Raman bands zeolites

Curiously, however, spectra due to some of the intermediates were not observed using both spectroscopic techniques, and the authors discuss possible causes for this. As with the previously discussed acetylene-zeolite systems (30,31), very strong Raman bands were observed for the v(ChC) modes which are very weak in the infrared (and forbidden in C2H2). 

The low intensity of the Raman bands intrinsic to zeolite structures is an advantage when attempting to observe adsorbed molecules (in contrast to the situation in infrared spectroscopy, where large regions of... 

The dependence of the framework vibrational frequencies on the Si/Al ratio and aluminosilicate ring size were also examined with Raman spectroscopy, shown in Figure 22. The Si/AI ratio was varied from 1.0 to 2.7 in a series of zeolite A materials. The low-frequency bands at 337 and 410 cm were not found to change with the Si/Al ratio. The strong band at 489 cm exhibits a weak dependence on Si/Al ratio. The 700 cm band, however, shows the most rapid and almost linear increase in frequency with Si/Al ratios. The bands in the 900 to 1100 cm region exhibit a complicated dependence on the Si/Al ratio. The strong Raman band at about SOO cm, which possesses a weak dependence on the Si/Al ratio, however, is very sensitive to the... 

Molecular dynamics calculations have been made on the motions of the Na+ ion in mordenite zeolites.9 Selenium clusters doped with Na (i.e. Na2Sen) show a Raman band in the range 165-225 cm 1 due to Na-Se motions.10 Raman microspectra have been reported for caesium oxides, e.g. an alg mode of Cs20 was seen at 103 cm 1.11... 

We assume that (i) the 960 cm band does not saturate for the TS-1 sample with lowest Ti content and (ii) the true Full Width at Half Maximum (FWHM) of this band is constant over all set of TS-1 samples. The former assumption is supported by the fact that the intensity of the 960 cm band, for the x = 1.0 sample, is less than 1.5 in absorbance units, while the latter is exactly what expected for a band associated to different concentrations of a unique species. Assumption (i) allows us to measure the true FWHM of the 960 cm band in TS-1 samples (27 cm ). Now, following assumption (ii), we were able to estimate for all samples reported in Fig. 3, the absorbance (W) at the height where the width of the band corresponds to 27 cm. This method minimize the errors due to the instrument sensibility and allows a quantitative estimation of the band intensity. Such obtained values plotted against x in the Fig. 3d ( data), give a high linear correlation (r = 0.9998) and validate assumption (i) and (ii). A further, and definitive, validation comes from the similar linearity found for the intensities of the Raman band, which can not be affected by saturation problems, see Fig. 3d ( data). These results implies that the 960 cm band is well a fingerprint of the insertion of Ti in the zeolitic framework, even if not... 

Lattice-dynamical calculations for unit cells of natrolite and edingtonite were performed. It was shown that strongest Raman bands of natrolite at 534 cm" and edingtonite at 530 cm are related to breathing modes of 4-membered rings. Assignment of vibrational spectra of used zeolites, presented here for symmetric modes of natrolite, may provide a base for interpretation of vibrations in other zeolites. Calculated natrolite crystal structure exhibits instability at about 5.5 GPa, which corresponds to amorphization observed at pressure range of 4-7 GPa. 

Fig. 67. (cont.) Zeolite ECR-18. (b) Raman spectra between 200 and 3200 cm [05K1]. The structural Raman band of each material is marked by an asterisk. Samples ... 

Fig. 2 The mainly Q connected siliceous zeolites [22,23] are poor Raman absorbers in the 700 to 1300 cm" Vs and Vas Si-O vibration range (2/a) and the 1140 cm Raman band of a gel made from Kasil-1624 (K/Si 0.76) [8, 10, 11] substantially decreases upon drying which makes its Q assignment dubious (lib).

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Recently, UV laser stimulation of catalyst samples has been developed to overcome the problem of interference by coke (carbon deposition) on catalysts.Fig. 9 shows a typical Raman data set that was obtained for carbon deposition as a function of temperature. To explore different coke formation behavior, the reaction of propene on a zeolite was performed. The spectra obtained were (A) C3H6/He flow at 773 K for 3 h (B) O2 flow at 773 K for 1 h and (C) O2 flow at 873 K for 1 h. This data shows that most of the carbon, identified as polyaromatic and pregraphite, can be removed at 773 K with oxygen. However there is still carbon present as identified by the broad band at 1610 cm suggesting that carbon is in a more inert form such as coke. Not until the temperate is taken to 873 K with oxygen is that carbon removed. 

The formation of peroxide and superoxide on Fe,H/MFI compared with Fe/MFI also shows two distinguishing features. First, the amount of peroxide on Fe,H/MFI at room temperature is significantly greater than on Fe/MFI, as determined by the peroxide peak intensity relative to the intensities of the zeolite bands (52). Second, on Fe,H/MFI, peroxide is converted to superoxide when the sample temperature is lowered to 93 K, and then restored when the temperature is returned to 300 K. Figure 8 shows an overlay of Raman spectra characterizing Fe,H/MFI measured at 300 and at 93 K using 2. The band at 703 cm ( 02 ) decreases at 93 K relative to its intensity at 300 K, whereas the intensity near 1090cm ( Oj) shows the opposite behavior with temperature. In the spectrum of Fe/MFI, the relative peak intensities of peroxide and superoxide remain constant with temperature between 93 and 300 K. (A specialized variable-temperature fluidized-bed Raman cell was constructed for these experiments.)... 

Introduction of aluminium into a zeolite lattice broadens the lattice modes, but also introduces additional bands in the Raman spectra at low frequencies due to cation vibrations, completely analogous to the far infrared bands described in section 3.3. Figure 18 shows, for example, Raman spectra taken from the work of Bremard and Le Maire [53] of zeolite Y exchanged with different alkali metal cations. The arrows indicate bands assigned to translational modes of the cations these move to lower frequency as the mass of the cations increases, just as in the far infared spectra. 

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