Redox Catalysis in Zeolites - ACS Symposium Series (ACS Publications)

Chapter 6

Redox Catalysis in Zeolites 1

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J. A. Dumesic and W. S. Millman 1

Laboratory for Surface Studies, University of Wisconsin, P.O. Box 413, Milwaukee, WI 53201 Department of Chemical Engineering, University of Wisconsin, 1415 Johnson Drive, Madison, WI 53706

Downloaded by UNIV OF MINNESOTA on May 5, 2013 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch006

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The active sites for redox reactions carried out using zeolitic catalysts are the exchangeable cations introduced into the zeolite matrix. These cations may occupy a number of different exchange sites, and in general, the catalytic activity of a cation depends upon its site location. This leads to the possibility of altering the catalytic activity by changing the site population of the exchangeable cations. We discuss two methods which have been used to alter the sites occupied by Fe /Fe cations in Y zeolite structures: increasing the silicon-to-aluminum ratio via silicon-substitution and co-exchanging Fe together with a second cation (Eu). 2+

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The effect on the catalytic activity forN Odecomposition on increased silicon content of the zeolite is to provide nearly 2 orders of magnitude increase in the turn-over-frequency (TOF) of the Fe cations. The increased TOF is accompanied by a change in the Mössbauer spectra of the Fe cations. These cations exhibit spectra which are indicative of cations moving from sites of high coordination (Site I) t sites of lower coordination (SitesI'/II'and/or II). In addition, the rate of oxidation of the cations decreases with increasing Si content of the lattice. When Eu and Fe are coexchanged into a Y zeolite, which hasn't had its silicon-to-aluminum ratio increased, the Fe Mössbauer spectrum is intermediate between those of the Fe exchanged Y-zeolites having silicon-toaluminum ratios of 4.5 and 6.2 as determined by Si NMR. The catalytic activity forN Odecomposition, while greater than normal Fe-Y, is less than expected from correlation with Mössbauer spectra. This observation is discussed in terms of interaction between Eu and Fe. When Eu is present alone it does not undergo redox until the temperature is increased to 873 K. When both cations are present Eu can undergo redox half reactions at 723 K. EPR spectra show that different sites may be involved when both cations are present as opposed to when Eu is present alone. 2

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0097-6156/90/0437-0066$06.00/0 © 1990 American Chemical Society In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Redox Catalysis in Zeolites

DUMESIC AND MILLMAN

Catalytic oxidation-reduction (redox) reactions in zeolites are generally limited to reactions of molecules for which total oxidation products are desired. One important class of such reactions falls under the category of emission control catalysis. This encompasses a broad range of potential reactions and applications for zeolite catalysts. As potential catalysts one may consider the entire spectrum of zeolitic structural types combined with the broad range of base exchange cations which are known to carry out redox reactions.


Transition metals exchanged into Y-zeolite offer a basis upon which to build an understanding of the important parameters involved in designing zeolitic redox catalysts. Yzeolite was chosen for this study because it is the most thoroughly characterized catalytic zeolite. Thus, one can address such questions as what non-framework cation sites are occupied, whether the cations move between sites, whether interactions between the cations themselves are important and how these factors relate to the kinetics of catalytic reactions. The choice of exchange cation is limited to those which do not undergo reduction to the metal, as redox reactions using these systems are generally irreversible (1). A criterion for long term catalytic stability of exchangeable cations in zeolites is the ability to undergo repeated oxidation and reduction cycles and to reach a reproducible stable state after each cycle. This criterion provides the possibility of a catalyst not only showing good activity, but also of having a long catalytic life. Two redox couples which will be discussed here are Fe - Fe and Eu Eu . 3+

2+

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The reversibility of the redox cycle involving H and 0 was established for Fe by Boudart and co-workers (2) using Môssbauer spectroscopic techniques. They proposed that the oxygen was held between two Fe cations. Fu et al. (3) showed that Fe-Y acted as a redox catalyst for reactions of CO with NO, CO with Ο» and N 0 with CO. The ability of Fe-Y to decompose N 0 into its elements was established in the work of Hall and co-workers (4), who also showed that Fe-Mordenite was as active as Fe-Y, despite containing only 16% as much Fe as its Y-zeolite counterpart. This difference in catalytic activity was thought to result from differences in the environments of the Fe within the zeolite structures. The objective of the present study was to alter the cation environment and relate that environment to the catalytic activity; this was accomplished by varying the silicon-to-aluminum ratio of Y-zeolite and by coexchanging Fe with Eu. 2

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EXPERIMENTAL The starting material for preparation of the samples was Linde Na-Y (SK-40, lot 1280-133). Before ion-exchange, the samples were washed with a pH 5.0 buffer solution. The procedures used to introduce Fe , Eu and other ions have been described earlier (5). The iron containing samples were stored under vacuum after preparation and drying under N at 400 Κ for 5 h followed by oxidizing in 25% 0 in He at a final temperature of 773 Κ for 2 hr. Gases were purified by standard means (6). 2+

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Spectroscopic characterization of the zeolites was carried out using a variety of techniques. The equipment is listed below: 1) Austin Science Associates Model S-600 Môssbauer Spectrometer equipped with a TracorNorthern Model N6-900 multi-channel analyzer. 2) Varian Model E-115 ESR spectrometer equipped with variable temperature accessories, covering a range from 2.4 Κ to 600 K.

In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS 3) A Spectrascan IV plasma spectrophotometer used for elemental analysis in determining the unit cell compositions. 4) A Phillips X-ray powder diffractometer equipped with scintillation counter and computer interface was used for crystallinity determinations.

Redox reactions and corresponding half-reactions were carried out in a Cahn Model C-2000 electrobalance operated in a flow mode to determine the oxidation state from weight changes. Catalytic activity was determined using a single pass differential flow reactor. Products were analyzed by a UTI100-C quadrupole mass spectrometer using continuous sampling. This system is described more completely elsewhere (6).


The catalysts used in this study are described in Table 1 by their unit cell compositions. Table 1. Unit Cell Composition of Catalysts Sample* Unit Cell Composition

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FeY(2.4) FeY(3.5) FeY(4.5) FeY(6.2) EuY EuFeY

Hs^Na^FeÂlO^SiO^^ Hâ^FeÂlOîSiOj)^ HâJFeÂlOûiSiO ^ H^Na Fe . (AlO ) 3^(SiO ) NanEuÂlO^SiO^ Ho^^FeÂlO^SiOj)^ 54

8 0

2 2

2 168k2

2 y

a) The value in parentheses is the Si/Al ratio from S i N M R . b) Cell compositions based on elemental analysis. The Si substituted samples are normalized to Si + A l = 192.

The location of the exchangeable cations within the zeolite matrix has been the subject of study by different techniques. First the use of xray diffraction has been sucessfully applied for single cations in some cases (10). Another technique which has had some recent sucess to univalent and divalent cations is Far IR(ll). Mid-IR has been used to follow the change in location of cations as a function of time when exposed to different gas phase environments (9), however this method is a rather indirect method for determining the location of the cations. The technique which we will rely on here is principally Môssbauer spectroscopy. While this technique is an indirect method it provides coordination environment information and oxidation state information in the short range (next nearest neighbor). Particularity with the silicon substituted zeolites this is the only information which can be used to infer cation location because the substitution of Si for Al is random (12) and no long range order is present either within the βcages of a unit cell or between unit cells, thus making it difficult to extract reliable information from techniques which rely on long range ordering. Thus, Môssbauer together with ESR spectroscopy will be used there to indirectly infer changes in cation location. RESULTS AND DISCUSSION Môssbauer spectra of the ^Fe in these samples following oxidation at 725 Κ are shown in Figure 1. The spectra show that as the silicon-to-aluminum ratio increases the spectra change from two doublets having isomer shifts of 0.38 and 0.33 mm/s to three doublets having isomer shifts of 0.36, 0.34 and 0.95 mm/s and associated quadrupole splittings of about 1.85,1.08 and 0.68 mm/s. Thefirsttwo doublets are associated with Fe(III) while the third having higher isomer shift is associated with Fe(II) which has not been completely oxidized, as will be seen in the spectra of the reduced samples. It is evident that the contribution of the Fe(II) species in creases with the silicon-to-aluminum ratio. The spectrum of the EuFe-Y sample (Fig. IE) is similar to that of the normal Fe-Y (Si/Al = 2.4).



(2.4);

Figure 1. Môssbauer Spectra of Fe in oxidized samples, a) FeY b) FeY (3.05); c) FeY (5.0); d) FeY (6.2); e) EuFeY. All samples oxidized inflowing0 at 725 Κ diluted 75% with He. 2


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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS

Hydrogen reduction of the above samples results in the Môssbauer spectra shown in Figure 2. It is apparent that as the Si/Al ratio increases the spectral contribution of the Fe with the smaller quadrupole splitting (QS) increases at the expense of the signal with the larger QS. These signals are denoted as the inner and outer doublets, respectively. As shown in Fig. 2E, incorporation of Eu together with Fe results in spectra nearly identical to that in Fig. 2C, which has a Si/Al ratio of 6.2.


Because quadrupolar interactions depend on the coordination environment of the ion, changes in Môssbauer quadrupole splittings are sensitive to the sites at which the cations are located, whereas differences in the isomer shift are sensitive to the oxidation state of the ion. For reduced samples, the ferrous cations in site I have been assigned to the outer doublet, while those in sites Γ, IF and/or II have been assigned to the inner doublet (2). The changes in the Môssbauer spectra as the Si/Al ratio increases are interpreted as iron cations moving out of site I into the B-cages and possibly the supercages. Thus, at the higher Si/Al ratios the fraction of Fe in more accessible sites is greater. This change with Si/Al ratio is also consistent with what has been observed for the exchangeable ions in going from X-zeolite to Y-zeolite. It is also consistant with calculations of the charge density at the different locations as a function of increasing Si content (13), ie., the charge density at Site I decreases with increasing Si content. The changes in the location of iron cations as one changes the Si/Al ratio or introduces a large, high-valent cation discussed above are reflected in the ability of the zeolites to carry out redox catalysis. Figure 3 shows the turnover frequency (TOF) as a function of the silicon-toaluminum ratio of the zeolite (as determined by ^Si NMR) for the decomposition of N 0 into its elements. The EuFe-Y sample (shown as a solid square) clearly has a TOF corresponding to a sample having a Si/Al ratio of about 3, i.e., its TOF is about 10 times greater then normal Fe-Y. Yet, a comparison of the Môssbauer spectrum for the EuFe-Y sample with spectra for the silicon-substituted Fe-Y samples suggests that the activity of the former sample should have been about 100 times greater than normal Fe-Y, based on the catalytic activity results for the silicon-substituted Fe-Y series of samples. Thus, there is clearly some other factor involved in addition to location of the cations. 2

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Eu-Y shows essentially no activity for N 0 decomposition, and Eu Môssbauer spectroscopy indicates that the Eu does not undergo reduction in 1 atm of flowing H at 700K for 5 h. However, when Fe is present, reduction of Eu does occur. Unfortunately, Eu does not have a large quadrupolar splitting so that no information concerning the location of these cations is available from the Môssbauer spectroscopy. 2

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Iton (7) has shown that ESR spectroscopy provides information on the location of Eu cations in Y-zeolite. The ESR spectra of Eu in EuFe-Y are shown in Figure 4A, following reduction in CO for 50h at 770 K. The spectra are characterized by three effective g-values at 6.0, 4.9, and 3.0. The first two resonances are associated with Eu at sites F, IF and II (7). Treatment in a 3:1 mixture of CO and 0 leads to a significant, and rapid decrease in the intensity of the resonance at g^ = 4.9, indicating a rapid conversion to Eu . This is in agreement with the changes in EPR spectra observed upon treatments of reduced samples with 0 . Of the sites Iton assigned to this resonance, site II is the most accessible; therefore, we interpret the resonance at 4.9 to Eu cations at this site. Unlike site IF, Eu cations at site II do not have the possibility of interacting with another ion at site F. This leaves the resonance at 6.0 to be associated with sites F and IF. Recalling the Môssbauer spectra of EuFe-Y, Fe was also shown to be located at Sites I, and II, and thus the possibility exists for interactions between the two different cations. 2+

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DUMESIC AND MILLMAN

Redox Catalysis in Zeolites


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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS

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Si to AI Ratio ( 29S

Redox Catalysis in Zeolites - ACS Symposium Series (ACS Publications)

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