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

Abstract

Physisorption isotherms and separation factors of n-C₆–n-C₁₂ alkanes on zeolite Y with Si/Al of 2.7 and on ultrastable Y (USY) zeolites with Si/Al ratios of 13 and 30 were determined using perturbation chromatography, at 506 K and for the hydrocarbon pressure range relevant to hydroisomerisation catalysis 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 interaction factor accounting for adsorbate–adsorbate interactions and surface heterogeneity 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 interaction 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 adsorption equilibria were used to extract intrinsic kinetic constants of individual n-alkanes from experimental catalytic conversion data of pure compounds. It is demonstrated that the conversion of the individual components of a four component n-alkane mixture can be accurately predicted by a model combining intrinsic reaction constants for the individual components with the multicomponent adsorption isotherms. The approach was equally successful with the H-Y zeolite as with the strongly dealuminated USY zeolites exhibiting strongly different acidity, catalytic activity and adsorption behaviour.