Depolymerization of enzymatic hydrolysis lignin: Review of technologies and opportunities for research

Recent years witnessed the dawn of the cellulosic ethanol industry based on enzymatic hydrolysis of carbohy-drates in lignocellulosic biomass. Meanwhile, new concepts have emerged to depolymerize lignin and upgrade the obtained lignin fragments into value-added fuels and chemicals. This review analyzes the convergence of these two fields, presenting the opportunities for applying such depolymerization technologies to the residual enzymatic hydrolysis lignin (EHL) now available at an industrial scale. Key characteristics of EHL are presented together with the most investigated technologies for EHL depolymerization (pyrolysis, hydrothermal, and sol-volysis) to obtain lignin bio-oils. The review also discusses the prospects for upgrading lignin bio-oils for use as aviation and marine biofuels. Furthermore, it presents a more long-term vision that progressively valorizes the biochar by-product and adds the co-production of chemicals harnessing the aromaticity and oxygenation of the lignin moieties. In conclusion, EHL has a unique set of characteristics (cost, availability, transportability, degrees of oxygenation and purity, reactivity, among others) to produce lignin bio-oils, instigating research questions dedicated to this type of industrially relevant feedstock.