Fig. 2. FTIR spectra of the solid aliquots from a series of
experiments at different intervals.
The aliquots prepared for 0.5-8.5 h show broad absorption bands in the
range of 400-1200 cm-1, which are associated with
amorphous sodium aluminosilicate gels [31]. Structure sensitive
vibrations of aliquots with obvious change occurs near 530-560 and 710
cm-1. The band at 530-560 cm-1 is
indicative for D5 ring units, which is typical of dominating secondary
structural units in the MFI-type structure [29,32,33]. This weak
broad band appears after the 0.5 h RPB premix process. After 1.5 h
dynamic hydrothermal treatment, this band becomes sharper with the
crystallization time prolonged, indicating the occurrence of the zeolite
nucleus [31, 34]. Moreover, the gradual redshift of this peak
position indicates that the SiO4 tetrahedra in the D5
ring unit are substituted continuously by AlO4tetrahedra [35,36], which leads to the formation of Si-OH-Al,
corresponding to Brønsted acid sites [37]. The band at 710
cm-1 is indicative for T-OH bending vibrations (T =
Si/Al), which represents the occurrence of hydrolysis reaction between
Si4+ and Al3+ [28] and the
formation of TO4 tetrahedral assembled into zeolite
skeleton [38]. This band has occurred at 0.5 h, indicating the
hydrolysis reaction has taken place in the RPB premix process at room
temperature. The gradual disappearance of the 710 cm-1peak is consistent with an increase in the intensity of the 530-560
cm-1 band, suggesting that the amount of framework
atoms connected by oxygen bridges increases constantly. This indicates
the amorphous gels have transformed gradually into zeolite crystals
[39].
Fig. 3. XRD patterns of solid aliquots obtained by (A) RPB and
(B) STR premix at different intervals.
Fig. 3A shows the XRD patterns of solid aliquots obtained by 30 min RPB
premix and dynamic crystallization at different intervals. X-ray
detection limit has no impact on the analysis result [28-30]. The
first crystalline phase with the crystallinity of 30% was detected at
5.5 h, and the crystallinity is 50% after 6.5 h, indicating that the
crystallization of the amorphous gel into long-range order zeolite
skeleton has occurred at 5.5 h of hydrothermal treatment.