Zeolite Raman spectra

Samples of natural zeolite, clinoptilolite, both with and without pore-incorporated Cu clusters were investigated by Raman spectroscopy and Diffuse Reflectance Spectroscopy (DRS). The reduction of Cu cluster incorporated in zeolite pores was carried out by heating of samples in dry H2 flow at temperatures from 150 to 450°C for 4 hrs. The comparison of Raman and DRS spectra of all samples evidences the essential role of incorporated in pores and reduced Cu clusters for registration of zeolites Raman spectra under low-intensive (mW/cm ) visible excitation. Possible chemical enhancement mechanisms of Raman scattering cross section for SiO atomic groups in close proximity to self-assembling Cu clusters are discussed. 

There have been some investigations into adsorption on zeolites (1, 2), and Greenler and Slager (3) have outlined a method for obtaining the Raman spectrum of a thin solid film deposited on a reflecting silver surface. 

The most significant changes associated with adsorption which have been observed to date were the displacements (45) of Raman fundamentals of ethyne on adsorption on zeolite 4A (see Table IX). Such changes constitute a useful monitor of adsorbate-adsorbent interaction for various adsorbents. The appearance of the Raman spectrum of ethyne on zeolites A suggests an... 

Figure 3. Raman spectrum of pyridine in the region of the v, (991—1016 cm1) and vie (1030-1036 cm 1) fundamentals (A) liquid pyridine, (B) pyridine adsorbed on a NaX zeolite, and (C) pyridine adsorbed on a Zri exchanged (78% exchange) NaX zeolite. The vertical bar represents (A) 2000 Hz, (B) 100 Hz, (C) 50 Hz.

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. 

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... 

Si-O-Al angle in the zeolite structure. It appears that as the Si-O-Al angle increases, the frequency of the bending mode decreases for zeolites built up of four-membered rings. Thus, the Raman spectrum is sensitive to the arrangement of the Si and A1 atoms and the coupling between them. 

One more advantage of Raman spectroscopy is due to the fact that the Raman spectrum of water exhibits only a few signals of low intensity. Thus, careful dehydration of zeolites, which is crucial in many IR experiments, does not play the... 

The adsorption of O2 on iron supported on zeolite MFl produced an IR band at 730 cm (698 cm in 02) assigned to the bridging peroxo-species Fe(02)Fe. " The resonance Raman spectrum of a peroxide intermediate derived from iron diazacyclononane includes vOO of an iron(III) peroxide complex at 854 cm (consistent with side-on geometry). ... 

Due to a splitting ascertained from the Raman spectrum for the vn CHi wagging mode of cyclopropane in zeolite NaX Cooney et al. inferred an edge-on arrangement [126]. As in the case of zeolites A a splitting of the P9 CHi deformation mode was not observed, a face-on configuration was favoured, which was supported by inelastic neutron scattering experiments on zeolites CoNaA and MnNaA [127]. 

Fig. 4.12 (a) HRTEiM image of as-prepared -Co(OH)2 supported on zeolite Y. (b) Photocatalytic O2 evolutirm using as-prepared and recycled -Co(OH)2/zeolite Y. (c) Raman spectrum of the reused / -Co(OH)2/zeolite Y (Reprinted with permission from Ref. [51] Copyright 2014, American Chemical Society). (d)TEM images of TSl with 1,5,10, and 20 wt% of graphene (Reprinted with permissimi liom Ref. [52] Copyright 2011, The Royal Society of Chemistry), (e) Scheme of the ene reaction and cycloaddition reaction... 

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. 

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).

The formation of peroxide and superoxide on Fe,H/MFI compared with Fe/MFI also shows two distinguishing features. First, the amount of peroxide on Fe,H/MFI at room temperature is significantly greater than on Fe/MFI, as determined by the peroxide peak intensity relative to the intensities of the zeolite bands (52). Second, on Fe,H/MFI, peroxide is converted to superoxide when the sample temperature is lowered to 93 K, and then restored when the temperature is returned to 300 K. Figure 8 shows an overlay of Raman spectra characterizing Fe,H/MFI measured at 300 and at 93 K using 2. The band at 703 cm ( 02 ) decreases at 93 K relative to its intensity at 300 K, whereas the intensity near 1090cm ( Oj) shows the opposite behavior with temperature. In the spectrum of Fe/MFI, the relative peak intensities of peroxide and superoxide remain constant with temperature between 93 and 300 K. (A specialized variable-temperature fluidized-bed Raman cell was constructed for these experiments.)... 

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). 

The reaction of water with low-loaded [Ru(bpy)3] + entrapped in zeolite Y has been reported [152]. Since translational mobility of the Ru molecules cannot occur in the zeolite, the multimolecular degradation step observed in solution is no longer possible. Instead, O2 was found to be formed from the reaction of [Ru(bpy)3] with water. It was possible to examine the evolution of this reaction at various pHs by spectroscopic methods, such as EPR, diffuse reflectance and Raman spectroscopy. Figure 30 shows the evolution of the diffuse reflectance spectra after exposure of Ru(bpy)3 +-zeolite Y to water at pH 7 [152]. Trace e is the spectrum of the... 

This review is concerned primarily with optical spectroscopic methods for characterising zeolites and molecules adsorbed in zeolites. The electromagnetic spectrum spans the range from radiofrequencies to X-radiation. Spectroscopic techniques included in this range are, in order of increasing frequency, NMR, EPR, infrared, UV-VIS and Raman, XPS, XAS and Mossbauer spectroscopies. 

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