Semiclathrate Hydrates in Tri-n-butylphosphine Oxide (TBPO)-Water and TBPO-Water-Methane Systems

In the present work, we studied semiclathrate hydrates in the TBPO-H₂O and TBPO-H₂O-CH₄ systems. The stoichiometry, temperature, and enthalpy of dissociation of TBPO semiclathrate hydrate crystals formed in the TBPO-H₂O binary system were found to be TBPO·33.6 ± 0.9H₂O, 280.0 K, and 253.1 ± 4.7 J g^-1, respectively. The crystal structure determined by single crystal XRD analysis (150 K) was the orthorhombic with space group Pbam and unit cell parameters a = 19.9313(8) Å, b = 23.4660(7) Å, and c = 12.1127(5) Å. The structural stoichiometry is TBPO·34.5H₂O. The TBPO guest molecules arrangement within the host water framework has been refined for the first time. The discrepancy between the analytically measured and structural stoichiometry is likely to be attributed to the structure defects, which cannot be revealed by the routine single-crystal XRD analysis. The methane capacity of TBPO + CH₄ double hydrate was measured by the thermovolumetric method in the range 14.9-55.8 wt % TBPO aqueous solution at a methane pressure of 8.5 ± 0.5 MPa and temperature of 274 ± 1 and 286 ± 1 K. The maximum included methane volumes of 61.6-74.6 mL/g were observed for the TBPO + CH₄ double hydrates synthesized from ∼26-30 wt % TBPO aqueous solutions. Powder X-ray diffraction measurements of the hydrate samples used in the thermovolumetric experiments revealed that the TBPO + CH₄ double hydrate has the same structural characteristics as the simple TBPO hydrate. The study of the Raman spectra of the TBPO + CH₄ double hydrate and TBPO simple hydrate showed that in the TBPO + CH₄ double hydrate CH₄ molecules selectively occupy the small D cages. The results of the present study did not confirm the earlier suggestion of the formation of several structural types of hydrates in the TBPO-H₂O system. The obtained results indicate that the TBPO-H₂O binary system has a potential for application in gas separation and as cold storage and transportation media.