Crystallization of zeolite X
After aging, hydrothermal treatment was applied on the same reaction
system (reactor) with amorphous aluminosilicate precursors to
crystallize zeolite X at elevated temperature. To reveal the
compositional and structural evolutions the precursors undergo duringin situ conversion to zeolite X, samples with different
crystallization times (tc ) were collected and
thoroughly characterized with structural, spectroscopic and microscopic
methods. Specifically, when tc = 4 h, preliminary
phase transition from the amorphous aluminosilicate aggregates to
zeolite X crystals was observed (20.5 % relative crystallinity)
according to the XRD data shown in Figure 3. The morphological changes
of crystallization initiated from the gel precursors are revealed by SEM
and TEM images presented in Figures 4 and 5 respectively. It appears
that the silica nanoparticles become denser aggregates, and
partially-crystallized particles from these aggregates were observed attc = 4 h.
The Raman bands at 290, 366, 504, 1000 and 1063 cm-1(see Figure 6a) belonged to the framework structure of zeolite X are
detected at tc = 4 h.52Specifically, the sharp band at 504 cm-1 is assigned
to the bending mode of T-O-T bond of the 4R in the zeolite X
structure.50 The double bands at 290 and 366
cm-1 can be attributed to the bending modes of T-O-T
bond of D6R, which indicate initial crystallization of zeolite X. In
contrast, the absence of these signals for sample attc = 0 h strongly suggests no zeolite X
formation. Additionally, the bands at 1000 and 1063
cm-1 are ascribed to the asymmetric stretching
vibration modes of the T-O-T bonds in zeolite X
framework.50 Moreover, the Raman results also
suggested formation of second structure units of 4R, 6R and D6R whentc = 4 h, implying the existence of ordered
crystalline zeolite X framework.