Efficient isomerization of glucose to fructose over Al-loaded functional lignin biopolymer

Abstract

Al-loaded functional lignin biopolymer (Alx-Lbp) catalysts were synthesized using H2O2 oxidized alkali lignin and aluminum acetate via mechanochemical modification without post-calcination treatment. The Alx-Lbp catalysts exhibited higher hydrophilicity and lower surface energy than lignin-free Al-activated carbon catalysts. Unprecedented high yields (58.5 %) of fructose from glucose were obtained over Al1-Lbp in ethanol at 140 ºC in 30 min reaction time. Al1-Lbp catalyst had Lewis acid sites (47.2 μmol/g), Brønsted acid sites (6.9 μmol/g) and a surface energy of 39.1 mN/m that facilitated glucopyranose ring-opening and gave a glucose isomerization activation energy of 65.2 kJ/mol, which was lower than values reported for Lewis acid-catalyzed systems. Active sites of Al2O3 and Al(OH)3 formed on the Al1-Lbp catalyst in ethanol promoted the formation of an enediol intermediate as confirmed with isotope experiments to achieve efficient isomerization of glucose to fructose. The Al1-Lbp catalyst demonstrated recyclability and maintained its initial activity for eight times reuse.