Triple Guest Occupancy and Negative Compressibility in Hydrogen-Loaded β-Hydroquinone Clathrate

Upper storage-capacity limit and multiple occupancy phenomena in H2hydroquinone clathrates using Monte Carlo and DFT simulations

The upper hydrogen-storage capacity limit of the β-hydroquinone clathrate has been investigated using hybrid Grand-Canonical Monte Carlo/Molecular Dynamics simulations, for temperatures ranging from 77 K to 300 K. The evolution with pressure of the cage occupancies has been monitored in detail, describing the progressive nature of the uptake process. It is found that the storage capacity of the pure β-HQ + H2 clathrate could reach 0.6 wt% (weight percentage) only for pressures above 1400 bar, at ambient temperature. The enhancement of the storage capacities by the multiple occupancy phenomenom was accordingly shown to be very limited by the need for extreme conditions. Following this observation, an unmodified version of the van der Waals & Platteeuw theory was applied allowing for the prediction of experimentally accessible formation pressures. Density functional theory calculations were addittionnaly performed to comprehensively characterize the hydrogen diffusion process within the clathrate crystalline structure, considering different occupancy scenarios.

Inflation Negative Compressibility during Intrusion-Extrusion of a Non-Wetting Liquid into a Flexible Nanoporous Framework

Negative compressibility (NC) is a phenomenon when an object expands/shrinks in at least one of its dimensions upon compression/decompression. NC is very rare and is of great interest for a number of applications. In this work a gigantic (more than one order of magnitude higher compared to the reported values) NC effect was recorded during intrusion-extrusion of a non-wetting liquid into a flexible porous structure. For this purpose, in situ high-pressure neutron scattering, intrusion-extrusion experiments, and DFT calculations were applied to a system consisting of water and a highly hydrophobic Cu₂(tebpz) metal-organic framework (MOF), which upon water penetration expands in a and c directions to demonstrate NC coefficients more than order of magnitude higher compared to the highest values ever reported. The proposed approach is not limited to the materials used in this work and can be applied to achieve coefficients of negative linear compressibility of more than 10³ TPa^-1.

Novel Hydroquinone-Alumina Composites Stabilizing a Guest-Free Clathrate Structure: Applications in Gas Processing

Organic clathrates formed by hydroquinone (HQ) and gases such as CO₂ and CH₄ are solid supramolecular host-guest compounds in which the gaseous guest molecules are encaged in a host framework of HQ molecules. Not only are these inclusion compounds fascinating scientific curiosities but they can also be used in practical applications such as gas separation. However, the development and future use of clathrate-based processes will largely depend on the effectiveness of the reactive materials used. These materials should enable fast and selective enclathration and have a large gas storage capacity. This article discusses the properties and performance of a new composite material able to form gas clathrates with hydroquinone (HQ) deposited on alumina particles. Apart from the general characterization of the HQ-alumina composite, one of the most remarkable observations is the unexpected formation of a guest-free clathrate structure with long-term stability (>2 years) inside the composite. Interestingly enough, in addition to a slight improvement in the enclathration kinetics of pure CO₂ compared to powdered HQ, preferential capture of CO₂ molecules is observed when the HQ-alumina composite is exposed to an equimolar CO₂/CH₄ gas mixture. In terms of gas capture selectivity toward CO₂, the performance of this new composite exceeds that of pure HQ and HQ-silica composites developed in a previous study, opening up new opportunities for the design and use of these novel materials for gas separation.

Simulation of Capture and Release Processes of Hydrogen by β-Hydroquinone Clathrate

Using molecular simulation techniques, we investigate the storage capabilities of H₂ gas by the clathrate of hydroquinone (HQ). Quantum mechanics calculations have been used to assess structure and interactions at the atomic scale and molecular dynamics to model the HQ clathrate at successive equilibriums during the processes of capture and release of H₂, as well as the diffusion of H₂ inside the clathrate structure. The thermodynamic conditions of the simulations performed try to reproduce closely the corresponding experimental procedures, with results that are in good agreement with literature observed trends. The results obtained contribute to depict a more complete and better substantiated image of the mechanisms involved in stability and in the processes of capture and release of H₂ by the HQ clathrate.

Quantum Dynamics of H_{2} Trapped within Organic Clathrate Cages

The rotational and translational dynamics of molecular hydrogen trapped within β-hydroquinone clathrate (H_{2}@β-HQ)-a practical example of a quantum particle trapped within an anisotropic confining potential-were investigated using inelastic neutron scattering and Raman spectroscopy. High-resolution vibrational spectra, including those collected from the VISION spectrometer at Oak Ridge National Laboratory, indicate relatively strong attractive interaction between guest and host with a strikingly large splitting of rotational energy levels compared with similar guest-host systems. Unlike related molecular systems in which confined H_{2} exhibits nearly free rotation, the behavior of H_{2}@β-HQ is explained using a two-dimensional (2D) hindered rotor model with barrier height more than 2 times the rotational constant (-16.2  meV).

Revisiting the thermodynamic modelling of type I gas–hydroquinone clathrates

Under specific pressure and temperature conditions, certain gaseous species can be engaged in a host lattice of hydroquinone molecules, forming a supramolecular entity called a gas hydroquinone clathrate.

Negative Linear Compressibility in Organic Mineral Ammonium Oxalate Monohydrate with Hydrogen Bonding Wine-Rack Motifs

Negative linear compressibility (NLC) is a relatively uncommon phenomenon and rarely studied in organic systems. Here we provide the direct evidence of the persistent NLC in organic mineral ammonium oxalate monohydrate under high pressure using synchrotron X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculation. Synchrotron X-ray powder diffraction measurement reveals that ammonium oxalate monohydrate shows both positive and negative linear compressibility along b-axis before 11.5 GPa. The red shift of the external Raman modes and abnormal changes of several selected internal modes in high-pressure Raman spectra further confirmed the NLC. DFT calculations demonstrate that the N-H···O hydrogen bonding "wine-rack" motifs result in the NLC along b-axis in ammonium oxalate monohydrate. We anticipate the high-pressure study of ammonium oxalate monohydrate may represent a promising strategy for accelerating the pace of exploitation and improvement of NLC materials especially in organic systems.

Observation of interstitial molecular hydrogen in clathrate hydrates

The current knowledge and description of guest molecules within clathrate hydrates only accounts for occupancy within regular polyhedral water cages. Experimental measurements and simulations, examining the tert-butylamine + H2 + H2O hydrate system, now suggest that H2 can also be incorporated within hydrate crystal structures by occupying interstitial sites, that is, locations other than the interior of regular polyhedral water cages. Specifically, H2 is found within the shared heptagonal faces of the large (4(3)5(9)6(2)7(3)) cage and in cavities formed from the disruption of smaller (4(4)5(4)) water cages. The ability of H2 to occupy these interstitial sites and fluctuate position in the crystal lattice demonstrates the dynamic behavior of H2 in solids and reveals new insight into guest-guest and guest-host interactions in clathrate hydrates, with potential implications in increasing overall energy storage properties.