Figure 5. TEM images of the synthesized samples at different crystallization times.
The 29Si MAS NMR spectrum of the sample withtc of 4 h presents chemical shifts at δ = -86.82, -91.00, -96.02 and -100.44 ppm (see Figure 6b), which correspond to the Si-(O-Al)4-, Si-(O-Al)3-, Si-(O-Al)2- and Si-(O-Al)- (Si linking to different number of Al atoms by oxygen bridges) species, respectively.53 The 27Al MAS NMR spectra shown in Figure 6c present significantly enhanced chemical shift at ~61 ppm owing to the binding of Al intermediate species with different number of -O-Si- groups.35
On the other hand, the development of zeolite X phase was monitored by XRD as a function of crystallization time. First, characteristic patterns of zeolite X were confirmed for all samples investigated.54 As crystallization proceeds, the relative crystallinity of zeolite X gradually increases until reaching the maximum at 24 h. Specifically, the samples with crystallization times of 6, 8 and 18 h have relative crystallinity of 48.1 %, 77.8 % and 95.0 %, respectively. Eventually, the relative crystallinity of sample crystallized up to 24 h is 99.6 %.