Silica infrared spectra

On silica, infrared spectra 146, 141 > 161) have shown clearly that two general sites of adsorption exist. Type I comprises the OH sites that remain firmly bound to the surface even after long times of evacuation at 450° C., and the other (type II) is the oxygen (or possibly the silicon) atoms in the surface. It is probable that the surface of most oxides consists of oxygen, but little is known in detail about this. Models of the arrangement of the OH groups have been suggested 152,153). 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

An aerosil sample was methoxylated at 400 C to examine the effect of surface composition on the infrared spectrum. The difference spectrum between the methoxylated silica and the dried silica is shown in Figure 2b. Comparing this with the difference spectrum for hydroxylated silica (2a) several changes are apparent. First, the band due to the hydroxyl stretches at 3744 cm is diminished and replaced by bands at 2958 and 2856 cm" due to the asymmetric and symmetric CH stretching modes of the adsorbed methoxy. 

Fig. 8. Infrared spectrum of silica measured at room temperature (a) undried silica (b) silica dried at 200°C for 3 hr (16).

Fig. 9. Reaction of silica dried at 200°C with Zrfallylh. Infrared spectrum measured at room temperature,---SiCb,----Si02/Zr(allyl)i (16).

Figure 8.9 Diffuse reflectance infrared spectrum of a silica support, showing silica vibrations at frequencies below 1300 cm1, overtones and combination bands between 1700 and 2050 cm-1, and various hydroxyl groups at frequencies above 3000 cm 1. The sharp peak at 3740 cm"1 is due to isolated OH groups, the band around 3550 cm 1 to paired, H-bonded OH groups, and the band around 3660 cm 1 to hydroxyls inside the silica (courtesy of R.M. van Hardeveld, Eindhoven).

Figure 2.18 Fourier transform infrared spectrum of sol-gel film composed of three layers and containing 10% vancomycin after 14 days of immersion in phosphate-buffered saline (weight measurements and release study showed 90% weight loss and 80% release). Intense silica bands and the bands associated with vancomycin (centred at 1660,1500 and 1397 cm-1) suggest the presence of a sol-gel film with vancomycin that remains after 14 days of immersion. (Reproduced from ref. 13, with permission.)...

Fig. 14. (A) Infrared spectrum of CO chemisorbed on silica-supported platinum (B) after treatment with hydrogen. [Eischens, R. P., J. Chem. Educ. 36, 385 (1968).)...

The oily liquid was purified by column chromatography (silica gel, 80 mesh eluant, petroleum ether) and distilled. A colorless oily liquid (b.p., 123°-125°C./0.5 mm. Hg, nD20 1.5530) was obtained. The infrared spectrum was that of 3-cyclohexylcyclohexene. 

The loss on ignition for the SC-155 material was 26.34 %, while the value for AC-ref was 96.80 % then the composite showed 3.67 times less carbon than the reference. Infrared spectrum of SC-155 showed exclusively the characteristic bands corresponding to silica. 

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 methoxylation can be carried out by reacting silica with methanol vapor at 300-400°C, or by refluxing silica in methanol (21,36). Because the infrared spectrum of the modified surface is well understood (36) we chose to use this system as a model to test the feasibility of using Raman spectroscopy (21 ) for studying such surface modification procedures. 

The infrared spectrum provides information on the presence and physical interactions of functional groups. Complementary information on the chemical environment and coordination of the Si and C atoms in the coating can be obtained from CP MAS NMR.4 19,20,21 22 23 24 Thus a combination of both techniques allows a clear description of the physical and chemical interactions and coordination of silanes on silica. 

The reaction with C02, as reported for the dried aminosilane polymer also occurs with immobilized aminosilane molecules. Culler43 reported that approximately half of the amine groups are reacting with C02 when silica samples modified with APTS in aqueous solution, are dried in air. Comparison with AEAPTS and a triaminosilane showed that only primary amines react with C02. The reaction product is evidenced by FTIR bands at 1630, 1575, 1488 and 1332 cm 1. Also after modification in dry conditions and drying at room temperature in humid air, the reaction with C02 may be observed. Characteristic infrared bands appear upon modification at high concentrations of APTS. Figure 9.32 shows the infrared spectrum of dry silica, modified with a 10% APTS/toluene solution, after air drying for 30 minutes. 

This could explain the statement of some authors that chlorosilanes react exclusively with free silanols. Most of cited studies described the chlorosilylation of a silica gel, pretreated at 673 K. After the reaction, the infrared spectrum indeed shows only a... 

Figure 12.2 shows the infrared spectrum of a Kieselgel 60, thermally pretreated at 973 K, reacted with ammonia at 373 K for 3 h and subsequently degassed at reaction temperature. A very small band around 3419 cm"1 can be observed, proving that the remaining ammonia on the silica surface is mainly physisorbed, and not chemisorbed. 

In the low reaction temperature region (273 - 423 K), there is at first sight no reactivity difference between the silylated and the boranated sample. In both cases, an equal amount of -NH2 species and NH4C1 species is formed. This is confirmed by the infrared spectrum of trichloroboranated silica, ammoniated at room temperature (figure 12.26). This spectrum is very similar to the spectrum of the ammoniated silica (figure 12.6), suggesting an identical reaction mechanism ... 

Figure 12.26 Infrared spectrum of trichloroboranated silica, ammoniated at room temperature.

Spectra of Xe-PtFt adducts.— The infrared spectrum of material deposited on silver chloride windows in a nickel-bodied gas cell was recorded. The composition of the adduct was Xe(PtF6)i,72. Only two peaks in the region 400-4000 cm. were assignable to the adduct 652 vs, 550 s. cm. h The visible and ultraviolet spectrum of material deposited on the windows of a silica gas cell was recorded. A single peak at 3825 A. was observed. The material absorbed strongly beyond 4000 A. No differences in the absorption spectra were noted for several separate adduct samples. 

The nonlocalized benzene adsorption on silica aerogel has been also inferred from the infrared spectrum of the surface silanol groups 43). It appears that the adsorbed molecules are executing a two-dimensional translatory motion with a free rotation in the molecular plane. The large bandwidth, mentioned above cannot be, however, ascribed to the mobility of the adsorbed molecule, since it is not appreciably affected by a lowering of the temperature down to 77 K, the shift of the spectrum being additionally increased by <30 cm. ... 

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