Zeolite overtones

In addition, there are some weaker broad bands that are typically observed between 1900 and 1500 cm" that have been assigned to overtones of framework vibrations. The IR spectra in Figure 4.22 are truncated at 1300 cm" because the absorbance of the sample is too high to measure at lower frequencies (<1200 cm" ). This is due to the very strong T-O-T stretching vibrations of the zeolite as mentioned in the previous section on framework IR measurements. 

Figure 3. Overtone and combination band spectrum oj ethylene and water adsorbed on Mn"A zeolite. The ethylene bands lie close to the gaseous (V -f- vn), (vt + vs), 2vlu and 2vs vibrational modes, indicating that the ethylene molecule has retained its chemical composition and structural integrity (-, 1) MnA 4- ethylene (-------------------,2) MnA hydrated CtHt (g) bands.

Absorbance Spectra In Figures 3 and 4, spectra of the freshly activated, unloaded zeolite sample A, as well as of the otherwise unchanged sample after admitting a partial pressure of 3.1 mbar of n-hexane are given. The experimental resolution was 2 cm. Although 25 spectra were accumulated, the signal-to-noise ratio is, due to the small sample area of about 20 x 20 pm rather low. The band at 2350 cm is attributed to COj present in the beam path within the IR microscope. The spectrum of the freshly activated sample exhibits IR bands at 2007, 1882 and 1644 cm which may be attributed to overtones of zeolite lattice vibrations [11]. The broad structure at 3500 cm is due to SiOH groups of lattice defect sites [12]. After equilibration of the sample with 3.1 mbar of n-hexane, the positions and relative intensities of the IR... 

Brdnsted acid sites) ratioed against the 1870 cm zeolite lattice overtone band. These results are shown in Table 1. 

Figure 22.4 IR spectrum of and H2 0 adsorbed on H-ZSM-5 (adapted from Fig. 2 in Ref [37]). Shown is the assignment based on frequency calculations for models of the neutral adsorption complex [34]. The overtone of the in-plane SiOH bending ( zqh) hilh ooto the red-shifted OH stretching frequency of the bridging hydroxy group of the zeolite, vOH. ...

Spectroscopic characterization of the zeolites. Diffuse reflectance spectroscopy reaches the spectral range comprised between 4000 and 40000 cm-1. Two types of transitions are of interest. In the first place, 4000-10000 cm-1 region contains vibrations associated with the OH groups combination bands near 4500-5000 cm-1 and the first overtone 2v0h near 72 00 cm-1. Secondly, the d-d transitions of the nickel ions show up between 4000 and 30000 cm-. ... 

Figure 4. Reflectance spectra of HY zeolites and silica gel dehydrated at 793 K OH combinations (left side) and overtones (right side).

Similarly, the low frequency overtone at 6950 cm-1 associated with acidic OH vanishes, while the silanol overtone band develops at 7325 cm-1 (9) and the ( v + 6) combination shifts to 4540 cm-1. These observations are consistent with the creation of silicon defects in the structure of dealuminated Y zeolites (10) while the weak overtone band at 7240 cm 1 is probably related to hydroxylated aluminium species extracted from the lattice (11, 12). Thus, the near-IR spectra give evidence for the decrease of the number of Bronsted acid sites as a result of dealumination. 

Figure 5. Reflectance spectra of NiHY15 zeolites OH overtones on dehydrated samples.

Forster H and Frede W (1984), Induced IR overtone and fundamental bands of isotopic H2 molecules adsorbed in NaCaA zeolites . Infrared Phys, 24, 151. 

Kazansky V B, Borovkov V Y and Karge H G (1999b), Diffuse reflectance IR study of molecular hydrogen and deuterium adsorbed at 77 K on NaA zeolite. Part 2. Overtone, combination and vibration-rotational modes and thermodesorption from NaA zeolite , ZPhys Chem, 211, 1. 

Kazansky and co-workers have studied the diffuse reflectance i.r. spectra of -OH groups in decationized forms of X-, Y-, and mordenite zeolites, as well as in the forms exchanged with alkaline and alkaline-earth ions, in a wide spectral range, covering fundamental stretchs, their overtones, and combination bands. They have shown that the combination bands of... 

Diffuse reflectance spectroscopy (DRS) has been frequently employed in UV-Vis spectroscopy of zeolites (cf. Volume 4, Chapter 4 of the present series). More recently, it became also popular in IR investigations (cf., e.g., [160, 161]). The DRIFT technique is advantageous because it is successfully applicable not only in the mid infrared but also in the near infrared (NIR) region of 4000-10,000 cm where the transmission technique usually fails because of severe scattering through the absorbent particles. The NIR range, however, is very important with respect to the study of overtone and combination modes of vibrations. Moreover, diffuse reflectance IR experiments can be carried out not only with pressed wafers as in transmission spectroscopy, but also with zeolite pow-... 

Hydroxy Groups of Zeolites Characterized by Deformation, Overtone and Combination Bands... 

The IR range of combination and overtone modes hes above ca. 4000 cm". It is also extremely difficult to investigate this region by transmission IR spectroscopy because of the weak intensities of the respective bands which usually are only 3 to 5% of that of the fundamentals. In this context one has to keep in mind that the absolute transmission of zeolite wafers in the region of fundamental OH stretching modes is sometimes only 1% or even less (vide supra, e.g.. Sect. 5.4.1.2.3). 

Deo et al. observed a weak combination band, Vi -1- V3, at 2470 cm and the more intense asymmetric stretching band with V3=1330 cm" shifted by A (V1-1-V3) = -42 and Av3=-31 cm with respect to the gas phase wavenumbers (Vi -h V3 = 2512, V3 = 1361 cm )> respectively. No further band appeared, in particular no V2 and no overtone band 2Vi. On admission of H2S to the S02-loaded Y-zeolite sample, a band around 6h2o=1650 cm developed originating from the deformation mode of H2O, which had formed according to Eq. (29). 

The bridging hydroxyls in zeolites (3610-3600 cm ) also manifest a pronounced (Voh + Sqh) mode, which is observed at higher frequencies than the combination modes of the silanol groups, namely in the region of4654-4661 cm (1.S3,312,323,411j. Thus, like the overtones, the combination modes of these different bands are well separated (see Figure 2.50). 

The KBr spectrum is employed in the internal-reference technique. In this case, a weak absorption band of the adsorbent (e.g., the overtones of the silica or zeolite lattice vibrations, 2100-1600 cm [119]) acts as the internal reference, and the ratio of adsorbate absorption to the reference absorption is proportional to the quantity of molecules adsorbed. The advantage of this is that it does not require to control precisely the sample weights [120]. 

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