Assessment of dominant factors affecting liquid phase hydroisomerization on bifunctional zeolites A. Fu ´ nez a , J.W. Thybaut b, *, G.B. Marin b , P. Sa ´ nchez a , A. De Lucas a , J.L. Valverde a a Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Avd. Camilo Jose ´ Cela s/n, 13071 Ciudad Real, Spain b Ghent University, Laboratory for Chemical Technology, Krijgslaan, 281-S5, B-9000 Ghent, Belgium 1. Introduction Environmental regulations place strong restrictions on the aromatic content of fuels in general and unleaded gasoline in particular. As a result, there has been an urgent need to develop efficient technologies for octane number enhancement processes in order to meet the demand for high octane number unleaded gasoline with low total aromatics content. Boosting the octane number of a gasoline fraction by increasing the content of branched alkanes is an environmentally acceptable alternative compared to other technologies, such as blending with oxygenates and aromatics [1,2]. Hence, the hydroisomerization of normal paraffins to branched paraffins is of considerable interest and has been studied intensively [3–7]. Hydroisomerization is a bifunctional process requiring metal as well as acid sites. Saturated hydrocarbons are dehydrogenated on the metal sites producing unsaturated hydrocarbons which in turn undergo protonation on the acid sites of the zeolite with formation of carbenium ions as reaction intermediates. These carbenium ions undergo skeletal rearrangements and b-scission reactions fol- lowed by deprotonation and hydrogenation of the resulting olefins [8]. Prior to these chemical steps, physisorption occurs in the micropores of the catalyst [9,10]. The balance between the number and the activity or strength of the metal and the acid sites plays a key role in the product selectivities [11–14]. Bifunctional zeolite catalysts exhibit high activity in the hydroisomerization of alkanes [15–17]. The zeolite provides the acid function where the isomerization reaction occurs, whereas a metal provides the hydrogenation–dehydrogenation capability. Noble metal-zeolite catalysts, in particular Pt or Pd loaded beta, mordenite and Y possess a high activity and selectivity for hydroisomerization of n-alkanes [18]. A large number of studies on n-alkane hydroisomerization over bifunctional zeolite catalysts have been reported in the literature, some of them studying the effect of the reaction conditions on the hydroisomerization process. Chao et al. [19], for example, studied the effect of the reaction pressure on the n-heptane hydroconver- sion on platinum-loaded mordenite and beta zeolites. de Lucas et al. [20] reported the effect of key reaction conditions, such as the temperature and the pressure, the hydrogen/n-octane molar ratio, the space time and the time on stream on the hydroisomerization of n-octane over beta zeolite. As expected, an increase in the Applied Catalysis A: General 349 (2008) 29–39 ARTICLE INFO Article history: Received 17 January 2008 Received in revised form 14 June 2008 Accepted 10 July 2008 Available online 19 July 2008 Keywords: n-Octane Hydroisomerization Liquid phase conditions Experimental investigation Kinetic model Zeolite USY Beta Mordenite ABSTRACT The hydroisomerization of n-octane in the liquid phase was investigated over beta, USY and mordenite zeolites loaded with 1 wt% Pt in a stirred semi-batch microautoclave. The total pressure ranged form 5 to 9 MPa and the temperature from 523 to 563 K with an initial catalyst/n-octane ratio of 7 g catalyst =mol n-C 8 . PtBETA was the most active catalyst at all operating conditions, followed by PtMOR and PtUSY. The isomer yields on PtMOR were somewhat lower than on PtBETA and PtUSY. Increasing the total pressure always resulted in a decrease in the n-octane conversion, which is indicative of so-called ideal hydroisomerization. The n-octane hydroisomerization experiments were simulated with a kinetic model based on a parallel/consecutive reaction scheme involving reversible mono- and multibranching and irreversible cracking from mono- as well as multibranched isomers. The model fitted adequately the experimental data on PtBETA and PtUSY. However, more dispersion was observed with catalyst PtMOR. The ratio of the composite rate coefficients for cracking to that for monobranching was significantly higher on PtMOR than on PtUSY and PtBETA. The composite activation energy for monobranching was 20 kJ mol 1 higher on USY if compared to that of mordenite and beta. These modelling results were related to pore sizes and geometry and average acid strength. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +32 9 264 45 19; fax: +32 9 264 49 99. E-mail address: Joris.Thybaut@UGent.be (J.W. Thybaut). Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.009