Conversion of Lignin to Value‐added Chemicals via Oxidative Depolymerization

Lignin depolymerization has been studied for more than 80 years; however, it has garnered considerable attention over the past two decades. Various depolymerization routes have been reported in the literature (i.e. hydrogenolysis, pyrolysis, solvolysis, etc.). However, catalytic oxidative depolymerization remains one of the most promising routes due to the inherent ability of oxidative methods to retain monomer functionality under relatively mild conditions. Indeed, while reductive routes utilize extreme temperatures and pressures (>200 °C and >30 atm of pressure), oxidative routes are mainly performed under 100 °C at or near atmospheric pressure. Unfortunately, in planta lignin serves as a defense mechanism against chemical and biological attack. This results in the repolymerization of depolymerization products, especially when one-electron-type oxidations are performed. Thus, oxidative depolymerization of lignin remains a challenge. The following chapter reviews the current state of the art in the catalytic oxidation of lignin with a special emphasis on the use of selective stepwise oxidative depolymerization. Additionally, the chapter highlights the difficulty of comparing lignin depolymerization strategies due to the vast differences in extracted lignins as well as the need for realistic model compounds and reaction conditions.