Hydrogenation kinetics of toluene on Pt/ZSM-22

Abstract

Kinetic experiments on the hydrogenation of toluene were performed on 0.5 wt.% Pt/ZSM-22 at temperatures in the range 423–498 K, H 2 inlet partial pressures of 100–300 kPa and toluene inlet partial pressures of 10–60 kPa. Construction of a kinetic model was based on a critical evaluation of available literature data on the hydrogenation of aromatic components together with physicochemical studies on the interaction of aromatic components and related hydrogenated products with metal surfaces as well as on quantumchemical calculations. This lead to a general kinetic model, analogous to the Horiuti Polanyi mechanism for ethylene hydrogenation, with the first four H atom addition steps not in quasi-equilibrium. Chemisorption of H 2 and toluene was assumed to occur on identical sites. No dehydrogenated surface species was taken into account. The preexponential factors were calculated using transition state theory. A model with equal surface reaction rate coefficients for the H addition steps was selected as the best model. The estimated toluene and H 2 chemisorption enthalpies amounted to −70 and −42 kJ mol −1. An activation energy in the range of 40–50 kJ mol −1 was found. Under typical reaction conditions, 60% of the surface is covered by toluene and 20% by H atoms. The remaining 20% are free. Negligible amounts of partially hydrogenated species were found to be present on the catalyst surface.