Phosphorus stably bonded to a silica gel matrix through niobium bridges

Abstract

For the first time niobium–phosphorus–silicon mixed oxide gels xNb₂O₅·xP₂O₅·(100 − 2x)SiO₂, with x = 2.5 (2.5PNb), 5 (5PNb), 7.5 (7.5PNb) and 10 (10PNb), were synthesized at room temperature by an innovative sol–gel route from phosphoryl chloride, niobium chloride and tetraethoxysilane. Thermogravimetry/differential thermal analysis, X-ray diffraction, solid state ²⁹Si and ³¹P NMR, Raman and Fourier transform infrared spectroscopy were used to examine the structure of dried and heat-treated (1 h at 500 °C) gels. The synthesis procedure developed in this work allowed obtaining, for each studied composition, transparent chemical gels by means of careful control of the precursor reactivity. Amorphous dried gels were obtained which were characterized by a very high degree of silicon cross-linking. Moreover, Si–O–Nb bridges were formed which allowed phosphorus to be anchored stably through Nb–O–P bonds within the gel. All heat-treated gels retain their amorphous nature with a high content of OH groups that are mainly linked to phosphorus, making them strong acidic solids.