Synthesis and Characterization of Silicophosphates Using Phosphoric Acid and Silica Gel Prepared from Tunisian Sand

Silicophosphates were synthesized with different amounts of P 2 O 5 by mixing phosphoric acid and silica gel, prepared by precipitation of sodium silicate and HCl. The materials were characterized by XRF, XRD, FTIR, SEM and NMR. XRF results showed that the weight ratio (P/Si) decreases for the samples prepared at 1000 °C. XRD results showed the formation of Si 5 O 25 P 6 for the samples containing 37 and 54 wt% of P 2 O 5 independently of the annealing temperature. Si 5 O 25 P 6 and SiP 2 O 7 were formed when the amount of P 2 O 5 was 64 wt% while SiP 2 O 7 disappeared at 1000 °C. 29 Si and 31 P NMR investigations clearly indicated the presence of both four and six coordinated silicon linked to PO 4 tetrahedral and their arrangement was strongly influenced by the amount of P 2 O 5 and the annealing temperature. Silicophosphates with 37 and 54 wt% of P 2 O 5 annealed at 800 °C presented high negative values of zeta potential leading to a great pH range of stability.