## 2 Results and Discussion
### Catalytic Performance of Nb/HMS and Nb/MCS Catalysts
#### 2.2.2 Effect of Reaction Conditions on Glucose Conversion over10-Nb/MCS Catalyst  
Figure 6 shows the effect of reaction temperature and time on the glucose conversion and products distribution over 10-Nb/MCS. The glucose conversion was increased with reaction temperature (Figure 6A) because the main catalytic reactions (isomerization and dehydration) in this system are endothermic nature [42]. The fructose concentration was observed at low concentration in all experiments, which suggested a high affinity of catalyst surface with fructose molecule, as well as a fast dehydration and/or condensation of fructose to other products (Figure 6B). The maximum HMF yield was a function of reaction time as shown in Figure 6C. At 190 \({}^{\circ}\)C, the HMF yield increased to the maximum value within 1 h, and then decreased when the reaction time was extended to 4 h. [PERSON] et al. [43] reported that a decrease in the HMF yield with increasing reaction time was explained by the propensity of humins formation via polymerization of glucose, fructose, HMF, and other reactive intermediates. This result was consistent with an increased humins yield with prolonged reaction time (Figure 6D). The suitable conditions for HMF synthesis over 10-Nb/MCS were 30 wt.% catalyst loading at 190 \({}^{\circ}\)C for 1 h, giving the highest HMF yield of 51.3% at 98.5% glucose conversion. Effect of catalyst loading level on the glucose conversion, HMF yield, and humins yield in the presence of 10-Nb/MCS is shown in Figure 7. When the amount of catalyst was increased, the glucose conversion was gradually increased and near completion at a 30 wt.% loading level. However, the HMF yield decreased concomitantly with an enhanced humins formation. It was attributed to an excess amount of Bronsted and Lewis acid sites available in the reaction system, which not only promoted the glucose dehydration into HMF but also the condensation of formed HMF to humins. The observed result was in accord with the previous work by [PERSON] et al. [44]. At the catalyst loading of 5 wt.%, the highest HMF yield (57.5%) was achieved at 93.2% glucose conversion. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & \multirow{2}{*}{**Glucose**} & \multicolumn{3}{c}{**Yield (\%)**} & \multicolumn{2}{c}{**HMF**} \\  & **Conversion (\%)** & **Fructose** & **HMF** & **Furfural** & **Others\({}^{\text{b}}\)** & **Selectivity (\%)** \\ \hline HMS & 68.9 \(\pm\) 0.6 & 3.8 \(\pm\) 0.3 & 30.2 \(\pm\) 0.7 & 0.3 \(\pm\) 0.0 & 34.6 \(\pm\) 0.2 & 43.9 \(\pm\) 0.6 \\
10-Nb/HMS & 78.6 \(\pm\) 0.8 & 4.3 \(\pm\) 0.1 & 29.4 \(\pm\) 0.8 & 0.6 \(\pm\) 0.0 & 44.3 \(\pm\) 1.6 & 37.4 \(\pm\) 1.4 \\
20-Nb/HMS & 74.3 \(\pm\) 1.0 & 5.4 \(\pm\) 0.1 & 27.0 \(\pm\) 0.8 & 0.4 \(\pm\) 0.0 & 41.5 \(\pm\) 1.7 & 36.3 \(\pm\) 1.6 \\
30-Nb/HMS & 68.0 \(\pm\) 0.8 & 8.1 \(\pm\) 0.4 & 24.1 \(\pm\) 0.5 & 0.3 \(\pm\) 0.0 & 35.5 \(\pm\) 0.7 & 35.5 \(\pm\) 0.3 \\ MCS & 69.2 \(\pm\) 0.6 & 5.4 \(\pm\) 0.3 & 32.5 \(\pm\) 0.5 & 0.3 \(\pm\) 0.0 & 31.1 \(\pm\) 0.4 & 47.0 \(\pm\) 0.3 \\
10-Nb/MCS & 78.9 \(\pm\) 0.7 & 4.8 \(\pm\) 0.5 & 35.0 \(\pm\) 1.0 & 0.6 \(\pm\) 0.1 & 38.6 \(\pm\) 1.1 & 44.3 \(\pm\) 1.5 \\
20-Nb/MCS & 78.1 \(\pm\) 0.2 & 5.0 \(\pm\) 0.0 & 30.0 \(\pm\) 0.9 & 0.5 \(\pm\) 0.0 & 42.5 \(\pm\) 0.7 & 38.5 \(\pm\) 1.0 \\
30-Nb/MCS & 76.6 \(\pm\) 0.8 & 5.1 \(\pm\) 0.0 & 28.4 \(\pm\) 0.9 & 0.5 \(\pm\) 0.0 & 42.7 \(\pm\) 0.4 & 37.0 \(\pm\) 1.1 \\ \hline \hline \end{tabular}  
* Reaction conditions: Catalyst amount, 0.1 g glucose concentration, 0.07 M; solvent volume (1:2 \(\
icefrac{v}{v}\) water saturated with NaCl: THF), 30 mL; temperature, 170 \({}^{\circ}\)C; time, 1 h; N\({}_{2}\) pressure, 10 bar. \({}^{\text{b}}\) Others are mainly humins.  
\end{table} 
Table 2: Glucose conversion and products distribution obtained from glucose conversion \({}^{\text{a}}\) over the HMS and MCS catalysts with different Nb loading amount.