Experimental Study on Hydrate Structure Transition Using an In Situ High-Pressure Powder X-ray Diffractometer: Application in CO<inf>2</inf> Capture

Clathrate (gas) hydrates as materials have received great interest due to their high-density gas storage potential and separation applications for their ability to preferentially separate a targeted component such as CO2 from waste streams. Among the three clathrate hydrate structures, only sH (structure H) hydrates require a large molecule such as neohexane (NH) or tert-butyl methyl ether (TBME) as well as a "help-gas"molecule such as methane (CH4) to form a stable structure. However, attempts to use CO2 as a help-gas came up with mixed results where it appeared that the sH hydrate formed was stable only at temperatures below the ice point, whereas the compound formed with CH4 was considerably more stable. sH hydrates have considerable potential for gas separation, for instance, of CH4-CO2 mixtures. Thus, in this work, several compositions were tested for their hydrate forming ability. The large cage guests tested were NH and TBME, the help-gas mixtures were a CO2-rich mixture (76% CO2 and 24% CH4), and a CO2 lean mixture (24% CO2 and 76% CH4). The phase behavior of hydrates formed from the various combinations was tested by measuring the endo- and exotherms associated with hydrate formation and decomposition in a high-pressure differential calorimeter. The different phases indicated from the DSC results were identified by employing an in situ high-pressure cell on a powder X-ray diffractometer. Powder patterns were recorded to identify the crystal phase arising from nucleation and possible re-crystallization events after annealing. It was confirmed that the CO2 lean mixture (24% CO2 and 76% CH4) forms structure H (sH) hydrate, while the CO2-rich mixture (76% CO2 and 24% CH4) forms structure I (sI) hydrate presenting a structure transition pattern across the gas mixtures investigated. Further, it was observed that the CO2 lean mixture, which forms sH hydrate, also starts as sI hydrate and gradually converts to the thermodynamically stable sH hydrate. This study, in essence, helps to understand the preference of CH4 and CO2 for three different types of cages in sH hydrates and presents a design framework for a suitable gas separation mechanism for this gas mixture of interest.