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in Ti-free silicalite-1, Figure 7d), four vibrations associated with the TiIV active sites are visible at 523, 640, 960 and 1125 cm−1. The first band (532 cm−1) has previously been assigned to the symmetric stretching vibration of framework Ti–O–Si species [22,23]. Both the 960 and 1125 cm−1 bands have also been attributed to the TiIV active sites of TS-1. These arise from the combination of three asymmetric stretching modes of a tetrahedral [Ti(OSi)4] unit (960 cm−1), and a totally symmetric stretching mode, i.e., breathing mode, of the same unit (1125 cm−1), respectively [11]. Accordingly, these final two stretches are directly related to the tetrahedrally-coordinated Ti atom present in the [Ti(OSi)4] active sites. We note that 1125 cm−1 band is also known to undergo significant resonance enhancement as the laser is progressively tuned into the LMCT modes, in good agreement with our data (Figure 10) [11]. Accordingly, the 530, 960 and 1125 cm−1 UV-Raman bands provide a unique opportunity to monitor the Ti site speciation in TS-1 and TS-1B as a function of post-synthetic modification. Figure 8 displays the UV-Raman (325 nm) spectra of TS-1 (blue) and TS-1B (red), with the right inset showing the relative intensities of the 960 and 1125 cm−1 bands. As can be seen, NH4HF2 treatment of TS-1 leads to significant changes in the UV Raman spectrum. The 1125 (inset), 960 (inset), and to a lesser extent the 530 cm−1 band (not shown) of TS-1 experience a significant decrease in relative intensity upon NH4HF2 treatment. Since the 960 and 1125 cm−1 bands have been assigned to stretching modes of a purely tetrahedral [Ti(OSi)4] unit [11], it can be proposed that a general decrease in tetrahedral, framework [Ti(OSi)4] atoms occurs upon post-synthetic modification, i.e., extraction of framework Ti occurs. This is in line with the decrease in the intensity of the 210 nm band in the UV-Vis spectra (ESI Figure S2). Based on the relative intensities of the 960 and 1125 cm−1 bands before and after treatment, we estimate the concentration of the original [Ti(OSi)4] atoms have decreased by approximately 25%–50% following NH4HF2 treatment, in further agreement to the decreased intensity of the Si–O–Ti band in the FTIR spectra (ESI Figure S3). Concurrently, a new Raman stretch at 695 cm−1 is observed. The generation of a new Raman band is in good agreement to the original UV-Vis data, which demonstrated that a new LMCT band was formed (270 nm) following NH4HF2 treatment. Given that this Raman stretch is in the framework, i.e., fingerprint, region of the Raman spectrum, it is likely associated with the TiIV active sites. However, to verify if this new species was indeed responsible of the absorption band at 270 nm (ESI Figure S2), and is therefore related to the extraction of the [Ti(OSi)4] units, an additional Raman experiment at 266 nm excitation wavelength was performed (Figure 8, left inset). As can be seen, irradiating TS-1B with a 266 nm laser significantly increases the intensity of this stretch by several orders of magnitude. The 266 nm excitation laser overlaps perfectly with the 270 nm absorbance band, thus allowing true resonance conditions to be met, strongly indicating that this Raman stretch 109PDF Image | Zeolite Catalysis
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