Journal of Catalysis 221 (2004) 241–251 www.elsevier.com/locate/jcat Alkane hydrocracking: shape selectivity or kinetics? Theo L.M. Maesen, b,∗ Sofia Calero, a Merijn Schenk, a and Berend Smit a a Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands b ChevronTexaco, Energy Technology Company, 100 Chevron Way, Richmond, CA 94802-0627, USA Received 21 May 2003; revised 23 July 2003; accepted 24 July 2003 Abstract A critical evaluation of published alkane hydrocracking product distributions shows that the kinetic network shifts from predominantly ααγ -trimethylalkane to predominantly αα- and αγ -dimethylalkane hydrocracking when the acid sites are insufficiently covered with alkenes. Since ααγ -trimethylalkane hydrocracking has a higher symmetry than αα- and αγ -dimethylalkane hydrocracking, this alteration in the pre- dominant hydrocracking pathway changes the product distribution from a histogram with a single sharp maximum irrespective of the alkane length to histograms with several maxima depending on the feed alkane length. Thermodynamic, kinetic, and mechanistic considerations are presented to explain both types of histograms in great detail. These largely kinetic explanations supplant earlier attempts at linking the features of the hydrocracking product distributions to features of the topologies of the various (zeolite-based) catalysts employed.  2003 Elsevier Inc. All rights reserved. Keywords: Amorphous aluminosilicates; MOR-, FAU-, EMT-, BEA-type zeolites; Alkane hydrocracking; Kinetics 1. Introduction Alkane (hydro)conversion has been extensively studied due to its importance in the refinery and petrochemical processes of FCC [1,2], hydrocracking [1–4], light naphtha hydroisomerization [2,5], and hydrodewaxing [6,7]. Zeolites play an important role in the catalysts used in these processes because they improve catalytic activity, selectivity, or stabil- ity by imparting shape selectivity [1–7]. Shape selectivity is best described as the unambiguous effect of zeolite pore topology on catalytic selectivity [8]. As part of an effort to gain a fundamental understanding of shape selectivity we have employed molecular simu- lations to elucidate the relevant processes at a molecular level [9–13]. Research so far suggests that the fate of a mole- cule depends on its Gibbs free energy of adsorption and the relative heights of the Gibbs free energy barriers to ad- sorption, reaction, diffusion, and desorption [11]. Transition- state shape selectivity occurs when the zeolite topology af- fects the fate of an adsorbed molecule by modifying the Gibbs free energy barriers to reaction [14,15]. When the mass transfer rate between the gas phase and the adsorbed * Corresponding author. E-mail address: tmaesen@chevrontexaco.com (T.L.M. Maesen). phase limits the reaction rate, four additional forms of shape selectivity can occur: 1. Zeolites preferentially consume molecules that combine a low Gibbs free energy of adsorption with a low Gibbs free energy barrier to diffusion (reactant shape selectiv- ity [16]); 2. Zeolites preferentially yield molecules that combine a high Gibbs free energy of adsorption with a low Gibbs free energy barrier to diffusion (product shape selectiv- ity [16]) [11]; 3. Zeolites preferentially form reaction intermediates that combine a low Gibbs free energy of adsorption (and for- mation in the adsorbed phase) with a high Gibbs free energy barrier to diffusion (reaction intermediate shape selectivity) [10,11]; 4. Zeolites preferentially process reactants at exterior sur- face pockets or pore mouths if they exhibit too high a Gibbs free energy of adsorption [17] or too high a Gibbs free energy barrier to diffusion [18–20] to fully penetrate the adsorbate (in so far as this phenomenon is indeed shape selectivity (as in [17]) it goes by a plethora of names [8]). If only adsorbate-adsorbent interactions are considered, the Gibbs free energy of adsorption is determined by the site 0021-9517/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2003.07.003