Hexamethyleneimine, 1-methylpiperidine, 2-methylpiperidine, 3-methylpiperidine, and 4-methylpiperidine as isomers of C6H13N were revealed as new sH clathrate hydrate forming molecules. They show fully soluble characteristics to water, whereas already known sH formers such as methylcyclohexane and 2,2-dimethylbutane (neohexane) are immiscible or very slightly soluble to water. The L–H–V equilibrium P–T behavior of these new sH clathrate hydrates shows a tendency to shift to much milder conditions than already known ones. We particularly note that 1-methylpiperidine appears to be the best for promotion. To verify the distribution of CH4 molecules and crystal structure of clathrate hydrates, 600 MHz solid-state NMR, Raman spectroscopy, and XRD pattern analysis were conducted. These noticeable properties of new formers are expected to open new research fields to the hydrate community and contribute to hydrate-based technological applications with high energy efficiency.

No abstract

No abstract

Guest−host hydrogen bonding strongly affects the physical properties of clathrate hydrate, such as the thermodynamic stability, water dynamics, and dielectric properties, but attempts to quantify the effects of hydrogen bonding on these properties are rare thus far. As a preliminary work, this study investigates methane clathrate hydrates with three diazine isomers, pyrazine, pyrimidine, and pyridazine, which expect nearly the same van der Waals volumes due to their similar molecular shapes and sizes, and their guest−host hydrogen-bonding behaviors. The crystal structures of all three binary diazine + CH 4 hydrate phases were identified as a cubic Fd3̅ m structure, including diazine molecules in the 5 12 6 4 cavity, commonly termed as structure II hydrate, by a high-resolution powder diffraction pattern analysis. The phase equilibrium curves of their clathrate hydrates were obtained by the P−T trajectory of the hydrate formation and dissociation process, and the thermodynamic stability trend was well-explained by the guest−host hydrogen bonding behavior as evaluated by the molecular polarities, proton affinities, and ring-breathing vibration frequencies of the three diazine isomers obtained from Raman spectroscopy. This study provides useful information that contributes to the realization of the expansion of the thermodynamics of clathrate hydrates to include guest−host hydrogen-bonding interactions.

The unexpected formation of gas hydrates during production and transportation processes in petroleum industries is known as a serious problem. To deal with this problem, the oil and gas industry has been searching for hydrate inhibitors that have great performance and cost effectiveness. Recently, ionic liquids (ILs) have been suggested as novel hydrate inhibitors that are able to act in both thermodynamic and kinetic ways (so-called dual-function inhibitors). In this paper, we suggest a non-ionic liquid compound, morpholine, as a dual-function inhibitor by measuring hydrate-phase equilibria and a series of microscopic analyses [powder X-ray diffraction, solid-state 13 C nuclear magnetic resonance (NMR), and Raman spectroscopy]. Moreover, the formation kinetics of gas hydrates in the presence of morpholine was found to be better than two comparators, 1-ethyl-3-methylimidazolium tetrafluoroborate and polyvinylpyrrolidone. Such inhibition effects of morpholine are thought to be mainly attributed to the nucleophilicity of the ring compound forming hydrogen bonds between surrounding water molecules.