Aluminosilicates, absorption bands

Flanigen monitored the changes in the IR spectra that occur during the synthesis of NaX zeolite from a sodium aluminosilicate gel. The appearance of absorption bands due to the formation of structural units in the zeoUte as the crystallization of NaX proceeded were observed [93]. In particular, the growth of a band around 575 cm indicated the formation of double six-rings which is one of the structural sub-units of X zeolite. 

Dissolved carbon dioxide also causes an infrared absorption band in glasses. A narrow absorption band due to dissolved CO2 molecules is found at 4.26//m in sodium aluminosilicate and heavy metal fluoride glasses. Bands due to carbonate species formed by reaction of carbon dioxide with oxide melts have also been reported. 

The IR spectra for samples prepared from aluminum acetate also show a significant difference in the function of pH (Fig. 8). The new absorption maximum at 627 cm on the curve for the sample prepared in an acidic medium can be interpreted as Al-O stretching vibrations of condensed octahedral AlOe, which forms a separate phase from the silica network. The absorption band at 627 cm disappears after washing the gel sample with water. The band of Si-O-Si can be found at 1,095 cm similarly to that of pure silica gel. The IR spectrum of the sample obtained in basic conditions differs strongly from that for other aluminosilicate gel samples. The position of the peak for the Si-O-Al bond is at a low wavenumber, at 1,047 cm corresponding to the A1 NMR result, which indicates 100% A1 incorporation. The vibrational bands at 1,412 and 1,553 cm can be associated with the acetate groups coordinated to A1 atoms [20]. 

Water incorporated into fused silica is of considerable importance because the resulting silanol groups affect the NIR transmission of silica optical fibers and other optical components. Silanols are discussed in the OH section. In mixed glasses, such as those containing borosilicates, aluminosilicates, and so on, associated water OH groups give rise to diffuse absorption bands. The absorption bands of water molecules on silica surfaces have been described by Klier et al. " ... 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

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