Abstract
Lignin is a highly abundant polyphenolic polymer that imparts mechanical strength to plant biomass. Transition-metal complexes can catalyze lignin oxidation to produce value-added products, but low catalytic efficiency has hampered their use in industry. Identifying the chemical and structural factors that govern catalytic activity is a prerequisite to rational design of catalysts with improved activity. Here, we combine computational and experimental approaches to investigate the mechanism of Co(salen)-catalyzed oxidation of the monomeric lignin models syringyl (S), vanillyl (G), and 4-hydroxybenzyl alcohol (H) to produce benzoquinone and benzaldehyde products. Experimentally, S oxidation to form dimethoxybenzoquinone proceeded efficiently with a Co(salen) catalyst coordinated by a pyridine ligand, but G and H did not undergo oxidation. Density functional theory calculations reveal that catalyst regeneration is energetically unfavorable in the presence of H, which prevents oxidation. In contrast, S readily facilitates catalyst regeneration. Formation of methoxybenzoquinone from G was achieved experimentally by adding bulky, noncoordinating bases. These findings provide a fundamental baseline for enhancing the activity of Co-Schiff base catalysts toward lignin-like molecules by adding sterically hindered nitrogenous bases or potentially by including a cocatalyst that promotes catalyst regeneration.
Keywords
shiff base,
valorization,
biomass,
transition metal,
density functional theory,
catalysis
Citation
Cooper, Connor J.; Alam, Shahrina; Nziko, Vincent de Paul N.; Johnston, Ryne C.; Ivanov, Alexander S.; Mou, Zhongyu; Turpin, David B.; Rudie, Alan W.; Elder, Thomas J.; Bozell, Joseph J.; Parks, Jerry M. 2020. Co(salen)-catalyzed oxidation of lignin models to form benzoquinones and benzaldehydes: A computational and experimental study. ACS Sustainable Chemistry & Engineering. 8(18): 7225-7234. https://doi.org/10.1021/acssuschemeng.0c01970.
