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

Abstract

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.

Ethylene Epoxidation with Nitrous Oxide over Fe–BTC Metal–Organic Frameworks: A DFT Study

“…This functional has been shown to be suitable for investigating adsorption and reaction mechanisms of hydrocarbons in zeolites and also for MOFs in which van der Waals interactions predominate. In a previous study we tested the M06‐L and M06‐2X functionals and compared them with the B3LYP functional for the adsorption of hydrocarbons on the large zeolite clusters . The adsorption energies obtained from M06‐L and M06‐2X were in agreement with experimental values.…”

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

“…In geometry optimizations and frequency calculations, the high level (HL) is typically described using a hybrid DFT functional, while the low level (LL) is treated by semi-empirical methods, e.g., AM1 [83], MNDO [84] or PM3 [85], or a molecular mechanics method, e.g., the universal force field (UFF) [86]. Some authors allow the entire cluster model to relax during geometry optimizations, keeping the terminating hydrogens fixed in space to prevent the cluster from collapsing [87,88], while others only relax the RR containing the active center [52,[89][90][91]. Following geometry optimizations at the ONIOM level, single-point energy refinements are sometimes performed on the entire cluster to minimize potential artifacts arising from the specific partitioning of the system [76,87,88,92].…”

Applications and validations of the Minnesota density functionals

“…Maihom et al [131] investigated the effects of the zeolite framework on the mechanism of n-hexane monomolecular cracking with M06-2X calculations. The calculated adsorption energies of hexane in the H-FAU and H-ZSM-5 zeolites are in good agreement with experiments.…”