Influence of Lewis acidity and CaCl2 on the direct transformation of glucose to 5-hydroxymethylfurfural

Nb-Zr oxides were evaluated for glucose dehydration to 5-hydroxymethylfurfural. A linear relationship was found between the amount of acid sites and niobium content. Pure Nb2O5 showed the highest acid concentration and surface area and both Bronsted and Lewis acid sites, whereas Zr-containing catalysts mostly showed Lewis acid sites. Thus, a correlation between the amount of acid sites and catalytic performance was demonstrated, in the absence of metal salt, being Nb2O5 the most active catalyst. However, the catalytic results obtained by the combination of CaCl2 and Zr-containing catalysts, which only showed Lewis acid sites, suggested the direct formation of HMF, without fructose formation, unlike that observed for Nb2O5, which possesses both Bronsted and Lewis acid sites. A HMF yield of 44% was attained by using Nb3Zr7 as acid catalyst and CaCl2 after 90 min at 175 oC. These catalysts can be reused at least 3 catalytic runs without any pretreatment.