Entropy considerations in monomolecular cracking of alkanes on acidic zeolites

doi:10.1016/j.jcat.2007.11.003

Journal of Catalysis

Volume 253, Issue 1, 1 January 2008, Pages 221-224

https://doi.org/10.1016/j.jcat.2007.11.003 Get rights and content

Abstract

Compensation between adsorption entropies and enthalpies results in less than a two-fold variation in adsorption equilibrium constants for C₃–C₆ alkanes at temperatures relevant for monomolecular cracking; the size-independent activation energy for C single bond C bond activation in C₃–C₆ alkanes indicates that the marked increase in monomolecular cracking turnover rates observed with alkane chain size reflects a concurrent increase in activation entropies. Thermodynamic treatments for non-ideal systems rigorously describe confinement effects within zeolite channels and show that pre-exponential factors depend on solvation effects of the zeolite-host environment through variations in the thermodynamic activity of the zeolitic proton. Observed differences in rates and selectivities of monomolecular alkane activation with zeolite structure, after normalization to intrazeolitic concentrations, reflect differences in intrinsic rate constants.

Section snippets

Results and discussion

Monomolecular cracking of alkanes via the Haag–Dessau mechanism prevalent at high temperature and low conversions (<1%) is purported to occur via cationic transition states with three-center/two-electron bonds that include both carbon atoms in the C single bond C bond being cleaved and a proton acting as a Brønsted acid (Scheme 1; Ref. [3]) [1], [2], [3], [4]. In cracking of n-alkanes, Haag et al. [5], Narbeshuber et al. [6], Kotrel et al. [2], Babitz et al. [7], and Ramachandran et al. [8] attributed

Acknowledgements

The authors acknowledge Chevron Energy Technology Company for financial support.

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Research NoteEntropy considerations in monomolecular cracking of alkanes on acidic zeolites

Abstract

Section snippets

Results and discussion

Acknowledgements

J. Phys. Chem. B

Catal. Lett.

Microporous Mesoporous Mater.

Stud. Surf. Sci. Catal.

J. Catal.

Appl. Catal. A Gen.

J. Catal.

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

Research Note
Entropy considerations in monomolecular cracking of alkanes on acidic zeolites