Phase Behavior and 13C NMR Spectroscopic Analysis of the Mixed Methane + Ethane + Propane Hydrates in Mesoporous Silica Gels

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.

CO Separation from H2 via Hydrate Formation in Single-Walled Carbon Nanotubes

Hydrogen is an alternative fuel without generating greenhouse gas or other harmful emissions. Industrial hydrogen production, however, always contains a small fraction of carbon monoxide (CO) (∼0.5-2%) that must be removed for use in fuel cells. Here, we present molecular dynamics simulation evidence on facile separation of CO from H₂ at ambient pressure via the formation of quasi-one-dimensional (Q1D) clathrate hydrates within single-walled carbon nanotubes (SW-CNTs). At ambient pressure, Q1D CO (or H₂) clathrates in SW-CNTs are formed spontaneously when the SW-CNTs are immersed in CO (or H₂) aqueous solution. More interestingly, for the CO/H₂ aqueous solution, highly preferential adsorption of CO over H₂ occurs within the octagonal or nonagonal ice nanotubes inside of SW-CNTs. These results suggest that the formation of Q1D hydrates within SW-CNTs can be a viable and safe method for the separation of CO from H₂, which can be exploited for hydrogen purification in fuel cells.

Enhanced CH₄ Recovery Induced via Structural Transformation in the CH₄/CO₂ Replacement That Occurs in sH Hydrates

The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the (13)C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 4(3)5(6)6(3) cages of sH hydrates or the large 5(12)6(2) cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.

Experimental verification of methane-carbon dioxide replacement in natural gas hydrates using a differential scanning calorimeter

The methane (CH4) - carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 - CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 - CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation.

Spectroscopic methods in gas hydrate research

Gas hydrates are crystalline structures comprising a guest molecule surrounded by a water cage, and are particularly relevant due to their natural occurrence in the deep sea and in permafrost areas. Low molecular weight molecules such as methane and carbon dioxide can be sequestered into that cage at suitable temperatures and pressures, facilitating the transition to the solid phase. While the composition and structure of gas hydrates appear to be well understood, their formation and dissociation mechanisms, along with the dynamics and kinetics associated with those processes, remain ambiguous. In order to take advantage of gas hydrates as an energy resource (e.g., methane hydrate), as a sequestration matrix in (for example) CO(2) storage, or for chemical energy conservation/storage, a more detailed molecular level understanding of their formation and dissociation processes, as well as the chemical, physical, and biological parameters that affect these processes, is required. Spectroscopic techniques appear to be most suitable for analyzing the structures of gas hydrates (sometimes in situ), thus providing access to such information across the electromagnetic spectrum. A variety of spectroscopic methods are currently used in gas hydrate research to determine the composition, structure, cage occupancy, guest molecule position, and binding/formation/dissociation mechanisms of the hydrate. To date, the most commonly applied techniques are Raman spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy. Diffraction methods such as neutron and X-ray diffraction are used to determine gas hydrate structures, and to study lattice expansions. Furthermore, UV-vis spectroscopic techniques and scanning electron microscopy (SEM) have assisted in structural studies of gas hydrates. Most recently, waveguide-coupled mid-infrared spectroscopy in the 3-20 μm spectral range has demonstrated its value for in situ studies on the formation and dissociation of gas hydrates. This comprehensive review summarizes the importance of spectroscopic analytical techniques to our understanding of the structure and dynamics of gas hydrate systems, and highlights selected examples that illustrate the utility of these individual methods.

Guest gas enclathration in semiclathrates of tetra-n-butylammonium bromide: stability condition and spectroscopic analysis

In this study, guest gas enclathration behavior in semiclathrates of tetra-n-butylammonium bromide (TBAB) was closely investigated through phase equilibrium measurement and spectroscopic analysis. The three-phase equilibria of semiclathrate (H), liquid water (L(W)), and vapor (V) for the ternary CH(4) + TBAB + water and CO(2) + TBAB + water mixtures with various TBAB concentrations were experimentally measured to determine the stability conditions of the double TBAB semiclathrates. Equilibrium dissociation temperatures for pure TBAB semiclathrate were also measured at the same concentrations under atmospheric conditions. The dissociation temperature and dissociation enthalpy of pure TBAB semiclathrate were confirmed by differential scanning calorimetry. The experimental results showed that the double CH(4) (or CO(2)) + TBAB semiclathrates yielded greatly enhanced thermal stability when compared with pure CH(4) (or CO(2)) hydrate. The highest stabilization effect was observed at the stoichiometric concentration of pure TBAB semiclathrate, which is 3.7 mol%. From the NMR and Raman spectroscopic studies, it was found that the guest gases (CH(4) and CO(2)) were enclathrated in the double semiclathrates. In particular, from the cage-dependent (13)C NMR chemical shift, it was confirmed that CH(4) molecules were captured in the 5(12) cages of the double semiclathrates.