Gas hydrates are crystalline solids comprised of a three-dimensional network of hydrogen-bonded water cages (host) that trap small (guest) molecules. The common structures of hydrates are cubic crystals known as structure I (sI) and structure II (sII). The structure type is largely determined by the size of the guest molecule (methane forms sI hydrate, while larger molecules such as propane form sII, as they are too large to fit into the large cage of sI). These structures can be distinguished using Solid-State Nuclear Magnetic Resonance instrument with Magic Angle Spinning probe (SSNMR-MAS). Specifically, the NMR chemical shifts provide a unique signature of what is occupying the small and large cages.  Hydrate crystallization, growth, and dissociation mechanism(s) within nanopores is not well understood, yet is crucial to advance the understanding of hydrate stability in nature, as well as for the development of hydrate-based technologies. In this work, NMR measurements were used to understand the mechanism of hydrate formation and dissociation. Measurements were conducted for two different systems: (1) bulk and (2) with nanoporous materials (e.g., SBA-15) using Tetrahydrofuran (THF) hydrate/ carbon dioxide hydrates. The chemical shifts during the formation and dissociation process obtained in these measurements enable the evaluation of the kinetics of hydrate formation and dissociation. Additionally, the NMR chemical shifts were also evaluated to determine occupancy of the small and large cages. Finally, the NMR chemical shift was also evaluated to provide insight into the possible selective dissociation of the hydrate cages (small vs. large cage decomposition).

Mechanism of hydrate formation and dissociation in nanopores using solid state nuclear magnetic resonance

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