Raman spectra of zeolites

The variation of the far-infrared and Raman spectra of zeolites, induced by the exchange of cations, clearly indicates that the cationic degrees of freedom manifest themselves in that frequency region where large-amplitude deforma-... 

J. Turkevich (Princeton University, Princeton, N. J. 08540) Beuch-ler and I have also had difficulty in observing laser Raman spectra of zeolites and finely divided aluminas. It seems that the ease of obtaining Raman spectra increases markedly as one goes down the periodic table. Thus, finely divided molybdenum oxide or uranium oxide are easy to observe. 

In the development of zeolite science, infrared spectroscopy has been one of the major tools for structure and reactivity characterization. However, the field of zeolite Raman spectroscopy is gaining importance. The Raman effect is an intrinsically weak phenomenon, and Raman spectra of zeolites are often obscured by a broad fluorescence. Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore, Raman spectroscopy has been used to examine zeolite synthesis mixtures with ex-situ methods (with separation of solid and liquid) and in-situ methods. In this work we give an overview of the zeolite framework vibrations, zeolite synthesis, adsorption on zeolites and metal substitution and ion exchange in zeolites. 

The spectra were taken from the hydrated samples without any chemical treatment. For excitation an Nd-YAG laser was used at 1064 nm with a laser power of 450 mW. As a general feature of all Raman spectra of zeolites, the stretching modes above 600 cm are of lower intensities and give less structured bands than the bending modes in the range below 600 cm k As in the corresponding infrared... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Angell (1) has investigated the Raman spectra of acetonitrile, propylene, and acrolein on a number of zeolites and found that physical adsorption occurred. There are sufficient differences between the spectrum of the liquid and of the adsorbed species (e.g. the carbon-carbon double bond stretching in the case of propylene and the carbon-nitrogen triple bond stretching in the case of acetonitrile) to make it quite clear that it was not merely a case of condensation in the pores of the solid adsorbent. 

The crystallization of zeolites from alkaline aluminosilicate gels was studied by luminescence and Raman spectroscopy. Trace amounts of Fe3+ ions substituted for Al3+in the tetrahedral aluminosilicate gel framework exhibit characteristic phosphorescence spectra, which have been used to follow the buildup of the zeolite framework. Phosphorescence spectra of exchanged Eui+ cations and Raman spectra of (CH N+ cations present in the solid phase of the gel indicate that no zeolitic cages exist in this phase during the induction period. Raman spectra of the liquid phase of the gel system show only the presence of Si02-(0H)2 and Al(OH)a anions. Our results demonstrate that crystallization of zeolites occurs within the solid phase of the gel, which is believed to consist of amorphous tetrahedral alumino-... 

Figures 4.24 and 4.25 show the FTIR spectra and FT Raman spectra of two samples, that is, a mesoporous molecular sieve (MMS) and a Ni-Y zeolite where aniline was incorporated and polymerized [67],...

FIGURE 4.25 FT Raman spectra of the adducts polyaniline-hosts in KBr pellets (a) MCM-41 MMS and (b) Ni-Y zeolite. 

Fig. 2. Spectra of zeolite H-MFI showing the effects of excitation wavelength and calcination on the masking of Raman spectral features by background fluorescence (24).

Fig. 6. Raman spectra of (a) liquid benzene and (b—d) benzene adsorbed in an ultrastable Y-zeolite. In spectra (b) and (c) the zeolite powder was pressed into a pellet. Spectrum (d) demonstrates the absence of benzene decomposition under the ultraviolet laser beam in the fluidized-bed reactor (25).

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. 

For the most part, the UV Raman spectra of adsorbed molecules are similar to those of their free-molecule counterparts. A significant exception, reported here, is that of benzene adsorbed in silicalite (the all-silica form of zeolite MFI) (56). 

Fig. 11. UV Raman spectra of coke formed during the methanol-to-hydrocarbons reaction catalyzed by zeolite H-MFI and during propane dehydrogenation catalyzed by chromia supported on alumina (66).

The NMR and XPS techniques are discussed elsewhere in Chapter 4 (B and C respectively). This review will give an overview of recent advances in EPR, infrared, Raman, UV-VIS and X-ray absorption spectroscopies as applied to zeolitic materials. Brief mention will also be made of new techniques recently reported for obtaining mass spectra of zeolites and adsorbed species. 

Figure 17. Comparison of calculated and experimental Raman spectra of siliceous zeolites.(a) sodalite (calculated), (b) zeolite A (experimental), (c) zeolite A (calculated), (d) FAU (calculated), (e) FAU (experimental), (f) silicalite (calculated).Reproduced with permission from reference 51.

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