Competing Occupation of Guest Molecules in Hydroquinone Clathrates Formed from Binary C₂H₄ and CH₄ Gas Mixtures

When reacted with pure ethylene (C₂H₄) and pure methane (CH₄) at 2.0 and 4.0 MPa, respectively, pure hydroquinone (HQ) was converted into β-form clathrate compounds. Experimental solid-state ¹³C NMR spectra and powder X-ray diffraction patterns provided direct evidence of C₂H₄ and CH₄ enclathration in the β-form HQ clathrates. On the basis of cage occupancy from the solid-state ¹³C NMR spectra, C₂H₄ (cage occupancies of 0.81–0.88) molecules are more likely to occupy the clathrate cages than CH₄ molecules (cage occupancies of 0.38–0.39). The selective occupation by C₂H₄ was also observed for HQ clathrates formed from C₂H₄ and CH₄ gas mixtures of 10, 30, 50, 70, and 90 mol % concentrations of C₂H₄. The experimental results from this study could be applied to a clathrate-based process for separating and concentrating C₂H₄ from gas mixtures

Removal of Vapor-Phase Elemental Mercury by Oil-Fired Fly Ashes

The vapor-phase elemental mercury removal efficiency of the heavy-oil-fired fly ash (HOFA) discharged from heavy-oil-fired power plants was first tested to evaluate its suitability as a base material for the development of a low-cost novel sorbent to capture vapor-phase mercury in coal combustion flue gases. Raw, CO2-activated, and sulfur-impregnated HOFAs were prepared and tested. The morphology, specific surface area, particle size, and chemical composition were analyzed for the tested samples. A bench-scale fixed-bed reactor system was used to determine the mercury removal efficiencies of the HOFAs and commercially available activated carbons for comparison. The CO2-activated HOFA showed slightly higher mercury removal efficiency than the raw HOFA, resulting from the increase of active sorption sites by the enlarged surface area. The mercury removal efficiencies of the HOFAs modified by the sulfur impregnation process significantly increased with increasing sulfur content and were comparable to those of the commercially available sulfur-impregnated activated carbons, despite their much smaller surface area. These results suggest that the sulfur sites formed on the surface of the HOFAs during the impregnation process are highly active in capturing vapor-phase elemental mercury

Role of Cation–Water Disorder during Cation Exchange in Small-Pore Zeolite Sodium Natrolite

By combining X-ray diffraction with oxygen K-edge absorption spectroscopy we track changes occurring during the K⁺–Na⁺ cation exchange of Na-natrolite (Na-NAT) as tightly bonded Na⁺ cations and H₂O molecules convert into a disordered K⁺–H₂O substructure and the unit cell expands by ca. 10% after 50% cation exchange. The coordination of the confined H₂O and nonframework cations change from a tetrahedral configuration, similar in ice <i>I</i>_<i>h</i>, with Na⁺ near the middle of the channels in Na-NAT to two-bonded configuration, similar in bulk water, and K⁺ located near the walls of the framework in K-NAT. This is related to the enhanced ion-exchange properties of K-NAT, which, in marked contrast to Na-NAT, permits the exchange of K⁺ by a variety of uni-, di-, and trivalent cations

Freshwater Recovery and Removal of Cesium and Strontium from Radioactive Wastewater by Methane Hydrate Formation

As human society has advanced, nuclear energy has provided energy security while also offering low carbon emissions and reduced dependence on fossil fuels, whereas nuclear power plants have produced large amounts of radioactive wastewater, which threatens human health and the sustainability of water resources. Here, we demonstrate a hydrate-based desalination (HBD) technology that uses methane as a hydrate former for freshwater recovery and for the removal of radioactive chemicals from wastewater, specifically from Cs- and Sr-containing wastewater. The complete exclusion of radioactive ions from solid methane hydrates was confirmed by a close examination using phase equilibria, spectroscopic investigations, thermal analyses, and theoretical calculations, enabling simultaneous freshwater recovery and the removal of radioactive chemicals from wastewater by the methane hydrate formation process described in this study. More importantly, the proposed HBD technology is applicable to radioactive wastewater containing Cs+ and Sr2+ across a broad concentration range of low percentages to hundreds of parts per million (ppm) and even subppm levels, with high removal efficiency of radioactive chemicals. This study highlights the potential of environmentally sustainable technologies to address the challenges posed by radioactive wastewater generated by nuclear technology, providing new insights for future research and development efforts

Role of Salts in Phase Transformation of Clathrate Hydrates under Brine Environments

Although ion exclusion is a naturally occurring and commonly observed phenomenon in clathrate hydrates, an understanding for the effect of salt ions on the stability of clathrate hydrates is still unclear. Here we report the first observation of phase transformation of structure I and structure II clathrate hydrates using solid-state ¹³C, ¹⁹F, and ²³Na magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, combined with X-ray diffraction and Raman spectroscopy. The phase transformation of clathrate hydrates in salt environments is found to be closely associated with the quadruple point of clathrate hydrate/hydrated salts and the eutectic point of ice/hydrated salts. The formation of the quasi-brine layer (QBL) is triggered at temperatures a little lower than the eutectic point, where an increasing salinity and QBL does not affect the stability of clathrate hydrates. However, at temperatures above the eutectic point, all hydrated salts and the QBL melt completely to form brine solutions, destabilizing the clathrate hydrate structures. Temperature-dependent in situ NMR spectroscopy under pressure also allows us to directly detect the quadruple point of clathrate hydrates in salt environments, which has been determined only by visual observations

Enhanced Hydrogen-Storage Capacity and Structural Stability of an Organic Clathrate Structure with Fullerene (C₆₀) Guests and Lithium Doping

An effective combination of host and guest molecules in a framework type of architecture can enhance the structural stability and physical properties of clathrate compounds. We report here that an organic clathrate compound consisting of a fullerene (C₆₀) guest and a hydroquinone (HQ) host framework shows enhanced hydrogen-storage capacity and good structural stability under pressures and temperatures up to 10 GPa and 438 K, respectively. This combined structure is formed in the extended β-type HQ clathrate and admits 16 hydrogen molecules per cage, leading to a volumetric hydrogen uptake of 49.5 g L^–1 at 77 K and 8 MPa, a value enhanced by 130% compared to that associated with the β-type HQ clathrate. A close examination according to density functional theory calculations and grand canonical Monte Carlo simulations confirms the synergistic combination effect of the guest–host molecules tailored for enhanced hydrogen storage. Moreover, the model simulations demonstrate that the lithium-doped HQ clathrates with C₆₀ guests reveal exceptionally high hydrogen-storage capacities. These results provide a new playground for additional fundamental studies of the structure–property relationships and migration characteristics of small molecules in nanostructured materials