*3.1. Catalyst Synthesis*

Table 1 summarizes the conditions used to prepare the catalysts as well as their performance to convert glucose into HMF in terms of HMF yield (YHMF) and glucose conversion (XGlu). As it can be seen, HMF yield ranged from 0.7% to 9.5% and glucose conversion from 65.3% to 93.6% according to the conditions established to prepare the catalyst. These results show that preparation conditions had an important influence on the e ffectiveness of glucose conversion into HMF. In some cases, the produced catalyst allowed achieving high glucose conversion rates, i.e., about 90% (assays 1, 3, 5, and 7). However, the highest HMF yield, about 9.5%, was achieved in assay 2, in which glucose conversion was only 75%. A high glucose conversion without a proportional HMF yield, as observed in other cases, suggests the formation of reaction by-products. In fact, the formation of humin, levulinic acid, furfural, and formic acid, which have been reported in literature as by-products of a glucose dehydration reaction to HMF, is often associated with reaction conditions, mainly to the use of solvents and high temperatures [5,26].


**Table 1.** Taguchi's L8 orthogonal array to evaluate the e ffect of calcination temperature, support and active phase on heterogeneous catalysts preparation on 5-Hydroxymethylfurfural yield (YHMF) and glucose conversion rate (XGlu).

1 YHMF: 5-Hydroxymethylfurfural yield (%). 2 XGlu: Glucose conversion (%). 1,2 All results are in duplicate.

The statistical significance of main e ffects and their interactions on response variables was verified by the analysis of variance (ANOVA). As shown in Table 2, the variation percentages explained by HMF yield and glucose conversion achieved a high coe fficient of determination (R<sup>2</sup> = 99.8% and 98.6%, respectively). These results reveal that the variations observed for response variables (HMF yield and glucose conversion) can be e ffectively explained by catalyst preparation conditions. With respect to HMF yield, calcination temperature, support, active phase, and interaction effects (AB, AC, BC, and ABC) were significant at confidence level of 95%; moreover, glucose conversion, calcination temperature, support, active phase, and AB and AC interactions were also significant at 95% confidence level.


**Table 2.** Analysis of variance of the main effects and their interactions on 5-hydroxymethylfurfural (HMF) yield (YHMF) and glucose conversion (XGlu) based on Taguchi's L8 orthogonal array.

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

The interaction effects of support and temperature for different active phases on HMF yield and glucose conversion are shown in Figure 1. Note that the highest HMF yields (Figure 1b) were obtained from reactions in which the catalyst was prepared by using HPW active phase supported on Nb2O5 and calcined at 300 ◦C. For the HPMo active phase, the type of support (Al or Nb) had no influence on final HMF yield, on the other hand, for this active phase, the catalysts calcined at 500 ◦C achieved higher HMF (7.0%) yields if compared to those calcined at 300 ◦C (3.5%). For the HPW active phase calcined at 500 ◦C, the type of support (Al or Nb) exerted no influence on the final HMF concentration. However, as for catalyst preparation using the same active phase calcined at 300 ◦C, the catalyst supported in Nb was more effective at HMF production and achieved yields of over 9%. These results sugges<sup>t</sup> that the catalyst produced using HPW as active phase can achieve greater HMF production; however, such a result can only be reached when using Nb2O5 as support at 300 ◦C of calcination temperature. The highest HMF yield reached with Nb2O5 is probably associated with the presence of Lewis and Brönsted acid sites in this support, while Al2O3only has Lewis sites on the surface [27].

For glucose conversion (Figure 1b), the highest conversions (above 85%) were obtained by using catalyst with HPMo as active phase, regardless of the type of support or calcination temperature conditions. Regarding the HPW active phase, the catalyst supported in Nb2O5 and calcined at 300 ◦C resulted in higher glucose conversion (75%) than the one calcined at 500 ◦C (65%); while for the catalyst supported in Al2O3, calcination temperature had no influence on glucose conversion (70%). Despite the high glucose conversion for catalysts prepared with HPMo active phase, HMF production was low, which suggests that reactions using catalyst with HPMo as active phase result in higher by-product formation and lower HMF selectivity. Thereby, the HPW/Nb2O5-300 ◦C catalyst was selected as the most desirable catalyst for converting glucose into HMF.

**Figure 1.** Interaction effect between temperature and support for different active phases (HPW and HPMo) on HMF yield (YHMF) (**a**) and glucose conversion (XGlu) (**b**).
