Figure 6. (a) UV-Raman and (b and c) NMR spectra of the
hydrothermally synthesized samples with different crystallization times.
(d) The mechanism deduced for in situ binderless zeolite X synthesis
from silica gel precursors.
From the SEM and TEM images presented in Figures 4 and 5, attc = 8 h, a number of crystals, from 300 to 500
nm in size, are observed within the gel precursors. This suggests the
formation of zeolite X crystals accompanied by consumption of silica
nanoparticles. Additionally, the Raman signals of 4R and D6R become
stronger as crystallization time increases, clearly suggesting formation
of zeolite X structure. Meanwhile, the chemical shift at about -96 ppm
in the 29Si MAS NMR spectra becomes stronger astc increases suggesting increase of the
Si-(O-Al)2- species population in the zeolite X
framework. Moreover, all peaks in the 29Si and27Al MAS NMR spectra retain their positions, while
their intensities increase due to the increase of structural Si and Al
species in zeolite X framework. According to these results, the
formation of zeolite X framework is enabled through the self-aligned
assemblage of the 4R, 6R and D6R, accompanied with consumption of Si
species in the silica gel precursors and transportation of Al species to
the silica gel pellets.
According to these results, we deduce the mechanism of in situhydrothermal conversion of silica gel precursors to binderless zeolite X
pellets (see Figure 6d). First, in the low temperature aging period, the
precursors containing silica nanoparticles are converted to amorphous
aluminosilicate aggregates by reacting with the Al species to form the
primary building units of [SiO4] and
[AlO4], in which the Al species diffuse from the
bulk solution to the silica gel precursors accompanied with simultaneous
formation of second building units (4R and 6R). During the
crystallization process, D6R are created by interconnection between 6R
and 4R, while the β cages are developed via self-assemblage of the
second building units. Finally, the zeolite X framework is constructed
through the reorganization of β cages with D6R. In our study, all these
transformations take place in situ from the initial prepared
silica gel precursors.
Kinetic analysis of
crystallization
To further understand the crystallization kinetics, the relative
crystallinity of the synthesized samples is examined as a function oftc . The aging time was fixed to be 6 h. The
crystallization kinetic curve is well described by using the
Avrami
- Erofe’ev (A-E) expression (see
Equation (1)).55,56
γ= [1-exp(- (k (tc-θ ))n )]×100%
(1)
where γ is the relative
crystallinity.k is the apparent rate constant of crystallization.tc and θ are the times of crystallization
and induction, respectively. And n is the A-E exponent, a
parameter related to the mechanism for nucleation and crystal growth.
By correlating the crystallization kinetics (see Figure S1 for the
crystallization kinetics curve) with the A-E model, parameters including
apparent rate constant k , induction time θ and Avrami -
Erofe’ev exponent n were extracted from the fitting results (see
Table 1). Under the synthesis condition described in the experimental
section, the induction time θ is determined to be 0.81 h. As a
result, the TEM image at tc = 4 h (see Figure 5)
presents partially-crystallized morphology, because the sample has
already experienced the induction period.51
Table 1. Avrami-Erofe’ev model parameters for in situsynthesis of zeolite X.