The dehydrogenation and cracking reactions of isobutane over the ZSM-5 zeolite

The dehydrogenation and cracking reactions of isobutane over zeolite HZMS-5 were studied at the DFT/B3LYP level of calculation. The zeolite was represented by the‘double-ring’ 20T cluster. The activation energies for the reactions were 9–12 kcal/mol lower than those obtained with the linear 5T cluster. In both cases the attack of the acid site proton was directly on a carbon atom of the substrate, and not on the C–H and C–C bonds, evidencing carbonium-ion-type transition states. The results suggest that the reactions should be competitive, although the more hindered acid sites should favor the dehydrogenation over the cracking reaction.