Comparing the influence of framework type on H₂ absorption in hypothetical and existing clathrasils: A grand canonical Monte Carlo study

The influence of the framework type, in particular the influence of the type of cage, on the hydrogen absorption capacity in a range of clathrasil structures (frameworks having Si₆O₆ as their largest ring aperture) is investigated by means of grand canonical Monte Carlo simulations. Our systematic analysis includes twelve existing clathrasils and for the first time we also calculate practical properties for seven energetically viable hypothetical clathrasil frameworks. Hydrogen absorption isotherms are calculated for every material at 78, 85, and 90 K for 0-12 bar, and at 273 and 573 K for 0-3000 bar. At all investigated conditions, cages with a small volume (<400 ų) give rise to high absorption energies due to the larger attractive contact area between the cage wall and hydrogen. Despite this higher absorption energy, the H₂ loading on a weight basis is relatively low in these small cages, because of the high internal surface-to-volume ratio, leaving little void space for the H₂ molecule. As a result, the existing and hypothetical structures with the largest cages, LTN and Dt3_819, exhibit the highest absorption capacities. All investigated hypothetical clathrasils show similar absorption behavior to the existing clathrasils, supporting other evidence that these currently hypothetical materials are likely to be synthesized.