Ab Initio investigation of the interaction of H₂ with lithium exchanged low-silica chabazites

The interaction of molecular hydrogen with the polarizing centers of lithium-exchanged chabazites with low Si/Al ratio (CHA-5/1 and CHA-3/1) was theoretically studied within a periodic approach at the B3LYP level of theory. The cation site preferences in both zeolites were determined and the H ₂ interaction was then studied by adding a molecule in the proximity of the different polarizing centers. The energetic features of the H ₂-Li₂CHA-5/1 complex were also refined by using a cluster model cut out from the periodic structure in which the contribution of the dispersive forces to the interaction (absent at the B3LYP level of theory) were estimated at the MP2 level by means of an ONIOM2-like approach. Overall results show that the position of the polarizing center in the structure and its stability in the site is crucial for determining the sorption capacity of the zeolite. Lithium exchanged low-silica zeolites are shown to be only effective for hydrogen storage when the Al loading is at least 3:1. However, the highest predicted enthalpy of adsorption is around 10 kJ/mol and this indicates that they are far from fulfilling the conditions for practical applications.