Effect of precursors on the structure of phosphosilicate gels: 29Si and 31P MAS-NMR study

Abstract

Gels were prepared using phosphoric acid, triethyl phosphate and trimethyl phosphite as the precursors of phosphorus. The upper limit for P2O5 for gel formation is 90 mol% for gels using phosphoric acid and trimethyl phosphite, and 70 mol% for gels using triethyl phosphate. Gels prepared from phosphoric acid with more than 10 mol% of P2O5 partially crystallize to Si5O(PO4)6 after heat treatment at 200°C for 10 h. Under the same heat treatment condition, all the gels prepared from trimethyl phosphite crystallize. Gels prepared from triethyl phosphate are amorphous even after heat treatment at 800°C for 10 h. However, chemical analysis indicates that most of the phosphorus in the triethyl-phosphate-prepared gels is driven off during the heating process; no more than 8 mol% of the initial amount of P2O5 is retained in the gels. The effects of phosphorus precursors on the phosphosilicate gels were studied by X-ray diffraction, and 29Si and 31P magic angle spinning (MAS)-nuclear magnetic resonance (NMR). 29Si and 31P spectra show that SiOP and POP linkages do not form in the xerogels (i.e. gels heat treated to ∼ 60°C). The gels heat treated at 200°C or above (those that did not crystallize) show evidence of SiOP and POP network formation.