Investigation of small inhibitor effects on methane hydrate formation in a carbon nanotube using molecular dynamics simulation

In this work, we simulated water molecules in fixed and rigid (15,0) CNTs and the confined water molecules formed a hexagonal ice nanotube in the CNT. After the addition of methane molecules in the nanotube, the hexagonal structure of confined water molecules disappeared and were replaced by almost all the guest methane molecules. The replaced molecules formed a row of water molecules in the middle of the hollow space of the CNT. We also added five small inhibitors with different concentrations (0.8 mol% and 3.8 mol%) to methane clathrates in CNT: benzene, 1-ethyl-3-methylimidazolium chloride ionic liquid ([emim⁺][Cl^-] IL), methanol, NaCl, and tetrahydrofuran (THF). We investigated the thermodynamic and kinetic inhibition behaviors of the different inhibitors on the methane clathrate formation in the CNT using the radial distribution function (RDF), hydrogen bonding (HB), and angle distribution function (ADF). Our results showed that the [emim⁺][Cl^-] IL is the best inhibitor from both aspects. It was also shown that the effect of THF and benzene is better than that of NaCl and methanol. Furthermore, our results showed that the THF inhibitors tended to aggregate in the CNT, but the benzene and IL molecules were distributed along the CNT and can affect the inhibitor behavior of THF in the CNT. We have also examined the effect of CNT chirality using the armchair (9,9) CNT, the effect of CNT size using the (17,0) CNT, and the effect of CNT flexibility using the (15,0) CNT by the DREIDING force field. Our results showed that the IL has stronger thermodynamic and kinetic inhibition effects in the armchair (9,9) and the flexible (15,0) CNT than the other systems, respectively.

Gas hydrates in confined space of nanoporous materials: new frontier in gas storage technology

Gas hydrates (clathrate hydrates, clathrates, or hydrates) are crystalline inclusion compounds composed of water and gas molecules. Methane hydrates, the most well-known gas hydrates, are considered a menace in flow assurance. However, they have also been hailed as an alternative energy resource because of their high methane storage capacity. Since the formation of gas hydrates generally requires extreme conditions, developing porous material hosts to synthesize gas hydrates with less-demanding constraints is a topic of great interest to the materials and energy science communities. Though reports of modeling and experimental analysis of bulk gas hydrates are plentiful in the literature, reliable phase data for gas hydrates within confined spaces of nanoporous media have been sporadic. This review examines recent studies of both experiments and theoretical modeling of gas hydrates within four categories of nanoporous material hosts that include porous carbons, metal-organic frameworks, graphene nanoslits, and carbon nanotubes. We identify challenges associated with these porous systems and discuss the prospects of gas hydrates in confined space for potential applications.