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

摘要

Amorphous sodium gallium silicate materials are isolated after short periods of heating a gel composition Ga_2O_3:SiO_24NaOH:80H_2O at 100 deg C. On extended heating (periods i excess of 24 hours), the crystalline zeolite gallium hydroxosodalite is produced from the reaction mixture. We have studied the materials using ~(23)Na (I = 3/2) MAS and multiple-quantum MAS (MQMAS) NMR spectroscopy in order to extract information about the local environemnt of the sodium in the amorphous solids, and thereby gain information about the role of sodium during the crystallisation of the zeolite. THe average ~(23)Na isotropic chemical delta_(iso), and the quadrupolar parameter, P_Q = C_Q ( + eta~2/3)~(1/2), were evaulated 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 enviroment of the sodium atoms. Although no simple correlation is known between ~(23)Na NMR parameters and the coordination number or average sodium-oxygen interatomic distance, a general trend of increasing delta_(iso) 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 sosium is very similar to that found in the crystalline zeolite product. Additional ~(71)Ga (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.