EFFECT OF PRECURSORS ON THE STRUCTURE OF PHOSPHOSILICATE GELS - SI-29 AND P-31 MAS NMR-STUDY

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-degrees-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-degrees-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 Si-29 and P-31 magic angle spinning (MAS)-nuclear magnetic resonance (NMR). Si-29 and P-31 spectra show that SI-O-P and P-O-P linkages do not form in the xerogels (i.e. gels heat treated to approximately 60-degrees-C). The gels heat treated at 200-degrees-C or above (those that did not crystallize) show evidence of Si-O-P and P-O-P network formation.