Combined Density Functional Theory and Monte Carlo Analysis of Monomolecular Cracking of Light Alkanes Over H-ZSM-5

Density functional calculations applying periodic boundary conditions have been performed to investigate adsorption and cracking of light alkanes (C 3-C 6) on zeolite H-ZSM-5. Intrinsic energy barriers were obtained from single-point calculations by applying the revised form of the PBE functional (RPBE) to structures optimized on the PBE potential energy surface. Dispersion interactions were accounted for by adding a damped dispersion term to the PBE energies. The dependence of the adsorption enthalpy on the carbon number is in agreement with experimental observation. From intrinsic energy barriers, intrinsic rate coefficients were calculated by means of transition state theory. The dependence of the intrinsic enthalpy and entropy of activation on the carbon number is discussed and compared to experimental observations. Transition path sampling was employed to unravel qualitatively the reaction mechanism for cracking of butane. Monte Carlo simulations in the canonical ensemble were conducted to estimate the temperature dependence of the adsorption enthalpy and entropy of propane to n-hexane. These quantities are not constant, as is often assumed in the interpretation of experimental data but become less negative with increasing temperature. It is shown how the selection of adsorption parameters influences the extraction of intrinsic rate parameters from experimental rate data. Based on the present analysis, an alternative partitioning of the experimentally accessible apparent entropy of activation into contributions from adsorption and intrinsic reaction is proposed. © 2012 American Chemical Society.