Selective Production of Syringaldehyde and Vanillin from Acetal-Stabilized Lignin

Industrial lignin oxidation has been used for vanillin production but is largely uncontrolled, not fully understood mechanistically, and leads to low yield. Here, we have demonstrated high-efficiency vanillin and syringaldehyde production from lignin via guaiacyl and syringyl α-diketones. These molecules can be selectively produced from lignin, but their valorization has been poorly explored. We therefore explored their conversion into aldehyde equivalents with proven market demand to expand their use. Aryl-α-diketones were hydrogenated to produce the corresponding glycol with a Pt/C catalyst under mild conditions (80 °C, 20 bar H2). This intermediate was then cleaved oxidatively to produce aromatic aldehydes with the VO(acac)2 catalyst using air as the oxidant at 80 °C. Both steps give quantitative yields and are performed in the same solvent (MeTHF) allowing simple processing. By applying this conversion on a stream derived from birch AAF lignin, we were able to produce 4 wt % of aromatic aldehydes on a dry biomass basis.