Dong-Yeun Koh

Reverse osmosis molecular differentiation of organic liquids using carbon molecular sieve membranes

Carbon sieving to separate the similar Separating organic molecules, particularly those with almost equal sizes and similar physical properties, can be challenging and may require energy-intensive techniques such as freeze fractionation. Taking inspiration from reverse osmosis of aqueous fluids, Koh et al. describe the synthesis, characterization, and mass transport performance of carbon molecular sieve membranes for the separation of liquid-phase organic molecules at room temperature. This technique is capable of separating very similar isomers, such as ortho- and para-xylene, on an industrial scale. Science, this issue p. 804 Carbon membranes efficiently separate similarly sized organic liquid molecules and isomers. Liquid-phase separations of similarly sized organic molecules using membranes is a major challenge for energy-intensive industrial separation processes. We created free-standing carbon molecular sieve membranes that translate the advantages of reverse osmosis for aqueous separations to the separation of organic liquids. Polymer precursors were cross-linked with a one-pot technique that protected the porous morphology of the membranes from thermally induced structural rearrangement during carbonization. Permeation studies using benzene derivatives whose kinetic diameters differ by less than an angstrom show kinetically selective organic liquid reverse osmosis. Ratios of single-component fluxes for para- and ortho-xylene exceeding 25 were observed and para- and ortho- liquid mixtures were efficiently separated, with an equimolar feed enriched to 81 mole % para-xylene, without phase change and at ambient temperature.

Spectroscopic Observation of Atomic Hydrogen Radicals Entrapped in Icy Hydrogen Hydrate

A hydrogen molecule entrapped in the cages of icy hydrogen hydrate is confined in host water framework and thus behaves unlike pure solid or liquid hydrogen. The gamma-irradiated hydrogen radicals are for the first time observed from ESR and solid-state MAS 1H NMR spectra to stably exist in the icy hydrate channels without any collapse of the host framework, confirming the chemical shift consistency of ionized hydrogen derivatives. We discuss the confined icy hydrate channels, which can act as potential storage sites for simultaneously imprisoning both molecular and ionized hydrogen and further as icy nanoreactors.

Nanoporous graphene: Membranes at the limit

Water desalination membranes can be created by etching nanometre-sized pores in a single layer of graphene.

Effect of Interlayer Ions on Methane Hydrate Formation in Clay Sediments

Natural methane hydrates occurring in marine clay sediments exhibit heterogeneous phase behavior with high complexity, particularly in the negatively charged interlayer region. To date, the real clay interlayer effect on natural methane hydrate formation and stability remains still much unanswered, even though a few computer simulation and model studies are reported. We first examined the chemical shift difference of 27Al, 29Si, and 23Na between dry clay and clay containing intercalated methane hydrates (MH) in the interlayer. We also measured the solid-state 13C MAS NMR spectra of MH in Na-montmorillonite (MMT) and Ca-montmorillonite (MMT) to reveal abnormal methane popularity established in the course of intercalation and further performed cryo-TEM and XRD analyses to identify the morphology and layered structure of the intercalated methane hydrate. The present findings strongly suggest that the real methane amount contained in natural MH deposits should be reevaluated under consideration of the compositional, structural, and physical characteristics of clay-rich sediments. Furthermore, the intercalated methane hydrate structure should be seriously considered for developing the in situ production technologies of the deep-ocean methane hydrate.

Nondestructive natural gas hydrate recovery driven by air and carbon dioxide

Current technologies for production of natural gas hydrates (NGH), which include thermal stimulation, depressurization and inhibitor injection, have raised concerns over unintended consequences. The possibility of catastrophic slope failure and marine ecosystem damage remain serious challenges to safe NGH production. As a potential approach, this paper presents air-driven NGH recovery from permeable marine sediments induced by simultaneous mechanisms for methane liberation (NGH decomposition) and CH4-air or CH4-CO2/air replacement. Air is diffused into and penetrates NGH and, on its surface, forms a boundary between the gas and solid phases. Then spontaneous melting proceeds until the chemical potentials become equal in both phases as NGH depletion continues and self-regulated CH4-air replacement occurs over an arbitrary point. We observed the existence of critical methane concentration forming the boundary between decomposition and replacement mechanisms in the NGH reservoirs. Furthermore, when CO2 was added, we observed a very strong, stable, self-regulating process of exchange (CH4 replaced by CO2/air; hereafter CH4-CO2/air) occurring in the NGH. The proposed process will work well for most global gas hydrate reservoirs, regardless of the injection conditions or geothermal gradient.

Atomic Hydrogen Production from Semi-clathrate Hydrates

Atomic hydrogen has received recent attention because of its potential role in energy devices, silicon devices, artificial photosynthesis, hydrogen storage, and so forth. Here, we propose a highly efficient route for producing atomic hydrogen using semi-clathrate hydrates. Two major hydrogen radical sources, derived from guest/host materials, are closely examined.

Abnormal methane occupancy of natural gas hydrates in deep sea floor sediments

Natural gas hydrates were recovered from near-seafloor sediments and analysed to compare two distinctive methane inclusion phenomena. We document the first observation of abnormal methane occupancy in sediment-rich NGH deposits.

Superoxide Ions Entrapped in Water Cages of Ionic Clathrate Hydrates

In the present work, we first described the stable entrapment of the superoxide ions in gamma-irradiated (Me(4)NOH + O(2)) clathrate hydrate. Owing to peculiar direct guest-guest ionic interaction, the lattice structure of gamma-irradiated (Me(4)NOH + O(2)) clathrate hydrate shows significant change of lattice contraction behavior even at relatively high temperature (120 K). Such findings are expected to provide useful information for a better understanding of unrevealed nature (such as icy nanoreactor concept, ice-based functional material synthesis and lattice tuning by specific ionic guests) of clathrate hydrate fields.

Multiple guest occupancy in clathrate hydrates and its significance in hydrogen storage

We report a new concept of structural transformation combined with tuning phenomena which together result in a significant increase in the hydrogen storage capacity in an icy material. It is necessary to investigate the use of a fully water-soluble structure H (sH) former so as to observe how hydrogen molecules are stably loaded into hydrate cages.

Thermal expansivity of ionic clathrate hydrates including gaseous guest molecules.

Although thermal expansion is a key factor in relation to the host-guest interaction of clathrate hydrates, few studies have investigated the thermal behavior of ionic clathrate hydrates. The existence of ionic species in these hydrates creates a unique host-guest interaction compared to that of nonionic clathrate hydrates. It was revealed that X-ray diffraction cannot be used for research of tetramethylammonium hydroxide clathrate hydrates due to damage of the cations by the X-ray, which results in abnormal thermal expansion of the ionic clathrate hydrates. Hence, in the present work, the thermal expansivities of binary sII Me(4)NOD·16D(2)O and sI DClO(4)·5.5D(2)O were measured by neutron powder diffraction (NPD) in order to shed light on their thermal behavior. General correlations for the thermal behaviors of given structures were established and lattice expansions depending on the guests were compared between ionic and nonionic clathrate hydrates. The peculiar change in the thermal expansivity of binary DClO(4)·5.5D(2)O was also considered in relation to the host-guest configuration.