Mixed NbO and NbP as effective catalysts in the direct inulin conversion to 5-hydroxymethylfurfural (HMF)

5-Hydroxymethylfurfural (HMF) is a well-known biomass-derived platform molecule that can be converted into a large number of fine chemicals, polymeric materials, solvents and fuels1. Although monosaccharides (glucose or fructose) are the most commonly studied substrates for the synthesis of HMF, the direct conversion of polysaccharides (e.g. inulin, cellulose, amylose, etc.) is a more sustainable and convenient strategy for HMF production from an industrial point of view. When glucose-containing polysaccharides are used as raw-materials for HMF synthesis, three distinct acid-catalyzed actions are involved: hydrolysis of polysaccharides, isomerization of the formed glucose monomers to fructose, and cyclo-dehydration of fructose to HMF. Consequently, the control of the selectivity to HMF is the result of a delicate balance between acid site number, strength and nature (Lewis, LAS, and Brønsted, BAS). Niobium-containing solid acids, such as niobium oxide (NbO) and niobium phosphate (NbP), owing to their strong acidic properties and water-tolerance have been successfully employed as catalysts in the transformation of biomass2. Both NbO and NbP possess BAS and LAS, with NbP showing a higher ratio of BAS to LAS than NbO3. Herein, the combination4 of NbO and NbP has been investigated in the direct inulin transformation to HMF. The composition of physical mixtures of NbO and NbP has been optimized to properly modulate the acidity and then maximize the HMF yield. Three physical mixtures with NbO:NbP mass ratio of 1:3, 1:1, and 3:1 have been prepared. The sample acidity has been characterized by liquid-solid titrations carried out in a modified liquid-chromatograph (HPLC) line by basic solutions of phenylethylamine (PEA) in an apolar-aprotic solvent (intrinsic acidity) and in water-isopropanol mixture (effective acidity). The LAS and BAS nature and relevant ratios of the acid sites were determined by infrared spectroscopy (FT-IR) analysis by pyridine adsorption under dry condition and in the presence of water. In the view to optimize the catalyst(s) able to directly convert inulin to HMF, the catalytic performances (activity, selectivity and stability) in the two single-step reactions of inulin hydrolysis to fructose/glucose and of fructose/glucose dehydration to HMF were, at first, separately investigated. Catalyst deactivation, which is the most severe limitation to the high catalytic performances in these reactions, has been deeply examined by carrying out TGA analysis on the used catalysts and by evaluating the deactivation kinetics. The comparison of catalytic behavior allowed to individuate the most promising candidates for the direct conversion of inulin to HMF. Direct synthesis of HMF starting from inulin was then studied with the selected catalysts in a two-fixed-bed reactor.