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.     

Some structural and thermodynamic studies of clathrate hydrates

X-ray and neutron diffraction studies show argon and krypton to preferentially form clathrate hydrates of structure II, rather than structure I as previously assumed; methane and hydrogen sulphide do form structure I. Re-examination of solid-solution thermodynamic theory shows that structure II is basically the more stable; structure I is generally formed only when the guest molecule is in the size range that favours occupancy of the 14-hedral over the 12-hedral cages. For molecules too large to enter the 12-hedra the relative stability of structure II is greatest at 0°C, in agreement with the observed sequence of change of stability of cyclopropane hydrate: I to II at –16° and II to I at 1.5°. Carbon dioxide hydrate is observed to decompose on prolonged standing at 105K in accord with the low-temperature instability predicted by Miller.     

On the thermodynamic stability of clathrate hydrates IV: double occupancy of cages

We have extended the van der Waals and Platteeuw theory to treat multiple occupancy of a single cage of clathrate hydrates, which has not been taken into account in the original theory but has been experimentally confirmed as a real entity. We propose a simple way to calculate the free energy of multiple cage occupancy and apply it to argon clathrate structure II in which a larger cage can be occupied by two argon atoms. The chemical potential of argon is calculated treating it as an imperfect gas, which is crucial to predict accurate pressure dependence of double occupancy expected at high pressure. It is found that double occupancy dominates over single occupancy when the guest pressure in equilibrium with the clathrate hydrate exceeds 270 MPa.     

Dynamic and thermodynamic properties of clathrate hydrates

Vibrational spectra and thermodynamic properties of ices and the cubic structure I (CS-I) clathrate hydrate have been studied by the lattice dynamics method. The phonon density of states for the empty hydrate framework and for xenon hydrate have been determined; the vibrational frequencies of the guest molecules in large and small cavities have been found. The stability of the hydrate with respect to the external pressure at low temperatures and its thermodynamic stability at temperatures around 0°C have been studied. It has been found that the empty hydrate framework is unstable in certain temperature and pressure regions. A definite degree of occupation of the large cavities by the guest molecules is necessary for the hydrate to become stable. It has been found that there is a maximum of the critical temperature at which the hydrate exists, which is a function of the external pressure.Dedicated to Dr. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Kinetic and thermodynamic behaviour of CF4 clathrate hydrates

This study presents experimental kinetic and thermodynamic data for CF₄ clathrate hydrates. Experimental measurements were undertaken in a high pressure equilibrium cell with a 40 cm³ inner volume. The measurements of experimental hydrate dissociation conditions were performed in the temperature range of (273.8 to 278.3) K and pressures ranging from (4.55 to 11.57) MPa. A thermodynamic model based on van der Waals and Platteeuw (vdW–P) solid solution theory was used for prediction and comparison of hydrate dissociation conditions and the Langmuir constant parameters for CF₄ based on Parrish and Prausnitz equation are reported. For the kinetics, the effect of initial pressure and temperature on the induction time, CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion during the hydrate formation were studied. Kinetic experiments were performed at initial temperatures of (275.3, 276.1 and 276.6) K and initial pressures of (7.08, 7.92, 9.11, 11.47 and 11.83) MPa. Results show that increasing the initial pressure at constant temperature decreases the induction time, while CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion increase. The same trends are observed with a decrease in the initial temperature at constant pressure.     

Using structural data for estimating the stability of water networks in clathrate and semiclathrate hydrates

Methods are discussed for estimating the energy of formation of a clathrate network from ice Ih based on the structural data for clathrate and semiclathrate hydrates. The energies of real networks in clathrate hydrates and of ideal undistorted networks (8 structures) are calculated from the crystal structure data using the Zimmerman-Pimentel quasiharmonic potential. For semiclathrate hydrates, two versions of estimation are suggested-Estimations for 7 hydrates of amines and quaternary ammonium base salts showed that formation of semiclathrate hydrates requires much energy for network deformation. Two methods are described for estimating the network energies according to topological information. The first technique involves calculating the number of cycles with different numbers of sides per unit cell; this method is applicable to networks involving strained cycles with 4, 7, and 8 sides. The second method represents the lattice energy via the partial energies of polyhedron units forming the lattice; it is applicable to the most widespread class of networks constructed from Allen’s polyhedra. The energies of the tetragonal and orthorhombic networks belonging to this class are estimated. A relative stability series involving 9 structures of empty undistorted networks is formed. Translated fromZhurnal Struktumoi Khimii, Vol. 40, No. 2, pp. 287–295, March–April, 1999.     

Molecular dynamics study of the stability of methane structure H clathrate hydrates

Molecular dynamics simulations are used to study the stability of structure H (sH) methane clathrate hydrates in a 3 x 3 x 3 sH unit cell replica. Simulations are performed at experimental conditions of 300 K and 2 GPa for three methane intermolecular potentials. The five small cages of the sH unit cell are assigned methane guest occupancies of one and large cage guest occupancies of one to five are considered. Radial distribution functions, unit cell volumes, and configurational energies are studied as a function of large cage CH(4) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies. Large cage occupancy of five is the most stable configuration for a Lennard-Jones united-atom potential and the Tse-Klein-McDonald potential parametrized for condensed methane phases and two for the most stable configuation for the Murad and Gubbins potential.     

The influence of pressure on the stability of clathrate hydrates of hydrogen and tetrahydrofuran

Changes in the Gibbs energy of hydration of molecular hydrogen and tetrahydrofuran (THF) at pressures of 0.1, 6.0, and 12.0 MPa over the temperature range 230–300 K were studied by the molecular dynamics method. The Gibbs energy of hydrogen in water-tetrahydrofuran-hydrogen solutions passed minima over the temperature range 235–265 K, which were indicative of a comparatively stable clathrate hydrate state. The Gibbs energy of the hydrogen molecule at the local minimum at 262 K was ∼4.5 kJ/mol; at atmospheric pressure and room temperature, it was ∼2 kJ/mol. An analysis of the radial distribution function and the coordination number of the THF molecule showed that, at 240–257 K, a clathrate hydrate of THF with the structure close to clathrate sII was predominantly formed.     

Thermodynamic studies of clathrate hydrates

Thermodynamic studies of clathrate hydrates, mainly of structures I and II, are considered in this review which is based on 147 references. There are two main subjects. The first is the host lattice energy and the guest-host interaction energy, both of these quantities being related to the enthalpy of dissociation and composition of the hydrates. The second subject concerns orientational ordering phenomena occurring in both host and guest, as reflected in the low temperature heat capacity. The classical theoretical treatment of clathrate formation has been reconsidered on the basis of recent experimental results. Particular emphasis has been given to orientational ordering since this topic is undoubtedly central to clarifying the nature of clathrate hydrates.Ausgehend von 147 Literaturangaben wurden in diesem Review thermodynamische Untersuchungen von Klathrathydraten hauptsächlich der Struktur I und II betrachtet. Es gibt zwei Hauptaugenmerke. Als erstes die Wirtsgitterenergie und die Gast-Wirt-Wechselwirkungsenergie, beide bezogen auf die Dissoziationsenthalpie und die Bildungsenthalpie der Hydrate. Das zweite Hauptaugenmerk betrifft Orientierungs-Konditionierungserscheinungen sowohl in Wirt als auch Gast, wie in den Wärmekapazitäten bei niedrigen Temperaturen widergespiegelt wird. Auf der Basis jüngster experimenteller Ergebnisse wurde die klassische theoretische Betrachtung über die Bildung von Klathraten überprüft. Der Orientierung-Konditionierung wurde besonderer Nachdruck verliehen, da dies zweifellos eine entscheidende Rolle bei der Klärung der Natur der Klathrathydrate spielt. 147 I II. . «» « — », . «» « », . . , .

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Contribution No. 155 from the Chemical Thermodynamics Laboratory.     

Structure transition and swapping pattern of clathrate hydrates driven by external guest molecules

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.     

Heat capacity and transition phenomena of structure I and II trimethylene oxide clathrate hydrates

Heat capacities of structure I and II trimethylene oxide (TMO) clathrate hydrates doped with small amount of potassium hydroxide (x=1.8×10^–4 to water) were measured by an adiabatic calorimeter in the temperature range 11–300 K. In the str. I hydrate (TMO·7.67H₂O), a glass transition and a higher order phase transition were observed at 60 K and 107.9 K, respectively. The glass transition was considered to be due to the freezing of the reorientation of the host water molecules, which occurred around 85 K in the pure sample and was lowered owing to the acceleration effect of KOH. The relaxation time of the water reorientation and its distribution were estimated and compared with those of other clathrate hydrates. The phase transition was due to the orientational ordering of the guest TMO molecules accommodated in the cages formed by water molecules. The transition was of the higher order and the transition entropy was 1.88 J·K^–1(TMO-mol)^–1, which indicated that at least 75% of orientational disorder was remaining in the low temperature phase. In the str. II hydrates (TMO·17H₂O), only one first-order phase transition appeared at 34.5 K. This transition was considered to be related to the orientational ordering of the water molecules as in the case of the KOH-doped acetone and tetrahydrofuran (THF) hydrates. The transition entropy was 2.36 JK^–1(H₂O-mol)^–1, which is similar to those observed in the acetone and THF hydrates. The relations of the transition temperature and entropy to the guest properties (size and dipole moment) were discussed.Contribution No 57 from the Microcalorimetry Research CenterThe authors would like to express their sincere thanks to the Nissan Science Foundation for their financial support.     

Thermal expansion of structure-H clathrate hydrates

The temperature dependence of the unit cell parameters of two newly identified hexagonal structure clathrate hydrates of hexamethylethane (HME) and 2,2-dimethylbutane (DMB) have been measured by X-ray powder diffraction. The thermal expansion of the two distinct crystallographic axes was found to be inequivalent. However, the coefficients of cubic expansion are comparable to that in the cubic structure I and II hydrates. The larger thermal expansivity in the clathrate hydrates relative to ice is attributed to the weakening of the host lattice due to the internal pressure generated by the rattling motions of the encaged guests.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

The thermodynamic stability of clathrate hydrate. Encaging non-spherical propane molecules

The thermodynamic stability of a clathrate hydrate encaging non-spherical molecules has been investigated by examining the free energy of cage occupancy. In the present study, a generalized van der Waals and Platteeuw theory is extended to treat the rotational motion of guest molecules in clathrate hydrate cages. The vibrational free energy of both guest and host molecules is divided into harmonic and anharmonic contributions. The anharmonic free energy associated with the non-spherical nature of the guest molecules is evaluated as a perturbation from the spherical guest. Predicted thermodynamic properties are compared with measured values. It is shown that this anharmonic contribution is important in the free energy of the hindered rotation of the guests.     

Thermodynamic stability of type-I and type-II clathrate hydrates depending on the chemical species of the guest substances

The free energy differences are calculated for various type-I and type-II clathrate hydrates based on molecular-dynamics simulations, thereby evaluating the thermodynamic stability of the hydrates depending on the chemical species of the guest substances. The simulation systems consist of 27 unit cells, that is, 1242 water molecules and 216 guest molecules for type-I hydrates, and 3672 water molecules and 648 guest molecules for type-II hydrates. The water molecules are described by TIP4P potential, while the guest molecules are described by one-site Lennard-Jones potential, U=4epsilon{(sigma/r)12-(sigma/r)6}, where U is the potential energy, r is the particle distance, sigma is the particle diameter, and epsilon is the energy well depth. The optimal values of sigma that yield the minimum free energy (the best thermodynamic stability) were determined to be 0.39 nm for the type-I hydrates and 0.37 nm for the type-II hydrates.     

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.     

Thermodynamic modeling for clathrate hydrates of ozone

We report a theoretical study to predict the phase-equilibrium properties of ozone-containing clathrate hydrates based on the statistical thermodynamics model developed by van der Waals and Platteeuw. The Patel–Teja–Valderrama equation of state is employed for an accurate estimation of the properties of gas phase ozone. We determined the three parameters of the Kihara intermolecular potential for ozone as a = 6.815 · 10⁻² nm, σ = 2.9909 · 10⁻¹ nm, and ε · k_B⁻¹ = 184.00 K. An infinite set of ε–σ parameters for ozone were determined, reproducing the experimental phase equilibrium pressure–temperature data of the (O₃ + O₂ + CO₂) clathrate hydrate. A unique parameter pair was chosen based on the experimental ozone storage capacity data for the (O₃ + O₂ + CCl₄) hydrate that we reported previously. The prediction with the developed model showed good agreement with the experimental phase equilibrium data within ±2% of the average deviation of the pressure. The Kihara parameters of ozone showed slightly better suitability for the structure-I hydrate than CO₂, which was used as a help guest. Our model suggests the possibility of increasing the ozone storage capacity of clathrate hydrates (∼7% on a mass basis) from the previously reported experimental capacity (∼1%).     

Crystal lattice size and stability of type H clathrate hydrates with various large-molecule guest substances

To gain a better understanding of the effects of guest molecules on the lattice and stability of type H hydrates, we performed powder X-ray diffraction (PXRD) measurements and semiempirical molecular orbital calculations. The unit cell parameters and cohesive energies of various type H hydrates that contain methane (CH4) were analyzed. PXRD measurements indicated that an increase in the large-molecule guest volume caused the unit cell volume to increase. It was also indicated that a large-molecule guest substance caused the a-axis-direction of the unit cell to increase with little decrease in the c-axis direction. Calculations of cohesive energy by means of a semiempirical molecular orbital method indicated that the functional group and configuration of large-molecule guest substances affects the stability of type H hydrates. It was concluded that the icosahedron (5(12)6(8)) cages do not easily increase in length along the c-axis direction when larger guest molecules are used to form the hydrate, but the 5(12)6(8) cage and the layer of dodecahedron (5(12)) cages can easily increase in length along the a-axis direction due to interactions of the guest-host molecule.