Catalytic reforming in petroleum refining aims at transforming naphta fractions into high octane gasoline containing aromatics or branched alkanes and to produce simultaneously hydrogen. The catalyst used is composed of platinum-based sub-nanometric clusters highly dispersed on gamma-alumina. These clusters exhibit fluxional character to hydrogen partial pressure. We investigate experimentally and theoretically the key reaction of the dehydrogenation of methyl-cyclohexane into toluene for which a detailed atomic scale understanding involved mechanisms and related kinetic parameters is required to enable a better control of dispersed platinum clusters.
We undertook periodic DFT calculations (PBE-dDsC) on a model of Pt13 clusters supported on γ-alumina, in order to identify the most relevant intermediates and transition states and calculate free activation energies in the dehydrogenation mechanism. The reaction path was explored by assuming sequential C-H breaking steps. A reconstruction of the nano-particles occurs during the reaction, found by Molecular Dynamics, confirming the high fluctionality of platinum clusters. All free energy activations for C-H bond breaking, H migration and cluster reconstruction were systematically determined at T=625 K. The highest activation free energy (ΔrG‡=96 kJ/mol) is found for the third C-H bond breaking on methyl-cyclohexene, while the most stable intermediate is the (Toluene+H2) adsorbed product.
We also achieved catalytic tests on Pt/γ-alumina (0.3 wt% Pt) at various temperatures (593-653 K) to determine the apparent activation free energy. DFT data are used to determine rate constants of elementary steps for introducing in kinetic models of increasing complexity (Energy span, Langmuir-Hinshelwood (LH), micro-kinetics) to predict kinetic parameters and the activity. A simple LH model reproduced the activation enthalpy relevantly: 196 by calculations versus 195 kJ/mol experimentally.