Valorization of Bark Using Ethanol-Water Organosolv Treatment: Isolation and Characterization of Crude Lignin

Bark serves multiple functions for a tree by combining protection and fluid transport. The tissue itself is composed of multiple types of material that includes typical cell wall structural polymers of cellulose and lignin, as well as extractives. With suitable fractionation, these bark constituents may serve to help valorize biomass when converted into feedstocks for polymeric precursors, such as aromatic polyols. In this study, organosolv processing was used to solubilize around 50% of pine bark (PB) and oak bark (OB), followed by the recovery of around 20% crude lignin. Two-dimensional H-1-C-13 heteronuclear single quantum coherence (2D HSQC) NMR and C-13 NMR revealed the crude lignin also contained mixed polyphenolics, suberin-compounds, and carbohydrate compounds, especially for PB. To optimize the extraction process, a two-level factorial design was used to study the impacts of temperature, ethanol concentration, and solid loading on the yield and related characteristics of crude lignin. Increased temperature and ethanol concentration during processing allowed more solubilization of the components along with the extraction of crude lignin with higher yield. The correlation analysis found that the thermal stability of crude lignin had a direct relationship with aromatic hydroxyl group content, while the glass transition temperature was impacted by the lower molar mass components contained in the crude lignin. Overall, the study showed promise to solubilize a high portion of bark material and provided insight into structure-property relationships of crude lignin-derived from an important, yet underutilized resource as a function of processing conditions.