(63b) Selective Oxidative Conversion of Acetylated Lignin Towards Aromatic Acids

A variety of lignin valorization methods have been employed to convert lignin to value-added biochemicals to promote biorefinery profitability. However, a route that can isolate a single type of value-added biochemicals from a mixture of lignin-depolymerized products has become a fascinating challenge. Herein, we propose a new three-step route to depolymerize the industrial lignin into a decent yield of targeted aromatic acids including p-hydroxybenzoic acid, vanillic acid and syringic acid using low-cost GO-UHP catalyst system and under mild conditions (atm, <120 C). In this route, metal-free graphene oxide selectively depolymerizes the acetylation-induced lignin towards aromatic aldehydes/acids. After that, a protective oxidizing upgrading method using UHP as the oxidizer converts the aromatic aldehydes in the lignin depolymerized mixture to targeted aromatic acids with an excellent yield (up to 18%). This route achieves an overall 95% organic oil yield, more importantly, a decent yield of AA can be isolated through a simple base/acid extraction method. Mechanisms related to each step are proposed to elucidate the role of catalyst, solvent and the impurities in the process, and quantum chemistry calculations are also adopted to elucidate the mechanism of this three-step oxidative lignin depolymerization reactions using a GO-UHP (Urea hydrogen peroxide) catalyst system.