Zeolites have been often proposed as microporous
materials for hydrogen storage, exploiting either their ability to physisorb the gas or to trap it in their smaller cavities. Although the obtained capacities (about 2 mass%) are well below the accepted target for mobile applications, they are still in a range deserving interest for special applications for the low cost and high stability of several siliceous structures. Moreover, the
interaction of H2 with porous materials is very important for its purification both in industrial plants and for FC fuelling.
To model these interactions, it is important to understand
the nature of the different interactions of H2 with zeolite cavities, i.e. with cationic sites, polar zeolite walls and other H2 molecules. A preliminary investigation was conducted, based
on ab initio computational techniques, of the interactions described above.
At first, the interaction between a H2 molecule and positively and negatively charged species was studied with the use of bare alkaline cations and halide anions at different levels of theory. The results obtained were compared with a large set of results from the literature, highlighting a linear correlation between the frequency shift and the binding enthalpy. Then, the cation model has been improved to simulate the zeolitic counterions, embedding the cations in 6-, 5- and 4-rings of aluminosilicate composition.
Finally, the importance of van der Waals interactions and
their dependence on the zeolite structure have been studied with classical atomistic simulations. Molecular dynamics of the diffusion of H2 in different zeolitic frameworks were conducted to estimate the maximal storage capacity. This analysis indicates a best loading of about 2.5 mass%.