A multiple-quantum Na-23 MAS NMR study of amorphous sodium gallium silicate zeolite precursors

Amorphous sodium gallium silicate materials are isolated after short periods of heating a gel of composition Ga2O3 : SiO2 : 4NaOH : 80H(2)O at 100 degreesC. On extended heating (periods in excess of 24 hours), the crystalline zeolite gallium hydroxosodalite is produced from the reaction mixture. We have studied the materials using Na-23 (I = 3/2) MAS and multiple-quantum MAS (MQMAS) NMR spectroscopy in order to extract information about the local environment of the sodium in the amorphous solids, and thereby gain information about the role of sodium during the crystallisation of the zeolite. The average Na-23 isotropic chemical shift, delta(iso), and the quadrupolar parameter, P-Q = C-Q (1 + eta(2)/3) K, were evaluated by analysis of the two-dimensional MQMAS spectra. In addition to the sodium gallium silicate materials, a number of crystalline sodium silicates, aluminates and gallates were also studied and their NMR parameters compared with literature values and related to crystallographic data describing the local environment of the sodium atoms. Although no simple correlation is known between Na-23 NMR parameters and the coordination number or average sodium-oxygen interatomic distance, a general trend of increasing diso with decreasing coordination number and interatomic distance is apparent. The data from the amorphous sodium gallium silicates strongly suggest that even after very short periods of heating the local environment of sodium is very similar to that found in the crystalline zeolite product. Additional Ga-71 (I = 3/2) MAS NMR spectra were also recorded from the amorphous samples to provide information on the local gallium environment and these show the presence of tetrahedral gallium, as in the crystalline zeolite, even in the samples prepared after the shortest period of heating.