Influence of the Nb/P ratio of acidic Nb-P-Si oxides on surface and catalytic properties

In this work, two acidic Nb-P-Si mixed oxide gel-derived materials characterized by Nb/P molar ratios equal to 2 (5Nb2.5P) and 1 (2.5NbP) were investigated for their surface and bulk properties in relation with the catalytic performances in the fructose dehydration reaction. The structural characteristics of the studied samples and the changes occurring after water treatment and after reaction were investigated by 29Si and 31P solid state nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron (XPS) spectroscopies, while the characterization of their acidic properties was performed by base (2-phenylethylamine) adsorption in liquid phase. MAS-NMR showed that the phosphorus remains firmly anchored into the siloxane matrix after exposure to cold water for 5Nb2.5P sample and XPS confirmed the homogeneity of the sample composition. Both samples exhibited good intrinsic acidity and maintained significant effective acidity in polar-protic liquids; 2.5NbP manifested a double amount of acid sites compared to 5Nb2.5P, when 2-phenylethylamine is used as probe. Fructose dehydration to 5-(hydroxymethyl)furfural (HMF) on the two gel-derived catalysts was performed in water and in water-isopropanol solution under mild conditions (130 °C) working in a recirculation reaction line comprising a tubular catalytic reactor. In water-isopropanol solution, the samples displayed good performances, as expected thanks to the lively effective acidity. Around 45–50% fructose conversion was attained on both samples, with selectivity to HMF equal to about 50% on 2.5NbP gel-derived catalyst. Recycling tests showed satisfactorily stable activity during three consecutive runs.