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Black plate (250,1) storage capacity achieved using a zeolite material at ambient temperature but still the material is nowhere near to providing conditions for a practical storage device74. Calculations were performed for twelve known clathrasils (a clathrasil is a framework with Si6O6 as its largest ring aperture) and seven hypothetical energetically stable analogues. Under all temperature and pressure conditions investigated, high adsorption energies were predicted for small volume cages (5400A3) in consequence of the larger contact area between the cage wall and H2. However, H2 loading into the material remains relatively low in consequence of the large internal surface-to-volume ratio which means there is little void space for the H2 molecules to occupy, and therefore clathrasils are unlikely to become of practical use as hydrogen storage media75. In conclusion, the prospect of using zeolites for practical hydrogen storage appears limited. Some zeolite-templated porous carbons seem to offer greater promise, and for one example a hydrogen uptake of 4.5wt% and 45gyL weight and volumetric densities, was reported at 77 K and 20 atmospheres (2 MPa) pressure76. Greater capacities still are reported for porous coordina- tion-framework materials giving uptakes for H2 of up to 6 wt%, at 78K and pressures less than 20atm77. Hence these will almost certainly be further investigated as potential practical hydrogen storage systems. However hydrogen might be used, either as a pure substance or as adsorbed into zeolites or other porous materials, the energy costs of cryogenic cooling and compression must be born and factored into the energy balance equation for the hydrogen economy. All such efforts to find a substitute for hydrocarbon fuels have brought home exactly how ideal the latter are as fuels, both in terms of energy density and their handling properties, and finding a substitute for them will be a hard act if it can be done at all5. 6. Chemical oxidations Various metal ion complexes have been incorporated into constrained environments to mimic the kind of chemistry known for related reagents in solution phase78. Oxidation fundamentally, provides the means by which hydrocarbons may be functionalised to produce both bulk and fine chemicals but when free radicals are involved the approach is usually messy and leads to multiple substitution and mixtures of products. When transition metal cations are present, cheap oxidants like O2, H2O2 or organic 250 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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