Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of n-Alkanes on Bronsted Acid Sites in Zeolites
Experimental' measurements of the rate coefficient (k(app)) and apparent enthalpies and entropies of activation (Delta H-app arid Delta S-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, Delta H-int(double dagger) and Delta S-int(double dagger). To determine Delta H-int(double dagger) and Delta S-int(double dagger), enthalpies and entropies of adsorption, Delta Hads-H+ and Delta Sads-H+, must be determined for alkane molecules moving from the gas phase to Bronsted add sites at reaction temperatures (>673 K). Experimental values of Delta H-app and Delta S-app must also be properly defined in terms of Delta Hads-H+ and Delta Hads-H+. We report here a method for determining Delta Hads-H+ and Delta Sads-H+ in which the adsorption site is represented by a fixed volume that includes the proton. Values of Delta Hads-H+ and Delta Sads-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 ineteasing temperature. Values of Delta H-int(double dagger) and Delta S-int(double dagger), for the cracking of propane through n-hexane, determined from measured values of k(app) and Delta H-app and simulated: values of Delta Hads-H+ and Delta Sads-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 kapp with increasing n-alkane size is due primarily to a decrease in Delta H-int(double dagger) with increasing chain length and that Delta S-int(double dagger) is independent of chain length.