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Black plate (249,1) The zeolite chabazite (CHA) was rated as having the largest capacity of any zeolite as a storage medium for hydrogen68,73. An adsorption capacity of 1.28% was determined at 77 K for a sample of H-SSZ-13 (SiyAl1⁄411.8),and slightly above that for zeolite-A (1.24 wt%) and for H-CHA itself (SiyAl ratio 1⁄4 2.1) at 1.10 wt%68. The paper describes the zeolite-adsorbed hydrogen as ‘‘liquid hydrogen’’, and it is shown that the working volume of a chabazite (H-SSZ-13) cage can contain seven hydrogen molecules at the density of liquid hydrogen. Experimental results indicate that at 77K, which is 57K above the boiling point of liquid hydrogen, around five hydrogen molecules are confined to each cage, which indicates that conditions close to the liquefaction of hydrogen gas are met when hydrogen is adsorbed into H-SSZ-13 zeolite at 77 K68. The volumetric uptake of H2 was determined at 77 K and a transmission IR measurement of H2 absorption made at 15 K, in H-SSZ-13, (which is an isostructural silico-aluminophosphate mate- rial with the same Brønsted site density), H-SAPO-34, and H-CHA itself. It was found that the H2 uptake increased as the Brønsted sites became more strongly acidic (in the order H-SAPO-34 to H- SSZ-13). Conversely, increasing the density of Brønsted sites (in the order H-SSZ-13 to H-CHA) is found to hamper the adsorption process. The latter result is counter-intuitive but possibly the additional Brønsted sites interact concertedly via H-bonds inside the small chabazite cages and the energy required to displace the adjacent OH ligand is greater than the adsorption enthalpy gained in forming the OH2H2 complex73. The 1.28wt%73 at 77K and one atmosphere pressure world record set by H-SSZ-13 has been broken using low silica type-X zeolites (LSX, SiyAl1⁄41) which are fully exchanged with alkali-metal cations (Li þ , Na þ , K þ ) 74. Hydrogen adsorption isotherms were determined separately at 77K and pressures below 1 atmosphere, and at 298K and less than 10MPa (about 100atm). The adsorption capacity of the zeolite depended strongly on the radius of the exchange-cation and on the density of cations located on the exposed sites. The interaction energies between H2 and the cations follow the series Li þ 4Na þ 4K þ , in order of the increasing cation radii: 0.068, 0.097, 0.133nm, respectively. Li-LSX had an adsorption capacity of 1.5 wt% at 77 K and 1 atmosphere pressure, and a capacity of 0.6 wt% at 298 K and 10 MPa pressure, which places it among the best of known sorbents for H2. By bridged hydrogen spillover onto carbon bridges between the dissociation catalyst and the zeolite, the hydrogen storage capacity could be increased to 1.6 wt% at 298 K and 10 MPa pressure of hydrogen gas. This is by far the greatest hydrogen www.scienceprogress.co.uk Properties and applications of zeolites 249PDF Image | Properties and applications of zeolites
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