Constant Pressure CO₂ Replacement of CH₄ in Different Hydrate Environments: Structure and Morphology

Gas hydrates (GHs) are solid, ice-like compounds composed of water molecules forming a lattice structure that hosts gas molecules, produced under high pressure and low temperature. The structure of the hydrate structure is affected by the surrounding environment, and in this context, a structural characterization of GHs prepared in different environments, ultrapure (UP) water, seawater, synthetic sand, natural sand, and sodium dodecyl sulfate, has been proposed. In particular, the Raman spectroscopy has been used to investigate the structural changes in the water cages, the gas uptake in the hydrate structure, the CH₄ cage occupancies, the hydration number, and the yield of carbon dioxide replacement at constant pressure. For this comparison, CH₄-hydrates, CO₂-hydrates, and CH₄/CO₂-hydrates (obtained from an implemented replacement process) were prepared in five different experimental conditions mentioned above and structurally characterized. From Raman investigation, pure CH₄-hydrates displayed almost full (>95%) occupation in the large cage and a significant change in the small cage occupation related to the different tested media. The cage occupancy calculation of CO₂/CH₄-hydrates showed that a higher yield of replacement can be obtained in UP water and that CH₄-hydrates in natural sand and in seawater, which are the most representing of natural environments, displayed a good replacement of CH₄ with CO₂. Additionally, the ex situ morphological characterization of the GHs by scanning electron microscopy (SEM) allowed the highlighting of morphological differences among the investigated samples.