Extension of the Single-Event Microkinetic Model to Alkyl Substituted Monoaromatics Hydrogenation on a Pt Catalyst

Abstract

The Single-Event MicroKinetic (SEMK) methodology, which had been successfully applied to benzene hydrogenation on a Pt catalyst, has now been extended toward substituted monoaromatics, that is, toluene and o-xylene. The single-event concept combined with thermodynamic constraints allowed to significantly reduce the number of adjustable parameters. In addition to the number of unsaturated nearest neighbor carbon atoms, H-atom addition rate and equilibrium coefficients were assumed to depend on the carbon atom type, that is, secondary or tertiary. This leads to three additional reaction families compared to benzene hydrogenation. Gas phase toluene and o-xylene hydrogenation experiments were performed on 0.5 wt % Pt/ZSM-22 in a temperature range from 423 to 498 K, a total pressure range from 1 to 3 MPa, H 2 inlet partial pressures between 100 and 600 kPa and aromatic inlet partial pressures between 10 and 60 kPa. A simultaneous regression of the SEMK model to an experimental data set consisting of 39 toluene and 37 o-xylene hydrogenation experiments resulted in activation energies of H additions to tertiary carbon atoms that are 10.5 kJ mol −1 higher than to secondary carbon atoms. This can be related to the steric hindrance experienced during H addition to a carbon atom bearing a substituent. The presence of a substituent on the aromatic ring was found not to affect the chemisorption enthalpies. The reaction path analysis has been carried out via differential contribution analysis and identified that the hydrogenation first occurs at secondary carbon atoms prior to the hydrogenation of the tertiary carbon atoms in the hydrogenation sequence. This is in line with the distribution of hydrocarbon species on the catalyst surface.

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“…−20 kJ/mol) in relation to the chemisorption of an aromatic cycle (adsorption enthalpy, ca. −70 kJ/mol) [50]. This presumption is in agreement with the non-coordinated mechanism proposed by Sautet and co-workers for the hydrogenation of exocyclic C=C bonds [49].…”

Escaping the trap of complication and complexity in multiscale microkinetic modelling of heterogeneous catalytic processes

“… 2 In fact, even though the surface structure has been a key factor in catalysis science since the discovery of structure sensitive reactions in single crystal studies, 46 the effect of the structure of the catalyst on reactivity and selectivity is at present neglected in state-of-the-art microkinetic modelling. 19 , 47 – 49 This is mainly due to the enormous complexity associated with the accounting for the nature and the structural dynamics of the active site under given local operating conditions in the reactor. As such, these models rely on an abstract and static concept of the “catalyst material”, which is often modelled as a generic free site “*” at the catalyst surface, without accounting for the effect of the actual orientation and positioning of the atoms at the surface.…”

Balance between model detail and experimental information in steam methane reforming over a Ni/MgO‐SiO2 catalyst

“…Generally kinetic models, which take more details into account and explicitly account for reactions at the elementary level, have a better performance in describing the mechanism, yet a microkinetic analysis is not always required. Depending on the catalyst and the range of operating conditions, it is not uncommon that only a limited number of reactions and species significantly contribute to the overall reaction . The inclusion of irrelevant elementary steps makes the determination of the corresponding parameters very challenging, not to say, impossible.…”