Structural and dynamical properties of methane clathrate hydrates

Equilibrium molecular dynamics (MD) simulations have been performed in both the NVT and NPT ensembles to study the structural and dynamical properties of fully occupied methane clathrate hydrates at 50, 125, and 200 K. Five atomistic potential models were used for water, ranging from fully flexible to rigid polarizable and nonpolarizable. A flexible and a rigid model were utilized for methane. The phonon densities of states were evaluated and the localized rattling modes for the methane molecules were found to couple to the acoustic phonons of the host lattice. The calculated methane density of states was found to be in reasonable agreement with available experimental data.     

The heats of fusion of tetrabutylammonium fluoride ionic clathrate hydrates

Two ionic clathrate hydrates with different structures are formed in the binary system tetrabutylammonium fluoride–water, namely tetragonal structure-I hydrate (TS-I) (n-С₄H₉)₄NF · 32.8H₂O, and cubic superstructure-I hydrate (CSS-I) (n-С₄H₉)₄NF · 29.7H₂O. The heats of fusion (ΔH_f) of these polyhydrates were measured calorimetrically with differential scanning calorimeter. For TS-I polyhydrate ΔH_f = (204.8 ± 2.3) kJ/mol hydrate, for CSS-I hydrate ΔH_f = (177.5 ± 3.1) kJ/mol polyhydrate. The change of water molecules energy state in the water lattices of TS-I and CSS-I polyhydrates are discussed.     

Encapsulation of saline solution by tetrahydrofuran clathrate hydrates and inclusion migration by recrystallization

Encapsulation of saline solution as an impurity in tetrahydrofuran clathrate hydrates grown in a stoichiometric solution with 3 wt % NaCl and the release of a saline solution during melting along with inclusion migration by hydrate recrystallization during annealing are studied using a directional growth apparatus in combination with a Mach-Zender interferometer. Interferometric observation revealed that the salt concentration increased locally in the solution near the growth interface. The time evolution of salt concentration in the solution was in accordance with the numerical results obtained from the diffusion equation for salt, assuming perfect rejection of salt by the hydrate. However, after the interfacial pattern developed into a serrated pattern (periodical array of trough and crest), the salt concentration in the solution ceased to increase, deviating from the theoretical value. This indicates that the saline solution was encapsulated by the growth hydrate. On the other hand, upon melting of the slowly grown hydrate, the salt concentration near the interface was observed to be locally high at the location of the trough during growth, whereas it was dilute at the location of the growth crest. Furthermore, when the hydrate was annealed under an applied temperature gradient, the inclusions (encapsulated saline solution) in the hydrate migrated toward the bulk solution and were finally expelled by hydrate recrystallization. The migration speed of the inclusions increased with a larger temperature gradient. By melting the sample over a sufficiently long anneal time, the melt was determined to be completely desalinated.     

Polymorphism in Br2 clathrate hydrates

The structure and composition of bromine clathrate hydrate has been controversial for more than 170 years due to the large variation of its observed stoichiometries. Several different crystal structures were proposed before 1997 when Udachin et al. (Udachin, K. A.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1997, 119, 11481) concluded that Br2 forms only the tetragonal structure (TS-I). We show polymorphism in Br2 clathrate hydrates by identifying two distinct crystal structures through optical microscopy and resonant Raman spectroscopy on single crystals. After growing TS-I crystals from a liquid bromine-water solution, upon dropping the temperature slightly below -7 degrees C, new crystals of cubic morphology form. The new crystals, which have a limited thermal stability range, are assigned to the CS-II structure. The two structures are clearly distinguished by the resonant Raman spectra of the enclathrated Br2, which show long overtone progressions and allow the extraction of accurate vibrational parameters: omega(e) = 321.2 +/- 0.1 cm(-1) and omega(e)x(e) = 0.82 +/- 0.05 cm(-1) in TS-I and omega(e) = 317.5 +/- 0.1 cm(-1) and omega(e)x(e) = 0.70 +/- 0.1 cm(-1) in CS-II. On the basis of structural analysis, the discovery of the CS-II crystals implies stability of a large class of bromine hydrate structures and, therefore, polymorphism.     

Localized oscillators and heat conduction in clathrate hydrates

The glassy behaviour of the thermal conductivities of structure I xenon and structure II hydrate at high temperature are found to be described surprisingly well by a localized oscillator model. This observation leads to the suggestion of strong coupling between the localized vibrations of the guest with the lattice acoustic phonons. This conjecture is confirmed by a phenomenological calculation using the Anderson-Fano resonant scattering model.Published as NRCC 37248.     

Energetics of framework formation in cubic clathrate hydrates

Geometrical parameters and energies of CS-III (143d) and CS-IV (Im3m) gas hydrate frameworks are calculated. For the four cubic framework, the dependences of the energies on the sizes of large cavities are compared. Due to this, energetically favorable regions of each framework have been isolated on the abscissa. The CS-I and CS-II frameworks show satisfactory agreement between the sizes of guest molecules and the structures of clathrate hydrates. The experimental lattice parameters of the hydrates vary within narrower ranges than the values calculated for the empty frameworks. An explanation is given for these contradictory results was well as for the discrepancies between the guest sizes and the boundaries between the energetically favorable regions of CS-III and CS-IV. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 494–500, May–June, 1995. Translated by L. Smolina     

Molecular hydrogen storage in binary THF-H2 clathrate hydrates

The hydrogen storage capacity of binary THF-H(2) clathrate hydrate has been determined as a function of formation pressure, THF composition, and time. The amount of hydrogen stored in the stoichiometric hydrate increases with pressure and exhibits asymptotic (Langmuir) behavior to approximately 1.0 wt % H(2). This hydrogen concentration corresponds to one hydrogen molecule occupying each of the small 5(12) cavities and one THF molecule in each large 5(12)6(4) cavity in the hydrate framework. Contrary to previous reports, hydrogen storage was not increased upon decreasing the THF concentration below the stoichiometric 5.6 mol % solution to 0.5 mol %, at constant pressure, even after one week. This provides strong evidence that THF preferentially occupies the large 5(12)6(4) cavity over hydrogen, for the range of experimental conditions tested. The maximum amount of hydrogen stored in this binary hydrate was about 1.0 wt % at moderate pressure (<60 MPa) and is independent of the initial THF concentration over the range of conditions tested.     

On the thermodynamic stability of hydrogen clathrate hydrates

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.     

CS-II binary clathrate hydrates at pressures of up to 15 kbar

The pressure dependence of the decomposition temperatures of binary clathrate hydrates of tetra-hydrofuran with xenon and methane as well as of chloroform and carbon tetrachloride clathrates with xenon has been studied. The absence of phase transitions at pressures from 1 to 15,000 bar indicates that the structure of all the hydrates remains constant (CS-II). The decomposition temperatures of the binary hydrates of tetrahydrofuran and carbon tetrachloride with xenon at 15 kbar (above 124ℴC) are exceedingly high for polyhedral clathrate hydrates because the guest molecules are highly complementary to the cavities of the clathrate lattice. The paper also considers the packing density effect in the crystal structure of hydrates on the behavior of the latter at elevated pressure. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 582-589, May–June, 2000.     

Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates

Tetrahydrofuran (THF) is one of the most widely used analogues for gas hydrates as well as a commonly used additive for reducing the formation pressure of a given hydrate process. Hydrates are also currently being investigated as storage materials for hydrogen as well as materials for hydrogen separations. Here we present a thermodynamic model, based on the CSMGem framework, that accurately captures the phase behavior of various hydrates containing THF and hydrogen. The model uses previously regressed parameters for components other than THF and H₂, and can reproduce hydrate formation conditions for a number of hydrates containing THF and/or hydrogen (simple THF, THF + CH₄, THF + N₂, THF + CO₂, THF + H₂, CH₄ + H₂, C₂H₆ + H₂ and C₃H₈ + H₂). The incorporation of THF and H₂ within this model framework will serve as a valuable tool for hydrate scenarios involving either of these components.     

Liquid clathrate formation in ionic liquid-aromatic mixtures

1-Alkyl-3-methylimidazolium containing ionic liquids with hexafluorophosphate, bis(trifyl)imide, tetrafluoroborate, and chloride anions form liquid clathrates when mixed with aromatic hydrocarbons; in the system 1,3-dimethylimidazolium hexafluorophosphate-benzene, the aromatic solute could be trapped in the solid state forming a crystalline 2:1 inclusion compound.     

Applying the Z method to estimate temperatures of melting in structure II clathrate hydrates

Equilibrium melting temperatures for structure II THF hydrate and argon/xenon (Ar/Xe) binary hydrate have been calculated using molecular dynamics using two melting techniques, namely the Z method [Belonoshko et al., Phys. Rev. B, 2006, 73, 012201] (applied for the first time to complex molecular solids) and direct phase coexistence simulations. The two methods give results in moderate agreement: calculations with the Z method give T(fus) to be 250.7 K (0.77 katm) for THF and 244.3 K (1.86 katm) for Ar/Xe hydrate respectively; the corresponding direct phase coexistence calculations give T(fus) in the range 235-240 K (0.77 katm) for THF and 240-252.5 K (1.86 katm) for Ar/Xe hydrate. The Z method was found to define the key thermodynamic states with high precision, although required long simulation times with these multicomponent molecular systems to ensure the complete melting required by the method. In contrast, the direct phase coexistence method did bracket the equilibrium temperature with little difficulty, but small thermodynamic driving forces close to phase equilibrium generated long-lived fluctuations, that obscured the precise value of phase coexistence conditions within the bracketed range.     

Nuclear magnetic resonance parameters for methane molecule trapped in clathrate hydrates

The calculations of the nuclear shielding and spin-spin coupling constants were carried out for two models of clathrate hydrates, 5(12) and 5(12)6(8), using the density functional theory three-parameter Becke-Lee-Yang-Parr method with the basis set aug-cc-pVDZ (optimization) and HuzIII-su3 (NMR parameters). Particular attention has been devoted to evaluate the influence of a geometrical arrangement, the effect of long-range interactions on the NMR shielding of methane molecule, and to predict whether (13)C and (1)H chemical shifts can distinguish between guests in two clathrate hydrates cages. The correlation of the changes in the (17)O shielding constants depend strongly on the hydrogen-bonding topology. The intermolecular hydrogen-bond transmitted (1h)J(OH) spin-spin coupling constants are substantial. The increase of their values is connected with the elongation of the intramolecular O-H bond and the shortening of the intermolecular O···H distance. These data suggests that hydrogen bonds between double donor-single acceptor (DDA)-type water molecules acting as a proton acceptor from single donor-double acceptor (DAA)-type water molecules are stronger than ones formed by DAA-type water molecules acting as an acceptor for a DDA water proton. These state-of-the-art calculations confirmed the earlier experimental findings of the cage-dependency of (13)C chemical shift of methane.     

Allokiric solutions based on clathrate hydrates in tetrabutylammonium fluoride-ammonium fluoride-water system

Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 5, pp. 88–95, September–October, 1992.