Co(salen)-Catalyzed Oxidation of Lignin Models to Form Benzoquinones and Benzaldehydes: A Computational and Experimental Study

• Published in: ACS sustainable chemistry & engineering Vol. 8; no. 18; pp. 7225 - 7234
• Main Authors: 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
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 11.05.2020; American Chemical Society (ACS)
• Subjects: biomass; catalysis; density functional theory; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Schiff base; transition metal; valorization; Schiff base; catalysis; valorization; biomass; transition metal; density functional theory
• ISSN: 2168-0485; 2168-0485
• DOI: 10.1021/acssuschemeng.0c01970

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.