AN AB-INITIO QUANTUM MECHANICAL STUDY OF A MICROPOROUS MATERIAL AS A CANDIDATE FOR HYDROGEN STORAGE: ACIDIC CHABAZITE

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.