## 3 Results
### Low-Acid-Catalyzed Hydrothermal Fractionation of Pine Wood  
Acid-catalyzed hydrothermal fractionation was performed by changing one of three operating parameters: reaction temperature, reaction time, and acid concentration. Figure 1 summarizes the behaviors of products decomposed by thermochemical fractionation and the recovery of xylose and glucose from pine wood according to the change in the sulfuric acid concentration under fixed conditions (reaction temperature 180 \({}^{\circ}\)C and reaction time 30 min). The XMG yield increased with increasing fractionation severity at a low sulfuric acid concentration but significantly decreased at a higher sulfuric acid concentration. The maximum XMG (mainly xylan) yield was 61.37% when pine wood was fractionated at 180 \({}^{\circ}\)C with 0.25 wt.% sulfuric acid for 30 min. As the fractionation severity, i.e., the acid concentration, increased, the XMG yield significantly decreased because hemicellulose degrades into XMG, which is further excessively decomposed into furfural and formic acid under more severe fractionation conditions. The xylose yield was 51.19% at 0.5 wt.% sulfuric acid and sharply decreased to 9.99% at 0.75 wt.% sulfuric acid, indicating that the acid-catalyzed hydrothermal fractionation performance is sensitive to the acid concentration. In addition, a little glucose was released as a liquid hydrolysate due to the breakdown of cellulose during pine wood fractionation. When the sulfuric acid concentration was 0.5 wt.% or more, the concentrations of 5-HMF and levulinic acid, known as representative glucose decomposition, considerably increased. Since the main goal of this fractionation process is the clean separation of XMG, the accompanying glucose release and over-decomposition of glucose are the main contributors to the loss of sugars from the solid residue as well as the lowering of the fractionated hydrolysate selectivity. Therefore, to achieve the goal of this study, we investigated low-acid-catalyzed fractionation, which is performed at a low sulfuric acid concentration of 0.5% or less. In addition, because the XMG yield was no longer high despite the increase in the sulfuric acid concentration and a decomposition reaction occurred, we believe that a reaction temperature higher than 180 \({}^{\circ}\)C should be applied to this fractionation. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Components (\%)**} & \multicolumn{2}{c}{**Dry Solids (_av/w_, \%)**} \\ \hline \multirow{4}{*}{Carbohydrate} & Glucan & 41.1 \(\pm\) 0.28 \\  & Xylan & 12.5 \(\pm\) 0.16 \\  & Mannan & 3.1 \(\pm\) 0.15 \\  & Galactan & 1.1 \(\pm\) 0.11 \\  & Arabinan & 0.5 \(\pm\) 0.09 \\ \hline \multirow{2}{*}{Lignin} & Acid Soluble & 8.6 \(\pm\) 0.49 \\  & Acid insoluble & 23.5 \(\pm\) 0.38 \\ \hline \multirow{2}{*}{Extractives} & Water & 5.8 \(\pm\) 0.13 \\  & Ethanol & 1.3 \(\pm\) 0.09 \\ \hline \multirow{3}{*}{Total} & Acetyl gr. & 6.0 \(\pm\) 0.11 \\  & Ash & 0.9 \(\pm\) 0.11 \\ \hline \multicolumn{2}{c}{Total} & 104.4 \\ \hline \hline \end{tabular} Note: The data in the table show the mean value (\(n\) = 10, SD: standard deviation). All numbers in the table are based on the oven-dry biomass.  
\end{table} 
Table 1: Initial composition of pine wood. Figure 2 shows changes in the glucan and XMG yields with reaction time when pine wood was fractionated at 0.1 (Figure 2a), 0.25 (Figure 2b), and 0.5 wt.% (Figure 2c) sulfuric acid at a solid-to-liquid ratio of 1:8 and a reaction temperature of 190 \({}^{\circ}\)C. In addition, the concentrations of acetic acid and furfural, which are products of xylose decomposition, were compared. As shown in Figure 2a, when fractionation was carried out using 0.1 wt.% sulfuric acid, the XMG yield increased with the reaction time, reaching 57.5% at 15 min, but tended to decrease slightly at a reaction time longer than 15 min, indicating the over-decomposition of xylose.