Computational Design of Metal-Exchanged ZSM-5 Catalyst For Propane Aromatization
Abstract
The computational study of propane aromatization starts with dehydrogenation over a choice metal exchanged zeolite like a gallium dihydride, and zinc hydride exchanged zeolite, ZSM-5. The cluster model (3 tetrahedral atoms, 3T) was used, and the reaction mechanism (as shown in Fig. (a)) follows the alkyl route, where the Lewis acid-base pair, created by the metal hydride interaction with the zeolite mainframe, polarizes the carbon-hydrogen bond of the propane and split the reactant heterolytically, forming the Bronsted acid site. The products, propene and hydrogen, evolved at different stages when the products from the two catalysts were compared. Hydrogen evolved first, followed by propene when GaH2/3T was used, whereas the reverse was the case with ZnH/3T. The activation energy (48.4 kcal/mol) and the heat of reaction (29.1 kcal/mol) of propane dehydrogenation over GaH2/3T using hybrid DFT, B3LYP with 6-31G(d) basis set agrees with the experimental data, 47.8 and 29.7 kcal/mol respectively. The computed activation energy and heat of reaction with the ZnH/3T catalyst were 45.3 kcal/mol and 29.7 kcal/mol. The rate-determining step was observed to change with the reaction conditions, 400-800 K and 101.3 kPa, using transition state theory, TST. The computational finding agrees with the experimental research [1] and other studies [2,3] where gallium oxide (gallium dihydride during the reaction) outperforms zinc oxide at a reaction temperature of 600-700 K. Besides activity, computational studies during oligomerization (reaction mainly on the Bronsted acid site) confirmed that gallium hydride is more stable than zinc-modified ZSM-5 even at standard temperature and pressure.
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