Zeolites hydroxyl stretching region

Infrared Measurements. OH Groups. Infrared spectra in the hydroxyl stretching region are shov n in Figure 2 for LaY zeolite. 

Figure 1. FTIR spectra recorded in the hydroxyl-stretching region on various exchanged Y zeolites.

Figure 2.16 Hydroxyl stretching region of FTIR spectra of ZSIVI-5 zeolites isomorphously substituted with iron, gallium, or aluminum. The position of the bridging hydroxyls is labeled. Reproduced with permission from Ref. (99). Copyright 1985 American Chemical Society.

Figure 2.38 Acid properties of AI-ITQ-33 zeolite measured by pyridine adsorption and subsequent (attempted) desorption at increasing temperatures. On the left is plotted the hydroxyl stretching region, where (a) is the IR spectrum after thermal treatment at 673 K under vacuum and (b) is the spectrum after adsorbing pyridine followed by desorption at 423 K. On the right is shown the C-C stretching region of the adsorbed pyridine after evacuation at (c) 423, (d) 523, and (e) 623 K. Reproduced from Ref. (462).

The spectrum of this zeolite is more complicated, In the OH stretching region three bands were observed after pretreatment at 573 or 673 K, at 3750, 3620 and 3540 cm", the former two resist pumping at 973 K, when the last one disappears. After CO adsorption at 77 K (fig. 2.) these bands diminish while two bands of perturbed hydroxyls show up at 3660 and 3330 cm . That at 3660 cm disappears after pumping off the gaseous CO at 77 K together with the band of adsorbed CO at 2160 cm and the restoration of the 3750 cm band, however, to remove all the CO bands and to recover the initial OH group spectrum, evacuation at room temperature is needed. 

Due to the limitations of diffraction experiments, and perhaps also due to the irregularity of their position, it is impossible to obtain experimental locations for these sites, however experimental evidence does exist for hydroxyl groups Inside zeolites in the form of i.r. spectra, which typically show bands in the region of >3700 cm. This allows calculated stretching frequencies to be compared with those from experiment, and for conclusions concerning relative... 

The v(OH) modes of surface hydroxyl groups are well distinguished in IR spectra, whereas in most cases, the M-O stretching vibrations and the deformation modes are masked by the strong absorbance of the bulk of the solid in the low-frequency region. However, in a few cases (predominantly concerning H-bonded hydroxyls), the direct detection of deformation modes is possible. This situation is encountered for metal oxides (10) and zeolites (11). 

---