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.

A hydrogen molecule entrapped in the cages of icy hydrogen hydrate is confined in host water framework and thus behaves unlike pure solid or liquid hydrogen. The gamma-irradiated hydrogen radicals are for the first time observed from ESR and solid-state MAS 1H NMR spectra to stably exist in the icy hydrate channels without any collapse of the host framework, confirming the chemical shift consistency of ionized hydrogen derivatives. We discuss the confined icy hydrate channels, which can act as potential storage sites for simultaneously imprisoning both molecular and ionized hydrogen and further as icy nanoreactors.

Smectite clays are widely dispersed in deep ocean sediments and can be subdivided into two representative clay types, Cheto-and Wyoming-montmorillonites. In this study, we measured the thermodynamic phase behavior of methane hydrates intercalated at various concentrations of these clays and found the relatively weak promotion tendency when compared to that of pure methane hydrate stability. The structure and morphology of intercalated methane hydrate (IMH) samples were analyzed using the MAS NMR, RAMAN, LT-XRD, and Cryo-FE-SEM. The 27 Al and 29 Si solid-state MAS NMR spectra of IMH Cheto and Otay clays represent that the structural stability is preserved during the IMH formation, which is also indicated with the XRD pattern showing no structural transformation but different d-spacing values due to clay-water suspension and IMH. In addition, Cryo-SEM images of IMH samples show that IMH Otay clays provide well-developed methane hydrate (MH) morphology, compared to the IMH Cheto clay.

Natural methane hydrates occurring in marine clay sediments exhibit heterogeneous phase behavior with high complexity, particularly in the negatively charged interlayer region. To date, the real clay interlayer effect on natural methane hydrate formation and stability remains still much unanswered, even though a few computer simulation and model studies are reported. We first examined the chemical shift difference of 27Al, 29Si, and 23Na between dry clay and clay containing intercalated methane hydrates (MH) in the interlayer. We also measured the solid-state 13C MAS NMR spectra of MH in Na-montmorillonite (MMT) and Ca-montmorillonite (MMT) to reveal abnormal methane popularity established in the course of intercalation and further performed cryo-TEM and XRD analyses to identify the morphology and layered structure of the intercalated methane hydrate. The present findings strongly suggest that the real methane amount contained in natural MH deposits should be reevaluated under consideration of the compositional, structural, and physical characteristics of clay-rich sediments. Furthermore, the intercalated methane hydrate structure should be seriously considered for developing the in situ production technologies of the deep-ocean methane hydrate.

Natural gas hydrates were recovered from near-seafloor sediments and analysed to compare two distinctive methane inclusion phenomena. We document the first observation of abnormal methane occupancy in sediment-rich NGH deposits.Natural gas hydrate (NGH) deposits in the deep sea floor are of interest to researchers because of their high potential as a next generation energy source. Among recovered NGHs, the structure I (sI) has been the most abundantly discovered hydrate worldwide. Regarding the existence of other NGH structures, Dr Ripmeester revealed that NGH samples collected from the Gulf of Mexico show sH and predicted that there are considerable sH NGH deposits in the ocean sediments. 1,2 However, we confirmed that sII and sH structures eventually transform to sI under severe conditions of continuous gaseous methane supply. 3 Marine sediments consist of a variety of geochemicals including clay minerals. Clay minerals comprise approximately 1 nm thick unit layers within micrometre-sized crystalline particles. This implies that the properties of gas hydrates confined in the clay interlayer are vastly different from those of gas hydrates in the bulk. 4 The interlayer mobile ions provide unique electrokinetic surroundings, where methane molecules are intercalated to form sI hydrate. Such heterogeneous complexity appearing in the mixed system of clay, water, light hydrocarbons, and cations are expected to strongly influence hydrate nucleation characteristics such as host water-lattice formation. Only a few studies have examined complex gas hydrate formation patterns in clay sediments, [5][6][7] with most works focusing on macroscopic identification of marine NGH sediments. In previous works, 8,9 we attempted to determine the phase behavior and cage occupancy of methane in clay sediments. Our analyses revealed that the gas hydrate formation in clays of Na-montmorillonite and Ca-montmorillonite differs greatly from that in pure gas hydrate. We also found that the interlayer cations might significantly affect the gas hydrate stability and cage occupancy of methane due to entrapment of cations in small cages.The purpose of the present investigation is to determine whether such abnormal methane occupancy significantly affects the overall stored methane amount in near-seafloor NGH. NGH samples recovered from three sites (06C, 08C, and 12C) in the East Sea (DongHae, 37 0 N and 132 0 S, 07GHP) located in the eastern part of the Korean Peninsula were tested to confirm the occurrence of abnormal methane population in marine NGH sediments. From these samples the clay-rich and NGH-rich parts were separated to characterize distinctive properties of each parts, as can be seen in the insets of Fig. 1.Solid-state 13 C MAS NMR spectroscopy was to evaluate the methane population at the recovered NGH, resulting in two peaks at À4.3 (sI-S) and À6.7 ppm (sI-L) in both clay-rich and NGH-rich parts, as shown in Fig. 1. The corresponding cage occupancy ratio, sI-L/sI-S, ranged from 3.5 to 3.9, which is quite close to the ideal ratio of ...

No abstract

We first report here that under strong surrounding gas of external CH4 guest molecules the sII and sH methane hydrates are structurally transformed to the crystalline framework of sI, leading to a favorable change of the lattice dimension of the host-guest networks. The high power decoupling 13C NMR and Raman spectroscopies were used to identify structure transitions of the mixed CH4 + C2H6 hydrates (sII) and hydrocarbons (methylcyclohexane, isopentane) + CH4 hydrates (sH). The present findings might be expected to provide rational evidences regarding the preponderant occurrence of naturally occurring sI methane hydrates in marine sediments. More importantly, we note that the unique and cage-specific swapping pattern of multiguests is expected to provide a new insight for better understanding the inclusion phenomena of clathrate materials.

The thermodynamic stability and storage capacity of the novel epoxycyclopentane hydrate are superior to those of THF or cyclopentane hydrate.