On Reaction Pathways and Intermediates during Catalytic Olefin Cracking over ZSM-5

The concept of catalytic olefin cracking is an alternative method to produce ethene and propene. Undesired higher olefins formed on site are converted using acid zeolites at temperatures higher than 600 K. Although the underlying elementary reactions can be explained by interconversion steps of carbenium ions, relatively little is known about the exact procedure of adsorption and the nature of intermediates. However, detailed knowledge about these topics is indispensable for a comprehensive theoretical description. In this work, a microkinetic single-event model for olefin cracking over ZSM-5 is analyzed in terms of reaction pathways and intermediates. An evaluation of adsorption states underlines the importance of differentiating between physisorption and π-complex formation because the latter leads to significantly higher accuracy when describing olefin cracking. A further investigation of protonation predicts the resulting intermediates to be of comparably low stability. Therefore, their total concentration is negligible, a conclusion that should nevertheless not be used for all approaches from literature. Finally, protonation enthalpies are estimated; the resulting values suggest carbenium ions as intermediates at least for tertiary species, which is in line with the stability order of the obtained activation energies. These findings can help to understand the interaction between olefins and acid zeolites, a topic of high importance in constructing exact and physically consistent theoretical descriptions.