The effect of acid strength on the mechanism of catalytic pyrolysis reaction of n-hexane in ZSM5: A DFT study

Abstract

Acid strength is an important factor affecting the reaction mechanism. The influences of different Brønsted acid strengths on the cracking path of n-hexane were systematically investigated using the density functional theory. Most of the hexane activation cracking transition states are carbenium ions with no intermediate products of five-coordinate hexanium due to the weak Brønsted acid sites were not conducive to the stabilization of hexanium ions. With increasing acid strength, the protonated cracking reaction (the rate-controlling steps of monomolecular paths) energy barrier was reduced by 38.0% and the β-scission reaction (the rate-controlling steps of bimolecular paths) energy barrier was reduced by 27.15%, where enhanced the selectivity of E&P by changing the ratio of monomolecular and bimolecular cracking. However, the optimal reaction paths, the rate-controlling steps and the order of the various reaction energy barriers haven’t been changed. In addition, decreasing the acid strength increased the energy barriers of aromatization reaction and inhibited the reaction. The effect of molecular size on aromatization was further explored. The intermolecular hydrogen transfer reaction between 2-propoxide and 3-ethylcyclohexene is the rate controlling step and the larger the cycloalkane molecule, the higher the reaction energy barrier. The catalyst design requires a reduced pore size to prevent the formation of cycloalkanes and thus inhibit side reactions to improve the selectivity of E&P.