Effect of Bronsted/Lewis Acid Ratio on Conversion of Sugars to 5-Hydroxymethylfurfural over Mesoporous Nb and Nb-W Oxides

A series of mesoporous Nb and Nb-W oxides were employed as highly active solid acid catalysts for the conversion of glucose to 5-hydroxymethylfurfural (HMF). The results of solid state P-31 MAS NMR spectroscopy with adsorbed trimethylphosphine as probe molecule show that the addition of W in niobium oxide increases the number of BrOnsted acid sites and decreases the number of Lewis acid sites. The catalytic performance for Nb-W oxides varied with the ratio of BrOnsted to Lewis acid sites and high glucose conversion was observed over Nb5W5 and Nb7W3 oxides with high ratios of BrOnsted to Lewis acid sites. All Nb-W oxides show a relatively high selectivity of HMF, whereas no HMF forms over sulfuric acid due to its pure BrOnsted acidity. The results indicate fast isomerization of glucose to fructose over Lewis acid sites followed by dehydration of fructose to HMF over BrOnsted acid sites. Moreover, comparing to the reaction occurred in aqueous media, the 2-butanol/H2O system enhances the HMF selectivity and stabilizes the activity of the catalysts which gives the highest HMF selectivity of 52% over Nb7W3 oxide. The 2-butanol/H2O catalytic system can also be employed in conversion of sucrose, achieving HMF selectivity of 46% over Nb5W5 oxide.