Nb₂O₅-γ-Al₂O₃ nanofibers as heterogeneous catalysts for efficient conversion of glucose to 5-hydroxymethylfurfural

One-dimensional γ-Al₂O₃ nanofibers were modified with Nb₂O₅ to be used as an efficient heterogeneous catalyst to catalyze biomass into 5-hydroxymethylfurfural (5-HMF). At low Nb₂O₅ loading, the niobia species were well dispersed on γ-Al₂O₃ nanofiber through Nb-O-Al bridge bonds. The interaction between Nb₂O₅ precursor and γ-Al₂O₃ nanofiber results in the niobia species with strong Lewis acid sites and intensive Brønsted acid sites, which made 5-HMF yield from glucose to reach the maximum 55.9~59.0% over Nb₂O₅-γ-Al₂O₃ nanofiber with a loading of 0.5~1 wt% Nb₂O₅ at 150 °C for 4 h in dimethyl sulfoxide. However, increasing Nb₂O₅ loading could lead to the formation of two-dimensional polymerized niobia species, three-dimensional polymerized niobia species and crystallization, which significantly influenced the distribution and quantity of the Lewis acid sites and Brönst acid sites over Nb₂O₅-γ-Al₂O₃ nanofiber. Lewis acid site Nb^δ+ played a key role on the isomerization of glucose to fructose, while Brønsted acid sites are more active for the dehydration of generated fructose to 5-HMF. In addition, the heterogeneous Nb₂O₅-γ-Al₂O₃ nanofiber catalyst with suitable ratio of Lewis acid to Brönsted sites should display an more excellent catalytic performance in the conversion of glucose to 5-HMF.