Rational tuning of monomolecular, bimolecular and aromatization pathways in the catalytic pyrolysis of hexane on ZSM-5 from a first-principles-based microkinetics analysis.

[Display omitted] • The dynamics of intermediates in the hexane-catalyzed pyrolysis pathway elucidated. • The rate controlling steps in the three paths are determined. • The major sources of ethylene and propylene were discovered. • The effect of temperature and pressure on the reaction process was studied. • The optimal temperature in three paths was established. It has been a challenge to accurately control the monomolecular pathways, bimolecular pathways and aromatization reactions in the catalytic pyrolysis of naphtha to further improve the selectivity of propylene. The dynamic variations of intermediate species in these three pathways were simulated using microkinetics under a wide range of operating conditions. The intermediate species in the active site and the rate controlling step as a function of temperature were identified. The optimum temperatures for the monomolecular, bimolecular and aromatization pathways were determined to be 1000 K, 950 K and 1200 K, respectively. Meanwhile, it is confirmed that most of the propylene originate from the bimolecular pathways. The results of the microkinetics were verified by DFT studies and experiments. The DFT results show that C5 and C6 carbeniums ions are more likely to occupy the active site for reaction. In the experiment, with increasing temperature, the proportion of the monomolecular pathways initially decreased and then increased, while the bimolecular pathways changes in the opposite direction. In addition, pressure affected the optimal temperatures but had insignificant impact on the maximum reaction rate and product selectivity for the three paths. As for the catalytic pyrolysis reaction of hexane, it is suggested that the optimal temperature and pressure be set at 950 K and 101.32 kPa, which promoted the occurrence of the main reaction path and inhibits the aromatization reaction, while maximized the selectivity of propylene. This study provides new insights into the control of the reaction pathway and optimization of operating conditions for the catalytic pyrolysis of n-hexane to produce propylene. [ABSTRACT FROM AUTHOR]