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.