FT-IR spectra (shown in Fig. 9b) also record on the variation of samples with different crystallization time. An intensive absorption band at 475 cm−1 assigning to internal vibrations of SiO4 tetrahedra in amorphous silica is still observed after crystallizing for 2 h [52]. Simultaneously, weak absorption bands at 1220 cm−1 and 550 cm−1, assigning to external asymmetric stretching vibration and external symmetric stretching vibration of the five-membered rings, are observed due to the presence of S-1 seeds. After crystallization for 6 h, the absorption band at 475 cm−1 disappears, while an intensive absorption band at 450 cm−1 assigning to Si-O-Si bending vibration of the five-membered rings appears. The intensities of the absorption bands at 1220 cm−1 and 550 cm−1 increase as the crystallization time is extended. As amorphous silica does not absorb at ca. 550 cm−1, therefore the optical density ratio of the absorption bands at 550 cm−1 and 450 cm−1 (I550/I450) is a good probe for the characterization of crystallinity [53]. The I550/I450 ratio for the sample of crystallization 12 h is 0.69, demonstrating that the synthesis of highly crystalline S-ZSM-5 zeolites is fast, as the I550/I450 ratio of well crystallized ZSM-5 sample is around 0.7 [53].