Modelling of sulfur deactivation of naphtha-reforming catalysts - Structure sensitivity in cyclopentane hydrogenolysis

The effect of particle size and the addition of Ir on the relative sulfur sensitivity of Pt-based catalysts has been studied. Cyclopentane hydrogenolysis, a structure-sensitive reaction, was employed as a test reaction and thiophene as the poisoning molecule. Fresh and sintered monometallic Pt/Al2O3 and bimetallic Pt-Ir/Al2O3 catalysts were used. Sulfur poisoning in the presence of simultaneous coke deactivation was characterised by two deactivation kinetic models. Model I assumes a single deactivation order for both deactivation causes, whereas in model II different deactivation orders were assumed (d(c) = 1, d(s) = 0.5). Thioresistance, calculated from the above models as the number of sulfur atoms initially needed to deactivate one atom of exposed Pt, was in the order: Pt-2 greater than or equal to Pt-1 > Pt-1A greater than or equal to Pt-Ir greater than or equal to Pt-2A. According to the deactivation models, thioresistance mainly depends on k(s), the specific rate constant of hydrogenolysis of adsorbed thiophene. The higher the hydrogenolytic constant, the lower the thioresistance. Moreover, both cyclopentane hydrogenolysis and sulfur poisoning depend on the mean particle size. When the particle size was increased, a higher hydrogenolytic activity and a lower thioresistance were observed. Thus sulfur deactivation is also a structure-sensitive reaction.