Metal/Acid Bifunctional Catalysis and Intimacy Criterion for Ethylcyclohexane Hydroconversion: When Proximity Does Not Matter

The apparent kinetics in metal/acid bifunctional catalysis is generally strongly affected by the metal to acid site ratio and their proximity. However, these two key parameters have not been systematically investigated in the scientific literature. Such a study is provided here for bifunctional catalysts using platinum as the metallic function and EU-1 zeolite as the acidic function. Two series of bifunctional catalysts with different metal to acid sites ratios and different metal to acid site distances were prepared and tested in ethylcyclohexane hydroconversion. By increasing the metal to acid sites ratio, the catalytic activity and isomerization selectivity increased until a plateau was reached, an observation that is in agreement with the classical bifunctional mechanism. At the same time, the intimacy criterion of Weisz was evaluated: strikingly, for a given metal to acid sites ratio, activities and selectivities are not affected by their distance (up to a micrometer scale). A dual-function kinetic model was successfully applied in order to quantify the effect of the metal to acid site ratio on the catalyst activity and isomerization properties. The application of this model showed that the metal to acid sites ratio needed to reach the catalytic activity plateau is higher than the ratio needed to reach the selectivity plateau. This was interpreted as a consequence of the lower kinetic rate constant for the naphthene ring-opening reaction in comparison to the naphthene isomerization reaction