In hydrogen technology, one of the most complicated
problems to be solved is the storage. Despite the weak
interactions between dihydrogen and solid surfaces, the
molecular adsorption in microporous materials, like zeolites, is an option which has been the object of intensive research. Among zeolites, Al-substituted chabazite in its acidic form has demonstrated to be an interesting candidate for this purpose. H2
can easily diffuse in its cavity, and infrared spectroscopy
studies have revealed a relatively strong interaction with the Bronsted sites.
A theoretical study was performed to characterize acidic
chabazite by using the periodic ab-initio program CRYSTAL.
Different Si/Al loadings were considered, namely: 1/1, 3/1, 5/1 and 11/1. In all cases, the most favorable distribution of the aluminum atoms and the most stable protonation site were identified. With the optimal structures, the energy of formation and the anharmonic OH stretching frequency were calculated, with the latter in good agreement with the experimental data. A hydrogen molecule was then inserted in the cavity of chabazite
with Si/Al = 11/1. The optimal position of the molecule was determined, and its influence over the neighbor atoms analyzed. Computed binding energy is definitely smaller than the experimental one.