Journal of Catalysis 221 (2004) 500–509 www.elsevier.com/locate/jcat Effect of confinement on the selectivity of hydrocracking H. Toulhoat, a,∗ P. Raybaud, b and Eric Benazzi c a Direction Scientifique, Institut Français du Pétrole, 1 & 4 avenue de Bois-Preau, 92852 Rueil-Malmaison cedex, France b Division Chimie et Physico-Chimie Appliquées, Institut Français du Pétrole, 1 & 4 avenue de Bois-Preau, 92852 Rueil-Malmaison cedex, France c Division Cinétique et Catalyse, Institut Français du Pétrole, 1 & 4 avenue de Bois-Preau, 92852 Rueil-Malmaison cedex, France Received 18 June 2003; revised 14 August 2003; accepted 10 September 2003 Abstract We present molecular simulation results for the adsorption of a set of hydrocarbons in model zeolite structures of various pore sizes. Average adsorption energies scale approximately linearly with the cubic root of the ratio of molecular volume to maximal molecular volume. The maximal molecular volume depends on the zeolite structure and characterizes a steric exclusion threshold. This behavior is consistent with the confinement theory introduced by Derouane and co-workers. We use this result in a simplified model of the effect of confinement in hydrocracking, in order to better understand the respective roles of acidity and confinement on the selectivity toward products of intermediate molecular weight. We show that selectivity is determined by the profile of activity as a function of pore size and/or maximal molecular volume, itself mostly determined by the confinement effect. These findings have important practical implications for the design of hydrocracking catalysts with a prescribed selectivity.  2003 Elsevier Inc. All rights reserved. Keywords: Mathematical model of hydrocracking; Bifunctional catalyst; Zeolites; Silica–alumina; Confinement; Adsorption; Molecular simulation 1. Introduction In a recent paper [1], it has been shown experimentally that the key factor determining the distribution of hydro- cracked products is the pore size of the bifunctional cata- lyst selected to convert a particular feedstock: indeed, un- der conditions of ideal hydrocracking, namely when the hydrogenating-dehydrogenating function is not limiting, at a given temperature and a given total conversion, the yield in the most valuable products of intermediate molecular weight, or selectivity, increases with the pore size. More- over, while the activity increases in proportion to the strength and density of acid sites, the selectivity is hardly affected by the acidity. The interpretation already presented in [1] points out the significance of the adsorption strength of primary cracking products within the catalyst’s pores, as overcrack- ing can result from a too strong adsorption. As shown by Derouane and co-workers [2–4], the adsorption strength is strongly dependent on the molecule to pore size ratio via the so-called “confinement effect.” We expect, therefore, that the problem of selecting the optimal pore size for a hydro- * Corresponding author. E-mail address: herve.toulhoat@ifp.fr (H. Toulhoat). cracking catalyst has no unique solution: each solution will actually depend on the feedstock’s average molecular weight and structure, and on the desired slate of products. The goal of the present communication is to help rational optimization on the basis of a simple mathematical model and molecular simulations of confinement effects. 2. Methods 2.1. Molecular simulation We have used the grand canonical Monte Carlo tech- nique, as implemented in the Sorption module of Cerius 2 release 4.0 [5], in order to evaluate adsorption loadings and enthalpies at equilibrium inside model microporous net- works, at a given temperature (T = 573 K) and pressure (P = 100 kPa), consistent with the conditions of catalytic experiments reported in [1]. Adsorption was simulated for hydrocarbons corresponding to intermediates and products in the hydrocracking of phenanthrene as observed in [1], in ideal siliceous frameworks for various relevant zeolitic structures. Some linear alkanes were added to this set of molecules. At high temperatures, hydrocarbon molecules are 0021-9517/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2003.09.014