Aromatics from Lignin through Ultrafast Reactions in Water

Abstract

Nowadays, the valorization of lignin, the major natural source of aromatics in earth, is being a challenge for the scientific community. In this study, kraft lignin is effectively converted into aromatic monomers by ultrafast depolymerization in hot and pressurized water. At reaction times below 500 ms, it is possible to avoid char formation originated from undesirable condensation reactions by controlling accurately the reaction time. Under alkaline medium, the reaction reaches an optimum point at 386ºC and 300 ms with a light oil yield of 60% with a concentration in key compounds such as guaiacol, creosol, vanillin and acetovanillone of around 20 %w/w. The char formation in this point was surprisingly low (4 %w/w). Analysis and quantification of the products allows to identify the evolution of the different reaction steps and propose plausible mechanism for the depolymerization and repolymerization stages. Furthermore, it is proven that the proposed technology is equally effective to treat directly industrial black liquors with a yield higher than 50% to light oil, containing as main monomers guaiacol (2.7%), syringol (3.0%) and syringaldehyde (7.3%).