Competitive physisorption effects in hydroisomerisation of n-alkane mixtures on Pt/Y and Pt/USY zeolite catalysts

Physisorption isotherms and separation factors of n-C-6-n-C-12 alkanes on z eolite Y with Si/Al of 2.7 and on ultrastable Y (USY) zeolites with Si/Al r atios of 13 and 30 were determined using perturbation chromatography, at 50 6 K and for the hydrocarbon pressure range relevant to hydroisomerisation c atalysis with platinum loaded versions of these zeolites. For the n-alkane pressure range from 0.3 to 0.9 bar, a refined Langmuir model with an intera ction factor accounting for adsorbate-adsorbate interactions and surface he terogeneity gave the best agreement with the experimental single component adsorption data. Expressions for multicomponent adsorption equilibria among the n-alkanes, their isomers and cracked products, and expressions with in teraction factors for the single components into multicomponent expressions were obtained. Separation factors among the n-alkanes and their variation with adsorbent loading derived from these multicomponent expressions are in agreement with experimental separation factors. The expressions for the ad sorption equilibria were used to extract intrinsic kinetic constants of ind ividual n-alkanes from experimental catalytic conversion data of pure compo unds. It is demonstrated that the conversion of the individual components o f a four component n-alkane mixture can be accurately predicted by a model combining intrinsic reaction constants for the individual components with t he multicomponent adsorption isotherms. The approach was equally successful with the H-Y zeolite as with the strongly dealuminated USY zeolites exhibi ting strongly different acidity, catalytic activity and adsorption behaviou r.