New Nb-P-Si ternary oxide materials and their use in heterogeneous acid catalysis

The surface and catalytic properties of oxide ternary materials consisting of niobium-phosphorus-silicon, Nb-P-Si, with Nb/P = 1 and general formula xNb2O5·xP2O5·(100–2x)·SiO2, with x = 2.5, 5, 7.5, and 10, and Si content ranging from 95 to 80 mol% are presented. Amorphous gel-derived samples were obtained, characterized by a very high degree of silicon cross-linking with presence of Si–O–Nb bridges, allowing phosphorus to be stably anchored to the matrix through Nb–O–P bonds. The morphology (surface area and porosity), the intrinsic acidity (density and average strength of the surface acid sites, by PEA-TPD in a TGA-DSC coupled instrument), and the nature of the acid sites were determined. The LAS/BAS ratio has been evaluated by FTIR of adsorbed pyridine in absence/presence of water to discover the differences between the intrinsic and effective nature of the acid sites. The amount of acid sites decreased with the Nb-content in the samples as well as the surface average acid strength. Both LAS and BAS are maintained in the presence of water, even if BAS are predominant in any case. The catalytic performances of the Nb-P-Si samples in the reaction of inulin hydrolysis were investigated in aqueous solution under mild conditions (below 100 °C).