Zeolite vibrational spectra

In summary, the direct quantum mechanical simulation of zeolite vibrational spectra is evidently a formidable task and is often severely hampered by limited computational resources. Pure ab initio methods are well-suited if local effects or groups with characteristic vibrational frequencies like Bronsted acidic OH groups are under study. In theoretical studies of vibrational spectra of zeolite frameworks and cations on extra-framework sites, QM calculations are of crucial importance in developing force field parameters which can be used in a subsequent step in MM, MD or NMA calculations. Due to the lack of sufficient exper-... 

Some Selected Examples of Modeling Zeolite Vibrational Spectra... 

UZM-5 samples show two distinct Si-OH-Al bands which are due to OH groups vibrating in the different size cages of these zeolites. Depending on the structure of the zeolite, the spectrum can have multiple Si-OH-Al vibrational bands and they can vary in position. 

The vibrational spectrum of the tetramethylammonium cation in the region 150 -550 cm l contains botii torsional and vibrational modes. The vg and V19 vibrational modes of E and T2 symmetry involve C-N-C bond angle bending. These modes are Raman active and have been studied for TMA+ in several zeolite environments, although little change in frequency is observed (51). The V4 and V12 torsional modes involve partial rotation about C - N bonds and form respectively a singlet (A2) and a triplet (Ti) which are both Raman inactive. These torsional modes are directly observed in the HNS spectra and prove to be sensitive to the character of the TMA+ cation (see Table 1) environment(52). 

Figure 12.16 EfTeci of dealumtnaiton on ihe vibration spectrum of a faujasite (O.S mg of zeolite in 300 mg of KBr>.

Vibrations of water in natrolite have been studied by Demontis et al.279 They were unable to accurately reproduce the vibrational spectrum and attributed their poor results to the influence of the zeolite s electric field on the water molecules. The authors later developed an electric field dependent force field for water2 to improve the simulation capabilities for hydrated zeolites. Water in ferrierite has also been studied in great detail by Leherte and coworkers234,257-262 using an ion pair potential. Finally, water complexes in cation-exchanged zeolites were investigated by Dil mukhambetov et al.28i... 

Another common method relies on the cluster approximation to study special sites in crystalline solids. This technique has been extensively employed to study acid catalyst sites in zeolites (e.g., Ba rtsch et al. 1994 Saer et al. 1999) and by Catlow and co-worker to study adsorption of organic molecules in zeolites (e.g.. Gale et al. 1993). In these types of studies, researchers are interested in particular vibrational modes observed in crystalline solids, so it is not necessary to compute the entire vibrational spectrum of a material. Hence, the cluster approximation is justified provided the cluster model is large enough to account for the solid environment surrounding the species of interest (see discussion above on gibbsite clusters). 

Infrared spectroscopy has proven to be a very informative and powerful technique for the characterization of zeolitic materials. Most infrared spectrometers measure the absorption of radiation in the mid-infrared region of the electromagnetic spectrum (4000-400 cm or 2.5-25 xm). In this region of the spectrum, absorption is due to various vibrational modes in the sample. Analysis of these vibrational absorption bands provides information about the chemical species present. This includes information about the structure of the zeolite as well as other functional... 

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. 

Infrared spectra of zeolite L in the range of frequencies of valence and deformation vibrations of water have not been studied much (8). Although we elucidate the bonds of water molecules with the frame, the nature of the residual water is of interest (23). Our studies of zeolite L (sample A) show that its spectrum in the range of frame vibration frequencies coincides with the data of Ref. 9. 

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.

The hydronium exchanged synthetic mordenite does have a band, as shown in spectrum 10 in Figure 1. The iron substituted mordenite samples (spectra 8 and 9) do not show the presence of the band it was "removed" as a result of the substitution reaction. An absorption band at 950 cm- is normally attributed to an Si-OH stretch vibration (14, 15), and is typically observed in some acid or hydrothermally treated zeolites. 

Typically, the UV Raman spectra of various hydrocarbons adsorbed in zeolites have been found to be similar to their spectra in solution, as a pure liquid, or as a pure solid (25). This is an important finding because the UV Raman spectra of free molecules (which are relatively quick and easy to measure) can be used for fingerprint identification of adsorbed species. One minor exception to this rule is the Raman spectrum of naphthalene, which shows some changes in the pattern of peak intensities between solid naphthalene and naphthalene adsorbed in ultrastable Y-zeolite. In this case, the adsorbed naphthalene spectrum more closely resembles that of the molecule in solution with benzene or CCI4, which suggests that interaction with the pore walls of the zeolite was similar to solvent interactions. The smaller pore diameters and pore intersections in zeolite MFI compared to Y-zeolite might be expected to produce more pronounced changes in molecular vibrational spectra as a consequence of steric interactions of the molecules with the pore walls. 

As shown in Fig. 37, the VS-2 catalyst degassed at 473 K exhibits a characteristic photoluminescence spectrum at about 450-550 nm with a vibrational fine structure attributed to the V=O bonds when the absorption band is excited at about 280 nm (202). As shown in Fig. 47, the addition of NO to the VS-2 catalyst leads to an efficient quenching of the photoluminescence in intensity and in lifetime, with the extent of both depending on the pressure of NO. These quenching results clearly indicate that the added NO molecules can approach the vanadium oxide sites located in the zeolite framework and that NO interacts readily with the vanadium oxide species in their excited state. 

The internal vibration modes of TO4 and more specifically the asymmetrical and symmetrical valence vibrations (1150-900 cm and 720-900 cm ) have a similar shape for all zeolites. The external vibration modes, however, which involve sequences of tetrahedra, arc characteristic of a given type of zeolite. Thus, the shape of the spectrum in the region 500-650 cm can be used to differentiate the various structures (Fig. 12.13). This highly empirical approach is sufficient in the majority of cases to distinguish two different structures from each other. 

The sensitivity of lattice modes to structural changes is illustrated by the recent study of Mueller and Connor [25] on the effects of cyclohexane adsorption on the structure of MFI zeolites. The adsorption of molecules such as paraxylene and benzene into MFI zeolites causes a structural transition from monoclinic to orthorhombic symmetry, which has been characterized by X-ray powder diffraction and 29 si NMR spectroscopy [26]. Cyclohexane has a slightly larger kinetic diameter than benzene or paraxylene (0.60 nm compared with 0.585nm), but does not cause the same structural transition. Cyclohexane adsorption does however affect the zeolite lattice mode vibrational frequencies. Figure 7 shows spectra taken from reference 25 before and after (upper spectrum) adsorption of cyclohexane in a low aluminium MFI zeolite. 

The infrared spectrum of adsorbed nitrogen can also be used to probe cation sites in zeolites. Zecchina et al [34] compared vibrational frequencies of CO and N2 adsorbed at low temperatures in mordenite containing different alkali metal cations. In both cases the vibrational frequencies could be correlated with (Rx + Rm) > where Rx is the cation radius and Rm the radius of the adsorbed molecule, suggesting a simple electrostatic field explanation for the frequency shifts between different cations. The appearance of a band due to N2 interacting with a particular zeolite cation will also mean that that particular cation is located in sites accessible to the N2 molecule. 

The frequencies calculated are similar even though the results of Refs. 97 and 98 were obtained with a GVFF model in contrast to a shell model used by Iyer and Singer. The vibration might be related to the band at 489 cm in the experimental Raman spectrum of zeolite... 

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