Multiple H2 occupancy of cages of clathrate hydrate under mild conditions

Experiments were carried out by reacting H(2) gas with N(2) hydrate at a temperature of 243 K and a pressure of 15 MPa. The characterizations of the reaction products indicated that multiple H(2) molecules can be loaded into both large and small cages of structure II clathrate hydrates. The realization of multiple H(2) occupancy of hydrate cages under moderate conditions not only brings new insights into hydrogen clathrates but also refreshes the perspective of clathrate hydrates as hydrogen storage media.     

Phase relations and binary clathrate hydrate formation in the system H2-THF-H2O

Experimentally determined equilibrium phase relations are reported for the system H2-THF-H2O as a function of aqueous tetrahydrofuran (THF) concentration from 260 to 290 K at pressures up to 45 MPa. Data are consistent with the formation of cubic structure-II (CS-II) binary H2-THF clathrate hydrates with a stoichiometric THF-to-water ratio of 1:17, which can incorporate modest volumes of molecular hydrogen at elevated pressures. Direct compositional analyses of the clathrate phase, at both low (0.20 mol %) and stoichiometric (5.56 mol %) initial THF aqueous concentrations, are consistent with observed phase behavior, suggesting full occupancy of large hexakaidecahedral (51264) clathrate cavities by THF, coupled with largely complete (80-90%) filling of small dodecahedral (512) cages by single H2 molecules at pressures of >30 MPa, giving a clathrate formula of (H2) < or =2.THF.17H2O. Results should help to resolve the current controversy over binary H2-THF hydrate hydrogen contents; data confirm recent reports that suggest a maximum of approximately 1 mass % H2, this contradicting values of up to 4 mass % previously claimed for comparable conditions.     

Double clathrate hydrates of tetrabutylammonium fluoride + helium, neon, hydrogen and argon at high pressures

Decomposition curves of double ionic clathrate hydrates of tetrabutylammonium fluoride with helium, neon, hydrogen and argon were studied at pressures up to 800 MPa. Formation of double hydrates with helium, neon and hydrogen does not lead to any significant increase of the temperatures of decomposition of these hydrates; at high temperatures the hydrates may decompose even at lower temperatures than the hydrate of pure tetraalkylammonium salt does. Decomposition temperatures of double hydrates with argon in all cases were 4–8 °C higher in comparison with the decomposition temperature of ionic clathrate hydrates of tetrabutylammonium fluoride. We suppose that this behavior is caused by simultaneous effect of three factors on hydrate decomposition temperature: (1) partial filling of the small cavities in the framework of the hydrate with water molecules, (2) weakness of the van der Waals interactions between the gas molecules and the host water molecules, and (3) dissolution of helium, hydrogen and neon in the solution of tetrabutylammonium salt causing a decrease of melting temperatures of the hydrates formed from these solutions.     

Phase equilibria of hydrogen sulphide clathrate hydrates in the presence of single or mixed salt aqueous solution

This work reports the dissociation pressures of hydrogen sulphide clathrate hydrates in the presence of single and mixed aqueous solutions of NaCl, KCl and CaCl₂ at different temperatures and various concentrations of salt(s) in aqueous solution. The equilibrium data were generated using an isochoric pressure-search method. These data are compared with some selected experimental data from the literature on the dissociation conditions of hydrogen sulphide clathrate hydrates in the presence of pure water to show the inhibition effects of the aforementioned aqueous solutions. Comparisons between our experimental data and the corresponding literature data show some disagreements in the literature data.     

High-pressure gas hydrates

It has long been known that crystalline hydrates are formed by many simple gases that do not interact strongly with water, and in most cases the gas molecules or atoms occupy 'cages' formed by a framework of water molecules. The majority of these gas hydrates adopt one of two cubic cage structures and are called clathrate hydrates. Notable exceptions are hydrogen and helium which form 'exotic' hydrates with structures based on ice structures, rather than clathrate hydrates, even at low pressures. Clathrate hydrates have been extensively studied because they occur widely in nature, have important industrial applications, and provide insight into water-guest hydrophobic interactions. Until recently, the expectation-based on calculations-had been that all clathrate hydrates were dissociated into ice and gas by the application of pressures of 1 GPa or so. However, over the past five years, studies have shown that this view is incorrect. Instead, all the systems so far studied undergo structural rearrangement to other, new types of hydrate structure that remain stable to much higher pressures than had been thought possible. In this paper we review work on gas hydrates at pressures above 0.5 GPa, identify common trends in transformations and structures, and note areas of uncertainty where further work is needed.     

Phase equilibrium measurements of structure II clathrate hydrates of hydrogen with various promoters

Phase equilibrium measurements of single and mixed organic clathrate hydrates with hydrogen were determined within a pressure range of 2.0-14.0 MPa. The organic compounds studied were furan, 2,5-dihydrofuran, tetrahydropyran, 1,3-dioxolane and cyclopentane. These organic compounds are known to form structure II clathrate hydrates with water. It was found that the addition of hydrogen to form a mixed clathrate hydrate increases the stability compared to the single organic clathrate hydrates. Moreover, the mixed clathrate hydrate also has a much higher stability compared to a pure hydrogen structure II clathrate hydrate. Therefore, the organic compounds act as promoter materials. The stabilities of the single and mixed organic clathrate hydrates with hydrogen showed the following trend in increasing order: 1,3-dioxolane < 2,5-dihydrofuran < tetrahydropyran < furan < cyclopentane, indicating that both size and geometry of the organic compound determine the stability of the clathrate hydrates.     

Physicochemical Properties of Ionic Clathrate Hydrates

Ionic clathrate hydrates are known to be formed by the enclathration of hydrophobic cations or anions into confined cages and the incorporation of counterions into the water framework. As the ionic clathrate hydrates are considered for their potential applicability in various fields, including those that involve solid electrolytes, gas separation, and gas storage, numerous studies of the ionic clathrate hydrates have been reported. This review concentrates on the physicochemical properties of the ionic clathrate hydrates and the notable characteristics of these materials regarding their potential application are addressed.     

A molecular dynamics study of ethanol-water hydrogen bonding in binary structure I clathrate hydrate with CO2

Guest-host hydrogen bonding in clathrate hydrates occurs when in addition to the hydrophilic moiety which causes the molecule to form hydrates under high pressure-low temperature conditions, the guests contain a hydrophilic, hydrogen bonding functional group. In the presence of carbon dioxide, ethanol clathrate hydrate has been synthesized with 10% of large structure I (sI) cages occupied by ethanol. In this work, we use molecular dynamics simulations to study hydrogen bonding structure and dynamics in this binary sI clathrate hydrate in the temperature range of 100-250 K. We observe that ethanol forms long-lived (>500 ps) proton-donating and accepting hydrogen bonds with cage water molecules from both hexagonal and pentagonal faces of the large cages while maintaining the general cage integrity of the sI clathrate hydrate. The presence of the nondipolar CO(2) molecules stabilizes the hydrate phase, despite the strong and prevalent alcohol-water hydrogen bonding. The distortions of the large cages from the ideal form, the radial distribution functions of the guest-host interactions, and the ethanol guest dynamics are characterized in this study. In previous work through dielectric and NMR relaxation time studies, single crystal x-ray diffraction, and molecular dynamics simulations we have observed guest-water hydrogen bonding in structure II and structure H clathrate hydrates. The present work extends the observation of hydrogen bonding to structure I hydrates.     

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.     

Hydrogen storage in sH hydrates: a Monte Carlo study

Grand canonical Monte Carlo simulations are performed to evaluate the hydrogen-storage capacity of the recently discovered hydrogen hydrates of the sH type, at 274 K and up to 500 MPa. First, the pure H2 hydrate is investigated in order to determine the upper limit of H 2 content in sH hydrates. It is found that the storage capacity of the hypothetical pure H2 hydrate could reach 3.6 wt % at 500 MPa. Depending on pressure, the large cavity of this hydrate can accommodate up to eight H2 molecules, while the small and medium ones are singly occupied even at pressures as high as 500 MPa. Next, the binary H2-methylcyclohexane sH hydrate is examined. In this case, the small and medium cavities are again singly occupied, resulting in a maximum H2 uptake of 1.4 wt %. Finally, the results from simulations on pure H2 and binary hydrates are utilized to investigate the potential of H2 storage in sH hydrates where the promoter molecules occupy the medium instead of the large cavities.     

Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature.     

A new method for screening potential sII and sH hydrogen clathrate hydrate promoters with model potentials

A new predictive computational method for classifying clathrate hydrate promoter molecules is presented, based on the interaction energies between potential promoters and the water networks of sII and sH clathrates. The motivation for this work is identifying promoters for storing hydrogen compactly in clathrate hydrates. As a first step towards achieving this goal, we have developed a general method aimed at distinguishing between molecules that form sII clathrate hydrates and molecules that can-together with a weakly interacting help gas-form sH clathrate hydrates. The new computational method calculates differences in estimated formation energies of the sII and the sH clathrate hydrate. Model interaction potentials have been used, including the electrostatic interactions with newly calculated partial charges for all the considered potential promoter molecules. The methodology can discriminate between the clathrate structure types (sII or sH) formed by each potential promoter with good selectivity, i.e., better than achieved with a simple van der Waals diameter criterion.     

High-pressure reactivity of clathrate hydrates by two-photon dissociation of water

It is shown that photoinduced reactions are observed at room temperature and pressure of few tenths of gigapascal in clathrate hydrates of CO and of model hydrocarbons under mild irradiation at 350 nm with power in the 50-610 mW range. The reactions are triggered by highly reactive OH radicals produced by two-photon excitation of the lowest electronic excited state of water having dissociative character. The formation of CO(2) is observed in all the reactions involving carbonaceous clathrate hydrates, and direct or indirect evidence for the formation of molecular hydrogen is obtained. The CO(2) produced in the reactions can be sequestered as a clathrate hydrate whose stability range seems to extend to room temperature at pressures of 0.5-0.6 GPa. Although the N(2) hydrate is stable up to 0.9 GPa under irradiation, a partial cleavage of the N-N triple bond is produced once the hydrate decomposes at 0.1 GPa.     

Molecular dynamics simulations of binary structure H hydrogen and methyl-tert-butylether clathrate hydrates

Binary structure H (sH) hydrogen and methyl-tert-butylether (MTBE) clathrate hydrates are studied with molecular dynamics simulations. Simulations on a 3 x 3 x 3 sH unit cell with up to 4.7 mass % hydrogen gas are run at pressures of 100 bars and 2 kbars at 100 and 273 K. For the small and medium cages of the sH unit cell, H2 guest molecule occupancies of 0, 1 (single occupancy), and 2 (double occupancy) are considered with the MTBE molecule occupying all of the large cages. An increase of the small and medium cage occupancies from 1 to 2 leads to a jump in the unit cell volume and configurational energy. Calculations are also set up with 13, 23, and 89 of the MTBE molecules in the large cages replaced by sets of three to six H2 molecules, and the effects on the configurational energy and volume of the simulation cell are determined. As MTBE molecules are replaced with sets of H2 guests in the large cages, the configurational energy of the unit cell increases. At the lower temperature, the energy and volume of the clathrate are not sensitive to the number of hydrogen guests in the large cages; however, at higher temperatures the repulsions among the H2 guest molecules in the large cages cause an increase in the system energy and volume.     

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.     

Gas hydrate formation in the system C2H6–H2–H2O at pressures up to 250 MPa

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.     

Water cavities of sH clathrate hydrate stabilized by molecular hydrogen: phase equilibrium measurements

In this experimental phase equilibrium study, we show for the first time that it is possible to stabilize structure sH of hydrogen clathrate hydrate with the help of some selected promoters. It was established that the formation pressures of these systems are significantly higher than that of structure sII of hydrogen clathrate hydrate when tetrahydrofuran (THF) is used as a promoter. Although no experimental evidence is available yet, it is estimated that the hydrogen storage capacity of structure sH can be as high as 1.4 wt % of H2, which is about 40% higher compared to the hydrogen storage capacity in structure sII.     

Stability of rare gas structure H clathrate hydrates

Molecular dynamics simulations are used to study the stability of structure H (sH) clathrate hydrates with the rare gases Ne, Ar, Kr, and Xe. Simulations on a 3 x 3 x 3 sH unit cell replica are performed at ambient pressure at 40 and 100 K temperatures. The small and medium (s+m) cages of the sH unit cell are assigned rare gas guest occupancies of 1 and for large (l) cages guest occupancies of 1-6 are considered. Radial distribution functions for guest pairs with occupancies in the l-l, l-(s+m), and (s+m)-(s+m) cages are presented. The unit cell volumes and configurational energies are studied as a function of large cage occupancy for the rare gases. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at 100 K and 1 bar and 20 kbar pressures. These studies show that the most stable argon clathrate has five guests in the large cages. For krypton and xenon the most stable configurations have three and two guests in the large cages, respectively.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.