Synthesis and Computational Investigation of Antioxidants Prepared by Oxidative Depolymerization of Lignin and Aldol Condensation of Aromatic Aldehydes

New antioxidants were prepared by oxidative depolymerization of lignin followed by aldol condensation to form conjugated structure with benzene ring. Kinetics of aldol condensation between lignin-derived aromatic aldehydes and methyl ethyl ketone was investigated. The increase in the antioxidation ability was confirmed by experimentation and interpreted by density functional theoretical calculation.

Abstract

Novel antioxidants are synthesized by CuSO₄-catalyzed oxidative depolymerization of lignin to form aromatic aldehydes followed by aldol condensation with methyl ethyl ketone (MEK). Aldol condensation greatly improves the antioxidation ability of lignin depolymerized products. Three lignin monomeric aromatic aldehydes, – p-hydroxybenzaldehyde, vanillin, and syringaldehyde – are further employed for aldol condensation with MEK, resulting in successful synthesis of new antioxidants 1-(4-hydroxyphenyl)pent-1-en-3-one (HPPEO), 1-(4-hydroxy-3-methoxyphenyl)pent-1-en-3-one (HMPPEO), and 1-(4-hydroxy-3,5-dimethoxyphenyl)pent-1-en-3-one (HDMPPEO), respectively. Kinetic modeling illustrates that p-hydroxybenzaldehyde has the highest rate of reaction with MEK, followed by vanillin and then syringaldehyde, which is probably affected by the presence of methoxy groups. The syringaldehyde-derived product (HDMPPEO) displays the best antioxidation ability. As revealed by density functional theory calculations, electron-donating groups, such as methoxy, and conjugated side chains effectively improve the antioxidation ability. A hydrogen atom transfer (HAT) mechanism tends to occur in nonpolar solvents, whereas a sequential proton-loss electron transfer (SPLET) mechanism is favored in polar solvents. This work thus can inspire new pathways for valorization of lignin to produce high value-added products.

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