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 ))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.