A fundamental Single-Event MicroKinetic (SEMK) model for the hydrogenation of aromatic components on a Pt catalyst has been developed. It is based on the Horiuti-Polanyi mechanism considering atomic hydrogen addition steps to the (partially hydrogenated) aromatic species on the catalyst surface. The reaction network used accounts for the position at which the hydrogen atoms are added to the ring. In accordance with a quantum chemical assessment of the reaction pathway it was assumed that the kinetic parameters only depend on the saturation degree of the nearest neighbor carbon atoms and the branching degree of the carbon atom involved in the hydrogen atom addition. Six reactions families, of which three occur in the reaction network for benzene, are considered. The total number of 18 model parameters was reduced to 7 by calculation of the pre-exponential factors and by accounting for thermodynamic constraints. Experimental benzene hydrogenation data measured at temperatures in the range from 423 to 498 K, benzene inlet partial pressures in the range from 10 to 60 kPa, and hydrogen inlet partial pressures from 100 to 600 kPa on Pt catalyst have been regressed. In accordance with quantum chemical, statistical, and thermodynamic calculations, the selected version of the model gives the best description of the data with an F value of 4150. According to this selected SEMK model, the activation energies for the hydrogen addition to a carbon atom between two unsaturated or two saturated carbon atoms are identical and lower than the activation energy for hydrogen addition to a carbon atom between an unsaturated and saturated hydrogen atom. The estimated chemisorption enthalpy of hydrogen amounts to À59.4 kJ mol À1 and corresponds with an average surface coverage of 30%. A value of À56.0 kJ mol À1 for the chemisorption enthalpy of benzene is obtained. The total surface coverage by hydrocarbon species amounts to 60% under typical reaction conditions, without a pronounced Most Abundant Surface Intermediate (MASI).

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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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