Isolation and further structural characterization of lignins from the valonea of Quercus variabilis

The isolation process of alkali lignin (AL) from the valonea of Quercus variabilis Blume was optimized (liquid/solid ratio, 12.21; isolation time, 4.21 h; isolation temperature, 42.21 °C; and alkali concentration, 0.85 mol/L) using the response surface method (RSM), with the highest isolation rate obtained being 22.67%. Then, the apparent structures of AL, enzymatic hydrolysis lignin (CEL) and milled wood lignin (MWL) were studied by scanning electron microscopy (SEM), which indicated that the isolation processes of AL and CEL caused some damage to the apparent structure of lignin. The comprehensive structure characteristics of lignin samples was studied using 1H, 13C and 2D-HSQC techniques based on former studies. It was found that (1) three lignins were GSH-type; (2) the relative content of β-O-4′ linkages in CEL (75.91%) was lower than those in AL (91.57%) and MWL (83.23%), suggesting that the β-O-4′ linkages were largely cleaved during the CEL isolation process. In addition, the existence of phenylcoumarane, ferulic acid, p-coumarates and p-hydroxycinnamyl alcohol end groups can be found; (3) The S/G ratios were estimated to be 8.72, 1.30 and 0.98 for AL, MWL and CEL, respectively, suggesting that the lignin fragment rich in S-units was easily released under the alkali conditions.