Mechanistic Insights into Formaldehyde-Blocked Lignin Condensation: A DFT Study

The formaldehyde (FA)-prevented lignin condensation reported by Luterbacher et al. [Science 2015, 354 (6310), 329-333] provides an efficient strategy for obtaining lignin monomers with near theoretical production. Our work presents a density functional theory calculation study that reveals the intrinsic mechanism why FA can efficiently hinder lignin condensation. Using veratrylglycerol-beta-guaiacyl ether (VG) as a lignin model compound, we found that the formation of C-C linkage between two VG units is highly exothermic (-43.7 kcal/mol) and involves a medium-heighted barrier (20.1 kcal/mol), explaining the extensive and irreversible lignin condensation observed during lignin depolymerization. In the presence of FA, VG prefers to react with FA with much lower barrier (13.1-20.7 kcal/mol) to form 1,3-dioxane structures, which protect the reactive sites of VG (1,3-diol groups and the ortho/para positions to methoxyl groups on lignin aromatic rings) and thereby hinder lignin condensation. The theoretical results provide a clear picture of how FA hinders lignin condensation.