Metal-Coordinated Lignosulfonate Catalysts for the Selective Conversion of Hexose: Active Site and Reaction Medium.

Sustainable lignosulfonate was employed as a key component for coordinating with various metal ions, affording metal-coordinated lignosulfonate catalysts (Hf-LigS) for hexose transformation. We first investigated the contribution of bare or tandem Lewis/Brønsted acid sites in each of the reaction steps. The results indicated that glucose isomerization was synergistically catalyzed by Lewis acid-base couple sites, and Brønsted acid sites were preferred for consecutive fructose dehydration to 5-hydroxymethylfurfural (HMF). DMSO and deep eutectic solvents enhanced the glucose dehydration and inhibited the isomerization reaction, leading to the formation of Levoglucosan (LGA) intermediate. Considering this, the Hf-LigS with moderate Lewis acid/base sites and Brønsted acid sites exhibited the best catalytic activity for hexose transformation, especially the isomerization reaction at high glucose concentration in ethanol (>20 wt%). This study established a fundamental understanding of the role of catalytic sites and solvent, guiding the selective transformation of hexose.