Comprehensive Understanding of the Role of Bronsted and Lewis Acid Sites in Glucose Conversion into 5-Hydromethylfurfural

The conversion of glucose and selectivity into 5-hydromethylfurfural (HMF) were investigated over various silica-alumina composite (AlSiO) catalysts. The type, amount, and strength of the acidic sites were characterized by using NH3 temperature-programmed desorption and FTIR spectroscopy and then correlated to the catalytic conversion of glucose into HMF to provide a quantitative relationship between the acidity and product selectivity. Lewis acid sites played an important role in glucose conversion, which can enhance the isomerization of glucose to fructose, whereas Bronsted acid sites had a detrimental effect. HMF selectivity had an almost linear relationship with the weak/total Lewis acid ratio (L*/L), indicating that weak Lewis acids could promote formation of HMF. The medium-to-strong Lewis acid sites can enhance the formation of undesired byproducts (levulinic acid, humins). The Bronsted to Lewis acid ratio (B/L) had an influence on the HMF selectivity; at similar L*/L ratios, volcano curves were obtained with the increase of the B/L ratio, but the influence was not as great as that of the L*/L ratio. Nb-doped AlSiO catalysts were prepared and used in the conversion of glucose into HMF, which also confirmed the above findings. Under the optimized conditions, the HMF selectivity can reach 71% at 92.6% conversion of glucose with no clear decline after four catalytic cycles.