Large-Cage Occupancies of Hydrogen in Binary Clathrate Hydrates Dependent on Pressures and Guest Concentrations

Balancing the formation and storage pressure with the storage capacity is one of the most significant steps toward developing H(2) storage in hydrates. The large-cage occupancies of hydrogen molecules in tetrahydrofuran (THF), acetone, cyclohexanone (CHONE), and methylcyclohexane (MCH) hydrates were investigated by Raman spectroscopy, volumetric gas release measurement, and X-ray diffraction analysis in a pressure region below the equilibrium pressure of pure H(2) hydrates at 255 +/- 2 K. The results from the measurements show that H(2) molecules occupy the large cage of the structure II THF+H(2), acetone+H(2), and CHONE+H(2) hydrates at the suitable pressures and concentrations of promoter guest species, while H(2) molecules do not occupy the largest cage of the structure H MCH+H(2) hydrates, even around 70 MPa. The present work reveals that the large-cage occupancy of H(2) strongly depends on the pressure and the concentration of promoter guest species. The maximum storage amount of H(2) in the acetone+H(2) hydrate was 3.6 +/- 0.1 wt %, similar to that of THF+H(2) hydrate, at 74 MPa and 255 +/- 2 K. Acetone is superior to THF and CHONE as the promoter based on the relation between pressure and the large-cage occupancy of H(2) molecules.