## 3 Results and Discussion
### RSM for Furfural Production from Xylose Standard Solution with Extracting Solvent  
Furfural was produced from the xylose standard by acid-catalyzed dehydration, as listed in Table 2. In run #2 with a reaction temperature of 180 \({}^{\circ}\)C, over a 60 min duration using a xylose concentration of 10 g/L, the maximum furfural yield was 69.87% with 96.02% xylose conversion. Although the maximum xylose conversion was 99.32% at run #12, the furfural yield under these conditions was 68.12%; this is lower than the yield in run #2. These results indicate that the more severe experimental conditions of run #12 as a result of a longer reaction time induced the degradation of the furfural that had been produced. The lower furfural yield due to furfural degradation under severe experimental conditions was also observed in runs #4, 8, and 10; these runs were also characterized by long reaction times at certain temperatures. It is inferred that these reaction conditions caused furfural degradation despite the presence of the extraction solvent to prevent furfural degradation. However, a previous study has reported that the amount of furfural degradation due to severe experimental conditions when using an extracting solvent is marginal compared to the amount of furfural degradation in conditions with no extracting solvent [24]. The condensation of furfural was suppressed through the addition of the THF; as such, there were no insoluble precipitates detected from furfural condensation in all experimental conditions. To evaluate the effect of each variable on furfural yield, regression analysis was undertaken using a 2\({}^{3}\) factorial design matrix with corresponding furfural yield (%). The following quadratic equation was generated (Equation (3)), based on the outcomes of the regression analysis:  
\[ \begin{split}\text{Furfural yield (\%)}&=-1122.2746+11.8539\; \text{X}_{1}+2.4079\;\text{X}_{2}+1.0355\;\text{X}_{3}-0.0091\;\text{X}_{1} \text{X}_{2}\\ &-0.0082\;\text{X}_{1}\text{X}_{3}+0.0015\;\text{X}_{2}\text{X} _{3}-0.0302\;\text{X}_{1}{}^{2}-0.0032\;\text{X}_{2}{}^{2}+0.0024\;\text{X}_{3 }{}^{2}\end{split}  \tag{3}\]  
In the equation, \(\text{X}_{1}\), \(\text{X}_{2}\), and \(\text{X}_{3}\) represent the actual reaction temperature, reaction time, and xylose concentration, respectively. The model had a high regression coefficient (R\({}^{2}\) = 0.95), indicating 95% variability in the response, while the \(p\)-value was extremely low (0.001), indicating that this regression model was significant. The coefficient of variation (CV) was 18.74%, which indicates the high precision and reliability of the experiments [25]. A three-dimensional (3D) plot and detailed contour of the RSM for furfural yield was established using Equation (3), by varying the three variables within the experimental range (Figure 1). As shown in Figure 1a, the furfural yield increased with reaction temperature and time to approximately 185 \({}^{\circ}\)C and 180 min, respectively. Once the temperature and time exceeded these points, there was a decrease in furfural yield due to its degradation under severe experimental conditions [26]. This phenomenon was clearly observed in Figure 1b,c. Figure 1b depicts the furfural yield based on the reaction temperature and xylose concentration with a fixed reaction time of 120 min. The furfural yield increased with temperature until approximately 165 \({}^{\circ}\)C, and then remained stable at temperatures of 165-188 \({}^{\circ}\)C, which was where the maximum yield was observed. Then, yield decreased when the temperature increased above 185 \({}^{\circ}\)C regardless of the xylose concentration. These results are similar to the findings from previous research where furfural production from biomass occurred using sulfuric acid as a catalyst [27,28]. Figure 1c depicts the relationship between the furfural yield, the reaction time, and xylose concentration with a fixed reaction temperature of 160 \({}^{\circ}\)C. The furfural yield increased with reaction time up to 180 min.