Refining biofuel combustion models: a high-level theoretical and kinetic modeling study of guaiacol

This work presents a high-level theoretical kinetic study on the pyrolysis and oxidation of guaiacol, a prototypical lignin-derived biofuel. Combustion mechanisms for such aromatic ethers often lack accuracy due to estimated rate constants for critical pathways. To resolve this fundamental issue, the present study provides a comprehensive characterization of the detailed potential energy surfaces (PESs) for guaiacol's initial consumption, including the unimolecular bond fissions (O–CH 3 , C–H, O–H), the hydrogen atom abstraction (HAA) by ȮH, HȮ 2 , and ĊH 3 radicals, Ḣ atom and O 2 , and the formation and isomerization of hydroxy‑methyl-cyclohexadienone (HMCHD) intermediates, using the M06–2X functional and the QCISD(T) methods. Crucially, the rate constants for the dominant, barrierless O–CH 3 bond fission and key radical recombination reactions were computed with rigorous variable reaction coordinate transition state theory (VRC-TST), a significant advancement over previous estimations. Substantial discrepancies, up to three orders of magnitude, are found between the present calculated rate constants and those estimated in the COLIBRI (Combustion of LIgnin derived Biofuel for Research and Innovation v2) mechanism (Delort et al., Fuel, 2024, 362, 130,832), particularly for the HAA reaction guaiacol + ȮH = HOA1OĊH 2 + H 2 O. The newly calculated parameters were integrated into the COLIBRI mechanism. The updated model demonstrates markedly improved predictive capability when validated against jet-stirred reactor (JSR) data for guaiacol pyrolysis and oxidation. It accurately captures guaiacol consumption and the formation profiles of major products including methylcatechol, hydroxybenzaldehyde, catechol and anisole. Rate of production (ROP) and sensitivity analyses trace this improvement to the refined description of the O–CH 3 fission and the subsequent recombination of the phenoxy radical (ȮA1OH) with ĊH 3 radical to form HMCHD isomers, which compete with and suppress the direct formation of methylcatechol, thereby steering the chemistry more accurately. Furthermore, the analysis identifies missing or inaccurately described ipso-addition reactions, particularly guaiacol + Ḣ → anisole + ȮH and guaiacol + ĊH 3 → cresol + CH 3 Ȯ, as key uncertainties requiring future investigation. This study establishes a high-accuracy kinetic framework for guaiacol consumption, providing fundamental advances in understanding the combustion chemistry of lignin-derived biofuels. Novelty and Significance Statement Prior kinetic models for guaiacol combustion have relied on rate constants estimated by analogy from anisole and phenol chemistry, introducing substantial uncertainties. This work provides the first high-level theoretical determination of rate constants for guaiacol-specific pathways, including the barrierless O–CH 3 bond fission and HAA reactions by ȮH, HȮ 2 , and ĊH 3 radicals, Ḣ atom and O 2 , using QCISD(T)/CBS calculations and VRC-TST. The present rate constants differ significantly from those estimated in the COLIBRI mechanism, with differences up to three orders of magnitude for the key HAA reaction guaiacol + ȮH = HOA1OĊH 2 + H 2 O. When incorporated into the COLIBRI mechanism, the updated model demonstrates markedly improved predictions for key aromatic products, including methylcatechol, hydroxybenzaldehyde, catechol, and anisole, in both guaiacol pyrolysis and oxidation. These improvements directly address a critical gap in the predictive capability of lignin-derived biofuel combustion models, with direct implications for soot precursor formation and the design of cleaner combustion systems.