Theoretical and kinetic analysis of anisole and cresol primary reactivity in pyrolysis and combustion

Anisole and ortho/para (o/p-) cresol are relevant oxygenated aromatic compounds representative of lignocellulosic biomass and potential octane boosters. Despite their significance, the existing literature fails to comprehensively interpret the fundamental aspects of their decomposition reactions. To bridge this gap, we employ high-level theoretical calculations to determine rate constants for key decomposition reactions of both C7H8O isomers and C7H7O resonance-stabilized radicals generated by bond-fission and abstraction reactions from C7H8O. The rate constant determined theoretically using Variable Reaction Coordinate Transition State Theory for the C6H5O + CH3 is in quantitative agreement with experimental data. In contrast to what is currently implemented in kinetic models, CH3 addition to the carbon sites of the aromatic ring is largely favored over anisole formation. Hence, large quantities of methyl-cyclohexadienones (MCHDs) form and stabilize at lower temperatures. Once formed, MCHDs can rapidly isomerize to cresols and eliminate CO. Cresols unimolecular decomposition was found to be slower than that of phenol, thus warning against rate rules adopted in kinetic models to estimate their reactivity. o/p-OC6H4CH3 are the main radicals produced and they decompose to C5H4CH3 + CO or to OC6H4CH2 + H. Notably, p-OC6H4CH3 exhibits faster H loss due to the higher stability of the product. Conversely, the ortho radical undergoes rapid interconversion with o-HOC6H4CH2 owing to the vicinal O and CH3 groups. Implementing the new theoretical calculations in the CRECK model, with appropriate automated lumping of o/p-MCHDs and o/p-C7H7O species, significantly impacts kinetic simulations of aromatics combustion, especially for anisole. Key findings include the substantial accumulation of MCHDs at lower temperatures and a larger fraction of ortho isomers, which suggests to reconsider lumping schemes for cresols adopted in the existing kinetic mechanisms. Finally, the underprediction of the high-temperature ignition of anisole, attributed to the insufficient consumption of cresols, advocates for a revision of their oxidation kinetics.