Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of N-alkanes on Brønsted-acid Sites in Zeolites

A. L. Janda, B. Vlaisavljevich, L.-C. Li, S. Mallikarjun Sharada, B. Smit, M. Head-Gordon, and A. T. Bell, Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of N-alkanes on Brønsted-acid Sites in Zeolites J Phys Chem C  19 (19), 10427 (2015) http://dx.doi.org/10.1021/acs.jpcc.5b01715



Abstract: Experimental measurements of the rate coefficient (kapp) and apparent enthalpies and entropies of activation (ΔHapp and ΔSapp) 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, 〖"ΔH" 〗_"int" ^"‡" and 〖"ΔS" 〗_"int" ^"‡" . In order to determine 〖"ΔH" 〗_"int" ^"‡" and 〖"ΔS" 〗_"int" ^"‡" , enthalpies and entropies of adsorption, ΔHads H+ and ΔSads H+, must be determined for alkane molecules moving from the gas phase to Brønsted-acid sites at reaction temperatures (> 673 K). Experimental values of ΔHads H+ and ΔSads H+ must also be properly defined in terms of ΔHapp and ΔSapp. We report here a method for determining ΔHads H+ and ΔSads H+ in which the adsorption site is represented by a fixed volume that includes the proton. Values of ΔHads H+ and ΔSads 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 is their ability to account for the redistribution of alkane adsorbed at protons in different locations with increasing temperature. Values of 〖"ΔH" 〗_"int" ^"‡" and 〖"ΔS" 〗_"int" ^"‡" for the cracking of propane through n-hexane, determined from measured values of kapp and ΔHapp and simulated values of ΔHads H+ and ΔSads 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 〖"ΔH" 〗_"int" ^"‡" with increasing chain length, and that 〖"ΔS" 〗_"int" ^"‡" is independent of chain length.

© Berend Smit 2019