Understanding the Distribution and Structural Feature of Eucalyptus Lignin Isolated by γ-Valerolactone/Water/Acid System

In this study, an effective isolation method with γ-valerolactone (GVL)/water/acid system was carried out to extract lignin from Eucalyptus camaldulensis Dehnh. The detailed spatial distribution and structural features of lignin were studied thoroughly. The Raman spectra demonstrated that a preferential delignification first occurred in the cell corner middle lamella (CCML) and then in the secondary wall (S) region. The isolated lignin yields increased with the increment of acid concentration and treatment time, and the obtained lignin had a relatively high purity by comparison with double enzymatic lignin (DEL). Moreover, the breakage of β-O-4′ linkages and the degree of condensation increased with the increment of the treatment intensity, resulting in a small amount of β-O-4′ linkages existing in the extracted lignin. There was an obvious decrease of aliphatic hydroxyl groups and an increment of phenolic hydroxyl groups in the lignin preparations with the elevation of acid concentration and the treatment time. These findings could be used for better understanding the structural changes of lignin during the GVL/water/acid isolation process and for improving the utilizations of lignin in the industrial biorefinery process.