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.  2005 Elsevier Inc. All rights reserved.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…The topologies in red have been patented by Shell 541 and ExxonMobil, 542 and GON has been "discovered" by simulations. 544 …”

“…For example, branched alkanes (and alkenes) can contribute the majority of the hydrocracking products to the desorbed product phase while the branched alkane portion of the desorbed phase remains minute. 359,504 Under such conditions, the gas phase and adsorbed phase can no longer equilibrate and the free-energy landscape imposed by the zeolite topology on the reacting system will leave its signature on the product distribution. Such a lack of equilibration between gas phase and adsorbed phase is endemic to larger molecules in industrial processes such as we consider here.…”

“…In the Henry regime the simulated parameter characterizing n-alkane reactivity (product of Henry constant and diffusion coefficient) reproduces the experimental reactivity remarkable well. 504 …”

Loading Dependence of the Diffusion Coefficient of Methane in Nanoporous Materials

“…The small entry pore is selective toward linear paraffins, and cracking can occur on sites within the supercage to produce smaller chain alkanes. 70 Zeolite A is also widely used in in ion-exchange separation. 71 We find a slightly smaller value for R ctw in chabazite-type (CHA) zeolite.…”

Computer Simulations of Shape Selectivity Effects

“…A lower free energy of adsorption is a measure of a higher concentration inside the zeolite, and a higher reactivity. Experimentally, insights into reactant shape selectivity due to differences in free energy of adsorption have been obtained in studies on the chain lengthdependence of the reactivity of n-alkanes [26,122,123], so that the discrimination of zeolites between n-alkanes of various lengths depends on the pore topology. To the extent that the reactivity of n-alkanes as a function of chain length varies with zeolite topology, it is -by definition [14] -an example of (reactant) shape selectivity.…”