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Black plate (248,1) loading, whereupon the quantity of hydrogen per unit volume is almost equal to that in liquid hydrogen, such that the limiting useful practical density of 65 kg H2 m 3 may be possible for most structures based on sodalite. For pure liquid hydrogen the density is 70.8 kg H2 m 3 which is the normal density of the liquid at a temperature of 20.28K. Liquefying hydrogen consumes around 30% of the energy that might be recovered from the material as a fuel, which is a considerable drawback to the strategy68. Despite the optimistic theoretical estimate of how much hydrogen can be put into a zeolite, the experimental results fall far short of it69, at 0.26 and 0.4wt% for all-Si (Si96O192) and AlP (Al48P48O192), given the calculated68 capacities of 4.8 and 5.2wt% respectively. The source of this discrepancy is that the theoretical maximum capacities are based solely on energetic considerations, and do not address influences from ions, water or other impurities which might serve to block access to portions of the overall internal volume of the sodalite cages. Nor do the calculations account for entropic effects, since in effect they refer to zero Kelvin. Adsorption isotherms of H2 in various sodalite materials have also been calculated using a grand canonical Monte Carlo method70. It is concluded that for loading capacities of technical interest, 573 K and 100 bar, a storage capacity of around 0.1wt% might be achieved for each type of sodalite structure. However, in the practical situation, capacities of greater than 4% will most likely be met only under conditions of extremely low temperature andyor extremely high pressure70. When hydrogen is absorbed in a zeolite, to all intents and purposes the adsorption may be considered as a facilitated liquefaction, where the solid-gas interaction aids liquid condensation at more readily accessible temperatures and pressures than are required to condense hydrogen gas into the bulk liquid phase. A theoretical study71 was made which employed the force-field method within the Discover module of the Materials Studio 2.2 package of Accelerys Inc.72 The progressive filling of twelve purely siliceous models of common zeolite frameworks with H2 was simulated in order to determine the effect of framework properties including flexibility on the maximum adsorption capacity for hydrogen. The conclusions are that the highest capacities, in the range 2.65–2.86wt% of H2, are exhibited by the flexible non- pentasil zeolites (RHO, FAU, KFI, LTA and CHA)71. A respect- able correlation was found between the calculated adsorption capacities and those determined experimentally at 77K. Nonetheless, these materials fall distinctly short of the 6.5wt% target limit agreed for practical hydrogen storage. 248 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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