Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of <i>n</i>‑Alkanes on Brønsted Acid Sites in Zeolites

Experimental measurements of the rate coefficient (<i>k</i>_app) and apparent enthalpies and entropies of activation (Δ<i>H</i>_app and Δ<i>S</i>_app) for alkane cracking catalyzed by acidic zeolites can be used to characterize the effects of zeolite structure and alkane size on the intrinsic enthalpy and entropy of activation, Δ<i>H</i>_int^⧧ and Δ<i>S</i>_int^⧧. To determine Δ<i>H</i>_int^⧧ and Δ<i>S</i>_int^⧧, enthalpies and entropies of adsorption, Δ<i>H</i>_ads‑H⁺ and Δ<i>S</i>_ads‑H⁺, must be determined for alkane molecules moving from the gas phase to Brønsted acid sites at reaction temperatures (>673 K). Experimental values of Δ<i>H</i>_app and Δ<i>S</i>_app must also be properly defined in terms of Δ<i>H</i>_ads‑H⁺ and Δ<i>S</i>_ads‑H⁺. We report here a method for determining Δ<i>H</i>_ads‑H⁺ and Δ<i>S</i>_ads‑H⁺ in which the adsorption site is represented by a fixed volume that includes the proton. Values of Δ<i>H</i>_ads‑H⁺ and Δ<i>S</i>_ads‑H⁺ obtained from Monte Carlo simulations are in good agreement with values obtained from experimental data measured at 300–400 K. An important feature of the simulations, however, is their ability to account for the redistribution of alkane adsorbed at protons in different locations with increasing temperature. Values of Δ<i>H</i>_int^⧧ and Δ<i>S</i>_int^⧧ for the cracking of propane through <i>n</i>-hexane, determined from measured values of <i>k</i>_app and Δ<i>H</i>_app and simulated values of Δ<i>H</i>_ads‑H⁺ and Δ<i>S</i>_ads‑H⁺, agree well with values obtained independently from quantum mechanics/molecular mechanics calculations. Application of our method of analysis reveals that the observed increase in <i>k</i>_app with increasing <i>n</i>-alkane size is due primarily to a decrease in Δ<i>H</i>_int^⧧ with increasing chain length and that Δ<i>S</i>_int^⧧ is independent of chain length