Molecular Hydrogen Storage in Binary THF−H₂ Clathrate Hydrates

Abstract

The hydrogen storage capacity of binary THF-H(2) clathrate hydrate has been determined as a function of formation pressure, THF composition, and time. The amount of hydrogen stored in the stoichiometric hydrate increases with pressure and exhibits asymptotic (Langmuir) behavior to approximately 1.0 wt % H(2). This hydrogen concentration corresponds to one hydrogen molecule occupying each of the small 5(12) cavities and one THF molecule in each large 5(12)6(4) cavity in the hydrate framework. Contrary to previous reports, hydrogen storage was not increased upon decreasing the THF concentration below the stoichiometric 5.6 mol % solution to 0.5 mol %, at constant pressure, even after one week. This provides strong evidence that THF preferentially occupies the large 5(12)6(4) cavity over hydrogen, for the range of experimental conditions tested. The maximum amount of hydrogen stored in this binary hydrate was about 1.0 wt % at moderate pressure (<60 MPa) and is independent of the initial THF concentration over the range of conditions tested.

Molecular-Dynamics and First-Principles Calculations of Raman Spectra and Molecular and Electronic Structure of Hydrogen Clusters in Hydrogen Clathrate Hydrate

“…37 Our calculated spectra are in qualitative agreement with the experimental observations in terms of relative peak positions and all the peaks from the S 1 and L 1-4 cages are red-shifted with respect to 4155 cm -1 of H 2 in the low-density gas phase. Absolute values of the frequencies are, however, underestimated by ∼20 cm -1 for S 1 and ∼15-5c m -1 for L [1][2][3][4] . The underestimations may originate from the isolation of the cages from the crystal, which causes the distances between guest gas and water molecules to be reduced because of neglecting intercage hydrogen bonding (more discussion later).…”

“…44,46 In hydrogen clathrate, H 2 vibrons in S 1 have the largest red shift in the spectra (Figure 2), indicating that the molecules are loosely confined in a highly attractive potential field with little intermolecular repulsion. In L 4 , a smaller red shift implies that the four H 2 molecules are still loosely confined and the repulsive interactions play a small role to offset the attraction. was observed for the C-O stretching mode.…”

“…The short intermolecular distances at 2.94 Å in L 4 and 2.51 Å in S 2 in the clathrate are a dramatic decrease from an intermolecular distance (3.78 Å) in solid H 2 at low temperature. 53 This is equivalent to a decrease of the intermolecular distances by ∼22% and ∼34% in L 4 and S 2 cages, respectively, or, corresponding volume compressions by 53% and 71%, indicating fundamentally different intrinsic intermolecular interactions in hydrogen clathrate as compared with those in solid H 2 . Interactions between H 2 and an electrostatic potential field from water oxygen and interactions arising from electron sharing among H 2 molecules within the H 2 clusters are important for confining them in the cages.…”

Solution behavior of reduced COH volatiles in silicate melts at high pressure and temperature

“…Those near 1.5 and 1.3 ppm could be due to H in CH 3 complexes, to CH 2 , CH 4 , or perhaps simply H 2 because the 1 H shift is very sensitive to environment near the 1 H nucleus. For example, local environmental variations have been reported to cause several ppm shifts in the 1 H resonance of H 2 in chlatrates as compared with the shift of isolated H 2 molecules (Florusse et al, 2004;Strobel et al, 2006). Even greater frequency changes can be caused by hydrogen bonding (e.g., Cody et al, 2005) NMR spectrum are not, therefore, certain and require substantiation with examination by other spectroscopic methods.…”

On the thermodynamic stability of hydrogen clathrate hydrates

“…14,15 The equilibrium ͑water͒ molar volume is obtained from GC/ NPT MC simulations. Pressure dependences of the molar volume are plotted at various temperatures in Fig.…”