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

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 Br & oslash;nsted 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 degrees C. The rate kinetics reveals that glucose degradation follows a second-order rate law with a rate constant (k(G)) of 2.4 x 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 x 10(-3) s(-1), but it also leads to significant unwanted side product formation (with a rate constant of k(2) = 4.7 x 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 Delta G degrees of 124.2-132.5 kJ mol(-1), with non-spontaneous reaction characteristics (Delta S-# of -109.0 to -259.6 J mol(-1) K-1).