More insight in the structure of silicophosphate gels by 31P–29Si CP MAS multidimensional experiments and 1H–31P–29Si triple resonance experiments

Abstract

Solid-state 29Si, 31P MAS and CP MAS NMR and X-ray powder diffraction have been used to study phosphosilicate gels of molar composition (TEOS/H3PO4 = 1) treated under various temperature conditions. It was found that the gel treated at 100 °C for 30 h had a structure consisting of a silica matrix including isolated molecules of phosphorous acid. In contrast, cross condensation (leading to the formation of Si–O–P bonds) was observed when the gel was treated at 136 °C for 6 days. Evidence for the formation of Si–O–P bonds was provided first by XRD, which showed the formation of the crystalline phosphosilicate phases Si5O(PO4)6 and Si(HPO4)2·H2O, and then by the use of two-dimensional (2D) NMR techniques: 31P–29Si heteronuclear correlation (HETCOR) experiments, based on cross polarization transfer. Very few data concerning this type of experiments are available in the literature: therefore, CP dynamics was carefully studied under MAS. To cite this article: C. Coelho et al., C. R. Chimie 9 (2006). Des analyses en RMN 29Si, 31P MAS à l'état solide ainsi qu'en diffraction des rayons X sur poudre ont été utilisées pour étudier des gels de phosphosilicates obtenus à partir de mélanges équimolaires de TEOS/H3PO4 et traités sous différentes conditions de température. Dans le cas d'un gel séché à 100 °C pendant 30 h, le spectre RMN 31P MAS montre un signal caractéristique de l'acide phosphorique piégé dans un réseau de silice amorphe. En revanche, le gel traité à 136 °C pendant six jours présente un réarrangement structural différent (phases cristallines Si5O(PO4)6 et Si(HPO4)2·H2O). L'utilisation de techniques RMN 2D (expériences 31P–29Si HECTOR) basées sur le transfert de polarisation croisée a permis de corréler les différents sites 31P et 29Si. Pour citer cet article : C. Coelho et al., C. R. Chimie 9 (2006).