Distribution of Butane in the Host Water Cage of Structure II Clathrate Hydrates

Geometric and Hydrophilic Effects of Oxirane Compounds with a Four-Carbon Backbone on Clathrate Hydrate Formation

The physicochemical properties of clathrate hydrates are influenced by the chemical nature and three-dimensional (3D) geometry of the added molecules. This study investigates the effects of five oxirane compounds: cis-2,3-epoxybutane (c23EB), trans-2,3-epoxybutane (t23EB), 1,2-epoxybutane (12EB), 1,2,3,4-diepoxybutane (DEB), and 3,3-dimethylepoxybutane (33DMEB) on CH4 hydrate formation. Despite having a four-carbon backbone, these compounds differ in their 3D geometries. The structures and stabilities of CH4 hydrates containing each compound were analyzed using high-resolution powder diffraction, solid-state 13C NMR, and phase equilibrium measurements. The experimental results revealed that c23EB, 12EB, and 33DMEB act as sII/sH hydrate formers and thermodynamic promoters, whereas t23EB and DEB have opposite roles. These results were analyzed in relation to the 3D geometries and relative stabilities of various rotational isomers using DFT calculations. Hydrate structure was influenced by both the length and thickness of the added compounds. Moreover, an appropriate level of (not excessive) hydrophilicity induced by an oxirane group appeared to enhance the thermodynamic stability of the hydrates. This study provides insights into how the chemical nature of additives influences the structure and stability of clathrate hydrates.

On the size, shape and energetics of the hydration shell around alkanes

A large number of clathrate-like cages have been proposed as the very first hydration shell of alkanes. The cages include canonical structures commonly found in clathrate hydrates and many others, not previously reported, derived from the carbon fullerene cavities. These structures have a rich and variegated form, which can adapt to the shape and conformation of the solute. They avoid "wasting" hydrogen bonds, while minimizing the volume cage and maximizing the solute-solvent van der Waals interactions. DFT/M06-2X and MP2 ab initio calculations give comparable structural and energetic results although the latter predicts slightly larger cages for a given solute. It is shown that the van der Waals interactions are substantial and the large exoenergetic values found for isobutane and cyclopentane provide an explanation for the surprising high melting points of related hydrates at room pressure. The encaging enthalpy for various hydrocarbons is similar to the enthalpy of solution measured at a temperature just above the melting point of aqueous hydrocarbon solutions, thus indicating that water molecules should not deviate too much from the configuration with O-H bonds tangentially oriented with respect to the solute surface. The computed trend differs from the enthalpy of solution measured at room temperature, thus the very first hydration shell departs, up to a certain degree, from the clathrate-like structures.

Dissociation kinetics of propane-methane and butane-methane hydrates below the melting point of ice

Understanding the dissociation mechanism of gas hydrates below the melting point of ice is crucial for expanding the practical applications of solid hydrates in gas storage. The kinetic processes for gas hydrates have not been clarified, except for those of pure CH4 hydrate and CO2 hydrates. In this study, using in situ X-ray diffraction analysis, the low-temperature onset of the dissociation of C3H8 and C4H10 hydrate fine particles encapsulating CH4 as a secondary guest was investigated during temperature ramping. At ∼200 K, the C3H8 + CH4 hydrate, n-C4H10 + CH4 hydrate, and iso-C4H10 + CH4 hydrate all dissociated in a single step, similar to pure C3H8 and pure iso-C4H10 hydrate. The dissociation of C3H8 hydrate was also found to accelerate the dissociation of CH4 hydrate. Based on the experimental results, it was confirmed that the C3H8 and C4H10 molecules released from the dissociating hydrates accelerated hydrate dissociation.

Managing hydrogen bonding in the clathrate hydrate of the 1-pentanol guest molecule

1-pentanol, long-chain alcohol molecule, can be encaged in the clathrate hydrate by managing the destabilizing influence of guest–host hydrogen bonding.

Temperature effects on the C-H symmetric stretching vibrational frequencies of guest hydrocarbon molecules in 512, 51262 and 51264 cages of sI and sII clathrate hydrates

C–H symmetric stretching vibrational frequencies of CH₄, C₂H₄ and C₂H₆ molecules encapsulated in 5¹², 5¹²6² and 5¹²6⁴ cages of structures I (sI) and II (sII) clathrate hydrates measured by Raman spectroscopy in the temperature range of 93–183 K was analysed. The slopes of the symmetric stretch vibrational frequencies under changing temperatures (Δv/ΔT) for CH₄, C₂H₄ and C₂H₆ molecules encapsulated in sII 5¹²6⁴ cages were smaller than those for molecules in sI 5¹²6² cages, although sI 5¹²6² cages are smaller than sII 5¹²6⁴ cages. We compared the results of Δv/ΔT in this study with the geometrical properties of each host water cage, and these comparisons suggest that the geometry of host water cages affects Δv/ΔT.

Temperature effects on the C–H symmetric stretch of hydrocarbons in various cages of sI and sII clathrate hydrates were observed.

Structure and Density Comparison of Noble Gas Hydrates Encapsulating Xenon, Krypton and Argon

Understanding the effect of guest species on the host framework is important for the development of structure-based properties of inclusion compounds. Herein, the crystal structures of the noble gas hydrates encapsulating Xe, Kr, and Ar were studied by powder X-ray diffraction measurements. The crystal structures and hydration numbers of these noble gas hydrates were solved by Rietveld refinements using optimized models with the direct-space technique. It was revealed that host cage size of these hydrates changed depending on the type of guest species even though their unit-cell parameters were the same. Based on the structure models obtained, the densities of Xe, Kr, and Ar gas hydrates were also determined to be 1.837, 1.445 and 1.097 g/cm³ at 93 K, respectively. Our findings, from a crystallographic point of view, may give insight into further understanding the thermodynamic stability and physical properties of gas hydrates encapsulating small guests.

Stability and characterization of the structure II binary clathrate hydrate of the refrigerant trans-1,3,3,3-tetrafluoropropene + methane

A new clathrate hydrate formed with trans-1,3,3,3-tetrafluoropropene and methane was characterized by phase equilibrium and PXRD measurements and MD simulations.

Thermodynamic stabilization of semiclathrate hydrates by hydrophilic group

Introducing hydrophilic groups into carboxylates is a way to modify semiclathrate hydrate frameworks and change the properties of the hydrates.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability

In this study, we investigate the crystal structures and phase equilibria of butanols+CH₄ +H₂ O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X-ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hydrates without any hydrogen-bonding interactions between the hydroxy group of the guests and the water-host framework. However, Raman spectra of the binary (1-butanol+CH₄ ) hydrate do not show the free OH signal, indicating that there could be possible hydrogen-bonding interactions between the guests and hosts. We also measure the four-phase equilibrium conditions of the butanols+CH₄ +H₂ O systems.