Aging of amorphous aluminosilicate precursor
To understand the aging process of silica gel precursors in sodium aluminate alkaline solution, the as-synthesized samples with different aging times were characterized using integrated structural, microscopic and spectroscopic methods. According to the XRD patterns in Figure 2a, the initially prepared silica gel precursors at the aging time of 0 h exhibit to be completely amorphous, which suggests the existence of initial silica nanoparticles in the precursors. The TEM images (see Figures 2b e) further confirm the presence of spherical silica nanoparticles (~20 nm) in the as-prepared precursors. The Raman bands at 436, 785, 964 and 1034 cm-1corresponding to the initial silica nanoparticles are observed in the Raman spectrum for the sample prior to aging (see Figure 2f). The broad band centered at 436 cm-1 is assigned to the bending mode of the Si-O-Si bond.46 The band at 785 cm-1 can be attributed to the symmetric stretching mode of the Si-O-Si bond,46 while the bands at 964 and 1034 cm-1 are associated with the asymmetric stretching mode of the Si-O-Si bond.47 Additionally, the 29Si MAS NMR spectrum in Figure 2g features a wide chemical shift at about -111.11 ppm, corresponding to the silica nanoparticles in the original silica gel precursors.35 The absence of27Al MAS NMR signal confirms a pure silica phase (see Figure 2h). Extended aging length leads to slight decrease in XRD peak intensity for amorphous silica, which is further supported by the TEM images clearly demonstrate the morphological transformations of silica nanoparticles in the sodium aluminate alkaline solution during the aging process. Specifically, as the aging length increases the dispersed silica nanoparticles gradually become coral-like amorphous aggregates without further crystallization. Such phenomenon is consistent with the XRD results showing no evidence of long range order.
As the aging time reaches 2 h, several weak Raman bands are detected in the region of 300 to 600 cm-1, which correspond to different ring sizes.48,49 The bands at 315 and 486 cm-1 can be ascribed to the bending modes of the T-O-T bond of the 6R and 4R, respectively, which indicate the formation of secondary building units (SBUs).50 The 4R suggested by the Raman band at 486 cm-1 is probably dissociative, which is distinctly different from the rings in zeolite X structure at around 500 cm-1. The band at 572 cm-1 is associated with the Al-O-Si stretches of the aluminosilicate anions formed in alkaline solution.51Moreover, the bands at 436, 964 and 1034 cm-1 (for the original precursor sample) shift to 449, 1013 and 1060 cm-1 (for the sample at 2 h of aging time), respectively. Extended aging length leads to stronger Raman absorption at 486 and 1060 cm-1, indicating gradual formation of 4R and T-O-T bonds during the aging process, respectively.