## 3 Results and Discussion
### Production of Furfural from Pentose in Dilute Acid Hydrolysate  
The optimal reaction conditions as predicted using the ANOVA (i.e., 170 \({}^{\circ}\)C, 120 min, and 10 g/L of xylose) were adopted to maximize furfural production from pentose in dilute acid hydrolysate. The xylose concentration was adjusted by mixing the hydrolysate, which had already contained 4% (w/w) sulfuric acid, with 4% sulfuric acid solution. The total volume of the aqueous phase was adjusted to 10 mL, similar to the xylose standard solution, and 20 mL of THF was added to extract furfural from the aqueous phase. Table 5 describes the pentose conversion and furfural yield from the xylose standard and dilute acid hydrolysate. The furfural yield and pentose conversion of hydrolysate were slightly lower, compared with the xylose standard. The presence of impurities such as hexoses, organic acids, and acid soluble lignin, may have impacted on the extraction efficiency of furfural from the hydrolysate [39]. The difference in furfural yield between the xylose standard and the hydrolysate was not as considerable as had been expected. This indicates that THF effectively prevents furfural loss by inhibiting the ring opening of furfural and condensation between furfural and acid soluble lignin to form insoluble precipitate [40]. To investigate the distribution of impurities in the organic and aqueous phases, NaCl was added to separate the hydrolysate into the organic and aqueous phases. Table 6 presents the change in the distribution of furfural and other chemicals in furfural that produced hydrolysate by phase separation. Most furfural produced was extracted in the organic phase with a partition coefficient of 8.43; this is slightly lower than that of the xylose standard solution (9.05). Sugars, such as glucose, XMG, and arabinse favor the aqueous phase owing to their hydroxyl group, while other chemicals such as furfural, organic acids, and ASL tend to be extracted by the organic phase of THF. It is known that THF is able to effectively dissolve lignin as it has a high affinity to phenolic compounds [41], thus, most ASL was extracted to THF. Approximately three-quarters of organic acids were extracted to the organic phase, and the THF was effective in extracting various organic acids; however, the detailed extraction mechanism of THF to organic acid continues to be unclear [42; 43]. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Solvent** & **THF** & **Toluene** & **DMSO** \\ \hline Spectroscopic polarity & 0.6 [36] & 0.55 [37] & 1 [38] \\ (Furfural: 0.426 [35]) & 9.05 & 5.82 & N/D ** \\ Partition coefficient * & 9.05 & 5.82 & N/D ** \\ Furfural yield (\%) & 72.39 \(\pm\) 0.50 & 58.01 \(\pm\) 0.00 & 38.28 \(\pm\) 0.00 \\ \hline \hline \end{tabular}  
* Partition coefficient = [Furfural]org/[Furfural]aq, ** N/D: Not detected.  
\end{table} 
Table 4: Properties of organic solvent and furfural yield from xylose standard based on the type of organic solvent utilized. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Pentose Conversion (\%)** & **Furfural Yield (\%)** \\ \hline Xylose standard solution & 100 \(\pm\) 0.00 & 72.39 \(\pm\) 0.50 \\ Liquid hydrolysate & 94.69 \(\pm\) 0.76 & 68.20 \(\pm\) 0.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Pentose conversion and furfural yield from xylose standard and dilute acid hydrolysate. THF, the separated organic phase, was removed, and 20 mL of fresh THF was added to 10 mL of furfural extracted aqueous phase to enhance the extraction rate of furfural to the organic phase. After each additional extraction, the amount of extracted furfural and other compounds were analyzed; the extraction rate was calculated as (Equation (4)): Extraction rate (%) = Amount of products extracted in THF phase (g)/Amount of products existed in furfural produced liquor before phase separation (g)  
Figure 2 shows the increase in the furfural extraction rate to THF with the number of extractions. Following the second additional extraction, the extraction rate increased from 86.03% to 94.63%.