OH-Initiated Reactions of -Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH Radicals.

Lignin is the most complex component of biomass, and development of a detailed chemical kinetic model for biomass pyrolysis mainly relies on the understanding of the lignin decomposition kinetics. -Coumaryl alcohol (-CMA, HOPh-CH═CH-CHOH), the focus of our analysis, is the simplest of the lignin monomers (monolignols) containing a typical side-chain double bond and both alkyl- and phenolic-type OH-groups. In parts I and II of our work (Asatryan, R. 2019, 123, 2570-2585; Hudzik, J. M. 2020, current issue), we created a detailed potential energy surface (PES) and performed a kinetic analysis of chemically activated, unimolecular, and bimolecular reactions pathways for -CMA + OH. Reaction pathways analyzed include dissociation, intramolecular abstraction, group transfer, and elimination processes. The α- and β-carbon addition reactions generate 1,3- () and 1,2-diol () adduct radicals, respectively. Well depths are approximately 29 and 41 kcal/mol below the -CMA + OH entrance level. Kinetic analysis aides in determining the major pathways for our conventional and fractional pyrolysis experiments. The current paper focuses on the H-abstraction reactions via H, OH, and CH light ("pool") radicals from -CMA. The thermochemical properties of all stable, radical, and transition-state species were determined using the ωB97XD density functional theory (DFT) and higher-level CBS-QB3 composite methods. Barrier heights from the prereaction complexes, for OH-radical abstractions, to the transition states for the propanoid side chain are compared to the model H-abstraction reactions of allyl alcohol (AA) with OH and -CMA with H and CH radicals. The lowest-energy, most stable, -CMA radical formed is at the C9 allylic position (-CMA-C9j) with exothermicity of 26.63, 41.32, and 27.34 kcal/mol for H, OH, and CH, respectively. For OH-radical abstraction at this position, our findings are consistent with corresponding data on AA + OH at 37.44 kcal/mol and similar to that of . A similar stable radical with an exothermicity of 34.95 kcal/mol occurs for the phenol hydroxyl group, generating the -CMA-O4j radical. H-abstraction pathways are considered in relation to other major pathways previously considered for -CMA + OH reactions including H-atom shifts, dehydration, and β-scission reactions. Derived rate coefficients for substituted phenols can be utilized in detailed kinetic models for lignin/biomass pyrolysis.