Abnormal thermodynamic promotion and tuning behavior of epoxycyclopentane for its implications in CO₂ storage

Clathrate hydrates have emerged as an attractive material with versatile energy and environmental applications, including carbon dioxide (CO₂) capture and separation process. To practically employ the hydrate-based CO₂ capture process, it is necessary to shift the thermodynamic phase equilibria of the clathrate hydrates to moderate pressure and temperature conditions. Introducing thermodynamic promoters such as cyclopentane and tetrahydrofuran could be a plausible option to improve the thermodynamic stability of these clathrate hydrates. Although the use of thermodynamic promoters often reduces gas storage capacity by filling the empty cages of the hydrate in place of CO₂, the tuning phenomenon induced by low-concentration promoters also enables higher CO₂ sorption capacity under mild formation conditions. Here, we focus on epoxycyclopentane (ECP) as a novel thermodynamic promoter, and the potential application to CO₂ capture was explored. The pressure (P)–temperature (T) phase equilibria were measured, and the results revealed that ECP acts as a strong thermodynamic promoter for CO₂ enclathration. The trade-off between maximal CO₂ capture capacity and mild formation condition was investigated by lowering the ECP concentration below the stoichiometric concentration so that more CO₂ molecules would have a chance to fill the large cages of the hydrate. Spectroscopic analyses, including PXRD, Raman, and NMR spectrometers, were performed, and the results provided strong evidence of the distinct tuning phenomena. These findings provide fundamental insights into the tuning phenomenon of CO₂ in sII hydrates and the development of clathrate hydrate-based CO₂ capture media.