gas-loading methods when investigating host-guest interactions. Specifically, loading the metal organic framework Sc2BDC3 with liquefied CO2 at 2 kbar reveals the presence of three adsorption sites, one previously unreported, and resolves previous inconsistencies between structural data and adsorption isotherms. A further study with supercritical CH4 at 3 -25 kbar demonstrates hyperfilling of the Sc2BDC3 and two high pressure displacive and reversible phase transitions are induced as the filled MOF adapts to reduce the volume of the system.Understanding how guest molecules in metal-organic framework (MOFs) interact with each other and the framework they occupy upon adsorption is vital for the development and commercial application of MOFs, for example in carbon capture and gas sequestration technologies. [1] Spectroscopic techniques, such as IR [2] and solid state NMR [3] are often used for probing these interactions. Crystallographic studies, both X-ray and neutron, utilizing various environmental cells have proven invaluable in determining the nature of host-guest interactions within MOFs and their guest-driven structural flexibility. [4] This is particularly important in the many cases where the uptake behavior, which can include conformational changes in the framework itself, can be perturbed by the type and amount of guest adsorbed. [5] One of the main reasons why so little data is available is that the experimental location of gas molecules in MOFs is challenging. The gas molecules are often disordered and exhibit thermal motion which is difficult to model crystallographically even when cooled close to the freezing temperature of the gas adsorbed. In-situ cells have been developed for collecting crystallographic data whilst exposing a MOF to a gaseous environment at tens, or even hundreds of bars of pressure in an effort to saturate the pores with gas molecules, though even here, the gas molecules are often difficult to observe experimentally. For these reasons, despite the intensive research in carbon capture technologies, relatively few crystallographic studies have been reported where CO2 molecules have been unambiguously located within MOFs. [6] The use of condensed gases as pressure transmitting media (PTM) in diamond anvil cells is commonplace in the fields of condensed matter physics and high-pressure mineralogy due to their excellent hydrostatic properties at very high-pressures (> 10 GPa) relative to common liquid PTMs. [7] He, Ar and Ne are typically used because they are chemically inert, whilst other gases have been studied for their diverse structural behavior. High temperature and pressure phases of CO2 and CH4 are of interest for understandin...

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