*Energies* **2020**, *13*, 655

The statistical significance of the main effects and interactions on response variables was found by the analysis of variance (ANOVA) (Table 5). Percentages of variation in HMF yield and glucose conversion showed high correlation coefficients (R2), i.e., over 95% confidence level. The statistical analysis showed that, for HMF yield, reaction temperature (A), acetone-to-water ratio (B), glucose (C) and catalyst concentration (D), and AC and AD interactions were significant at 95% confidence level. Regarding glucose conversion, reaction temperature (A), glucose (C) and catalyst concentration (D), and AB and AD interactions were significant at the same confidence level.


**Table 5.** Analysis of variance of response variables evaluated in the experimental design Taguchi L16.

\* Significant at 95% confidence level: *p* test < 0.05.

Figure 5 shows the signal-to-noise ratio (SN ratio) diagrams of HMF yield (A) and glucose conversion (B). It is possible to determine optimal conditions for HMF production from glucose thereof. According to Figure 5A, the highest HMF yield was achieved when reaction temperature and glucose concentration were at the lowest level (temperature of 160 ◦C and 50 g/<sup>L</sup> of glucose), while catalyst concentration was at the highest level (5% *w*/*v*). Acetone-to-water ratio was insignificant for HMF yield in the range of values studied herein.

With respect to glucose conversion (Figure 5B), the highest conversion rates were achieved when reaction temperature, acetone-to-water ratio, and catalyst concentration were at the highest level (200 ◦C, 3:1 *<sup>v</sup>*/*<sup>v</sup>*, 5% *<sup>w</sup>*/*<sup>v</sup>*, respectively), while glucose concentration was insignificant within the studied range of values. It is interesting to note that glucose conversion was the only response variable that obtained a better result when at high reaction temperatures. This suggests that, for the studied range of values, high reaction temperatures may favor by-product formation.

According to results of the statistical analysis and considering that this paper aimed to obtain the highest HMF yield, optimal conditions for HMF production from glucose in the evaluated range of values was temperature of 160 ◦C, 1:1 (*v*/*v*) acetone-to-water ratio, glucose concentration of 50 g/L, and catalyst concentration of 5% *<sup>w</sup>*/*<sup>v</sup>*.

**Figure 5.** Signal-to-noise ratio (SN ratio) of (**A**) HMF yield (YHMF %) and (**B**) glucose conversion (XGlu %) response variables.

The catalytic performance of HPW/Nb2O5 was compared with some recent reported studies in literature that produce HMF from glucose, as shown in Table 6. The HMF yield (40.8%) obtained in the present study using HPW/Nb2O5 as catalyst and water/acetone was higher than most studies reported in the literature which used heterogeneous catalyst [34–36], even when organic phase [37] or ionic liquid [11] was applied as solvent. Teimouri et al. [38], who used the same solvent as the present work (water/acetone), achieved lower HMF yield (34.6%), even using higher reaction temperature and time. Huang et al. [26], who used the same reaction temperature and time but different solvent (Water/γ-valerolactone), also obtained lower HMF yield compared to the present work. Shahangi et al. [9], Shen et al. [39], Zhang et al. [10], and Moreno-Recio et al. [40] achieved similar HMF yield obtained in this work, however all of them applied more drastic reaction conditions, as higher temperature and/or time. In general, even using acetone/water as a reaction medium, we achieved HMF yield equivalent to the highest values reported in the literature, but with milder reaction conditions.


**Table 6.** Comparison of the performance of HPW/Nb2O5-300 ◦C catalyst with some recent works reported in literature on HMF production from glucose.

a POM; Polyoxometalates. b GO; Graphene oxide. c ACBL2; Activated carbon with acid treatment 18M H2SO4 and activated carbon with 15 wt.% of zinc. d TSA350; Alumina-promoted sulfated tin oxide calcined at 350 ◦C. e Al-KCC-1; Aluminosilicate with Si/Al = 5. f S-TsC; Sulfonated tobacco stem-derived porous carbon. g HMOR\_20; zeolite with SiO2/AlO3 = 20. h ATP; Attapulgite. i Al-SPFR; Al3+-modified formaldehyde-p-hydroxybenzenesulfonic acid resin catalyst.
