Effect of the Zeolite Nanocavity on the Reaction Mechanism of n-Hexane Cracking: A Density Functional Theory Study

The effects of the zeolite framework on the mechanism of n-hexane monomolecular cracking have been investigated with M06-2X/6-311+G(2df,2p)//M06-2X/6-31G(d,p) calculations. M06-2X is a recently developed hybrid-meta functional that is parametrized to include the London dispersion energy. The 38T H-FAU and 34T H-ZSM-5 nanocluster models where T atoms are either Si or Al atoms are used to represent H-FAU and H-ZSM-5 zeolites. The adsorption energies of hexane are predicted to be -10.8 and -18.2 kcal/mol for H-FAU and H-ZSM-5, respectively, in good agreement with experimental measurements. This indicates that the confinement effects on different types of zeolites can be well represented by the M06-2X functional. The reaction is assumed to proceed in two steps. In the first step, the central C-C bond of adsorbed n-hexane is protonated to form a hexonium intermediate. The adsorbed 3-C-hexonium is highly unstable and can be rapidly decomposed to produce the products. The first step is found to be the rate-determining step with activation energies of 45.7 and 45.8 kcal/mol for H-FAU and H-ZSM-5, respectively. For step two, the activation energies are calculated to be 8.6 and 9.9 kcal/mol for H-FAU and H-ZSM-5, respectively. The results clearly demonstrate that the reaction of n-hexane cracking is intrinsically the same in these large- and medium-pore zeolites. The different apparent activities can be explained by the different adsorption energies which are mainly due to the van der Waals interactions with the zeolite walls.