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 (DHads H+ and DSads H+ of gas phase alkanes onto Bronsted protons. These parameters are used to extract intrinsic activation enthalpies (DHint+ entropies (DSint+ and rate coefficients (kint) from measured data. As DSads H+ decreases (i.e. as confinement increases) DHint+ and DSint+ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for DSint+ indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state) DHint+ is relatively constant, while DSint+ increases as DSads 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 DSint+ 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 DSint+ and DHint+ 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 DHint+ and DSint+ are structure insensitive and that kapp is dominated by Kads H+ They also suggest that DSads H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.
Journal of the American Chemical Society