The rehydration of 5-hydroxymethylfurfural (HMF), an important bio-based chemical building block, to levulinic acid (LA) and formic acid (FA) over Brønsted acid catalysts is the key block to the effective production of HMF from hexose. In this work, we develop a novel acidic solid organocatalyst, sulfonated polyaniline (SPAN), for the effective dehydration of fructose into HMF in the low-boiling water/1,4-dioxane cosolvent. The highest HMF yield of 71% is obtained from fructose with complete restriction of HMF rehydration to LA. We demonstrate that hydrogen bonds form between the ring-attached sulfonic acid group and the quinoid imine nitrogen as a result of internal doping, which confines the Brønsted acidity of the SPAN catalyst. The H-bonded sulfonic acid species is active for fructose-to-HMF dehydration and complete suppression on HMF rehydration. The chemical bonding of sulfonic acid groups on the backbone of the PAN chain allows stable recyclability of the polymer catalyst. This work highlights the potential importance of confining Brønsted acidity on a solid organocatalyst via H-bonding for transforming renewable carbohydrates into fine chemicals.

The synergetic action of tetrahydrofuran (THF) and low-pressure CO₂ (1.0 MPa) is favourable for effective inhibition of humin precursor (i.e., oligomer) formation in the conversion of glucose to 5-hydroxymethylfurfural (HMF).

We report a novel solid organocatalyst, a double-hydrogen-bonded sulfonated polymer catalyst (D-SPC), for the cascade conversion of fructose to 5-ethoxymethylfurfural (EMF) with a highest yield of 68.8 mol% and good catalyst recyclability. We demonstrate that the treatment of a sulfonated nitrogen-containing polymer catalyst (SPC) with dihydroxy acetone (DHA) molecules generates new double hydrogen-bonds (H-bonds) between the ring-attached sulfonic acid groups and DHA by deconstructing the existing H-bonds between the sulfonic acid groups and imine/amine nitrogens of the SPC, which is confirmed by FT-IR spectroscopy, elemental analysis, ¹H MAS and ¹³C cross-polarization MAS NMR, X-ray photoelectron spectroscopy (XPS), and quantum chemical calculations. The ³¹P MAS NMR, pH_surface, and NH₃-TPD of the catalysts reveal the weakening of acidity strength due to the stronger double H-bonding on the D-SPC. Notably, we figure out the inverse relationship between the acidity strength of the catalysts and the EMF yield in the one-pot fructose-to-EMF conversion, wherein the use of weaker acid catalysts accelerates the fructose dehydration while decelerating the side-reaction of furan ring opening, therefore avoiding the negative influence of water on the tandem reactions. Moreover, the existence of stable double H-bonds contributes to the good catalyst recyclability due to the stabilization of sulfonic acid groups. The acidic property of the solid catalysts could be adjusted by varying the DHA dosage and the preparation conditions of the SPC (i.e., monomer ratio and oxidant dosage), respectively. We show that the solid acid catalyst with weaker acidity strength and suitable acidity density (ca. 2.5 mmol g⁻¹) is favorable for the one-pot synthesis of EMF directly from fructose. This work highlights the application of novel sulfonated polymer catalysts with adjustable acidic property for the highly selective production of EMF, a liquid biofuel candidate, from biomass-derived carbohydrates.