A Novel Pretreatment for Eucalyptus by Cooking with Active Oxygen and Solid Alkali in γ-Valerolactone/H ₂ O Binary System: Experimental and Mechanism Study

Lignocellulosic biomass mainly consists of cellulose, hemicellulose, and lignin, which are tightly interconnected with each other. Reasonable pretreatment of biomass is necessary for separating its different components. In this work, cooking with active oxygen and solid alkali (CAOSA) in γ-valerolactone (GVL)/H2O was developed to remove lignin and hemicellulose from eucalyptus wood as raw material. It was found that utilizing GVL/H2O as a solution instead of neat water could effectively improve the CAOSA pretreatment effect. Under milder conditions, lignin and hemicellulose removal was achieved up to 90.2% and 90.9% with a 93.5% cellulose retention rate. It was found that CAOSA in GVL/H2O could promote reactive oxygen species to improve the pretreatment effect of CAOSA. The enhancement of reactive oxygen species promoted the detachment of hemicellulose, and the degradation degree of hemicellulose was higher than that in the neat water system with the gradual degradation of a large amount of hemicellulose into small molecule weight acids. Meanwhile, the lignin recovered from the separated aqueous phase was studied, indicating that the incorporation of GVL with H2O enabled more C–C bonds to be broken during the reaction process than C–O bonds under the CAOSA treatment. In addition, the mechanism of lignin fraction and cellulose protection by GVL/H2O CAOSA treatment was also elaborated. In conclusion, CAOSA in GVL/H2O can effectively improve the pretreatment effect of CAOSA for biomass. The CAOSA in GVL/H2O provided a more straightforward and more complete recovery method of magnesium, better degradation of hemicellulose, and better retention of the separated lignin. The lignin and cellulose obtained through CAOSA in GVL/H2O treatment have a better potential for downstream product conversion.