Hydrogen hydrates with tetrahydrofuran (THF) as a promoter molecule are investigated to probe critical unresolved observations regarding cage occupancy and storage capacity. We adopted a new preparation method, mixing solid powdered THF with ice and pressurizing with hydrogen at 70 MPa and 255 +/- 2 K (these formation conditions are insufficient to form pure hydrogen hydrates). All results from Raman microprobe spectroscopy, powder X-ray diffraction, and gas volumetric analysis show a strong dependence of hydrogen storage capacity on THF composition. Contrary to numerous recent reports that claim it is impossible to store H(2) in large cages with promoters, this work shows that, below a THF mole fraction of 0.01, H(2) molecules can occupy the large cages of the THF+H(2) structure II hydrate. As a result, by manipulating the promoter THF content, the hydrogen storage capacity was increased to approximately 3.4 wt % in the THF+H(2) hydrate system. This study shows the tuning effect may be used and developed for future science and practical applications.

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.