A mesoporous Ta2O5/Nb2O5 nanocomposite with Lewis/Brønsted acid sites to enhance stepwise glucose conversion to 5-hydroxymethylfurfural

Abstract

Herein, we report a higher formation of 5-hydroxymethylfurfural (HMF), a potential liquid fuel precursor, using glucose over a bifunctional Ta₂O₅/Nb₂O₅ composite. The mesoporous nanocomposite comprising 25% wt Nb₂O₅ provides both Lewis (24%) and Brønsted acid (76%) sites, with increased oxygen vacancies. The former sites can facilitate glucose isomerization to fructose, while the latter condition promotes fructose dehydration to HMF. Therefore, it enables the systematic conversion of glucose into HMF through the formation of fructose, resulting in 78.6% HMF and 86.5% selectivity after 3 h at 170 °C. The rate kinetics reveals that glucose degradation follows a second-order rate law with a rate constant (k_G) of 2.4 × 10⁻² L mol⁻¹ s⁻¹. On the other hand, fructose decomposition follows first-order kinetics with a rate constant (k_Fru) of 1.3 × 10⁻³ s⁻¹, but it also leads to significant unwanted side product formation (with a rate constant of k₂ = 4.7 × 10⁻⁵ s⁻¹). Both glucose and fructose decompositions to HMF exhibit temperature-dependent kinetics, as indicated by the Arrhenius plots. The calculated reaction thermodynamic parameters of glucose and fructose decomposition to HMF by employing the Eyring–Polanyi theory establish a comparable feasibility ΔG° of 124.2–132.5 kJ mol⁻¹, with non-spontaneous reaction characteristics (ΔS^# of −109.0 to −259.6 J mol⁻¹ K⁻¹).