Molecular Storage of Ozone in a Clathrate Hydrate Formed from an O₃+O₂+CO₂ Gas Mixture

Abstract

Owing to its strong oxidizing power, ozone (O 3 ) is utilized in various industrial processes and commercial activities, such as the decontamination of air and water, disinfection of medical instruments and hospital equipment, sterilization of perishables, and bleaching of organic compounds. However, ozone in the gaseous state reacts with itself and rapidly decomposes to oxygen (O 2 ). Owing to this reaction, it is generally considered that ozone cannot be stored and transported like other industrial gases.In 1964, McTurk and Waller [1] presented the idea of storing ozone in the form of a clathrate hydrate, a crystalline solid compound framed by interlinked cages made up of hydrogen-bonded water molecules, in which the ozone molecules could be separated from each other by the cage walls and thus prevented from mutually interacting to cause the ozone-to-oxygen reaction. In this pioneering study, the authors experimentally demonstrated the formation of an sII O 3 + CCl 4 double hydrate from pure ozone and carbon tetrachloride (CCl 4 ) and showed ten sets of four-phase (hydrate + O 3 -rich gas + CCl 4 -rich liquid + H 2 O-rich liquid) equilibrium pressure-temperature data, in which the CCl 4 served as the "help-guest substance" for decreasing the hydrate-forming pressure. However, they reported no actual test of preserving the enclathrated ozone. No subsequent study on ozone-containing hydrates has been reported in the literature for more than 40 years. In the patent application document released in 2007, Masaoka et al.[2] stated that they formed an O 3 + O 2 hydrate by spraying water into a reactor charged with an O 3 + O 2 gas mixture (containing O 3 at a mole fraction of about 5 %) and maintained at 13 MPa and À25 8C. Based on an analysis of the gas released from this hydrate while being decomposed, they estimated the ozone content in the hydrate to be 2.3 g L À1 (ca. 0.2 % in mass fraction). They also described that the hydrate could be preserved for 10 days in a closed container conditioned at 13 MPa and À25 8C without causing a substantial loss of its ozone content.More recently, we showed that hydrates formed from an O 3 + O 2 gas mixture and CCl 4 or xenon (Xe) and cooled to around À20 8C can preserve ozone at a mass fraction on the order of 0.1 % for over 20 days under atmospheric pressure. [3] It should be noted that such in-hydrate ozone concentrations are higher than the typical ozone concentration in "ozonated water" for disinfection use by three orders of magnitude. Besides the ozone preservation tests, we measured the fourphase equilibrium for the O 3 + O 2 + CCl 4 hydrate-forming system (O 3 mole fraction in the gas phase = 6.9 AE 0.8 %) in the temperature range from 2.4 to 4

Increasing molecular O3 storage capacity in a clathrate hydrate

“…As such help guests, carbon tetrachloride, xenon, 1,1-dichloro-1-fluoro-ethane and carbon dioxide were previously tested. [7][8][9] Due to the toxicity of carbon tetrachloride, the cost of xenon and the environmental impacts of 1,1-dichloro-1-fluoro-ethane, we selected carbon dioxide as the most promising substance in place of the other help guests.…”

Phase Transition of a Structure II Cubic Clathrate Hydrate to a Tetragonal Form

“…[2] These host structures are analogues of the semiconducting Group 14 clathrates [3] and silica clathrates (so-called clathrasils). [4] Natural-gas or methane hydrate is now seen as apossible global source of methane, [5,6] and the capability of gas hydrates to store large amounts of gas has opened up possibilities for potential industrial applications such as hydrogen [7,8] and ozone [9,10] storage.P hysical properties of gas hydrates are attributed to host-guest interactions such as rattling motions [11] or hydrogen bonding [12,13] of guest mole-cules.H ost-guest interactions also play ac rucial role,a sg as hydrate structures are thermodynamically stable only when am inimum number of cages are occupied by the guest molecules,depending on their nature.Phase changes of clathrate hydrates are known to take place with pressure,and cubic clathrate structures,sIand sII, at ambient pressure transform to hexagonal sH or astructure closely related to it at pressure above 0.5 GPa. [14] Up to now, however, few examples of the phase change with temperature have been reported in the common clathrate hydrate structural families sI, sII, and sH at constant composition.…”

Phase Transition of a Structure II Cubic Clathrate Hydrate to a Tetragonal Form

“…Natural‐gas or methane hydrate is now seen as a possible global source of methane, and the capability of gas hydrates to store large amounts of gas has opened up possibilities for potential industrial applications such as hydrogen and ozone storage. Physical properties of gas hydrates are attributed to host–guest interactions such as rattling motions or hydrogen bonding of guest molecules.…”