Dissociation and Nucleation of Tetra‑<i>n</i>‑butyl Ammonium Bromide Semi-Clathrate Hydrates at High Pressures

The equilibrium pressure–temperature relations of the tetra-<i>n</i>-butyl ammonium bromide (TBAB) semiclathrate hydrate were measured at pressures of up to 80 MPa by high-pressure differential scanning calorimetry. As a pressurizing medium, tetrafluoromethane (CF₄), which cannot occupy any hydrate cages in the TBAB semiclathrate hydrate at the present experimental pressures, was used. The dissociation temperature of tetragonal TBAB semiclathrate hydrate (TBAB·26H₂O) increases with the increase in pressure, whereas the dissociation enthalpy is (192 ± 3) J·g^–1 and almost constant at pressures of up to 80 MPa. The temperature difference between formation and dissociation at the same pressure, that is, the maximum allowable degree of supercooling, is (17.7 ± 0.7) K and independent of the pressure

Structure‑H Methane + 1,1,2,2,3,3,4-Heptafluorocyclopentane Mixed Hydrate at Pressures up to 373 MPa

Thermodynamic stability boundary of structure-H hydrates with large guest species and methane (CH₄) at extremely high pressures has been almost unclear. In the present study, the four-phase equilibrium relations in the structure-H CH₄ + 1,1,2,2,3,3,4-heptafluorocyclopentane (1,1,2,2,3,3,4-HFCP) mixed hydrate system were investigated in a temperature range of (281.05 to 330.12) K and a pressure range up to 373 MPa. The difference between equilibrium pressures in the structure-H CH₄ + 1,1,2,2,3,3,4-HFCP mixed hydrate system and the structure-I simple CH₄ hydrate system gets larger with increase in temperature. The structure-H CH₄ + 1,1,2,2,3,3,4-HFCP mixed hydrate survives even at 330 K and 373 MPa without any structural phase transition. The maximum temperature where the structure-H CH₄ + 1,1,2,2,3,3,4-HFCP mixed hydrate is thermodynamically stable is likely to be beyond that of the structure-H simple CH₄ hydrate

High-Pressure Phase Equilibria of Tertiary-Butylamine Hydrates with and without Hydrogen

Thermodynamic stability boundaries of the simple tertiary-butylamine (<i>t</i>-BA) hydrate and <i>t</i>-BA+hydrogen (H₂) mixed hydrate were investigated at a pressure up to approximately 100 MPa. All experimental results from the phase equilibrium measurement, in situ Raman spectroscopy, and powder X-ray diffraction analysis arrive at the single conclusion that the <i>t</i>-BA hydrates, under pressurization with H₂, are transformed from the structure VI simple <i>t</i>-BA hydrate into the structure II <i>t</i>-BA+H₂ mixed hydrate. The phase transition point on the hydrate stability boundary in the mother aqueous solutions with the <i>t</i>-BA mole fractions (<i>x</i>_{<i>t</i>‑BA}) of 0.056 and 0.093 is located at (2.35 MPa, 267.39 K) and (25.3 MPa, 274.19 K), respectively. On the other hand, in the case of the pressurization by decreasing the sample volume instead of supplying H₂, the simple <i>t</i>-BA hydrate retains the structure VI at pressures up to 112 MPa on the thermodynamic stability boundary

Effects of Halide Anions on the Electrical Conductivity in Single-Crystalline Tetra‑<i>n</i>‑butylammonium Salt Semiclathrate Hydrates

Semiclathrate hydrates are electroconductive materials for possible solid electrolytes and are also useful for monitoring their formation and dissociation processes. In the present study, the electrical conductivities and electrical relaxation times in the single-crystalline tetra-n-butylammonium (TBA) chloride and fluoride semiclathrate hydrates were measured and compared with those of the TBA-bromide semiclathrate hydrate. In the descending order of the electrical conductivity, the largest was the TBA-bromide semiclathrate hydrate, followed by TBA-chloride and TBA-fluoride semiclathrate hydrates. On the other hand, 2H NMR spin–lattice relaxation times in their deuterates were similar. Although the reorientation motion of water molecules should be a significant factor to govern the electrical conductivity in these semiclathrate hydrates, the present results reveal that the difference between the electrical conductivities in three TBA-halide semiclathrate hydrates would be caused by the concentration of the proton, a conduction carrier, rather than the diffusion processes. Additionally, electrical conductivity in the single-crystalline TBA-hydroxide semiclathrate hydrate was measured. The electrical conductivity even in single crystals was much higher than those in the TBA-halide semiclathrate hydrates

High-Pressure Phase Equilibrium and Raman Spectroscopic Studies on the 1,1-Difluoroethane (HFC-152a) Hydrate System

High-pressure phase equilibrium relations of the 1,1-difluoroethane (HFC-152a) + water binary system were investigated in a temperature range of (275.03 to 319.30) K and a pressure range up to 370 MPa. Four three-phase coexisting curves of hydrate + aqueous + gas phases, hydrate + HFC-152a-rich liquid + gas phases, hydrate + aqueous + HFC-152a-rich liquid phases, and aqueous + HFC-152a-rich liquid + gas phases originate from the quadruple point of hydrate + aqueous + HFC-152a-rich liquid HFC-152a + gas phases located at (288.05 ± 0.15) K and (0.44 ± 0.01) MPa. The structure of HFC-152a hydrate remains structure I (s-I) in the pressure range up to 370 MPa. Raman spectra of the HFC-152a molecule in the HFC-152a hydrate indicate that the HFC-152a molecules occupy only large cages of s-I HFC-152a hydrate in the presence of completely vacant small cages at a pressure up to 370 MPa

Investigating the Thermodynamic Stabilities of Hydrogen and Methane Binary Gas Hydrates

When hydrogen (H₂) is mixed with small amounts of methane (CH₄), the conditions required for clathrate hydrate formation can be significantly reduced when compared to that of simple H₂ hydrate. With growing demand for CH₄ as a commercially viable source of energy, H₂+ CH₄ binary hydrates may be more appealing than extensively studied H₂ + tetrahydrofuran (THF) hydrates from an energy density standpoint. Using Raman spectroscopic and powder X-ray diffraction measurements, we show that hydrate structure and storage capacities of H₂ + CH₄ mixed hydrates are largely dependent on the composition of the initial gas mixture, total system pressure, and formation period. In some cases, H₂ + CH₄ hydrate kinetically forms structure I first, even though the thermodynamically stable phase is structure II