Adsorption thermodynamics and intrinsic activation parameters for monomolecular cracking of n-alkanes on br?nsted acid sites in zeolites
Journal
Journal of Physical Chemistry C
Journal Volume
119
Journal Issue
19
Pages
10427-10438
Date Issued
2015
Author(s)
Janda A.
Vlaisavljevich B.
Lin L.-C.
Mallikarjun Sharada S.
Smit B.
Head-Gordon M.
Bell A.T.
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, (Formula presented.) and (Formula presented.). To determine (Formula presented.) and (Formula presented.), enthalpies and entropies of adsorption, ΔHads-H+ and ΔSads-H+, must be determined for alkane molecules moving from the gas phase to Br?nsted acid sites at reaction temperatures (>673 K). Experimental values of ΔHapp and ΔSapp must also be properly defined in terms of ΔHads-H+ and ΔSads-H+. 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, however, is their ability to account for the redistribution of alkane adsorbed at protons in different locations with increasing temperature. Values of (Formula presented.) and (Formula presented.) 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 (Formula presented.) with increasing chain length and that (Formula presented.) is independent of chain length. ? 2015 American Chemical Society.
Subjects
Adsorption
Chain length
Chains
Chemical activation
Cracks
Enthalpy
Entropy
Hexane
Intelligent systems
Molecular modeling
Monte Carlo methods
Quantum theory
Thermodynamics
Zeolites
Activation parameter
Adsorption thermodynamics
Entropy of activation
Experimental values
Important features
Increasing temperatures
Quantum mechanics/molecular mechanics
Reaction temperature
Paraffins
Type
journal article