Magnetic Transition and Long-Time Relaxation Behavior Induced by Selective Injection of Guest Molecules into Clathrate Hydrates

Oxabicyclic Guest Compounds as sII Promoters: Spectroscopic Investigation and Equilibrium Measurements

In this study, we investigate three oxabicyclic compounds, 3,6-dioxabicyclo[3. 1.0]hexane (C₄H₆O₂, ETHF), 7-oxabicyclo[2.2.1]heptane (C₆H₁₀O, 14ECH), and 7-oxabicyclo[4.1.0]heptane (C₆H₁₀O, 12ECH) as novel promoters for gas hydrates. According to the outcomes of powder X-ray diffraction (PXRD) and synchrotron high-resolution powder diffraction (HRPD), all CH₄ hydrates formed with ETHF, 14ECH, and 12ECH were identified to be sII (Fd-3m) hydrates with corresponding lattice parameters of 17.195, 17.330, and 17.382 Å, respectively. It was also clearly demonstrated that CH₄ molecules are accommodated in the sII-S cages through solid-state ¹³C NMR and Raman spectra. Consequently, we clarified that the three compounds observed are large guest molecules (LGMs) that occupy the sII-L cages. Moreover, the thermodynamic stability of each LGM + CH₄ (and N₂) hydrate system was remarkably improved compared to that of the simple CH₄ (and N₂) hydrate. In particular, 14ECH manifested several unique features compared to the other two promoters. First, the 14ECH + CH₄ hydrate did not dissociate up to room temperature (298 K), even at a moderate pressure of approximately 60 bar. At a given pressure, 14ECH increased the dissociation temperature of the CH₄ hydrate by ~18 K, indicating that its promotion capability is as strong as that of tetrahydrofuran (THF), currently considered to be the most powerful promoter. Second, more interestingly, it was revealed by further PXRD, NMR, and Raman analyses that 14ECH forms a simple sII hydrate in the absence of help gases. According to differential scanning calorimetry (DSC) outcomes, we revealed that the simple 14ECH hydrate dissociates at 270~278 K under ambient pressure. In addition to the thermodynamic stability, we also note that the 14ECH + CH₄ hydrate presented a sufficiently high temperature of formation, requiring little additional cooling. Given these promising features, we expect that the 14ECH hydrate system can be adopted to realize hydrate-based technologies. We also believe that the LGMs introduced here have considerable potential to serve as alternates to conventional promoters and that they can be widely utilized in both engineering and scientific research fields.

Epoxycyclopentane hydrate for sustainable hydrate-based energy storage: notable improvements in thermodynamic condition and storage capacity

We suggest epoxycyclopentane (ECP) as a novel guest compound for hydrate-based energy storage. All of the key properties of the ECP hydrate, including the thermodynamic stability, storage capacity, and formation condition, are notably superior to those of hydrates containing tetrahydrofuran (THF) and cyclopentane (CP), currently considered to be the most powerful promoters.