Effect of confinement on the selectivity of hydrocracking

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.