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 %.