171, 77–84 (1997)
Kinetics of Hydro-isomerization of n-Hexane over Platinum Containing Zeolites
A. van de Runstraat, J. A. Kamp, P. J. Stobbelaar, J. van Grondelle, S. Krijnen, and R. A. van Santen
Department of Inorganic Chemistry and Catalysis, Faculty of Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,The Netherlands Received November 27, 1996; revised March 1, 1997; accepted May 20, 1997
Kinetic hydro-isomerization experiments at atmospheric pressure using n-hexane as reactant were performed. Four zeolites with different pore structures but intrinsically equal acid strength (mordenite, ZSM-5, ZSM-22, and β) were used. Adsorption and diffusion effects were found to mainly determine thedifferences in activity of these zeolites. The general observation is that the higher the adsorption enthalpy of n-hexane, the higher the activity per acid site and the lower the activation energy. The latter effect is not very pronounced when the order of reaction in n-hexane is low. The true activation energy of isomerization with respect to the adsorbed alkoxy state is found to be approximately 125kJ/mol. ZSM-22 and mordenite were both relatively inactive compared to the other zeolites. In the case of ZSM-22 this was due to pore-mouth catalysis resulting in a 5 to 20% usage of the acid sites. The relatively low activity of mordenite occurs since two-thirds of the acid sites, those c 1997 located in the side-pockets, are not accessible to n-hexane.
The hydro-isomerizationmechanism can be divided in (de)hydrogenation, protonation and isomerization steps. This bifunctional mechanism is given schematically in Fig. 1 (1). These reactions take place in the adsorbed state. According to Kazansky, adsorption of an alkene to an acid site of a zeolite gives an alkoxy species (2, 3). This species is characterized by a covalent bond between an oxygen atom from the zeolitelattice and a carbon atom of the alkene. The elementary isomerization reaction step involves in this case a carbenium ion-like transition state; the n-alkoxy species are the stable intermediates. After a short summary of current thinking in hydroisomerization kinetics, details of the experiments and their results will be provided. Under Discussion we will evaluate the results of optimizing theplatinum content and the resulting “key” samples. The relation between performance of the zeolites and their adsorption behavior will be highlighted.
In this paper we will report the results of a study of the catalytic activity of acidic zeolites. It will be determined whether differences are solely due to differences in intrinsic zeolite proton properties ordifferences in adsorption characteristics. We used the hydro-isomerization of n-hexane as a model reaction since its low deactivation rate enabled study of the reaction under true steady-state conditions. Catalysts had to be chosen such that the platinum function was not rate determining. Hexane was chosen as reactant because of its low cracking rate. The rate of isomerization could therefore beapproximated by the rate of n-hexane conversion. Four zeolites with a Si/Al ratio larger than 10 were chosen. The catalysts had the same intrinsic acid strength, as determined by 1H NMR and IR spectroscopy. Therefore, comparison of their activities normalized per acid site was useful. From the data obtained on the optimized (“key”) samples an intrinsic activation energy of isomerization in zeolites could bededuced.
A Summary of Hydro-isomerization Kinetics When it is assumed that the isomerization reaction step is rate determining (ideal bifunctional behavior) and the hydrocarbon concentrations inside the zeolite are in equilibrium with the gas phase, the overall rate of isomerization is given by (4) R = kiso K dehydr · K prot · 1 + K dehydr · K prot ·
pnC6 pH2 pnC6 pH2 α
≈ kiso K...