Hydrogen in porous tetrahydrofuran clathrate hydrate

The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier.1 Storage within solid-state clathrate hydrates,2-4 and in the clathrate hydrate of tetrahydrofuran (THF), has been recently reported.5, 6 In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that-in contrast to previous reports([5, 6])-hydrogen (deuterium) occupies the small cages of the clathrate hydrate only to 30 % (at 274 K and 90.5 bar). Such a D(2) load is equivalent to 0.27 wt. % of stored H(2). In addition, we show that a surplus of D(2)O results in the formation of additional D(2)O ice Ih instead of in the production of sub-stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. 6). Structure-refinement studies show that [D(8)]THF is dynamically disordered, while it fills each of the large cages of [D(8)]THF17D(2)O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above approximately 220 K, the H-storage characteristics of the clathrate hydrate have similarities with those of surface-adsorption materials, such as nanoporous zeolites and metal-organic frameworks,7, 8 but at lower temperatures, the adsorption rates slow down because of reduced D(2) diffusion between the small cages.