Unraveling
Diffusion and Other Shape Selectivity Effects
in ZSM5 Using <i>n</i>‑Hexane Hydroconversion Single-Event Microkinetics
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Abstract
Potentially
dominant factors governing the shape selectivity in <i>n</i>-hexane hydroconversion over a Pt/H-ZSM5 catalyst were
evaluated by means of single-event microkinetic (SEMK) model regression
against experimental data. The observed product distribution could
be adequately modeled, and a corresponding physically meaningful interpretation
could be made only when accounting for intracrystalline diffusion
limitations for each hexane isomer involved in the reaction network,
rather than considering physisorption effects or transition-state
shape selectivity. Simultaneous diffusion and reaction inside the
catalyst crystallites were expressed via Fick’s second law,
while the alkane Fick diffusion coefficients were assessed by explicitly
accounting for mixture nonideality effects. A 3-fold lower diffusion
coefficient was found to be required for 3-methylpentane compared
with 2-methylpentane to explain the typically high selectivity toward
the latter alkane. Once formed inside the catalyst crystallite, dimethylbutane
isomers remained nearly immobile as was evident from their significantly
lower diffusion coefficients. Reaction at the crystallite external
surface was primarily responsible for the marginal conversion toward
the former species, as observed experimentally