Hydrogen storage in low silica type X zeolites

Low silica type X zeolites (LSX, Si/Al = 1) fully exchanged by alkali-metal cations (Li⁺, Na⁺, and K⁺) were studied for their hydrogen storage capacities. Hydrogen adsorption isotherms were measured separately at 77 K and <1 atm, and at 298 K and <10 MPa. It was found that the hydrogen adsorption capacity of LSX zeolite depended strongly on the cationic radius and the density of the cations that are located on the exposed sites. The interaction energies between H₂ and the cations follow the order Li⁺ > Na⁺ > K⁺, as predicted based on the ionic radii. Oxygen anions on zeolite framework were minor adsorption sites. Li-LSX had an H2 capacity of 1.5 wt % at 77 K and 1 atm, and a capacity of 0.6 wt % at 298 K and 10 MPa, among the highest of known sorbents. The hydrogen capacity in LSX zeolite by bridged hydrogen spillover was also investigated. A simple and effective technique was employed to build carbon bridges between the H₂ dissociation catalyst and the zeolite to facilitate spillover of hydrogen atoms. Thus, the hydrogen storage capacity of Li-LSX zeolite was enhanced to 1.6 wt % (by a factor of 2.6) at 298 K and 10 MPa. This is by far the highest hydrogen storage capacity obtained on a zeolite material at room temperature. Furthermore, the adsorption rates were fast, and the storages were shown to be fully reversible and rechargeable. Further optimization of the bridge building technique would lead to an additional enhancement of hydrogen storage.