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

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

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