Water cavities of sH clathrate hydrate stabilized by molecular hydrogen

X-ray diffraction and Raman spectroscopic measurements confirm that molecular hydrogen can be contained within the small water cavities of a binary sH clathrate hydrate using large guest molecules that stabilize the large cavity. The potential increase in hydrogen storage could be more than 40% when compared with binary sII hydrates. This work demonstrates the stabilization of hydrogen in a hydrate structure previously unknown for encapsulating molecular hydrogen, indicating the potential for other inclusion compound materials with even greater hydrogen storage capabilities.     

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.     

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.     

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.     

Micro-Raman investigations of mixed gas hydrates

We report laser Raman spectroscopic measurements on mixed hydrates (clathrates), with guest molecules tetrahydrofuran (THF) and methane (CH₄), at ambient pressure and at temperatures from 175 to 280 K. Gas hydrates were synthesized with different concentrations of THF ranging from 5.88 to 1.46 mol%. In all cases THF molecules occupied the large cages of sII hydrate. The present studies demonstrate formation of sII clathrates with CH₄ molecules occupying unfilled cages for concentrations of THF ranging from 5.88 to 2.95 mol%. The Raman spectral signature of hydrates with 1.46 mol% THF are distinctly different; hydrate growth was non-uniform and structural transformation occurred from sII to sI prior to clathrate melting.     

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.     

Communication: Single crystal x-ray diffraction observation of hydrogen bonding between 1-propanol and water in a structure II clathrate hydrate

Single crystal x-ray crystallography is used to detect guest-host hydrogen bonding in structure II (sII) binary clathrate hydrate of 1-propanol and methane. X-ray structural analysis shows that the 1-propanol oxygen atom is at a distance of 2.749 and 2.788 ? from the closest clathrate hydrate water oxygen atoms from a hexagonal face of the large sII cage. The 1-propanol hydroxyl hydrogen atom is disordered and at distances of 1.956 and 2.035 ? from the closest cage water oxygen atoms. These distances are compatible with guest-water hydrogen bonding. The C-C-C-O torsional angle in 1-propanol in the cage is 91.47° which corresponds to a staggered conformation for the guest. Molecular dynamics studies of this system demonstrated guest-water hydrogen bonding in this hydrate. The molecular dynamics simulations predict most probable distances for the 1-propanol-water oxygen atoms to be 2.725 ?, and the average C-C-C-O torsional angle to be ~59° consistent with a gauche conformation. The individual cage distortions resulting from guest-host hydrogen bonding from the simulations are rather large, but due to the random nature of the hydrogen bonding of the guest with the 24 water molecules making up the hexagonal faces of the large sII cages, these distortions are not observed in the x-ray structure.     

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.     

Spectroscopic signatures of halogens in clathrate hydrate cages. 2. Iodine

UV-vis and Raman spectroscopy were used to study iodine molecules trapped in sII clathrate hydrate structures stabilized by THF, CH(2)Cl(2), or CHCl(3). The spectra show that the environment for iodine inside the water cage is significantly less perturbed than either in aqueous solution or in amorphous water-ice. The resonance Raman progression of I(2) in THF clathrate hydrate can be observed up to v = 6 when excited at 532 nm. The extracted vibrational frequency omega e = 214 +/- 1 cm(-1) is the same as that of the free molecule to within experimental error. At the same time, the UV-vis absorption spectrum of I(2) in the sII hydrate exhibits a relatively large, 1440 cm(-1), blue-shift. This is mainly ascribed to the differential solvation of the I(2) electronic states. We conclude that iodine in sII hydrate resides in a 5(12)6(4) cavity, in which the ground-state I(2) potential is not significantly perturbed by the hydrate lattice. In contrast, in water and in ice, the valence absorption band of I(2) is dramatically broadened and blue-shifted by 3000 cm(-1), and the resonance Raman scattering is effectively quenched. These observations are shown to be consistent with a strong interaction between water molecule and iodine through the lone pair of electrons on water as in the case of bromine in the same media. The results presented here, and the stability of other halogen hydrates, were used to test the predictions of simple models and force-field calculations of the host cage-guest association energy.     

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.     

Time-resolved in situ neutron diffraction studies of gas hydrate: transformation of structure II (sII) to structure I (sI).

We report the in situ observation from diffraction data of the conversion of a gas hydrate with the structure II (sII) lattice to one with the structure I (sI) lattice. Initially, the in situ formation, dissociation, and reactivity of argon gas clathrate hydrate was investigated by time-of-flight neutron powder diffraction at temperatures ranging from 230 to 263 K and pressures up to 5000 psi (34.5 MPa). These samples were prepared from deuterated ice crystals and transformed to hydrate by pressurizing the system with argon gas. Complete transformation from D(2)O ice to sII Ar hydrate was observed as the sample temperature was slowly increased through the D(2)O ice melting point. The transformation of sII argon hydrate to sI hydrate was achieved by removing excess Ar gas and exposing the hydrate to liquid CO(2) by pressurizing the Ar hydrate with CO(2). Results suggest the sI hydrate formed from CO(2) exchange in argon sII hydrate is a mixed Ar/CO(2) hydrate. The proposed exchange mechanism is consistent with clathrate hydrate being an equilibrium system in which guest molecules are exchanging between encapsulated molecules in the solid hydrate and free molecules in the surrounding gas or liquid phase.     

Effect of small cage guests on hydrogen bonding of tetrahydrofuran in binary structure II clathrate hydrates

Molecular dynamics simulations of the pure structure II tetrahydrofuran clathrate hydrate and binary structure II tetrahydrofuran clathrate hydrate with CO(2), CH(4), H(2)S, and Xe small cage guests are performed to study the effect of the shape, size, and intermolecular forces of the small cages guests on the structure and dynamics of the hydrate. The simulations show that the number and nature of the guest in the small cage affects the probability of hydrogen bonding of the tetrahydrofuran guest with the large cage water molecules. The effect on hydrogen bonding of tetrahydrofuran occurs despite the fact that the guests in the small cage do not themselves form hydrogen bonds with water. These results indicate that nearest neighbour guest-guest interactions (mediated through the water lattice framework) can affect the clathrate structure and stability. The implications of these subtle small guest effects on clathrate hydrate stability are discussed.     

Hydrogen in Porous Tetrahydrofuran Clathrate Hydrate

The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier. ¹ Storage within solid‐state clathrate hydrates, ² – ⁴ and in the clathrate hydrate of tetrahydrofuran (THF), has been recently reported. ⁵ , ⁶ In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that—in contrast to previous reports^{[5, 6]}—hydrogen (deuterium) occupies the small cages of the clathrate hydrate only to 30 % (at 274 K and 90.5 bar). Such a D₂ load is equivalent to 0.27 wt. % of stored H₂. In addition, we show that a surplus of D₂O results in the formation of additional D₂O ice Ih instead of in the production of sub‐stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. ⁶ ). Structure‐refinement studies show that [D₈]THF is dynamically disordered, while it fills each of the large cages of [D₈]THF?17D₂O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above ≈220 K, the H‐storage characteristics of the clathrate hydrate have similarities with those of surface‐adsorption materials, such as nanoporous zeolites and metal–organic frameworks, ⁷ , ⁸ but at lower temperatures, the adsorption rates slow down because of reduced D₂ diffusion between the small cages.     

Molecular-dynamics simulations of binary structure II hydrogen and tetrahydrofurane clathrates

The binary structure II hydrogen and tetrahydrofurane (THF) clathrates are studied with molecular-dynamics simulations. Simulations are done at pressures of 120 and 1.013 bars for temperatures ranging from 100 to 273 K. For the small cages of the structure II unit cell, H2 guest molecule occupancies of 0, 16 (single occupancy), and 32 (double occupancy) are considered. THF occupancies of 0-8 in the large cages are studied. For cases in which THF does not occupy all large cages in a unit cell, the remaining large cages can be occupied with sets of four H2 guest molecules. The unit-cell volumes and configurational energies are compared in the different occupancy cases. Increasing the small cage occupancy leads to an increase in the unit-cell volume and thermal-expansion coefficient. Among simulations with the same small cage occupancy, those with the large cages containing 4H2 guests have the largest volumes. The THF guest molecules have a stabilizing effect on the clathrate and the configurational energy of the unit cell decreases linearly as the THF content increases. For binary THF + H2 clathrates, the substitution of the THF molecules in the large cages with sets of 4H2 molecules increases the configurational energy. For the binary clathrates, various combinations of THF and H2 occupancies have similar configurational energies.     

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.     

Structure transition and tuning pattern in the double (tetramethylammonium hydroxide + gaseous guests) clathrate hydrates

In this study, we present an extraordinary structural transition accompanying the occurrence of more than two coexisting clathrate hydrate phases in the double (CH4 + tetramethylammonium hydroxide (Me(4)NOH)) and (H2 + Me(4)NOH) ionic clathrate hydrates using solid-state NMR spectroscopy (high-powered decoupling and CP/MAS) and powder X-ray diffraction. It was confirmed that structure-I (sI) and structure-II (sII) hydrates coexist as the water concentration increases. In the Me(4)NOH-depleted region, the unique tuning phenomenon was first observed at a chemical shift of -8.4 ppm where relatively small gaseous CH4 molecules partly occupy the sII large cages (sII-L), pulling out large cationic Me(4)N+ that is considered to be strongly bound with the surrounding host lattices. Moreover, we note that, while pure Me(4)NOH.16H(2)O clathrate hydrates melted at 249 K under atmospheric pressure conditions, the double (CH4 + Me(4)NOH) clathrate hydrate maintained a solid state up to approximately 283 K under 120 bar of CH4 with a conductivity of 0.065 S cm(-1), suggesting its potential use as a solid electrolyte. The present results indicate that ionic contributions must be taken into account for ionic clathrate hydrate systems because of their distinctive guest dynamic behavior and structural patterns. In particular, microscopic analyses of ionic clathrate hydrates for identifying physicochemical characteristics are expected to provide new insights into inclusion chemistry.     

Thermal expansivity of tetrahydrofuran clathrate hydrate with diatomic guest molecules

The guest dynamics and thermal behavior occurring in the cages of clathrate hydrates appear to be too complex to be clearly understood through various structural and spectroscopic approaches, even for the well-known structures of sI, sII, and sH. Neutron diffraction studies have recently been carried out to clarify the special role of guests in expanding the host water lattices and have contributed to revealing the influence factors on thermal expansivity. Through this letter we attempt to address three noteworthy features occurring in guest inclusion: (1) the effect of guest dimension on host water lattice expansion; (2) the effect of thermal history on host water lattice expansion; and (3) the effect of coherent/incoherent scattering cross sections on guest thermal patterns. The diatomic guests of H 2, D 2, N 2, and O 2 have been selected for study, and their size and mass dependence on the degree of lattice expansion have been examined, and four sII clathrate hydrates with tetrahydrofuran (THF) have been synthesized in order to determine their neutron powder diffraction patterns. After thermal cycling, the THF + H 2 clathrate hydrate is observed to exhibit an irreversible plastic deformation-like pattern, implying that the expanded lattices fail to recover the original state by contraction. The host-water cage dimension after degassing the guest molecules remains as it was expanded, and thus host-guest as well as guest-guest interactions will be altered if guest uptake reoccurs.     

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.     

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.