Tuning surface acidic properties of NbP-based catalysts for biomass conversion

Solid acid catalysts are among the most used in industrial chemical processes. The tolerance to polar-protic liquids is an additional feature, which plays a key role when the use of water or other benign solvents (e.g. alcohols) is concerned. Niobium oxophosphate (NbP) is recognized as an efficient water-tolerant solid acid catalyst with high ratio of Brønsted (BAS, deriving from P-OH groups and in lesser extent from Nb-OH sites) to Lewis (LAS, typically associated with unsaturated Nb(V) sites) acid sites. For this reason, it represents a suitable catalyst for aqueous heterogeneous catalysis, in particular for carbohydrate biomass valorization [1]. Attempts to tailor the acid properties of NbP by chemical treatment or dilution in inert matrix to disperse active NbP phase have been recently proposed in the literature [2,3]. The obtained samples have been used with success in the hydrolysis reaction of inulin to fructose and in the hydrolysis plus dehydration of cellobiose to HMF. Herein calorimetric acid-titration measurements using 2-phenylethylamine (PEA) basic probe in various liquids (cyclohexane, water, isopropanol, and water-isopropanol mixtures) have been carried out to determine the intrinsic and effective acid strength of NbP based catalysts. Liquid-solid acid-base titrations have been also performed in a modified liquid-chromatograph (HPLC) through successive injections of phenylethylamine, PEA in cyclohexane (apolar-aprotic solvent) and protic solvents (water and water-isopropanol mixtures) to determine, respectively, intrinsic and effective acid site density. Furthermore, the Lewis or Brønsted nature and relevant ratios of acid sites of solid catalysts have been investigated by infrared spectroscopy (FT-IR), monitoring pyridine adsorption/desorption. The combination of these complementary analytical tools allowed to provide an overview of the surface acid properties of NbP and modified samples under different liquid environments in relation with their catalytic activity.