Ab Initio Investigation of Primary Fuel Reactions of Monoaromatic Hydrocarbons under Pyrolytic Conditions: Anisole, Phenetole, and the 2‑, 3‑, 4‑Methylanisole Isomers

Wooden biomass contains high amounts of lignocellulose,
which is
one of the main fuel components during wildfire events. Furthermore,
its properties in the context of alternative energy carriers are of
interest in recent research. In order to better analyze and understand
these highly complex molecules and their fundamental combustion properties,
a complexity reduction by using model compounds can be applied. Monoaromatic
oxygenated hydrocarbons (MAHs) are an option to map these systems
on a more accessible level. In the present study, the MAHs anisole,
phenetole, 2-methylanisole, 3-methylanisole, and 4-methylanisole were
investigated by means of quantum chemical calculations. To this end,
the DLPNO–CCSD(T)/CBS(cc-pVTZ, cc-pVQZ)//B3LYP-D3BJ/def2-TZVP
levels of theory were utilized to derive a range of important physical
and chemical quantities. These include bond dissociation energies
(BDEs), one-dimensional representations of the potential energy surface,
thermodynamic properties, and reaction rate parameters. As previously
demonstrated, reactions of the aromatic ring structure and the attached
hydrogen atoms are energetically unfavorable. This prompted the investigation
of only the reactions affecting the methyl and alkoxy side chains.
The reactions examined in this study are the primary fuel reactions
that are relevant to pyrolysis. This set of 47 reactions includes
the H atom abstraction by Ḣ and ĊH3, the
unimolecular bond fissions, and the internal H atom migration reactions
on the methoxy or ethoxy side chain. For all five molecules, the C–O
bond on the alkoxy side chain is the weakest bond by BDEs, and the
respective bond fissions are dominant. Besides the general importance
of H atom abstractions, these dominant bond fissions have the highest
impact on the overall reactivity among the investigated reactions.
Due to the comprehensive amount of available literature for anisole,
it is included as a benchmark molecule. The available literature on
phenetole is limited, and the present study provides fundamental data
for this species. For methylanisole, a recent publication focused
on experimental and modeling efforts for these isomers. The importance
of the C–O bond breaking, and the other determined reactions
in this work were tested by including the calculated rate parameters
in a validated chemical kinetic mechanism for methylanisole isomers
from literature. The modified model was subsequently assessed in comparison
to the initial version of the published model and experiments. Shock
tube and rapid compression machine experiments were performed in the
temperature range between 880 and 1220 K for pressures of 10 and 20
bar at stoichiometric conditions. This assessment yielded two notable
findings. First, it confirmed the significant impact of the C–O
bond fission. However, a comparison with recent high-level ab initio
calculations revealed significant deviations in the rate constants.
Second, it emphasized the importance of the subsequent phenoxy/methylphenoxy
radical chemistry and the associated thermodynamic properties. Further
refinement of the model descriptions of MAHs is warranted and necessary
to improve the understanding of these important reference molecules.