Oxidative Valorization of Spruce Bark to Yield Vanillin

A competitive and environmentally sustainable biorefinery should valorize all parts of biomass, including bark. Currently, bark, which constitutes 10 wt% of timber and pulp wood, is burned at a low value. Softwood bark comprises 38% lignin, suggesting that this component could be an important source of valuable aromatic compounds; this lignin also contains substantial amounts of interlinkages with strong C–C bonds, such as β-5 linkages, which are not cleaved during standard depolymerization methods. In contrast to wood lignin which is linear, bark lignin appears to be cyclic, which further hinders depolymerization. Here, we present a method using oxygen to oxidatively depolymerize spruce bark lignin to yield valuable monophenols at 30-fold higher levels than expected from nitrobenzene oxidation. This is an important step in making biorefineries more sustainable and competitive and a new source to green vanillin.