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.
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