Investigation of tuning behavior of trimethylene oxide hydrate with guest methane molecule and its critical guest concentration

Due to the promising storage capacity for CH₄, the method of natural gas storage and transportation using solid gas hydrate is recognized as an alternative to conventional liquefied natural gas process. To make this alternative more viable, however, it is necessary to further shift the formation temperature and pressure of natural gas hydrate closer to ambient conditions. Although there is a trade-off between storing more CH₄ and mitigating the formation conditions, it is known that adding specific liquid molecules such as tetrahydrofuran, as a secondary guest to CH₄ hydrate could lead to favorable thermodynamic stability while still affording promising CH₄ storage capacity via a mechanism known as the tuning phenomenon. Here, this study focused on trimethylene oxide (TMO) + CH₄ hydrate to reveal its unique guest occupation patterns for potential application to natural gas storage and transportation. The occurrence of the tuning behavior on the TMO + CH₄ hydrate was observed for the first time. Most importantly, the tuning factor referred to as the CH₄ occupation ratio between the large and small cages of sII hydrate, exhibited a value of 0.73 at the critical guest concentration, which is the largest value reported to date. The phase equilibria of the TMO + CH₄ hydrate were provided for TMO concentrations ranging from 1.0 to 5.56 mol%. The spectroscopic data obtained from synchrotron HRPD, dispersive Raman, and ¹³C solid-state HPDEC MAS NMR spectrometry provided experimental evidence of the distinct enclathration characteristics of CH₄ and TMO molecules during the tuning phenomenon. These findings provide fundamental knowledge on the tuning effect and lend useful insights for designing and developing gas hydrate-based natural gas storage and transportation process.