Synthesis of High Surface Area Phosphosilicate Glasses by a Modified Sol−Gel Method

Abstract

High surface area phosphosilicate glasses were synthesized by a new sol−gel method and characterized for structural, textural, and acid properties by X-ray diffraction, thermogravimetry/differential thermal analysis, IR spectroscopy (Fourier transform and diffuse reflectance Fourier transform), solid state 31P and 29Si NMR spectroscopy, N2 adsorption, and NH3 temperature programmed desorption. The glasses were synthesized by a sol−gel route hydrolyzing the precursors in an almost solely aqueous environment. Microstructural changes occurring during the conversion of the gels into the corresponding glasses were investigated. For all dried gels (383 K), the elimination of organic residues was complete at 673 K. After this heat treatment, ca. 80% of the phosphorus was present as free phosphoric acid except for the sample with the highest P content for which the percentage dropped to ca. 47%. Textural properties of samples treated at 673 K were greatly influenced by the composition. Surface area decreased and pore dimension increased with increasing phosphorus content, indicating a gradual transformation of the siloxane matrix from microporous to mesoporous. Brønsted acid sites of differing strength, mainly related to the presence of phosphoric acids, were found. The surface concentration and strength of acid sites increased with phosphorus content.