Revealing crucial role of fuel radicals in the pyrolysis chemistry of 1,2-dimethoxybenzene serving as a model system for lignin

Abstract

Understanding the pyrolysis mechanism of lignin, a key renewable biomass, is crucial for its effective utilization in mitigating environmental challenges. This work explored the pyrolysis mechanism of 1,2-dimethoxybenzene (DMOB), a lignin model compound, in a flow reactor at 0.04 atm and 873–1083 K. Using a synchrotron vacuum ultraviolet radiation photoionization mass spectrometer, various featured C₈C₆ products, including 2-hydroxybenzaldehyde and 2-methylanisole, as well as smaller C₅C₁ species, were detected. A detailed kinetic model was developed, revealing that DMOB mainly decomposes via unimolecular OCH₃ bond cleavage, forming a 2-methoxyphenoxy radical, which elucidates the formation of 2-hydroxybenzaldehyde. Additionally, bimolecular reactions, such as ipso-substitution and H-abstraction, play a pivotal role in DMOB decomposition and products formation, accounting for approximately 49 % of DMOB consumption at 963 K. These reactions contribute the formation of 2-methylanisole, anisole, and guaiacol. Moreover, DMOB-related reactions significantly contribute to the formation of smaller C₅C₁ products, with the unimolecular methyl elimination reaction exhibiting the highest sensitivity to the formation of methane and carbon monoxide. Finally, when comparing DMOB to two other lignin model compounds with distinct side chains, i.e., anisole and guaiacol, it was observed that the corresponding fuel radicals, formed during the pyrolysis of these three compounds, play a vital role in controlling the products distribution. In summary, this work provides valuable insights into the pyrolysis behaviors of lignin model systems, which have significant potential for elucidating the pyrolysis mechanism of lignin.