Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane

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.