Unraveling Diffusion and Other Shape Selectivity Effects in ZSM5 Using <i>n</i>‑Hexane Hydroconversion Single-Event Microkinetics

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

Integrated Stefan–Maxwell, Mean Field, and Single-Event Microkinetic Methodology for Simultaneous Diffusion and Reaction inside Microporous Materials

The assessment of intrinsic reaction kinetics in the presence of diffusion limitations within a porous material remains one of the key challenges within the field of catalysis. The model-guided design of medium-pore zeolite catalysts which typically give rise to mass transport limitations would offer a feasible alternative to conventional trial-and-error procedures. Intracrystalline diffusion limitations during <i>n</i>-hexane hydroconversion on Pt/H-ZSM5 were assessed using an integrated Stefan–Maxwell, mean field, and Single-Event MicroKinetic (SEMK) methodology. The former theory quantifies multicomponent diffusion through a microporous substituent from pure component properties, while framework parameters inherent to the ZSM5 topology are incorporated via a mean field approximation. The complex chemistry involved in <i>n</i>-hexane hydroconversion was described by an SEMK model which is based upon the reaction family concept. Model regression against experimental data resulted in excellent agreement between the model and experiment. In addition, the estimated values for, among others, the component diffusion coefficients were physically meaningful. A sensitivity analysis of the catalyst descriptors demonstrated that especially the total acid site concentration and the crystallite geometry impact the catalyst activity and product distribution, establishing them as critical catalyst design parameters