Terahertz Signatures of the Methane Replacement Reaction in Hydroquinone Clathrates

We report a comprehensive experimental and computational study into the low-frequency vibrational dynamics of hydroquinone clathrate during in situ gas loading, in order to monitor the replacement of carbon dioxide with methane in the atomic-level pores of the material. Specifically, terahertz time-domain spectroscopy is utilized, as the terahertz modes are highly-sensitive to the identity and structure of the enclathrated guest molecules. Through the use of ab initio simulations, it was clearly determined that the replacement reaction does not go to completion, and instead we observe the formation of a heterogenous material, with the methane molecules occupying approximately one third of the available adsorption sites. Additionally, while the structure of the methane-hydroquinone clathrate system has been previously determined, our observations suggest that the reported symmetry is incorrect due to methane molecules weakly-interacting with the framework, resulting in dynamic disorder (as opposed to positional disorder) of the guests, unlike the related carbon dioxide clathrate system that is fully-ordered. This work puts us on the path to quantitatively tracking gas loading in porous materials using terahertz spectroscopy.