Noble metal catalysts for the hydrocracking of FT waxes

Sammanfattning: Bifunctional catalysts consisting of palladium or platinum and supported on amorphous silica-alumina were prepared and tested in the hydrocracking of n-hexadecane, which is considered to be representative of n-paraffins in hydrocracker feeds. In addition to the evaluation of the  physicochemical properties, a comprehensive study on catalyst activity and selectivity has been conducted, in the full range of conversions. A theoretical model was proposed to fit the experimental conversion-selectivity data. The n-hexadecane reactivity pattern was expressed in terms of a reaction network involving lumps consisting of monobranched and multibranched n-hexadecane isomers, and cracking products. Pseudo first order kinetics and irreversible reaction steps were assumed in order to obtain the kinetic constants of each step. For the same metallic molar loading, a platinum-based catalyst proved more active than a palladium one. The reaction network model showed that cracking products were produced by means of a bifunctional mechanism on palladium catalysts, with n-hexadecane isomers as intermediates. However, on platinum catalysts, an additional monofunctional mechanism was observed. The noble metal catalyzes the hydrogenolysis of n-hexadecane without requiring any acid function. An increase in the platinum loading leads to an increase in the importance of this direct cracking route. The deactivation in the platinum-based catalysts is only due to coke formation, which deactivates the metal sites. The regeneration by means of a Temperature-Programmed Oxidation does not lead to a complete recovery of the metal function, according to the volumetric chemisorption measurements and the experimental selectivity  data. Further work is required to determine the real causes.