Hydroxyl group infrared absorption frequencies

Note that in all the examples discussed so far, infrared spectroscopy gives its information on the catalyst in an indirect way, via hydroxyl groups on the support, or via the adsorption of probe molecules such as CO and NO. The reason why it is often difficult to measure the metal-oxide or metal-sulfide vibrations of the catalytically active phase in transmission infrared spectroscopy is that the frequencies are well below 1000 cm-1, where measurements are difficult because of absorption by the support. Infrared emission and Raman spectroscopy, discussed later on in this chapter, offer better opportunities in this respect. 

The nature of the acidity of mordenite and its relation to catalytic activity have been investigated by Benesi (757), Lefrancois and Malbois (227) and Eberly et al. (225). Eberly et al. observed two absorption bands in the hydroxyl region of the infrared spectrum of H-mordenite. A band at 3740 cm-1 was attributed to silica-type hydroxyl groups, and a lower frequency band, 3590 cm-1, was thought to arise from hydroxyl groups associated with aluminum atoms in the structure. Acid extraction of the aluminum atoms from the framework, although leaving the structure intact resulted in a loss of the lower frequency hydroxyl band. 

The acid dissociation constant of the OH groups on polymeric silica has also been shown to be about by an entirely different method. Hair and Hertl (43) measured the frequency shift of the infrared absorption band of phenolic hydroxyl groups when adsorbed on the surface of silica and compared these with the shifts of phenol in the presence of alcohols of known acidity constants. Marshall et al. (44) concluded from a similar study that the p/To of some SiOH groups on the silica surface may be about 7.2. 

Haslewood (25) has discussed the value of infrared spectroscopy for the characterization of bile acids and has indicated that bands at about 9.3, 9.6, 10.2, 10.5, and 10.95 p are especially useful for detection of the cholic acid nucleus. Earlier he and Anderson (69) noted the importance of absorption bands at 10.4 and W.2 p for the identification of allocholic acid. He has pointed out the prominence of bands at about 9.2, 9.7, 9.9, 10.4, and 11.2 /i in the spectrum of allocholic acid. Those bands between 9.0 and 10.0 p due to the C-0 stretching frequency can be attributed to the axial hydroxyl groups at Cj, C, and Cjj. This is apparent from the data in Table IV which show the absorption bands for a series of substituted allo-acids in the vicinity of the bands reported for allocholic acid. The band found in the vicinity of 9.9 p can be attributed to the 3a-hydroxyl group replacement with a 3/9-hydroxyl group produces a hypsochromic shift. 

There have been several detailed and careful studies in this field recently. For example. Farmer and Russell [1964] have studied the infrared absorption bands arising from the structural hydroxyl groups of a number of layer silicates, i.e., in the region from 3500 to 3700 cm" . They attempted to assign the two frequencies they observed in terms of either octahedral substitutions or of Al-for-Si substitution tetrahedrally. They confirmed earlier work that in the dioctahedral micas such as muscovite, the OH dipole oscillations lie at around 16° to the plane of the sheets, whereas in the trioctahedral micas, the O—4 bond is normal to the plane of the sheets. 

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