Understanding decomposition and encapsulation energies of structure I and II clathrate hydrates

Structure and thermodynamics of empty clathrate hydrates below the freezing point of water

Recently prepared as a new H2O phase, ice XVI was obtained by degassing a Ne-sII clathrate hydrate under vacuum, however very little is known of that crystalline solid under temperatures (T ≤ 220 K) and pressures (p ≤ 5000 bar) relevant for the Earth's environment and geochemistry. In this work, atomically detailed calculations using long time-scale molecular simulations, seldom paralelled before, are employed to probe empty sII clathrate hydrates. It is found that the volumetric response to an applied pressure-temperature gradient is accurately described by the Parsafar and Mason equation of state with an accuracy of at least 99.7%. Structural deformation induced upon the crystals is interpreted by monitoring the unit cell length and isobaric thermal expansivity, whilst benchmarked against previous neutron diffraction measurements of ice XVI and hexagonal ice under room pressure conditions; a critical comparison is established with other sII guest occupied lattices (CH4, CO2 and CnH2n+2 with n = 2, 3, 4), often found in permafrost regions and in the margins of continental shelves. Such an analysis reveals that empty sII frameworks are slightly more stable to thermal deformation than their sI analogues and that hexagonal ice is the structurally most stable of the condensed H2O phases addressed here. Of paramount importance for the oil and natural gas industries, heat capacities obtained from enthalpy profiles are identical for the sI and sII empty clathrates up to 2000 bar and diverge by only ∼7.3% at 5000 bar. The canonical tetrahedral symmetry of water-bonded networks is analysed in terms of an angular and a distance order parameters, which are observed to decrease (increase) as pressure (temperature) increases (decreases). The results now being reported constitute a landmark for future studies dealing with high-pressure and very low-temperature conditions, characteristic of the Earth's permafrost environment and other planetary interiors, whilst contributing to expand our knowledge regarding the recently discovered ice XVI phase.

Thermophysical Property Measurements of Tetrabutylphosphonium Oxalate (TBPOx) Ionic Semiclathrate Hydrate as a Media for the Thermal Energy Storage System

With increasing global power demand, thermal energy storage technology could play a role ensuring a sustainable energy supply in power generation from renewable energy sources and power demand concentration. Hydrates have high potential as phase change materials (PCMs) for the use as a thermal energy storage medium. To develop thermal energy storage technology using a hydrate-based material, further investigation of thermophysical properties and the selection of a suitable hydrate are required. Tetrabutylphosphonium oxalate (TBPOx) ionic semiclathrate hydrate contains oxalic acid in salt form, as a guest compound, which is classified as carboxylic acid group with low environmental impact. In the present study, the phase equilibrium temperature and the dissociation heat of TBPOx hydrate were measured. The highest equilibrium temperature of the solid hydrate formed was 9.4°C at the mass fraction 0.35 of TBPOx in aqueous solution. The largest dissociation heat was 186.0 ± 0.5 kJ·kg^-1 at the mass fraction of 0.35. Comparing with other PCMs with close phase equilibrium temperatures, TBPOx hydrate is superior in safety and sustainability. These results indicate that TBPOx hydrate would be suitable as the thermal storage medium for the general air conditioning systems.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Stability and characterization of the structure II binary clathrate hydrate of the refrigerant trans-1,3,3,3-tetrafluoropropene + methane

A new clathrate hydrate formed with trans-1,3,3,3-tetrafluoropropene and methane was characterized by phase equilibrium and PXRD measurements and MD simulations.