Interplay of the adsorption of light and heavy paraffins in hydroisomerization over H-Beta zeolite
The effect of pressure on the activity and selectivity of well-balanced Pt/H-Beta bifunctional catalysts in n-hexadecane (n-C16) hydroisomerization was studied. The turnover frequency per Brönsted acid site of the catalyst decreased when increasing pressure due to the lower concentration in olefins at equilibrium, in line with the classical bifunctional mechanism. Conversely, when increasing the total pressure, the C16 isomer yield unexpectedly decreased in contradiction with pure kinetic/ thermodynamic effect of olefin pressure on the reaction rates. Thanks to Grand Canonical Monte Carlo (GCMC) calculations, non-idealities in adsorption behavior in the zeolite micropores were revealed when considering a representative reaction medium. Via mechanistic kinetic simulations combined with GCMC simulations for the relevant intermediates concentrations, the pressure effect on catalyst selectivity is proposed to be due to the interplay between the (light) cracked products and the (heavy) hexadecanes in the pores of Beta zeolite which leads to a pressure-dependency on the adsorption behavior.