The hydroisomerization of n-hexane over platinum-loaded
mordenite and ZSM-5 was simulated
using a microkinetics approach. Alkoxy species were assumed to be
the reactive intermediates.
Carbenium ions were considered to be part of the transition state.
This leads to true activation
energies of isomerization of approximately 130 kJ/mol for both
zeolites. The model explicitly
takes into account the variation in micropore filling. This
enables modeling of the orders of the
reaction over a wide pressure and adsorption enthalpy regime. The
resulting model describes
experimental measurements very well. It is shown by both methods
that ZSM-5 is more active
per acid site than Mordenite due to its higher adsorption enthalpy for
n-hexane. The effect is
small because of the low hexane order. The adsorption entropy
plays a decisive role in
determining the overall activity.
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