Zeolites electronic spectrum

Several reasons were proposed to explain the cleaner chemistry. First, the electrostatic fields (0.3 and 0.9 V for Na-Y and Ba-Y, respectively) in the zeolite stabilize the hydrocarbon-02 charge transfer, thereby shifting the transition to visible wavelengths. Second, the use of visible excitation suppresses the secondary photochemistry rampant with UV light. Third, the steric constraints imposed by the zeolite promote product selectivity. This chemistry can be demonstrated by the reaction of 2,3-dimethyl-2-butene (DMB). Figure 26a shows the electronic spectrum... 

The Effect of Adsorbed Molecules on the Spectrum of the Cu Cations in Zeolites. Figure 5 shows the change in the spectrum corresponding to the transitions between d-electron levels of Cu during dehydration of the zeolite. The spectrum of completely hydrated zeolite revealed a broad absorption band with a maximum at 12,100 cm" Thermal treatment of the zeolite at 100 °C resulted in the appearance of a new absorption band at approximately 15,500 cm" After vacuum treatment at high temperatures, there appeared in the spectrum an absorption band at 11,200 cm" The position of the absorption band due to Cu " in the spectrum of completely hydrated zeolite is close to that of the [Cu(H20)e] complex (12,600 cm" ) (2). This indicates that Cu " enters the hydrated zeolite structure as an octahedral hexaquo-complex. The same conclusion has been reached by other investigators 4, 18, 20) on the basis of e.s.r. spectroscopic measurements of the Cu " cations in completely hydrated zeolites. 

The recent EPR study by Kevan et al.[16] of cobalt substimted ALPO-5 illustrates the power of the EPR technique to characterise metal substituted zeolites. CoAPO-5 as synthesised is blue in colour the electronic spectrum is characteristic of tetrahedrally coordinated Co2+, suggesting that Co2+ has been incorporated into the AIPO4 lattice. The corresponding generation of Bronsted acid sites indicates that Co2 + substitutes for Al +.The material does however change colour to yellow-... 

As an illustration of the current state of the art for electronic spectroscopy of transition metal ions in zeolites, refer to the recent review by Schoonheydt of Cu2+ in different zeolites [56]. Schoonheydt shows that experimental measurement of diffuse reflectance spectra (and in the case of Cu2 + EPR spectra) must be combined with theoretical calculations if a complete interpretation is to be made. The exact frequencies of the d-d transitions in the electronic spectrum of Cu2+ are independent of the zeolite structure type, the Si Al ratio, and the co-exchanged cations, but depend solely on the local coordination environment. Figure 20 shows the diffuse reflectance spectrum of dehydrated Cu-chabazite the expanded portion reveals the three d-d transitions in the region around 15000 cm l. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

The spectrum of Mn2+ in zeolites has been used to study the bonding and cation sites in these crystalline materials. This is a 3d5 ion hence, one would expect a zero-field splitting effect. A detailed analysis of this system was carried out by Nicula et al. (170). When the symmetry of the environment is less than cubic, the resonance field for transitions other than those between the + and — electron spin states varies rapidly with orientation, and that portion of the spectrum is spread over several hundred gauss. The energies of the levels are given by the equation... 

The range of the gzz values is shown clearly by a comparison of the results for the NaY and NaX zeolites. Since the migration of Na+ ions is related to the presence of water (76), it is likely that the type of precursor (Na4)4+ -(H20)x complex formed after a proper degree of dehydration (278) will be strongly dependent on the pretreatment conditions. This will be reflected in the gzz values of the OJ produced during y irradiation by electron transfer from the precursor (278). It is also likely that the OJ can migrate after its formation as shown by Kasai and Bishop (264). These authors (272) have detected a superhyperfine interaction from Na nuclei (I = ) in the EPR spectrum of OJ formed in Na-reduced NaY zeolite and characterized by gzz = 2.113. This value is very close to those observed for alkalisuperoxides trapped in krypton matrices (Ref. 44, Appendix A). 

The electronic properties and location of the Cu ions in Type Y zeolite. (i) Hydrated CuIlY. The spectrum in Fig. 5 essentially agrees with earlier reported spectra of hydrated Cu Y zeolites (16, 17, 18), although some more structure is noted here. The near-infrared band system at 11000-15000 cm- has been discussed by de Wilde, Schoonheydt, and Uytterhoeven (18). Their spectrum of hydrated Cu Y is shown as a single banT at 12150 cm with width 5700 cm-, and for comparison they give analogous values for Cu in aqueous solution of 12300 cm- and 5000 cm-. ... 

The spectrum for LaY impregnated with vanadyl naphthenate shows a characteristic band at 365 nm that loses most of its intensity after calcination, Figs. 5a, 5b. This is not surprising since Pompe et al (30), using TGA/DTA data,have shown that the oxidative decomposition of the vanadyl naphthenate is complete at 500°C. Electron paramagnetic resonance (EPR) studies have shown that vanadium (after calcination) is stabilized mainly in the form of vanadyl (V02+) cations in the zeolite supercages (29). 

Direct observation of transition-state selectivity has been observed from the low-temperature cyclization of dienes inside H-mordenite and H-ZSM-5 (9). By using electron spin resonance (ESR) spectroscopy, it has been possible to explore radical formation upon the sorption of dienes on H-mordenite and H-ZSM-5. From the analysis obtained, it was found that the dienes are not very reactive for oligomerization inside H-mordenite channels. Heating H-mordenite with presorbed 1,4-pentadiene or 1,5-hexadiene yields selective cyclization of molecules via cycloalkenie radicals inside the H-mordenite channel. However, in the smaller pores of H-ZSM-5 (although die nature of both acid and redox sites in both zeolites are the same) no eyclo-olefinie radicals are formed as shown by the ESR spectrum. These experiments illustrate the reality of transition-state selectivity inside the pores of zeolites. 

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