Enhancing Aromatic Production from Reductive Lignin Disassembly: <i>in Situ</i> O‑Methylation of Phenolic Intermediates

The selective conversion of lignin into aromatic compounds has the potential to serve as a “green” alternative to the production of petrochemical aromatics. Herein, we evaluate the addition of dimethyl carbonate (DMC) to a biomass conversion system that uses a Cu-doped porous metal oxide (Cu₂₀PMO) catalyst in supercritical methanol (sc-MeOH) to disassemble lignin with little to no char formation. While Cu₂₀PMO catalyzes C–O hydrogenolysis of aryl–ether bonds linking lignin monomers, it also catalyzes arene methylation and hydrogenation, leading to product proliferation. The MeOH/DMC co-solvent system significantly suppresses arene hydrogenation of the phenolic intermediates responsible for much of the undesirable product diversity via O-methylation of phenolic −OH groups to form more stable aryl-OCH₃ species. Consequently, product proliferation was greatly reduced and aromatic yields greatly enhanced with lignin models, 2-methoxy-4-propylphenol, benzyl phenyl ether, and 2-phenoxy-1-phenylethan-1-ol. In addition, organosolv poplar lignin (OPL) was examined as a substrate in the MeOH/DMC co-solvent system. The products were characterized by nuclear magnetic resonance spectroscopy (³¹P, ¹³C, and 2D ¹H–¹³C NMR) and gas chromatography–mass spectrometry techniques. The co-solvent system demonstrated enhanced yields of aromatic products