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