Dynamic and Steady State Evolution of Active Sites in H-ZSM5

Abstract

Catalytic cracking of hexane over steamed ZSM-5 is studied under steady state and dynamic conditions to elucidate the role of the active sites on the product distribution. The product distribution from the riser simulator representing the dynamic state of the catalyst cannot be resembled from monocracking or bimolecular reactions by Bronsted acid sites alone. The catalyst promotes the hydride transfer function which controls the hexane conversion at 460–500 °C that flips into methanation function at 550 °C with a propene to ethene ratio of 1.04. In addition, hydrogen induction is observed in the first two pulses. Steady state data obtained from a fixed bed reactor, on the other side, shows that the product distribution is controlled by monomolecular cracking with low yield of methane and high propene to ethene ratio ranging from 4.3 to 3.3 depending on the temperature and conversion. We are not able to explain these data by considering the Bronsted acid sites alone and suggest that Lewis acid sites with short-lived activity are not inactive in the carbon-carbon activation before fading by coke deactivation. The reported findings are of importance to academia and industry and are very relevant to fluid catalytic cracking (FCC) processes.