Microkinetic Modelling of Toluene and o-Xylene Hydrogenation On a Pt Catalyst

The Single-Event MicroKinetic (SEMK) methodology, which had been successfully applied to benzene hydrogenation [1], has now been extended towards substituted aromatics, i.e., toluene and o-xylene. In addition to the number of unsaturated nearest neighbour carbon atoms, H-atom addition rate coefficients were assumed to depend on the carbon atom type, i.e., secondary or tertiary. A simultaneous regression of the SEMK model to an experimental data set consisting of 39 toluene and 37 o-xylene hydrogenation experiments measured at temperatures in the range from 423 K to 498 K, aromatic inlet partial pressures in the range from 10 kPa to 60 kPa and hydrogen inlet partial pressures from 100 kPa to 600 kPa on Pt catalyst resulted in activation energies of H-additions to tertiary carbon atoms that are 11 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.

In steady-state experimentation of aromatic hydrogenation no direct information is obtained on the intermediates involved in the complex surface reaction network. Benzene hydrogenation involves 13 surface species interconnected by 20 H-additions and abstractions, while for toluene and o-xylene, these numbers amount to 40 and 104, and 36 and 96 respectively. Six different reaction families are involved with potentially different activation energies and reaction enthalpies. In contrast to typical kinetic models for aromatic hydrogenation, the SEMK methodology specifically accounts for all partially hydrogenated species in the reaction network, including the position in which the hydrogen atoms are added. It provides a unique insight in potential dominant reaction pathways, most abundant surface intermediates as well as the identification of a rate-determining step. For simulation purposes, reactant chemisorption and product desorption were assumed to be quasi equilibrated, while the pseudo-steady state approximation was applied for the other hydrocarbon surface intermediates. By virtue of statistical thermodynamics and the implementation of thermodynamic consistency in the model, the ultimate number of adjustable parameters amounted to 13, i.e., 6 rate coefficients, 3 surface equilibrium coefficients and 4 chemisorption equilibrium coefficients.

The effect of the operating conditions on the aromatic hydrogenation behavior is consistent with the literature [2]. A maximum in the conversion as a function of the temperature is observed for all components, see Figure 1. The increase of the hydrogenation rate coefficient with the temperature is gradually overcompensated by the decrease of the surface concentrations of the hydrocarbon species. An increase in the hydrogen inlet partial pressure enhances the cycloalkane outlet flow rate, while the opposite is observed for the aromatic inlet partial pressure [1-2]. Aromatic hydrogenation conversions and, hence, rates, decreased in the following order: benzene > toluene > o-xylene.

The activation energies are in the range of 59 to 73 kJ mol^-1 and the surface reaction enthalpies correspond to slightly endothermic H addition reactions consistent with literature [1-5]. This indicates that on Pt the aromaticity is lost upon adsorption of the aromatic component on the surface.  Chemisorption enthalpies were estimated very close to each other for toluene and o-xylene. As a result, according to the SEMK model, differences in hydrogenation rates between the investigated monoaromatic components are primarily due to differences in H addition rate coefficients rather than in chemisorption strength. The simulated temperature effect is more pronounced than what is actually observed. A refinement in the fundamentals of the preexponential factor calculation is expected to further enhance the agreement between model simulations and experimental data.

  References

[1]            Bera, T.; Thybaut, J. W.; Marin, G. B., Ind. Eng. Chem. Res., In press,  doi: 10.1021/ie200541q,

[2]            Thybaut, J. W.; Saeys, M.; Marin, G. B., Chem. Eng. J., (2002) 90, 117.

[3]            Lin, S. D.; Vannice, M. A., J.Catal., (1993) 143, 563.

[4]            Saeys, M.; Reyniers, M. F.; Thybaut, J. W.; Neurock, M.; Marin, G. B., J.Catal., (2005) 236, 129.

[5]            Cooper, B. H.; Donnis, B. B. L., Appl. Catal. A Gen., (1996) 137, 203.