Spectroscopic methods in gas hydrate research

Abstract

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.

Insights into the Kinetics of Methane Hydrate Formation in a Stirred Tank Reactor by In Situ Raman Spectroscopy

“…The first experiment, which utilized a higher resolution, produced the three Raman signals at wavelengths close to the theoretical values. The splitting of the Raman peak from $\tilde \nu $ =2911 to 2905 and 2915 cm −1 illustrates the characteristic Raman shift for methane hydrate formation 11c. If the molecular interactions between water and methane change from solvent and solute to host and guest molecules of hydrates, the guest molecules enter two different chemical environments of large and small cages, each of which is responsible for the unique wavelength at which the Raman peak is observed.…”

From Infrared Spectra to Macroscopic Mechanical Properties of sH Gas Hydrates through Atomistic Calculations

“…IR spectroscopic studies of water are of great interest to research involving gas hydrates [16]; however, it presents a challenge to the field of computational spectroscopy [17]. IR spectroscopic studies of gas hydrates involve studies of molecular interactions, kinetic and structural analysis [16].…”

New opportunity to investigate physico-chemical phenomena: time-resolved X-ray and IR concurrent analysis

“…A rationale approach to characterize these systems and their dynamics is a combination of a structural probe sensitive to the guest metal such as X-ray absorption spectroscopy (Liu et al 2008) and a molecular probe such as IR spectroscopy (Marcelli et al 2009b). Many researches have been performed and characterizations of these systems are available (Rauh and Mizaikoff 2011). However, formation and decomposition of clathrate hydrates are not chemical reactions but first order phase transitions and changes are often discontinuous.…”