Zeolite vibrations

K. A. Iyer and S. J. Singer, /. Phys. Chem., 98, 12679 (1994). Local Mode Analysis of Complex Zeolite Vibrations Zeolite-A. 

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. 

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

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. 

Broclawik, E., Datka, J., Gil, B. et al. (2000) T-O-T skeletal vibration in CuZSM-5 zeolite IR study and quantum chemical modeling, Phys. Chem. Chem. Phys., 2, 401. 

IR and Raman spectroscopy can fulfill these requirements and they are also robust enough for in situ silicate analysis in plant reactors [7]. Both of these techniques have been used for identifying the symmetric (s) and asymmetric (as), stretching (va, vas) and bending (5a, 5as) O-Si-O vibrations in aqueous alkaline silicate solutions which are the cheapest hence most frequently used ingredients for zeolite synthesis [8, 9 and references herein]. However, this information has to be "translated" into siloxane ring... 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

Diffraction patterns and FTIR spectra of skeletal vibrations of the ZSM-5 and ferrierite zeolites indicated high crystallinity of the analyzed samples. The strong band with a chemical shift of about 55 ppm in the 27Al MAS NMR spectra of hydrated zeolites indicated the presence of more than 97 % Al in the framework in tetrahedral coordination the very low intensity of the peak at 0 ppm indicated less than 3 % rel. of Al in octahedral coordination. 

It is known [4] that methylacetylene can be adsorbed dissociatively or not. The dissociation of methylacetylene is characterized by the appearance of a typical v(OH) vibration. Therefore, the absence of any zeolitic v(OH) band for the different LSX samples indicates that methylacetylene adsorbs mainly without dissociation The presence of non-dissociated adsorbed methylacetylene is also evidenced by the detection of specific v(C=C) and v(=CH) vibrations band. As expected, their wavenumbers decrease with increasing basicity. Moreover, the complex shape of the v(=CH) band reveals different environments of basic sites. 

Vibrational dynamics of small molecules adsorbed on cation sites in zeolite channel systems IR and DFT investigation... 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Interaction energies, geometrical parameters and shifts in vibrational frequencies for various AN and CO adsorption complexes found in zeolites Na-A and Na-FER. 

Zeolite samples (NaY. Na-mordenite and Na-ZSM-5) were prepared in Research Institute for Petroleum and Hydrocarbon Gases in Bratislava. A mesoporous alumina, the carrier for reforming catalyst was used. Porosity of pure mesoporous alumina evaluated by t-plot method did not show the presence of micropores within the range of accuracy of 0.001 cm3/g. Mixtures of zeolites with mesoporous alumina were prepared on the base of dried samples in 5% steps. The prepared mixtures of alumina with zeolite were homogenized in vibration mill. 

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