Boosting the catalytic efficiency of sulfonated crosslinked-polyaniline organocatalyst for producing 5-hydroxymethylfurfural from fructose

Acid-catalyzed dehydration of fructose to 5-hydroxymethylfurfural (HMF) represents an effective method for valorizing the biomass-derived carbohydrates, however, always suffers from low catalytic efficiency of solid acid catalysts. In this work, we report the preparation of novel sulfonated crosslinked polyaniline (S-P-PAN) catalysts for high-efficient dehydration of fructose to HMF in low-boiling 1,4-dioxane and water. The skeletal structure of the organocatalysts was adjusted to a crosslinked form by co-calcination of PAN with KOH, which allowed the reduction of acid density but increment of acid strength and surface hydrophobicity after sulfonation and in turn increased the catalytic efficiency. The sulfonic acid groups hydrogen-bonded with adjacent benzenoid or tertiary amine nitrogen in the organocatalysts were found to act as the active sites, wherein the latter one were predominant. By tuning the mass ratio between PAN and KOH, as well as the temperature of co-calcination and sulfonation during the catalyst preparation, the S100-P1/2-PAN-400 catalyst with moderate acid density (1.71 mmol·g−1) and acid strength exhibited high turnover frequency (18.8 ∼ 80.6 h−1) in converting fructose in wide range of concentrations (4.5 ∼ 45.0 wt%). This work highlights the importance of deliberately adjusting the acid property of solid organocatalysts for enhancing the catalytic efficiency in HMF synthesis.