Temperature- and Pressure-induced Structural Transition of Binary Clathrate Hydrates

We discover new structure II (sII) hydrate forming agents of two C4H8O molecules (2-methyl-2-propen-1-ol and 2-butanone) and report the abnormal structural transition of binary C4H8O+CH4 hydrates between structure I (sI) and sII with varying temperature and pressure conditions. In both (2-methyl-2-propen-1-ol+CH4) and (2-butanone+CH4) systems, the phase boundary of the two different hydrate phases (sI and sII) exists at the slope change of the phase-equilibrium curve in the semi-logarithmic plots. We confirm the crystal structures of two hydrates synthesized at low (278 K and 6 MPa) and high (286 K and 15 MPa) temperature and pressure conditions by using high-resolution powder diffraction and Raman spectroscopy. 2-Methyl-2-propen-1-ol and 2-butanone can occupy the large cages of sII hydrate at low temperature and pressure conditions; however, they are excluded from the hydrate phase at high temperature and pressure conditions, resulting in the formation of pure sI CH4 hydrate.