Mossbauer zeolite

Nature of active sites in the Fe-TON Zeolites Mossbauer and IR studies... 

XL30). Mossbauer spectroscopy (KFKIj was applied to follow the state of Fe species in the zeolites. Carbon monoxide and ammonia adsorption (monitored with FTIR) (EQUINOX 55) was used to determine the nature, concentration and acid strength of the active sites in the Fe-TON zeolites. 

Fe Mossbauer study of iron distribution in zeolite A during zeolite crystallization process... 

Keywords Fe-exchanged zeolite A, 57Fe Mossbauer spectroscopy, hydrothermal crystallization... 

An iron-exchanged mordenite was also studied by Meisel et al. (182), who incorporated Fe3+ into the zeolite structure. Upon calcination at temperatures greater than 500 K the appearance of Fe2+ was noted in the Mdss-bauer spectrum, and for calcination temperatures higher than 770 K the formation of a-Fe203 was observed to take place inside the mordenite. For the iron- mordenite system, it can now be seen that the Mossbauer effect provides information about the chemical state, symmetry, interaction strength with the support, and location on the support of the resonant iron ions. This information enhances the understanding of the catalytic activity of this and other zeolites (178). 

Figure 7.1 Mossbauer spectra of FeZSM-5 zeolite (0.31 wt% Fe) after various types of activation.

Spectral bands in the 500-575 nm region are quenched when the LaY zeolite is loaded with 1% tin, suggesting that tin addition minimizes V205 formation. Mossbauer studies have indicated that tin is in the +4 state after calcination (26-31) the reduction in intensity seen when comparing Figures 6a and 6b could be due to the formation of V+5-0-Sn+4 linkages (26). 

Mossbauer spectrometry is a powerful means for the elucidation of the state of iron in materials [44,138-142,145]. Figure 4.63 [44] shows the 57Fe Mossbauer spectra of the natural zeolite rocks, such as MP, C2, Cl, and C4 (see Table 4.1). In Table 4.12, the Mossbauer parameters calculated with the help of the numerical resolution of the spectra presented in Figure 4.63 are reported. That is, with the help of the recorded spectra, the accurate peak positions, integrated intensities, as well as the FWHM of each peak were calculated. This calculation was carried out by fitting the spectra with three quadrupole doublets one for site 1, another for site 2, and a last one for site 3 [44], The peaks were simulated with Gaussian functions and the fitting process for the numerical resolution of the spectra was carried was carried out with a peak separation and analysis software, developed for this purpose [44,145] based on a least square procedure [48],... 

This method is applied for the determination of the amount of magnetite present in the natural zeolite rocks C2 and C4 (see Table 4.1). The magnetite present in this zeolite was magnetically separated from the grounded samples and identified by XRD and Mossbauer spectrometry [44],... 

It was shown (Ovanesyan et al., 2000) that iron complexes formed during the thermal treatment of FeZSM-5 zeolite perform single-turnover cycles of methane oxidation to methanol at ambient conditions when nitrous oxide is used as a source of oxygen. The long-living active intermediate is capable of transferring an accepted O atom into a C-H bond of methane to produce methanol at 100% selectivity. On the basis of joint Mossbauer and catalytic data, the structure and composition of iron active centers are suggested. 

The Mossbauer spectrum of ferrous Y-zeolite is somewhat similar to that of the reduced silica gel samples (103). The spectrum consists of two overlapping and partially resolved doublets with the inner doublet, 3 = 0.89 mm sec-1 and A = 0.62 mm sec-1, being attributed to the ferrous ion on the surface. In both the Y-zeolite and the reduced iron oxide on silica samples, the inner doublets representing surface ferrous states are the first to be affected by adsorption of polar molecules, but in the case of Y-zeolite the addition of excess amounts of water or ammonia causes the disappearance of the spectrum, and this has been interpreted in terms of "solvation of the ferrous ions by absorbate causing weakening of the bonding to the crystalline lattice. It is also possible that the spectrum is a composite representing a multiplicity of parameters. 

Mossbauer studies of the impregnation of silica with ruthenium chloride solution and subsequently dried at 383 K have reported (59) the presence of a ruthenium surface complex resembling RuC13 xH20. Recent work (128) has shown that Mossbauer spectra of "Ru supported on alumina, silica, activated charcoal, and X- and Y-zeolite are sensitive to the nature of the preparation and treatment of the samples. 

Supported non-framework elements, as well as substituted or doped framework atoms, have been important for zeolite catalyst regeneration. By incorporating metal atoms into a microporous crystalline framework, a local transition state selectivity can be built into the active site of a catalytic process that is not readily attainable in homogeneous catalysis. The use of zeolites for carrying out catalysis with supported transition metal atoms as active sites is just beginning. The local environment of transition metal elements as a function of reaction parameters is being defined by in situ Mossbauer spectroscopy, electron spin echo measurements, EXAFS, and other novel spectroscopic techniques. This research is described in the second part of this text. 

As time moved on, the Mossbauer studies of iron-containing zeolites started to involve different aspects of zeolite chemistry, most notably in the area of catalysis. In addition, other new preparations were revealed as well as experiments that were designed to obtain superparamagnetic iron oxide particles in zeolites. Garten, Gallard-Nechschein and Boudart (5) studied iron mordenites in the reverse of the water-gas shift reaction and the ammoxidation of propylene. Collins and Mulay (6) reported that Fe(C0)5 and FeClj when thermally treated in air would form fine particles of Fe2<)3. 

Other recent reports of the use of Mossbauer in the characterization of iron-containing zeolites involve the study of coordination complexes synthesized in zeolites. One such study carried out by Lunsford and coworkers (12) involved the study of tris(2,2 bipyridine) iron(II) complexes In zeolite Y. These materials were reacted with chlorine gas in order to oxidize the iron. Another study done by Banerjee (13) involved the Mossbauer analysis of lron(II)phenanthrollne complexes sorbed on zeolite 3A. These are most of the Mossbauer studies of iron-containing zeolites that we have found, although some earlier work has not been Included since reviews (14) are available. 

Photochemical activation (15) and thermal activation (11,16, 17) of iron carbonyl complexes In various zeolites have been reported. Part of our study Is to use Mossbauer spectroscopy to Investigate the behavior of Fe(C0)5 on several zeolites when activated photochemically and thermally. Another part of our study Is to Investigate the novel preparation method of Scherzer and Fort (18) that Introduces iron Into (in their study) zeolite NH Y as an anionic complex. Finally, we will report the preparation of ferrocene sublimed onto zeolite ZSM-5. The photochemical and thermal activation of these systems will be reported as well as preliminary results of the photochemical isomerization of olefins by Fe(C0)5 zeolites and the thermal activation of Fischer-Tropsch catalytic systems. It also should be noted here that our Mossbauer studies involve an in-situ pretreatment cell which can be heated to 500°C under various gaseous atmospheres. 

The Fe(C0)5, (C Hj Fe and mixed metal zeolites were reduced in hydrogen in our in-situ Mossbauer cell between temperatures of 300°C and 500°C for various times, 4 hours to 25 hours, depending on the form of iron introduced into the zeolites and depending on the particular zeolite. 

A Mossbauer transmission cell similar to that prepared by Delgass and coworkers (20) was used for all Mossbauer experiments. Zeolite pellets between 200 mg and 300 mg were used as samples. 

The Mossbauer spectra for Fe(C0)5 on zeolites A, X, Y, mordenite (Na+ and IT"), and ZSM-5(Na+) are all very similar with a quadrupole split doublet appearing at an isomer shift of 0.A0 mm/sec. The H" " form of ZSM-5, however, shows a broad singlet centered at 0.A0 mm/second. The quadrupole splitting for each of the iron carbonyl samples is between 0.8 mm/second and 1 mm/second. 

The ferrocene-exchanged ZSM-5 zeolite shows a Mossbauer spectrum that when analyzed gives a 40% ferrocinium ion signal with an isomer shift of 0.53 mm/second. It is likely that water in the zeolite is being reduced as the ferrocene is oxidized. Reduction of this sample in hydrogen causes a complete loss of both the ferrocinium ion and the ferrocene Mossbauer features and a new appearance of alpha iron(0) metal and a new quadrupole split doublet of 0.4 mm/second and a quadrupole splitting of... 

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