Figure 3. XRD patterns of the synthesized samples at different crystallization times.
In the 29Si MAS NMR spectra, the chemical shift at around -111 ppm gradually vanishes during the aging process because of the conversion of silica nanoparticles to aluminosilicate species. The27Al MAS NMR spectra show a chemical shift centered at ~63 ppm suggesting multiple chemical circumstances of Al species with the Al atom binding to different number of -O-Si- groups. As the aging time increases, the intensity of the peak at ~63 ppm increases, indicating continuous gelation of -O-Si- groups of silica precursors binding to Al species within the sodium aluminate alkaline solution. Such reaction is primarily dominated by the diffusion of Al species into the silica gel precursor pellets. Interestingly, for all samples collected during aging, there is no characteristic Raman band corresponding to the D6R of zeolite X framework. We conclude that there is probably no crystalline phase formed during the aging process, and all samples appear to be completely amorphous, evidenced by the XRD and TEM results.