Tertiary Amines from RCF Lignin Mono- and Dimers: Catalytic N-Functionalized Antioxidants from Wood

Functionalization of bio-based aromatics offers an appealing opportunity toward a renewable way of fulfilling our current needs for chemicals and materials. Here, an atom-efficient Cu-catalyzed hydrogen borrowing strategy is presented, which successfully functionalizes aliphatic alcohols in aromatic monomers and dimers, derived from lignin by the reductive catalytic fraction (RCF), into tertiary dimethylamines. Kinetic experiments and ToF-SIMS analysis of the supported copper catalyst demonstrated a reduced catalytic activity for monomeric methoxyphenolics, such as guaiacol and syringols, relative to phenolic and nonphenolic model compounds. This is explained by the formation through demethylation and the adsorption of strong coordinating catechol species. The nature of the catalyst support proved to be key to cope with the catechol deactivation and keep high catalytic activity, with Cu supported on SiO2 outperforming earlier reported CuZrO2. The hydrogen borrowing method was extended to real spruce wood-derived RCF lignin oil fractions, containing both phenolic mono- and oligomers. Special effort was done to identify the composition and molecular structure of the resulting phenolic dimer amines by GC × GC-TOF/MS and 1 H-13C-NMR techniques. The stable lignin-derived tertiary amines displayed excellent antioxidant activity during an ABTS assay, highlighting the added value of the products obtained by the hydrogen borrowing upgrading strategy.