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.
Highlights
Catalytic cracking of light liquid feed to produce light olefins has emerged as a new technology to compete with existing thermal cracking, that is, capex and energy intensive, technology
We argue that the Bronsted acid sites are not enough to explain all of these observations especially when considering the changes in the product distribution and analyzing the conversion mechanisms
By integrating the fixed bed data representing the steady state conditions and riser data representing the dynamic conditions of active sites, we suggested that Lewis acid sites are active in alkanes cracking and their activities change from promoting hydride transfer reactions, not elementary but concerted reactions, in the temperature range of 460 to 500 ◦ C to promoting methanation at 550 ◦ C
Summary
Catalytic cracking of light liquid feed to produce light olefins has emerged as a new technology to compete with existing thermal cracking, that is, capex and energy intensive, technology. The catalytic cracking technology of light liquid feed has the versatility to produce light olefins with a different ethene to propene ratio depending on the feed composition and operating conditions. The catalytic activities stem from the Bronsted acid sites (BAS) generated when the negative charge on alumina is compensated by a proton at the oxygen connecting the aluminum with the silicon (Al–OH–Si) [2]. The aluminum in the as-made zeolites has tetrahedral coordination but can be altered during the catalyst activation [3,4,5,6] steps or post treatment steps such as steaming and dealumination [7,8,9]. The structure of LAS are not resolved without ambiguity and several molecular structures are proposed such as three-coordinated aluminum units and others non-framework aluminum moieties
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