# The Impact of Thermal Treatment on Structural Changes of Teak and Iroko Wood Lignins
## 3 Results and Discussion
### Nitrobenzene Oxidation Products Analysis  
Teak has a GS-lignin, with a monomeric composition determined by Py-GC/MS(FID) of H:G:S 1:34:24 (sapwood) and 1:29:23 (heartwood) and S/G ratios of 0.71 and 0.81, respectively [28; 35]. Similar values--50.6% guiaacyl units, 46.0% syringl units, 3.4% \(p\)-hydroxyphenyl units, with S/G ratio 0.91--were found by nitrobenzene oxidation (Table 2). On the other hand, [PERSON] et al. (1987) [36] found up to 70% of guiaacyl structural units in teak lignin. The ratio of syringl units to guiaacyl (S/G) decreases with an increasing temperature, the yield of NBO products significantly increases except at the highest temperature. The results indicate the predominance of degradation reactions at lower temperatures and condensation reactions at the temperature of 210 \({}^{\circ}\)C.  
Like teak lignin, iroko has a GS-lignin, containing more guiaacyl units compared to syringl ones with an S/G ratio of 0.84 (Table 3). Some authors reported higher values of the S/G ratio for iroko lignin, 0.92 (by Py-GC-MS) and 1.005 (by FTIR) respectively [37; 38]. Differences may be caused by other methods of the isolation of lignin as well as different methods of analysis. The yield of NBO products gradually increased up to 180 \({}^{\circ}\)C, then decreased, however, the changes are mostly insignificant. The increase in NBO products may suggest that iroko lignin treated at lower temperatures  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Wood** & **20 \({}^{\circ}\)C** & **160 \({}^{\circ}\)C** & **180 \({}^{\circ}\)C** & **210 \({}^{\circ}\)C** \\ \hline Teak—KL & 35.42 \(\pm\) 0.04 & 39.30 \(\pm\) 0.15 & 39.52 \(\pm\) 0.04 & 40.52 \(\pm\) 0.06 \\ Iroko—KL & 29.03 \(\pm\) 0.25 & 29.00 \(\pm\) 0.11 & 29.90 \(\pm\) 0.18 & 36.92 \(\pm\) 0.19 \\ Teak—DL & 8.64 \(\pm\) 0.11 & 11.38 \(\pm\) 0.14 & 15.32 \(\pm\) 0.23 & 18.32 \(\pm\) 0.25 \\ Iroko—DL & 4.13 \(\pm\) 0.05 & 4.52 \(\pm\) 0.06 & 4.72 \(\pm\) 0.11 & 7.20 \(\pm\) 0.10 \\ \hline \end{tabular}
\end{table}
Table 1: Klason and dioxane lignin yields from untreated and thermally treated tropical wood species (mean \(\pm\) SD, % odw).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Wood** & **20 \({}^{\circ}\)C** & **160 \({}^{\circ}\)C** & **180 \({}^{\circ}\)C** & **210 \({}^{\circ}\)C** \\ \hline \(p\)-Hydroxybenzoic acid & 0.09 \(\pm\) 0.03 & 0.12 \(\pm\) 0.00 & 0.11 \(\pm\) 0.01 & 0.11 \(\pm\) 0.00 \\ \(p\)-Hydroxybenzaldehyde & 0.81 \(\pm\) 0.22 & 1.10 \(\pm\) 0.02 & 1.24 \(\pm\) 0.03 & 0.98 \(\pm\) 0.02 \\ Vanillic acid & 0.21 \(\pm\) 0.03 & 0.27 \(\pm\) 0.04 & 0.51 \(\pm\) 0.16 & 0.20 \(\pm\) 0.01 \\ Vanilline & 13.13 \(\pm\) 1.20 & 18.30 \(\pm\) 0.72 & 22.66 \(\pm\) 0.54 & 23.93 \(\pm\) 0.30 \\ Syringic acid & 3.10 \(\pm\) 0.45 & 2.45 \(\pm\) 0.16 & 3.74 \(\pm\) 0.15 & 3.20 \(\pm\) 0.04 \\ Syringaldehyde & 9.02 \(\pm\) 0.61 & 13.31 \(\pm\) 0.30 & 15.14 \(\pm\) 0.15 & 11.92 \(\pm\) 0.15 \\ Total yield on DL & 26.36 \(\pm\) 2.46 & 35.55 \(\pm\) 1.41 & 43.40 \(\pm\) 0.41 & 40.34 \(\pm\) 0.22 \\ S/G ratio & 0.91 \(\pm\) 0.00 & 0.85 \(\pm\) 0.01 & 0.81 \(\pm\) 0.01 & 0.63 \(\pm\) 0.01 \\ \hline \end{tabular}
\end{table}
Table 2: Nitrobenzene oxidation products of teak dioxane lignins (mean \(\pm\) SD, %).  
may be structurally modified to an easily degradable form. A similar trend was observed in the S/G ratio, indicating preferential contribution of G-type lignin to the condensation reaction as reported by [PERSON] et al. (2014) [39].