Dehydration of levoglucosan to levoglucosenone over solid acid catalysts. Tuning the product distribution by changing the acid properties of the catalysts

Abstract

The effect of the catalytic properties of various types of solid acids, including oxides (ZrO2, SOX/ZrO2, WOX/ZrO2, Nb2O5, NbOPO4, Al2O3, TiO2), silica–aluminas (ZSM5 and USY zeolites) and acidic ion-exchange resins (Amberlyst-70 and Dowex 50wx8) were investigated for liquid-phase levoglucosan (LGA) conversion. H2SO4 was also evaluated as a reference catalyst. The concentration of both Brønsted and Lewis acid sites in the catalysts was measured by FTIR of adsorbed pyridine and titration with NaOH. The catalysts were tested under different reaction conditions (110–170 °C and 30–60 bar of N2) for 1 h using tetrahydrofuran (THF) as solvent. The product distribution obtained from the LGA conversion can be tuned depending on the type of acid site of the catalyst or the reaction conditions used. Catalysts containing only Bronsted acid sites were selective to LA while those with both Bronsted and Lewis acid sites favored the formation of 5-hydroxymethylfurfural (HMF). LGO was mainly produced by the homogeneous catalyst. The USY and HZM5 (SiO2/Al2O3 ratio of 23) promoted the production of HMF and glucose, respectively. A reaction mechanism was proposed taking into account the acid properties of the catalysts. The reactions of dehydration of LGA to LGO or hydration of glucose requires Brønsted acid sites. Catalysts with only Lewis acid sites produced only humins. For the catalysts containing both Brønsted and Lewis acid sites, LGA is mainly hydrated to glucose on the Bronsted acid sites. Then, glucose is preferentially isomerized to fructose on the Lewis acid sites, followed by its dehydration to HMF. The low LA formation shows that the hydration of HMF to LA requires a higher concentration of Brønsted acid sites.