Single Crystals of Naturally Occurring Gas Hydrates: The Structures of Methane and Mixed Hydrocarbon Hydrates

Computational Analysis of Vibrational Spectra of Hydrogen Bonds in sII and sH Gas Hydrates

The amount of energy in natural gas hydrates is thought to be equivalent to twice that of all other fossil fuels combined. However, economic and safe energy recovery has remained a challenge till now. To develop a novel method of breaking the hydrogen bonds (HBs) surrounding the trapped gas molecules, we investigated the vibrational spectra of the HBs of gas hydrates with structure types II and H. Two models of 576-atom propane-methane sII hydrate and 294-atom neohexane-methane sH hydrate were built. A first-principles density functional theory (DFT) method was employed using the CASTEP package. The simulated spectra were in good agreement with the experimental data. Compared with the partial phonon density of states of guest molecules, we confirmed that the experimental infrared absorption peak in the terahertz region mainly arose from HB vibrations. By removing the components of guest molecules, we found that the theory of two kinds of hydrogen bond vibrational modes applies. The use of a terahertz laser to enable resonance absorption of HBs (at about 6 THz, to be tested) may therefore lead to the rapid melting of clathrate ice and release of guest molecules.

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

Studies revealed that gas hydrate cages, especially small cages, are incompletely filled with guest gas molecules, primarily associated with pressure and gas composition. The ratio of hydrate cages occupied by guest molecules, defined as cage occupancy, is a critical parameter to estimate the resource amount of a natural gas hydrate reservoir and evaluate the storage capacity of methane or hydrogen hydrate as an energy storage medium and carbon dioxide hydrate as a carbon sequestration matrix. As the result, methods have been developed to investigate the cage occupancy of gas hydrate. In this review, several instrument methods widely applied for gas hydrate analysis are introduced, including Raman, NMR, XRD, neutron diffraction, and the approaches to estimate cage occupancy are summarized.

Hydrogen Storage in Propane-Hydrate: Theoretical and Experimental Study

There have been studies on gas-phase promoter facilitation of H2-containing clathrates. In the present study, non-equilibrium molecular dynamics (NEMD) simulations were conducted to analyse hydrogen release and uptake from/into propane planar clathrate surfaces at 180–273 K. The kinetics of the formation of propane hydrate as the host for hydrogen as well as hydrogen uptake into this framework was investigated experimentally using a fixed-bed reactor. The experimental hydrogen storage capacity propane hydrate was found to be around 1.04 wt% in compare with the theoretical expected 1.13 wt% storage capacity of propane hydrate. As a result, we advocate some limitation of gas-dispersion (fixed-bed) reactors such as the possibility of having un-reacted water as well as limited diffusion of hydrogen in the bulk hydrate.

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.

Non-equilibrium molecular-dynamics study of electromagnetic-field-induced propane-hydrate dissociation

Non-equilibrium molecular-dynamics simulations have been performed for dissolution of planar propane-hydrate/water interfaces in externally-applied electromagnetic (e/m) fields in the microwave to far infrared range (∼2.45-200 GHz) at electric-field intensities up to 2.0 V/nm and at roughly 20 K over/under temperatures vis-à-vis the zero-field propane-hydrate melting point. Upon e/m-field application, there is a field-frequency threshold above which the dissociation rate drops significantly, with a plateau therein for larger-frequencies. It was found that higher intensity and lower frequency facilitates dissociation. Except in the presence of a thermal driving-force, the 10 GHz frequency shows more substantial rate-enhancement effect vis-à-vis static electric fields or, indeed, lower-frequency e/m fields.

Molecular-dynamics study of propane-hydrate dissociation: Fluctuation-dissipation and non-equilibrium analysis

Equilibrium and non-equilibrium molecular-dynamics (MD) simulations have been performed to investigate thermal-driven break-up of planar propane-hydrate interfaces in contact with liquid water over the 260-320 K range. Two types of hydrate-surface water-lattice molecular termination were adopted, at the hydrate edge with water, for comparison: a 001-direct surface cleavage and one with completed cages. Statistically significant differences in melting temperatures and initial break-up rates were observed between both interface types. Dissociation rates were observed to be strongly dependent on temperature, with higher rates at larger over-temperatures vis-à-vis melting. A simple coupled mass and heat transfer model, developed previously, was applied to fit the observed dissociation profiles, and this helps us to identify clearly two distinct hydrate-decomposition régimes; following a highly temperature-dependent break-up phase, a second well-defined stage is essentially independent of temperature, in which the remaining nanoscale, de facto two-dimensional system's lattice framework is intrinsically unstable. Further equilibrium MD-analysis of the two-phase systems at their melting point, with consideration of the relaxation times gleaned from the auto-correlation functions of fluctuations in a number of enclathrated guest molecules, led to statistically significant differences between the two surface-termination cases; a consistent correlation emerged in both cases between the underlying, non-equilibrium, thermal-driven dissociation rates sampled directly from melting with that from an equilibrium-MD fluctuation-dissipation approach.

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.

Calculations of NMR properties for sI and sII clathrate hydrates of methane, ethane and propane

Calculations of NMR parameters (the absolute shielding constants and the spin-spin coupling constants) for 5¹², 5¹²6² and 5¹²6⁴ cages enclathrating CH₄, C₂H₆ and C₃H₈ molecules are presented. The DFT/B3LYP/HuzIII-su3 level of theory was employed. The ¹³C shielding constants of guest molecules are close to available experimental data. In two cases (the ethane in 5¹² and the propane in 5¹²6² cages) the ¹³C shielding constants are reported for the first time. Inversion of the methyl/methylene ¹³C and ¹H shielding constants order is found for propane in the 5¹²6² cage. Topological criteria are used to interpret the changes of values of NMR parameters of water molecules and they establish a connection between single cages and bulk crystal.

Fundamentals and applications of gas hydrates

Fundamental understanding of gas hydrate formation and decomposition processes is critical in many energy and environmental areas and has special importance in flow assurance for the oil and gas industry. These areas represent the core of gas hydrate applications, which, albeit widely studied, are still developing as growing fields of research. Discovering the molecular pathways and chemical and physical concepts underlying gas hydrate formation potentially can lead us beyond flowline blockage prevention strategies toward advancing new technological solutions for fuel storage and transportation, safely producing a new energy resource from natural deposits of gas hydrates in oceanic and arctic sediments, and potentially facilitating effective desalination of seawater. The state of the art in gas hydrate research is leading us to new understanding of formation and dissociation phenomena that focuses on measurement and modeling of time-dependent properties of gas hydrates on the basis of their well-established thermodynamic properties.

Raman spectroscopic investigations on natural samples from the Integrated Ocean Drilling Program (IODP) Expedition 311: indications for heterogeneous compositions in hydrate crystals

Natural gas hydrates usually are found in the form of structure I, encasing predominantly methane in the hydrate lattices as guest molecules, sometimes also minor amount of higher hydrocarbons, CO2 or H2S. Raman spectroscopy is an approved tool to determine the composition of the hydrate phase. Thus, in this study Raman spectroscopic analyses have been applied to hydrate samples obtained from Integrated Ocean Drilling Program (IODP) Expedition 311 in two different approaches: studying the samples randomly taken from the hydrate core, and--as a new application--mapping small areas on the surface of clear hydrate crystals. The results obtained imply that the gas composition of hydrate, in terms of relative concentrations of CH4 and H2S, is not homogeneous over a core or even within a crystal. The mapping method yielded results with very high lateral resolution, indicating the coexistence of different phases with the same structure but different compositions within a hydrate crystal.