Adsorption isotherms of H2 in microporous materials with the SOD structure:: A grand canonical Monte Carlo study

The feasibility of the sodalite-type (SOD) structures, Si6O12:(all-Si), Na3Si3Al3O12:(Na-AlSi(3:3)), Al3P3O12:(AIP(3:3)), and Ge6O12:(all-Ge), as hydrogen storage media is assessed by calculating the hydrogen uptake isotherms by means of grand canonical Monte Carlo (GCMC) calculations for the range 73-773 K and 0-3000 bars. The calculated uptakes are compared to experimentally measured values at 573 K and 100 bars and show a good quantitative agreement. Additionally, the isotherms show a good qualitative agreement to experimentally determined H-2 isotherms at 77 K in other small pore zeolites, thus further proving the strength of the employed methodology. The isotherms are then fitted with Dubinin-Astakhov (DA) and Henry's isotherms to give the isosteric heats of adsorption Q(st), by means of Clausius/Clapeyron, and the micropore volumes of each SOD-type. These Q(st) values are compared to results we obtained previously in a molecular mechanics (MM) study and to the Q(st) values extracted directly from the GCMC simulation, showing good agreement. The predicted maximum storage capacities, however, follow a different trend than the ones predicted by MM, because of the assumption of a rigid framework in the DA model; an assumption only valid for loadings smaller than similar to 2.5 wt%. At technically interesting loading conditions, 573 K and 100 bar, a storage capacity of around 0.1 wt% is expected for each SOD-type. This lies far below the required capacity for a technically feasible H-2 storage medium. The more technologically interesting capacities of > 4 wt% are found only to be achieved under extremely low temperature and/or extremely high pressure conditions. (c) 2005 Elsevier Inc. All rights reserved.