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

Solid-state 29 Si, 31 P MAS and CP MAS NMR and X-ray powder diffraction have been used to study phosphosilicate gels of molar composition (TEOS/H 3 PO 4 = 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 Si 5 O(PO 4 ) 6 and Si(HPO 4 ) 2 ·H 2 O, and then by the use of two-dimensional (2D) NMR techniques: 31 P– 29 Si 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. .