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

Al-loaded functional lignin biopolymer (Al x -Lbp) catalysts were synthesized using H 2 O 2 oxidized alkali lignin and aluminum acetate via mechanochemical modification without post-calcination treatment. The Al x -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 Al 1 -Lbp in ethanol at 140 ºC in 30 min reaction time. Al 1 -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 Al 2 O 3 and Al(OH) 3 formed on the Al 1 -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 Al 1 -Lbp catalyst demonstrated recyclability and maintained its initial activity for eight times reuse.