*3.4. Catalyst Recycling Test*

Finally, the HPW/Nb2O5-300 ◦C catalyst stability was evaluated through a recycling test using optimal reaction conditions for HMF production from glucose. Figure 6 shows that the catalyst was capable of maintaining the HMF yield constant at approximately 40% during four runs. This means that a simple calcination process was sufficient to eliminate by-products (humin) that could have covered some active sites of the catalyst. This result indicates that the catalyst HPW/Nb2O5-300 ◦C has high stability, good recyclability, and low active phase leaching, thus confirming its effectiveness to be used in HMF production from glucose.

**Figure 6.** Catalytic performance of HPW/Nb2O5-300 ◦C during a recycling test. Reaction conditions were 160 ◦C, 5% *w*/*v* of catalyst, 50 g/<sup>L</sup> of glucose, 1:1 *v*/*v* acetone-to-water, 300 rpm, and reaction time of 30 min.
