Structure and Antioxidant Activity Investigation of Milled Wood Lignin from Furfural Residues, Xylose Residues, and Corncob

Xylose residue (XR) and furfural residue (FR) are byproducts of xylose and furfural produced from hemicellulose in corncob at different intensities of acid hydrolysis, resulting in structural changes of main contents, especially lignin. To reveal the structural changes and make efficient utilization of lignin, corncob milled lignin (CL), XR milled lignin (XRL), and FR milled lignin (FRL) were isolated and comparatively analyzed. The structural properties of CL, XRL, and FRL were investigated by 2D-HSQC NMR, FTIR, ³¹P NMR, gel permeation chromatography, and elemental analysis. Results showed that the structural units of CL, XRL, and FRL all contained guaiacyl (G), syringyl (S), and <i>p</i>-hydroxyphenyl (H) types, but the contents differed with various acid hydrolysis strengths, especially for the H and G units. CL contained the highest amount of the β-<i>O</i>-4' bond (44.59/100 Ar), but only 18.44/100 Ar of XRL and undetected in FRL. In addition, the total hydroxyl content decreased with acid hydrolysis strength increasing, with 2.43, 2.06, and 1.99 mM/g of CL, XRL, and FRL, respectively. The DPPH removal rates of CL, XRL, and FRL were 79.21%, 76.71%, and 80%, respectively, comparable to those of the commercial antioxidant, 2,6-Di<i>tert</i>-butyl-p-cresol (BHT). Therefore, the lignin in XR and FR has promising application as a potential antioxidant.