Adjusting the acidity of sulfonated organocatalyst for the one-pot production of 5-ethoxymethylfurfural from fructose

Abstract

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.