title: Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane.
abstract: The effects of zeolite structure on the kinetics of n-butane monomolecular cracking and dehydrogenation are investigated for eight zeolites differing in the topology of channels and cages. Monte Carlo simulations are used to calculate enthalpy and entropy changes for adsorption (ΔHads-H+ and ΔSads-H+) of gas-phase alkanes onto Brønsted protons. These parameters are used to extract intrinsic activation enthalpies (ΔHint‡), entropies (ΔSint‡), and rate coefficients (kint) from measured data. As ΔSads-H+ decreases (i.e., as confinement increases), ΔHint‡ and ΔSint‡ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for ΔSint‡, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), ΔHint‡ is relatively constant, while ΔSint‡ increases as ΔSads-H+ decreases because less entropy is lost upon protonation of the alkane. Concurrently, selectivities to terminal cracking and dehydrogenation decrease relative to central cracking because ΔSint‡ decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (kapp) with confinement are driven by changes in kint or by changes in the adsorption equilibrium constant (Kads-H+). Values of ΔSint‡ and ΔHint‡ are positively correlated, consistent with weaker interactions between the zeolite and transition state and with the greater freedom of movement of product fragments within more spacious pores. These results differ from earlier reports that ΔHint‡ and ΔSint‡ are structure-insensitive and that kapp is dominated by Kads-H+. They also suggest that ΔSads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California-Berkeley , Berkeley, California 94720, United States.
authors.forename: Amber
authors.initials: A
authors.lastname: Janda
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California-Berkeley , Berkeley, California 94720, United States.
authors.forename: Bess
authors.initials: B
authors.lastname: Vlaisavljevich
authors.affiliation: Department of Process and Energy, Delft University of Technology , Leeghwaterstraat 39, 2628CB Delft, The Netherlands.
authors.forename: Li-Chiang
authors.initials: LC
authors.lastname: Lin
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California-Berkeley , Berkeley, California 94720, United States.|Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , Rue de l'Industrie 17, CH-1951 Sion, Switzerland.
authors.forename: Berend
authors.initials: B
authors.lastname: Smit
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California-Berkeley , Berkeley, California 94720, United States.
authors.forename: Alexis T
authors.initials: AT
authors.lastname: Bell
mesh_terms: D000327:Adsorption; D002073:Butanes / Q000737:chemistry*; D003198:Computer Simulation; D006865:Hydrogenation; D007700:Kinetics; D009010:Monte Carlo Method; D013816:Thermodynamics; D014961:X-Ray Diffraction; D017641:Zeolites / Q000737:chemistry*
chemical_list: D002073:Butanes; D017641:Zeolites; C046888:butane