**4. Conclusions**

This study revealed that the conditions used for preparing the heterogeneous catalysts based on heteropolyacids, i.e., type of support, active phase, and calcination temperature, strongly affects catalyst performance to convert glucose into HMF. The heterogeneous catalyst prepared by using HPW as active phase, supported in Nb2O5, and calcined at 300 ◦C presented optimal performance at obtaining HMF yield (9.5%) with 1:1 *w*/*w* acetone-to-water ratio, 160 ◦C, 1% *w*/*v* of catalyst, 100 g/<sup>L</sup> of glucose and reaction time of 30 min. After optimizing reaction conditions (temperature at 160 ◦C, 1:1 (*v*/*v*) acetone-to-water ratio, glucose concentration of 50 g/L, and catalyst concentration of 5% *w*/*v*), HMF yield increased to 40.8%. The catalyst presented high stability and is capable of maintaining HMF yield at around 40%, even after four runs. This was the first time that a catalyst based on HPW and Nb2O5 was used for HMF production from glucose. The promising results here obtained will open up new opportunities to expand HMF production from lignocellulosic biomass. The possibility of recycling the catalyst is also a point to be highlighted contributing to the sustainability aspect of the proposed process.

**Author Contributions:** Conceptualization, J.P.A.S., S.I.M. and L.M.C.; Methodology, J.S.M.N., J.P.A.S. and L.M.C.; Software, J.S.M.N., J.P.A.S. and L.M.C.; Investigation, J.S.M.N., J.P.A.S and L.M.C.; Resources, J.P.A.S, S.I.M. and L.M.C.; Writing—Original Draft, J.S.M.N. and L.M.C.; Writing—Review and Editing, J.P.A.S, S.I.M. and L.M.C.; Supervision, J.P.A.S, S.I.M. and L.M.C.; Project Administration, J.P.A.S, S.I.M. and L.M.C.; Funding Acquisition, J.P.A.S, S.I.M. and L.M.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), project number 2017/24050-8 and 2018/03714-8, Brazil; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil; and Novo Nordisk Foundation, gran<sup>t</sup> number: NNF10CC1016517, Denmark.

**Acknowledgments:** We thank Companhia Brasileira de Metalurgia e Mineração (CBMM) for donations the materials used for experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
