Kinetic analysis of the pyrolysis of phenethyl phenyl ether: computational prediction of ?/?-selectivities
- ORNL
We have developed a scheme to predict / -product selectivities in the pyrolysis of model compounds for the -ether linkage in lignin. The calculation of the individual / -selectivities for hydrogen abstraction by chain carrying benzyl and phenoxyl radicals profits from error cancellation in the computation of relative rate constants of similar reactions with common reactants. The Arrhenius pre-factors depend strongly on the description of the low-frequency modes for which anharmonic contributions are important. We use density functional theory in combination with transition state theory in this analysis. Diagonal anharmonic effects for individual low-frequency modes are included by employing a second-order Wigner-Kirkwood expansion in a semiclassical expression for the vibrational partition function. The composite / -product selectivity is obtained by applying quasi-steady-state kinetic analysis for the intermediate radicals. We calculate an overall / -selectivity for the pyrolysis of phenethyl phenyl ether as a composite of the / -selectivities in the hydrogen abstraction reactions by the phenoxyl and by the benzyl radical that is in excellent agreement with experiment. The difference between the individual selectivities for these radicals is explained by analyzing the electronic structure of the transition states. Spin delocalization of the single electron favors the -pathways. An opposing effect occurs for polarized transition states, such as the transition states for the hydrogen abstraction by the electrophilic phenoxyl radical, where the adjacent ether oxygen in phenethyl phenyl ether stabilizes the -transition states. These results also indicate that theory will be able to provide excellent predictions of / -product selectivities for more complicated lignin model compounds bearing multiple substituents.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). National Center for Computational Sciences (NCCS)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 930959
- Journal Information:
- Journal of Physical Chemistry A, Vol. 111, Issue 48
- Country of Publication:
- United States
- Language:
- English
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