Preservation of CO₂ Hydrates with Cyclopentane and Ethylcyclohexane near the Melting Point of Ice under Atmospheric Pressure

In this study, we investigated the preservation of CO2 hydrates in organic liquids near the melting point of ice at atmospheric pressure under isothermal temperature conditions to enable CO2 storage and transportation using gas hydrates under milder temperature and pressure conditions. Cyclopentane (CP), ethylcyclohexane (ECH), and 2-ethyl-1-hexanol (2-EH) were used as organic liquids and their effects on the dissociation behaviors of the CO2 hydrate were evaluated. After 24 h, the preservation ratios of CO2 hydrate powders with CP or ECH were ∼70% at 265 K and ∼60% at 269 K. In contrast, the preservation ratios without organic liquids were ∼30% at 265 K and ∼10% at 269 K. The analysis revealed that CP promoted the formation of CP hydrate shells on the CO2 hydrate surface during dissociation, whereas ECH facilitated ice shell formation. However, the CO2 hydrate with 2-EH dissociated rapidly within approximately 1 h at 265 and 269 K because 2-EH inhibited ice shell formation. These results indicate that hydrophobic CP and ECH enhance the CO2 hydrate stability by enabling the formation of shell structures by CP hydrate or ice that encapsulates the hydrates and prevents CO2 gas release.