Phase relations and binary clathrate hydrate formation in the system H₂-THF-H₂O

Experimentally determined equilibrium phase relations are reported for the system H₂-THF-H₂O as a function of aqueous tetrahydrofuran (THF) concentration from 260 to 290 K at pressures up to 45 MPa. Data are consistent with the formation of cubic structure-II (CS-II) binary H₂-THF clathrate hydrates with a stoichiometric THF-to-water ratio of 1:17, which can incorporate modest volumes of molecular hydrogen at elevated pressures. Direct compositional analyses of the clathrate phase, at both low (0.20 mol %) and stoichiometric (5.56 mol %) initial THF aqueous concentrations, are consistent with observed phase behavior, suggesting full occupancy of large hexakaidecahedral (5¹²6⁴) clathrate cavities by THF, coupled with largely complete (80-90%) filling of small dodecahedral (5¹²) cages by single H₂ molecules at pressures of >30 MPa, giving a clathrate formula of (H₂)_≤2·THF·17H₂O. Results should help to resolve the current controversy over binary H₂-THF hydrate hydrogen contents; data confirm recent reports that suggest a maximum of ~1 mass % H₂, this contradicting values of up to 4 mass % previously claimed for comparable conditions.