Understanding cage effects in the n-alkane conversion on zeolites

Abstract

Molecular simulations are used to provide insight into published catalytic reactivity data for zeolites that exhibit a cage effect, the selective and preferential conversion of short-chain rather than long-chain n-alkanes. This paper demonstrates that understanding cage effects for ERI-, AFX-, and FER-type zeolites requires consideration of four components: (1) adsorption thermodynamics, (2) adsorption kinetics, (3) conversion at the exterior surface of the catalysts, and (4) coke-induced modifications to the pore texture. By breaking down the Gibbs free energy of adsorption into its enthalpic and entropic contributions, we can further elucidate the influence of the zeolite topology on the adsorption of n-alkanes with different hydrocarbon chain lengths. This analysis indicates that zeolite topologies with cages accessible through windows < 0.47 nm wide are particularly prone to exhibiting a cage effect, because they impose a high thermodynamic penalty on the adsorption of molecules that are too long to fit comfortably in a single cage. This improved understanding of the cage effect could facilitate its renewed use in commercial practice.