Spectroscopic and thermodynamic investigations of clathrate hydrates of methacrolein

Abstract

This study characterized new structure II (sII) clathrate hydrates, consisting of 136 H2O molecules with 8 large 5(12)6(4) cages and 16 small 5(12) cages, with methacrolein for the first time. The crystal structure and guest distributions of binary (methacrolein + gaseous guests) clathrate hydrates were identified using spectroscopic tools, namely powder X-ray diffraction (PXRD) and Raman spectroscopy. The PXRD and Raman results showed that the inclusion of methacrolein and gaseous guests including CH4, N-2, O-2 or CO2 could be monitored in the large and small cages of sII hydrates, respectively. The conformation of methacrolein in the large cages of sII hydrates was also analyzed via Raman spectroscopy, revealing an s-trans conformer of methacrolein in the large cages of sII hydrates. High-resolution powder diffraction (HRPD) and Raman spectroscopy were also used to identify the dissociation of binary (methacrolein + CH4) clathrate hydrate, showing that it was almost completely dissociated at 200 K. Finally, we measured the equilibrium conditions of four phases, clathrate hydrates, liquid water, liquid methacrolein, and the vapour phase, to check the thermodynamic stability of binary (methacrolein + gaseous guest) clathrate hydrates. The phase equilibria of binary (methacrolein + CH4, N-2, or O-2) clathrate hydrates showed that the addition of methacrolein to the hydrate phase increased the hydrate stability with a higher hydrate dissociation temperature when compared to the hydrate stability of pure (CH4, N-2, or O-2) clathrate hydrates. The thermodynamic stability of binary (methacrolein + CO2) clathrate hydrate exhibits a higher hydrate dissociation temperature when compared with that of the pure CO2 clathrate hydrate below 279 K.