Zeolite bands assignments

Figure 6 Typical IR spectra for a "structurally pure" H-Y zeolite. Band assignment Is given In text.

Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm 1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu+ and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex. 

They concluded that the infrared spectrum contained vibrational modes from both structure insensitive internal tetrahedra and structure sensitive external linkages. The exact frequency of these bands depends on the structure of the zeolite as well as its silicon to aluminum raho (Si/Al). A typical framework IR spectrum for a Y zeolite sample is shown in Figure 4.17. The accepted band assignments and frequency ranges are shown on the figure. 

Table 4.2 Zeolite vibrational band assignments as per Flanigen et al. [91].

Figure 13.20 IR spectra of tetramethylben- zeolite-only. Spectrum 13 tetramethylben-zene over H-form of Beta zeolite at 300°K and zene-only. The arrow points to the 1604cm 400°K. Spectrum 1 initial. Spectra 2-10 band assigned to the tetramethylbenzenium...

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. 

Ihe reaction of H2S with 0 over NaX zeolite causes the formation of different sulphate species. The IR spectrum of such sulphated zeolite shows new bands at 610, 705, 870, 1050 and -1140 cm" (Fig.l spectr.b). At the same reaction conditions oxidation of SD2 with O2 does not lead to the appearance of new IR bands, but in X-ray spectrum one can observe additional bands assigned to sodium suphate species. However after activation of NaX for Ih, when traces of water... 

Rgure 8 Si MAS-NMR spectrum of LZY-62 zeolite with Si/Al = 2JS. Band assignment is given in text. 

The coke formation in methanol to hydrocarbons conversion over zeolite H-MFI was also studied with UV-RS. To distinguish between the signals that correspond to CH deformations and to CC stretches, experiments were carried out with deuterated methanol (CD3OH). The bands assignments used in this study are summarized in Table 2. It is concluded that cyclopentadienyl species are intermediates in the formation of polyaromatic hydrocarbons. By comparison with pure polynuclear aromatics UV-RS spectra, it is suggested that coke... 

The observed bands were classified by Flanigen et al. [112] into two types, namely internal modes of the TO4 tetrahedra ( intra-tetrahedral modes, cf. Sect 5.2) and external modes ( inter-tetrahedral modes) of the zeolite fi-amework. Table 1 summarizes the zeolite infrared assignment according to the FKS correlation. The internal vibrations represent structure-insensitive modes, and no distinction has been made between the modes of Si04 and AIO4 tetrahedra. The bands of external modes were observed to be sensitive to the structure, and their positions in the spectra are shifted in dependence on the framework topology and on the nsi/n i ratio. Even though this classification proved to be very successful in many applications, from a theoretical point of view such a division into... 

The suggestions of band assignments to framework vibrations advanced in Refs. [112,114] were based on earlier IR investigations of silica and non-zeolitic aluminosilicate frameworks [238, 239]. These assignments are illustrated in Fig. 11. 

Fig. 16. (a) Extra-framework cation sites in X- and Y-type zeolites, (b) Far- infrared spectrum of Na-Y with band assignments to cation sites according to [232]. (c) Experimental IR spectrum in comparison to simulated spectra calculated according to the shell model and occupancy of different cation sites, (d) Experimental spectrum in comparison to power spectra simulated by MD at occupancy of different cation sites (parts c and d from [79] with permission)... 

The isomorphous substitution of tetrahedral A1 in zeolite structure with elements such as gallium, boron or iron has been described in a large number of papers. In the case of Fe for instance, the presence of Fe h jp the framework gives rise to new infrared bands assigned to Si-O-Fe bonds (8). Mossbauer spectroscopy, which is a very valuable tool in this case,... 

The IR spectrum of [Ali2Pi3052] includes bands due to AIO4 and AIO5 polyhedra, as well as PO4 modes7 UV-Raman spectra of aluminosilicate zeolites had bands assignable to bending modes of 4-, 5-, 6- and 8-membered aluminosilicate rings7 ... 

The variation in the lattice vibration of the solid products was examined by utilizing the FT-IR technique at successive DGC process times and the results are presented in Fig. 5. The absorption bands at 550 cm and 450 cm" are assigned to the vibration of the MFI-type zeolite and the internal vibration of tetrahedral inorganic atoms. The band 960 cm" has been assigned to the 0-Si stretching vibration associated with the incorporation of titanium species into silica lattice [4], This indicates that the amorphous wall of Ti-MCM-41 was transformed into the TS-1 structure. 

CoSx-MoSx/NaY exhibited doublet bands at 1867 and 1807 cm, accompanying a weak shoulder peak at ca. 1880 cm. These signals are apparently assigned to those of NO molecules adsorbed on Co sulfides. No peaks ascribable to e NO adsorption on Mo sulfide sites were detected at all. What is important in Fig.7 is that in CoSx-MoSx/NaY, coordinative unsaturation sites are present only on the Co sites in spite of the coexistence of the same amount of Mo sulfide species in the zeolite cavities. These results clearly support that the Co sites in CoSx-MoSx/NaY play major roles in the HYD and HDS reactions. 

Based on previous studies [15, 22-25], the band at 1941 cm-i is assigned to Co2+(NO), and the pair of bands at 1894 and 1815 cm-i, to Co2+(NO)2- The shoulders at 1874 and 1799 cm may be due to a second dinitrosyl species. While little is known about the location and coordination of the Co 2+ in ZSM-5, it is likely that cobalt ions are associated with both [Si-0-Al]- and [Al-0-Si-0-AI]2- structures in the zeolite. In the former case, the cobalt cations are assumed to be present as Co2+(OH-) cations and in the latter case as Co2+ cations. The presence of cobalt cations in different environments could account for the appearance of two sets of dinitrosyl bands. The band at 2132 cm-> is present not only on Co-ZSM-5 but also on H-ZSM-5 and Na-ZSM-5, and has been observed by several authors on Cu-ZSM-5 [26-28]. 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

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