Zeolite combination bands

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.

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-. ... 

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... 

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

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). 

Similar (Vqh + Sqh) combination bands have been detected for silanol groups in zeolites in the 4573-4540 cm region (183,312,411). 

IR diffuse reflectance spectroscopy seems to be the more appropriate method for observing OH stretch frequencies as well as their overtones and combination bands in the near infrared, especially with high-silica zeolites, i.e. at low hydroxyl concentrations, thus acquiring additional information on the shape of the potential curve [17,26,27]. In these papers an attempt is made to draw a unifying picture of the coinciding OH frequencies of silica-rich zeolites such as mordenites and ZSM type zeolites and the different and more complicated spectra of... 

Combination of IR spectroscopy and DFT calculations provides evidence that heterogeneous dual cation sites can be formed in zeolites. Bridged carbonyl complexes can be formed whenever two metal cations are at the right distance apart from each other and give rise to a low energy CO stretching band in IR spectra. 

Infrared spectroscopy can be used to obtain a great deal of information about zeolitic materials. As mentioned earlier, analysis of the resulting absorbance bands can be used to get information about the structure of the zeolite and other functional groups present due to the synthesis and subsequent treatments. In addition, infrared spectroscopy can be combined with adsorption of weak acid and base probe molecules to obtain information about the acidity and basicity of the material. Other probe molecules such as carbon monoxide and nitric oxide can be used to get information about the oxidation state, dispersion and location of metals on metal-loaded zeolites. 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

The much greater combined intensity of these bands versus that of the silanol stretch at 3745 cm-1 must be due in part to the low silicon-to-aluminum ratio for the H-offretite ( 4). However, when making such comparisons it is important to keep in mind that the silanol peak intensity can be as much a function of the surface area-to-volume ratio of the zeolite crystallites as it is of the silicon-to-aluminum ratio (.13). ... 

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. 

A careful X-band Al HYSCORE and W-band H ENDOR analysis showed that from the three Cu species found in Cu-containing Si Al zeolite Y (Si Al = 12 and 5), only one Cu was bound to the framework oxygens [139]. The other species consisted of a copper ion with a complete coordination sphere of water and no direct bonding with the zeolite framework. In a similar way, combined CW-EPR and Al HYSCORE provided evidence of the interaction of Cu with the framework in copper-doped nanoporous calcium aluminate (mayenite) [140]. In mayenite, the positively charged calcium aluminate framework is counter-balanced by extra-lattice O ions. Such free oxide ions are responsible for the ion conductivity of the materials and are readily replaced by various guest anions, such as O2 and OH. A native O2 species could indeed be identihed with EPR in the Cu-doped mayenite materials [140]. 

Attempts have been made to correlate this behavior with the ring sizes and average T-O-T angles present in zeolites of different topologies [300]. However, the observed frequency dependencies were not fairly uniform. In particular, for A-type zeolites a trend reversed to that for faujasites has been observed by changing the nsi/n i ratio systematically [290]. Hence, the frequency shifts obviously arise from different effects of structural changes, altered nature of normal modes, and electron density distribution and from combinations of these effects. Instead of the band shift observed for aliunimun-rich samples, for highly dealuminated faujasites the band width of the most prominent peak was discovered to reflect the aliuninum content at low levels [295]. 

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). 

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