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 (ΔH_ads-H+ and ΔS_ads-H+) of gas-phase alkanes onto Brønsted protons. These parameters are used to extract intrinsic activation enthalpies (ΔH_int ^‡), entropies (ΔS_int ^‡), and rate coefficients (k_int) from measured data. As ΔS_ads-H+ decreases (i.e., as confinement increases), ΔH_int ^‡ and ΔS_int ^‡ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for ΔS_int ^‡, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), ΔH_int ^‡ is relatively constant, while ΔS_int ^‡ increases as ΔS_ads-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 ΔS_int ^‡ decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (k_app) with confinement are driven by changes in k_int or by changes in the adsorption equilibrium constant (K_ads-H+). Values of ΔS_int ^‡ and ΔH_int ^‡ 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 ΔH_int ^‡ and ΔS_int ^‡ are structure-insensitive and that k_app is dominated by K_ads-H+. They also suggest that ΔS_ads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.