Towards a new understanding of the retro-aldol reaction for oxidative conversion of lignin to aromatic aldehydes and acids.

The retro-aldol reaction is one of the key steps involved in the oxidative conversion of lignin to aromatic aldehydes and acids. In principle, the retro-aldol reaction can proceed in the absence of oxygen. In this work, a new approach based on the influence of oxygen on the oxidation of lignin was investigated. In this approach, the duration of oxygen charged during the reaction was optimized to, for the first time, improve the yield of aromatic aldehydes and acids. The effect of reaction chemistry, time, temperature, and lignin feedstock plays a key role on the yield of aromatic aldehydes and acids. At 140 °C, oxidation of softwood Lignoboost kraft lignin for 40 min results in combined maximum yield of 5.17% w/w of vanillin and vanillic acid. In comparison, using the new approach in which oxygen was charged for only 20 min during the 40 min reaction improved this yield considerably to 6.95%. Further, yield improvement was obtained when applying this approach to different lignin feedstocks. Oxidation also increased the carboxyl content in lignin from 0.49 mmol/g to 1.41 mmol/g which represents a marked improvement. The current study provides new evidence showing that the oxidation reaction is a crucial pathway for lignin valorization.