In situ analysis of chemical changes of thermally compressed wood by 2D NMR

Scots pine was subjected to radial compression at 160 °C and heat treatment at 180–220 °C using hot pressing to produce compressed wood and thermally compressed wood (heartwood and sapwood). Then, the macromolecular structure changes of modified wood without destruction were analyzed using high-resolution 2D HSQC NMR. After heat treatment, the contour signals in NMR spectra evidently reduced. Its reduction mainly came from side-chain cleavage of O-acetylated galactoglucomannans (GGMs) and 4-O-methyl-gluconoxylans (MGXs) in hemicellulose. Moreover, the thermal stability of GGMs was lower than that of MGXs. Specifically, the thermal stability order of monosaccharides in heartwood and sapwood should be as follows: glucose > xylose > mannose > 2-O- and 3-O-Ac- Manp > galactose > 4-O-methyl-α-D-glucuronic acid ≥ arabinose. At 220 °C, hemicellulose only left minor xylan and mannan. Conversely, the change of cellulose structure was not obvious. NMR spectra indicated high temperature caused the breaking of β-O-4, β-5, and α-O-4 bonds, leading to the mass loss of lignin.