Aluminum doped solid acid with suitable ratio of Bronsted and Lewis acid sites synthesized by electric-flocculation of phosphotungstic acid via hydrothermal treatment for producing 5-hydroxymethylfurfural from glucose

Novel solid acid catalyst PWAl-200 was synthesized by a new green method of electric-flocculation of phosphotungstic acid (PW) with aluminum as electrodes to transform glucose to 5-hydroxymethylfurfural (5-HMF). Through electric-flocculation, PW was deposited in the hydrated aluminum received from electrolysis. Then the obtained floc was treated by hydrothermal process to get solid acid. Characterization results showed that after electric-flocculation, the electron-withdrawing influence of terminal W=O in the PW on hydrated aluminum formed Lewis (L) acid sites on these six-coordinated aluminum. The H+ derived from PW could supply Bronsted (B) acid sites. In the further hydrothermal treatment, hydrated aluminum dehydrated to produce reformed six coordinated aluminum. It was linked to heteropoly anions ligands through oxygen bridges, producing more L acid center. Besides, four-coordinated aluminum formed from the hydrated aluminum. The positive charge it produced increased L acid sites due to the strong effect of nearby tungsten species, further adjusting to a moderate ratio of Lewis/Bronsted (L/B) on solid acid for catalytic synthesis 5-HMF from glucose. The highest yield was 61.7% at 170 degrees C for 4 h and the catalyst could be recycled for four times and tend to stabilize.