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

HighlightsWhat are the main findings?Sustainable metal-coordinated lignosulfonate catalysts were selected as the model catalyst to reveal the role of active sites and solvents in hexose transformation.Lewis acid–base couple sites and Brønsted acid sites regulated the transformation process.What are the implications of the main finding?Isomerization or dehydration pathway of glucose was adjustable via different solvents.Hf-LigS showed the best catalytic activity in hexose transformation, especially isomerization reaction at high glucose concentration in ethanol.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.