Deactivation of USHY zeolite by coke in the cracking of n-nonane.

摘要

The mechanism of catalyst decay in the cracking reactions of n-nonane on USHY zeolite has been investigated. Using structural information, a link between the observed decline in catalyst activity and the coke formed on the catalyst is postulated for the cracking reactions of n-nonane.; The cracking reaction occurs by two mechanisms, monomolecular initiation involving the protolysis of C-C bonds of the reactant and bimolecular propagation which consists of the abstraction of a hydride ion from a reactant species by a surface carbenium ion. Termination involves the desorption of the carbenium ion from the active site. Modelling of the reaction kinetics indicates that despite the mechanistic evidence for the propagation mechanism, a three parameter model that theoretically accounts only for the initiation mechanism is statistically adequate to describe the data. Monte Carlo simulations show that the use of average conversion data, a high rate of catalyst decay, and experimental error combine to make the model insensitive to the kinetic effects of the postulated propagation reaction.; The electronic influence of coke on the catalyst was investigated by {dollar}sp{lcub}29{rcub}{dollar}Si MAS-NMR spectroscopy. The spectra were analyzed statistically and show that in the absence of paramagnetic oxygen and physisorbed water, the changes induced by the presence of the coke are small relative to the reproducibility of the NMR spectra. Interpretation of the results is made difficult by strong correlations between parameters in the model.; Examination by {dollar}sp{lcub}13{rcub}{dollar}C CP/MAS-NMR spectroscopy of the coke deposited on the catalyst at various times on stream, combined with the reaction chemistry and kinetic modelling of the system, indicates that the structure of the coke and of surface species in general, have an important role in the activity of the catalyst. This is manifested in a structure--reactivity relationship that governs the reactivity of the surface species to propagation and desorption reactions. As surface species become more dehydrogenated, they become less reactive to either a propagation reaction with a gas phase reactant species or to a desorption event. This results in a decline in the activity of the catalyst with increased time on stream.