Niobium oxide-phosphorylated carbon xerogel composites as solid acid catalysts for cascade conversion of glucose to 5-hydroxymethylfurfural (HMF) in pure water

Abstract

The cascade conversion of glucose to a key platform commodity, 5-hydroxymethylfurfural (HMF), was studied as a sustainable method for producing platform molecules from biomass. Bifunctional catalysts were prepared by the deposition/precipitation of Nb2O5 (containing NbO4 Lewis and Nb-OH Brønsted acid sites) onto the surface of phosphorylated carbon xerogels (CX_P) with Brønsted acid sites (BA) in the form of oxygen and phosphorus functionalities. The number of Lewis acid (LA) sites in the composites was tuned by varying the Nb loading (0–60%), and Brønsted acidity was modified by thermal (post)treatments. Catalytic tests carried out in pure water revealed that the LA sites played an important role in selective glucose conversion, enhancing the isomerization of glucose to fructose, whereas BA sites, in the absence of LA sites, catalyzed the degradation of glucose to humin. For bifunctional catalysts, the glucose conversion was proportional to the total Brønsted acidity of the catalysts. Moreover, the glucose conversion to fructose increased with higher Nb loadings, while the selectivity to HMF decreased, resulting in similar HMF yields obtained on the composites. The drop in selectivity to HMF was attributed to the degradation of HMF to humin catalyzed by high concentrations of BA sites. After the removal of strong BA sites originating from Nb2O5, the selectivity to HMF significantly increased, leading to the maximum HMF yield of 31% obtained by a composite containing 60% Nb2O5 and treated at 300 °C in air. Thus, a catalyst with poor Brønsted acidity of Nb2O5 origin (Nb-OH) was shown to be preferable to achieve moderate HMF yield without humin formation in an environmentally friendly and cost-effective process. It was also shown that the type and amount of BA sites from the carbon component affected the yield of HMF. BA in the form of oxygen functionalities increased the selectivity to HMF, whereas larger concentrations of P-OH showed the opposite effect.