Structure H (sH) Clathrate Hydrate with New Large Molecule Guest Substances

This study characterized new structure H (sH) clathrate hydrates with bromide large-molecule guest substances (LMGSs) bromocyclopentane (BrCP) and bromocyclohexane (BrCH), using powder X-ray diffraction (PXRD) and Raman spectroscopy. The lattice parameters of sH hydrates with (CH₄ + BrCP) and (CH₄ + BrCH) were determined from their PXRD profiles. On the basis of their Raman spectra, the M-cage to S-cage occupancy ratio (4³5⁶6³ and 5¹² cages, respectively), θ_M/θ_S, was estimated to be approximately 1.3, and the Raman shift of the symmetric C–H vibrational modes of CH₄ in S- and M-cages was 2911.1 and 2909.1 cm^–1, respectively. The phase-equilibrium conditions of sH hydrates with (CH₄ + BrCP) and (CH₄ + BrCH) were determined by an isochoric method. A comparison between the equilibria of sH hydrates with BrCP and BrCH and those with other typical nonpolar and polar LMGSs (methylcyclopentane, MCP; methylcyclohexane, MCH; neohexane, NH; and <i>tert</i>-butyl methyl ether, TBME) at the same temperature revealed that the equilibrium pressure increased in the order NH < MCH < BrCH < TBME ∼ MCP < BrCP. The phase stabilities of sH hydrates can be determined by not only molecular geometry but also their polar properties, which affect guest–host interactions

Phase Equilibrium Conditions for Clathrate Hydrates of Tetra-<i>n</i>-butylammonium Bromide (TBAB) and Xenon

Phase equilibrium pressure–temperature (<i>pT</i>) conditions for the xenon (Xe)–tetra-<i>n</i>-butylammonium bromide (TBAB)–water system were characterized by an isochoric method in the pressure range from (0.05 to 0.3) MPa using TBAB solutions with mole fractions ranging from (0.0029 to 0.0137). The phase equilibrium <i>pT</i> conditions in the system appeared at a lower pressure and higher temperature than in the pure Xe hydrate. Furthermore, under atmospheric pressure, the dissociation temperature in the Xe–TBAB–water system shifted to a higher region than in the pure TBAB hydrate. In the experimental TBAB concentration range, the powder X-ray diffraction patterns of the Xe–TBAB–water system revealed that the TBAB clathrate hydrate is TBAB·38H₂O

Structural Characterization of Structure H (sH) Clathrate Hydrates Enclosing Nitrogen and 2,2-Dimethylbutane

In this study, we characterized structure H (sH) clathrate hydrates (hydrates) containing nitrogen (N₂) and 2,2-dimethylbutane (neohexane, hereafter referred to as NH) molecules. On the basis of the powder X-ray diffraction profile, we estimated the unit cell dimensions of the sH hydrate of N₂ + NH to be <i>a</i> = 1.22342(15) nm and <i>c</i> = 0.99906(17) nm at 153 K. The <i>c</i> axis of this hydrate was slightly shorter (i.e., 0.00584 nm) than that of CH₄ + NH, whereas we observed no difference in the <i>a</i> axis between these two hydrates. We successfully observed a symmetric N–N stretching (N–N vibration) Raman peak with two bumps, and we determined that the N–N vibrational mode in the 5¹² and 4³5⁶6³ cages occurred at approximately 2323.8 and 2323.3 cm^–1, respectively. We found the cage occupancy ratio of the 4³5⁶6³/5¹² cages (θ_Mθ_S) of the sH hydrate of N₂ + NH to be approximately 1.30. From a comparison of the N–N vibrational modes in the 5¹², 4³5⁶6³, 5¹²6², and 5¹²6⁴ cages of the sI, sII, and sH hydrates, we determined that N₂ molecules in the distorted 4³5⁶6³ cages experience more <i>attractive</i> guest–host interaction than those in spherical 5¹²6⁴ cages, whereas the guest/cage diameter ratio of 4³5⁶6³ cages is larger than that of 5¹²6⁴ cages. We determined the L₁–L₂–H–V four-phase equilibrium pressure–temperature conditions in the N₂–NH–water system in the temperature range of 274.36–280.71 K. Using the Clausius–Clapeyron equation, we estimated the dissociation enthalpies of the sH hydrates of N₂ + NH to be 388.4 and 395.9 kJ·mol^–1 (per one molar of N₂ molecules) in the experimental temperature range

Phase Transition of Tetra‑<i>n</i>‑butylammonium Bromide Hydrates Enclosing Krypton

The phase equilibrium conditions for krypton (Kr)–tetra-<i>n</i>-butylammonium bromide (TBAB)–water systems were determined using an isochoric method. The pressure and temperature ranges were (0.06 to 1.0) MPa and (280 to 290) K, respectively, and TBAB solutions had TBAB molar fractions, <i>x</i>_TBAB, of 0.0062, 0.0138, 0.0234, and 0.0359. A second order transition of the TBAB hydrate was observed in all the Kr–TBAB–water systems. In the region at lower pressure than the phase transition point, the Kr–TBAB–water systems with low concentration (<i>x</i>_TBAB = 0.0062 and 0.0138) and high concentration (<i>x</i>_TBAB = 0.0234 and 0.0359) prefer to form TBAB·38H₂O and TBAB·26H₂O hydrates, respectively. However, a <i>new</i> TBAB hydrate was observed as a stable crystal structure in the higher pressure regions. Raman spectrum of the new TBAB hydrate shows band shapes remarkably similar to that of <i>pure</i> TBAB·38H₂O with the crystalline space group <i>Pmma</i> in the frequency ranges of the lattice for C–C stretching, C–H bending, the C–H stretching bands of the −CH₂ groups of TBA⁺ molecules, and the O–H stretching modes of water molecules, excluding the C–H stretching bands of the CH₃ groups of TBA⁺ molecules

Crystal Phase Boundaries of Structure‑H (sH) Clathrate Hydrates with Rare Gas (Krypton and Xenon) and Bromide Large Molecule Guest Substances

Phase equilibrium pressure–temperature (<i>pT</i>) boundaries of structure-H clathrate hydrates (sH hydrates) with rare gas (Kr and Xe)-bromide large molecule guest substances (LMGSs: bromocyclohexane, BrCH and bromocyclopentane, BrCP) were measured. The phase boundaries for the sH hydrates in the Kr–LMGS–water systems shifted to lower pressures than those for the <i>pure</i> Kr hydrate in the temperature range of (273.2 to 279.3) K. In this study, sH hydrate formation was not confirmed in the Xe–BrCP–water system, but sH hydrates were found in the Xe–BrCH–water system. At temperatures below 277 K, equilibrium conditions were observed at lower pressures for the Xe–BrCH–water system than for the <i>pure</i> Xe hydrate. However, the equilibrium <i>pT</i> curve for the Xe–BrCH–water system crossed over the equilibrium <i>pT</i> curve for the Xe hydrate at around 277 K. Intersections between the equilibrium <i>pT</i> curves for the Xe hydrates and the sH hydrates (Xe + LMGS) have also been found in Xe–methylcyclohexane–water systems. Using the Kr–and Xe–bromide LMGS–water systems showed that the sH hydrate phase stabilities are strongly related to the encaptured LMGS

In Situ Methane Hydrate Morphology Investigation: Natural Gas Hydrate-Bearing Sediment Recovered from the Eastern Nankai Trough Area

The hydrate morphology of natural gas hydrate-bearing (GH) sediments recovered from the eastern Nankai trough area was investigated under hydrostatic pressurized conditions that prevent dissociation of gas hydrates in a sediment. We developed a novel X-ray computed tomography system and an attenuated total reflection infrared (ATR-IR) probe for use in the Instrumented Pressure Testing Chamber for our set of Pressure-Core Nondestructive Analysis Tools (PNATs), which can measure the sediment structure, primary wave velocity (PWV), density, and shear strength under pressurized conditions. The hydrate saturation values estimated using the ATR-IR absorption bands of H₂O molecules strongly correlate with PWV. Assuming homogeneity of hydrate distribution in the planes perpendicular to the sample depth direction, the hydrate morphology of natural GH sediments in the eastern Nankai trough area demonstrated a load-bearing morphology type. The predicted hydrate morphology results are in good agreement with data reported in the literature. The combination of PNATs including ATR-IR spectroscopy can be used to estimate the properties of GH sediments without the release of pressure to atmospheric conditions in order to model gas hydrate reservoirs for natural gas production

Characteristics of Natural Gas Hydrates Occurring in Pore-Spaces of Marine Sediments Collected from the Eastern Nankai Trough, off Japan

Pore-space gas hydrates sampled from the eastern Nankai Trough area off of Japan were minutely characterized using several instrumental techniques. Gas chromatographic results indicated that the natural gas in the sediment samples studied comprises mainly CH₄. The concentrations of minor components varied according to depth. The powder X-ray diffraction patterns showed that the pore-space hydrates were of structure I (sI); the lattice constants were 1.183−1.207 nm. Both ¹³C NMR and Raman spectra confirmed that CH₄ molecules were encaged in sI hydrate lattice. The average cage occupancies were calculated, respectively, from the Raman data as 0.83 for small cages and 0.97 for large cages. The hydration numbers were determined as 6.1−6.2