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.