Cooperative action of Brønsted and Lewis acid sites of niobium phosphate catalysts for cellobiose conversion in water

The lively acidic properties in water of niobium phosphate (NBP) catalyst and NBP modified samples by acidic treatments in HCl solutions at 0.1, 1.0, and 10 M concentration (NBP01, NBP1, and NBP10, respectively) have been exploited for the direct conversion reaction of cellobiose to hydroxymethylfurfural (HMF) at temperature of 80-130 °C. Acidity of the samples was determined by liquid-solid titrations working in a modified HPLC line with basic solutions of phenylethylamine (PEA), used as a base probe. As solvents, an apolar-aprotic one, cyclohexane, was used for determining the intrinsic acidity and water for the effective acidity. In addition, pyridine/water co-adsorption FT-IR experiments complemented the acidity study, and the results obtained confirmed the presence of both water-tolerant Lewis acid sites (LAS) and some residual Brønsted acid sites (BAS) on all the NBP samples. On NBP, the LAS to BAS ratio was 1.69, measured in cyclohexane, and only slightly decreased to 1.30, when measured in water. Catalytic activity was studied in a liquid-solid reaction line with fixed bed flow reactor working in complete recirculation. The reaction of HMF formation from cellobiose consists of three distinct catalytic actions: hydrolysis of the ?-1,4-glycosidic bonds of cellobiose, isomerization of the formed glucose monomers to fructose, and cyclo-dehydration of fructose to HMF. Comparative catalytic results obtained with a sulfonic acid resin (Amberlite IR 120) showed that it was not able to form any HMF from cellobiose in the range of temperature studied, only cellobiose hydrolysis to glucose can be successfully achieved on the protonic acid resin.