Framework Fe Sites in Sodalite: A Model for Fe T Sites in Zeolites

J. Weitkamp, H.G. Karge, H. Heifer and W. HOlderich (Eds.) Zeolites and Related Microporous Materials: State of the Art 1994 Studies in Surface Science and Catalysis, Vol. 84 Q 1994 Elsevier Science B.V. All rights reserved.

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FRAMEWORK Fe SITES IN SODALITE: A MODEL FOR Fe T SITES IN ZEOLITES D. Goldfarb' , M. Bernardo, K. G. Strohmaier, D. E. W. Vaughan and H. Thomann2 Exxon Research and Engineering Co., Route 22 East, Annandale, NJ, 08801

The incorporation of Fe3+ into framework T sites of Sodalite was studied by EPR, pulsed electron-nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEIvl) spectroscopies. The EPR spectrum shows a powder pattern centered at g=2 indicative of a single Fe3+ site. The pulsed ENDOR spectrum of a 57Fe enriched sample consists of three major peaks at 15.4, 42.6 and 71.4 MHz from whch a hyperfine coupling of (28.61 MHz was obtained. We found a good correlation between the onset of the Sodalite structure during synthesis as obtained by X-ray diffraction results and the appearance of the ENDOR spectrum, supporting the assignment of the spectrum to 57Fe3+ in Sodalite T sites. 23Na127Al, 'H and 35Cl peaks were observed in ESEEM spectra of Fe-Sodalite. The 23Na and 35Cl peaks increased with the formation of Sodalite whereas the 'H peak of water decreased. The ESEEM results also confirm the assignment of the Fe3+ to framework T sites. The unique EPR, ENDOR and ESEEM characterists of 57Fe3+ in T sites of Sodalite make it a model to which Fe3+ in T sites of other zeolites can be compared.

INTRODUCTION The incorporation of transition metals into the framework of aluminosilicate and aluminophosphate molecular sieves represents one aspect of the search for new selective catalysts. A particular example is the successful incorporation of Fe into a number of zeolites such as L [l],ZSM-5 [2], Sodalite [3] and AlP04-5 [4]. A major problem with Fe zeolites is that the iron does not exclusively occupy framework T sites. It can also be present as extra-framework cations or exist as an interstitial phase of small particles located within the molecular sieve cavities or on the external surface [5,6]. Therefore, in order to understand the source of the catalytic activity of these materials an unambiguouse identification and characterization of the various Fe sites is required. Permanent address: Department of Chemical Physics, The Weizmann Institute of Science, Rehovot 76100, Israel also at Department of Chemistry, State University of New York, Stony Brook

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EPR spectroscopy of high spin Fe3+ in zeolites powders is a very useful tool for the determination of the number of the different types of Fe3+ present and it also provides general qualitative information regarding the symmmetry of each one. However, due to its low resolution, arising from the inhomogenous broadening caused by the zero field splitting (ZFS), it usually does not give detailed information on the Fe3+ local environment [7]. Such information can be obtained from the NMR spectrum of 57Fe3f and of the nuclei coupled to the Fe3+, as determined from pulsed electron-nuclear double resonance (ENDOR) spectroscopy. This method is highly advantageous as it combines the high sensitivity of EPR with the high resolution of NMR [8]. Another useful method for obtaining the NMR frequencies of nuclei in the vicinity of paramagnetic centers is the electron spin echo envelope modulation (ESEEM) technique which is particularly suitable for the measurements of weak superhyperfine interactions with surrounding nuclei [9,10]. The ESEEM method is complimentary to the ENDOR technique because it is most sensitive to nuclei with small couplings, which usually are hard to detect by ENDOR spectroscopy. In this work we present a detailed magnetic resonance characterization study of FeSodalite using X-band and &-band EPR, pulsed ENDOR and ESEEM spectroscopies. We chose Fe-Sodalite due to previous results showing that at low Fe loadings only a single Fe site, assigned to framework T sites, exists [3,7]. We present a new approach for characterizing Fe in zeolites using pulsed ENDOR of enriched 57Fe. The spectra observed yield the 57Fe hyperfine coupling which can be related to the Fe bonding characteristic. Although Mossbauer spectroscopy also provides the 57Fehyperfine couplings, the pulsed ENDOR technique has the advantage of the additional site selectivity provided by the EPR spectrum. Finally, we show that the combination of pulsed ENDOR and ESEEM is an effective tool for the investigation of synthesis mechanism during which the Fe is incorporated into the Soldalite framework. This in turn provides a new insight into the zeolite formation mechanism.

EXPERIMENTAL Synthesis. Iron was added as FeC13 dissolved in water as the final component to the synthesis gel, partly replacing the desired level of A13+ in otherwise conventional synthesis procedures [3,11]. Samples with the following compositions were prepared: FeSOD1- Feo.oolAll.oSio.g6Na1.32, FeSOD2- Feo.ozAlo.solSil.oNal.20, FeSOD3Feo.osAlo.93Sii.oNai.24. 57FeSOD1was obtained using an enriched 57FeC13solution made by dissolving 57Fe203 in excess 1N HC1 solution. The synthesis of 57Fe SOD1 was followed by placing the synthesis gel in a set of autoclaves and the synthesis products were studied by Xray diffraction and spectroscopic measurements as a function of the reaction times (at 150%). Prior to the measurement the products were washed, filtered and dried at 100-llO°C. The gel sample was measured as is without any treatment. In a second

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experiment, 57FeSOD1was obtained by the same synthesis procedure at 100°C. In this case only one container was used and small amounts of the products were removed at different times for analysis. EPR, ENDOR and ESEEM measurements. X-band EPR ( x 9.2 GHz) spectra were recorded on a Varian E-12 spectrometer and &-band EPR ( M 34.0 GHz) spectra were recorded using a Bruker ESP300 spectrometer. The pulse ENDOR and the ESEEM ( x 9.25 GHz) measurements were carried out at 1.2K on a homebuilt spectrometer described elsewhere [12). The pulse sequence used for the ENDOR and ESEEM measurements are given in Fig. l. The Davies ENDOR spectrum is obtained by recording the echo intensity as a function of the radio frequency (RF) whereas the ESEEM waveform is obtained by recording the echo intensity as a function of the time intervals r or T. The echo detected (ED) EPR spectra were obtained by measuring the two-pulse echo intensity, at a fixed 7 value, as a function of the external magnetic field. Typically microwave ~ / and 2 K pulses of 30 ns and 50 11s duration, respectively, were used for the ESEEM and Davies ENDOR experiments. The width of the RF pulse in the ENDOR experiment was 2.5 ps.

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Figure 1. (a) Two-pulse and (b) three pulse ESEEM pulse sequences. (c) Davies ENDOR pulse sequence.

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Figure 2. X-band EPR spectra, recorded at rooni temperature, (a) of FeSODl, (b) FeSOD2 and (c) FeSOD3.

RESULTS The room temperature X-band spectra of FeSOD1, FeSOD2 and FeSOD3 are shown in Fig. 2. While the spectra of FeSOD2 and FeSOD3 show a single signal at g = 2, the spectrum of FeSODl is better resolved and exhibit additional fine structure. It consists of two peaks at 9 = 2 and two broad shoulders, one on each side of the g = 2 peaks. The low field shoulder, at 9 = 2.32, is resolved whereas the other is partially buried under the center peak. The narrow g=2 signal corresponds to a very low spin concentration

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and we assign it to an "impurity". Based on comparison with the Q-band spectrum we attribute the shoulders and the central 9=2 peak to a powder pattern of a single Fe3+ site [7]. The better resolution of the FeSODl spectrum is due to very low Fe content where broadening due to Fe-Fe spin interactions is negligible. The same spectra were observed also at 140K. When the ZFS of Fe3+ is small compared to the electronic Zeeman interaction the EPR spectrum of orientationally disordered samples consists of four overlapping powder patterns due to the rns transitions I -512 >-+ - I 312 >, I -312 >-+I -112 1 112 >+I 312 > and 1 512 >-+I 312 > which show first order dependence on the ZFS, and a central, narrower 1-1/2>-+1 112 > transition broadened to second order by the ZFS [13]. Accordingly, the g = 2 line is assigned to the I -112 >+I 112 > transition and the broad soulders to singularities in the powder patterns of the other transitions. In the spectrum of FeSOD2 (Fe/(Fe+Al)=O.OP) the shoulders are not resolves due to the increasing linewidth, but the lineshape does indicate their existance. From the position of the shoulders we estimate the ZFS parameter, D, to be in the order of a few hundred Gauss. The Davies ENDOR spectrum of 57FeSOD1, recorded at 3317G (9 = 2), shown at the bottom of Fig. 3, comprises of three major peaks at 15.4, 42.6, and 71.4 MHz assigned to t,he 57Fe (I=1/2) transitions. The ENDOR spectrum of a similar nonenriched sample did not show any peaks. When the ZFS is smaller than the electronic Zeeman interaction the ENDOR spectrum of 57Fe3+ consists of three well separated pairs of peaks, given to first order by A / 2 fvn, 3A/2 fvn and 5A/2 fvn corresponding to the mS = f 1 / 2 . *3/2 and f5/2 manifolds, respectively. A is the hyperfine coupling, vn is the 57Fe Larmor frequency and A > vn. We assign the three observed peaks to these three groups respectively. The ENDOR spectrum shows also three shoulders at 14.3, 45.1, and 72.8 MHz. ) may originate These may either represent the 57FeLarmor frequency splittings ( 2 ~ n or from a powder pattern lineshape arising from the effect of the ZFS on the ENDOR frequencies or from hyperfine anisotropy (see next). The former seems unlikely since W away from the 42.6 MHz peak and the 45.1 MHz peak is more than ~ Z J ~ (1MHz) all three shoulders do not exhibit the expected magnetic field dependence. We exclude the possibility that these shoulders originate from a second type of Fe3+, at extra framework sites, since an identical ENDOR spectrum was obtained from 57FeSOD1 synthesized under different conditions (100OC). One would expect the relative intensities of the shoulders to vary with synthesis conditions if they were due to extra framework Fe3+. Moreover, only one Fe3+ site has been identified by EPR [7] and Mossbauer [3] spectroscopies.

To first order, A can be independently determined from the position of each of the ENDOR peaks yielding 130.81,127.71,and 128.61 MHz, respectively. The variation in the calculated A values is either due to the effects of hyperfine anisotropy combined with

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Figure 3. Davies ENDOR spectra (right) and ED-EPR spectra (left) of the synthesis gel of 57FeSOD1 (top), the products after 2 hr (middle) and the products after 4 hr (bottom). The ED-EPR spectra were a obtained with T = 0 . 1 9 ~ sand the ENDOR spectra with T = 0 . 2 8 ~ All ~ . spectra were recorded at 3317G (9 = 2).

orientation selectivity, which results in the excitation of different orientations in each mS manifold, or due to the effect of the ZFS. The effect of the former is expected to be small since the hyperfine anisotropy of 57Fe3+ has been observed to be very small ( M 0.5 MHz) in other oxides [14-161. A third order perturbation treatment of the spin Hamitonian yield ENDOR frequencies which depend on the ZFS parameters [17], introducing additional anisotropy which is a function of mS. The third order perturbation expression for the energies shows that the ENDOR frequencies corresponding to mS = f 5 / 2 are the least affected by the ZFS, therefore we take 128.61 MHz as the better value for A. The unique ENDOR spectrum of 57FeSOD makes it an exellent probe for the investigation of the Fe incorporation into the tetrahedral sites during synthesis. The use of 57Feeliminates the problem of the presence of Fe impurities in the starting matrials which contribute to the EPR signal but not to the ENDOR signals. Fig. 3 shows the Davies ENDOR and the ED-EPR spectra of the synthesis products as a function of synthesis time. The ED-EPR spectrum of the starting gel is very broad indicating a large ZFS and maybe overlapping Fe3+ signals. The ENDOR peaks, appearing at M 15 and M 42 MHz, are also very broad. After 2 hr of synthesis (at 15OoC), X-ray diffraction data showed that the product was crystalline and consisted of 90% zeolite A and only 10%Sodalite. The corresponding ED-EPR spectrum narrowed significantly as compared to that of the gel and although there is only 10% Sodalite at this stage the characteristic 57FeSOD ENDOR peaks are well apparent. A close look, however, reveals some broadening, particularly evident

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for the 42 MHz signal which seems to be superimposed on a broader peak. This is clearer in ENDOR spectra recorded at a higher field as shown in Fig.4. At this field the A / 2 peak (= 15 MHz) is not present since the appropriate EPR transitions were not excited. After 3.5 hr at 100°C the product consisted of zeolite A only. The ED-EPR spectrum of this sample was very broad and the ENDOR spectrum did not show any detectable "Fe signals. Hence, we cannot attribute the broad peak mentioned above to framework 57Fe in zeolite A and assign it to some extra framework 57Fe3+. After 4 hr of synthesis (150°C) the X-ray diffraction pattern indicated the formation of pure Sodalite and the ENDOR spectrum was similar to that obtained after 6 and 8 hr as expected. The intensity of the corresponding ED-EPR spectrum was significantly larger than that recorded after 2 hr at the same temperature.

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Figure 4. Davies ENDOR spectra, recorded at 3788 G (g=1.75) of (A) synthesis products after 2 hr (T = 0.45ps), (B) synthesis products after 6 hr (T = 0.47~s)(pure 57Fe-Sodalite).

The variation of the local environment of Fe3+ during the synthesis process was also investigated by ESEEM. Figure 5 shows the ESEEM waveforms of the gel, the synthesis products after 2 hr and after 4 hr, along with the corresponding Fouriertransformed (FT) ESEEM spectra. The spectrum of the gel shows two peaks, one due to 'H (14.06 MHz) and the second due to 27Al (3.71 MHz). We note however, that some 23Na contributions to the 3.71 MHz peak from Na cations in the gel cannot be excluded, since at the magnetic field used (- 3300 G) the Larmor frequency peaks of 27Al and 23Nacannot resolved. The changes the ESEEM waveform undergoes upon the generation of the crystalline matrial are very significant. The 27Al, 23Na modulation amplitude increases whereas that of the 'H decreases. This is better manifested in the FT-ESEEM spectrum which shows a decrease in the relative intensity of the 'H peak and the narrowing of the 27Al,23Na peak. The decrease in the protons peak as compared to the gel sample is expected since the 2 hr products were dried (at 100OC) prior to the

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measurement. The FT-ESEEM spectrum shows three additional low intensity peaks, two at 7.42 and 11.1 MHz which are due to combination harmonics of the 3.71 MHz peak [9,18], and a third at 1.37 MHz , assigned to 35Cl. After 4 hr of synthsis, the 27Al, 23Na peak narrows, the intensities of the corresponding combination harmonics increase and so does that of the 35Cl peak, whereas the l H peak has practically disappeared.

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w rl (MHz) Figure 5. (A) Two-pulse ESEEM waveforms (left) and the corresponding FT-ESEEM spectra (right) of (A). Synthesis gel (B) synthesis products after 2 hr, (C) synthesis products after 4 hr (pure FeSOD). 1

In order to asses the contribution of 23Na to the ESEEM of FeSOD we carried out similar measurements on Sodalite where Ga replaces the A1 (FeGaSOD) and the Fe content was less then 0.01%. The EPR spectrum of FeGaSOD was similar to that of FeSOD1, although it was slightly broader. Galium has two isotopes ( I = 3/2) that may show modulation, 6gGa (60.1% natural abundance) and 71Ga (39.9% natural abundance) with Larmor frequencies (at 3300G) of 3.38 MHz and 4.27 MHz repectively. The ESEEM of FeGaSOD showed modulation with a frequency of 3.71 MHz as well, as shown in Fig. 6. The modualtion amplitude is, however, significantly lower (- 50%) as demonstrated by the ratio of the ESEEM waveforms of FeSODl and FeGaSOD. The ratio method has been proven to be most effective in comparing ESEEM modulation amplitudes and frequencies since it eliminates the effect of different echo decays due the different relaxation times [7,19]. We note also that no peaks due to 69Ga or 71Ga could be detected in the corresponding FT-ESEEM. These results indicate that the eak at 3.71 MHz in the FeSODl spectrum consists of a significant contribution from %Na in addition to that of 27Al.

DISCUSSION Previous EXAFS, Mossbauer and chemical anlysis data of FeSOD with higher Fe

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Figure 6. Three-pulse ESEEM waveforms (recorded at g = 2 and FeSODl and FeGaSOD and their ratio.

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loadings (0.2,0.5 wt% Fe respectively) showed that at these levels the Fe occpies framework T sites "31. FeSOD2 and FeSOD3 are from the same batch on which these measurements were performed. We showed that the EPR spectra of these samples are essentially similar to that of the j7FeSOD1 (Fe/(Fe+A1)=0.001) excluding linewidth differences caused by the spin-spin interaction due to the higher Fe loadings in FeSOD2 and FeSOD3. Therefore, we conclude that the 57Fe ENDOR spectrum we observed is characteristic of Fe3+ in framework T sites of Sodalite. The observation of intense 23Na modulation provides additional supporting evidence for the framework location of Fe3+, substituting for A1 or Ga. Framework Fe3+ should show intense 23Na modulation due to the presence of Na'+ cations in the vicinity of the T sites (first shell consists of 4 cation at 3.1-3.3a). If the Fe3+ were in cation exchange sites then the 23Na modulation should have been significantly shallower since each Fe3+ replaces three Nal+ cations, thus reducing the distance and the number of Nal+ cations around it. The above assignment is further supported by the lack of Ga modulation in FeGaSOD and the comparable 27Al and 23Na modulation amplitudes in FeSOD, inspite of the 27Al higher nuclear spin, 5/2. In the case of framework Fe, 27Al and Ga are located four shells away from it (2 at 4.4A and 8 at 5.44a) thus deep modulation is not expected. The appearance of 35Cl modulation in the ESEEM of FeSODl is assigned to C1- anions, located in the center of the Sodalite cage. The presence of the 35Cl signal along with the absence of a protons (water) signal is again in good agreement with the assignment of the Fe to T sites. The value of A , I 28.6 I MHz, in j7FeSOD is somewhat lower then that of j7Fe3+ in CaO, MgO and A1203 (-29.811, -30.147 and -30.27 MHz respectively) where the 57Fe3+ has an octahedral symmetry. It is, however, similar to that of 57FeP04 (I 28.21 MHz)

41 1

which has a Quartz crystal structure where the Fe3+ is tetrahedrally coordinated. [20]. This is also in agreement with the observation that the isotopic hyperfine of tetrahedral 57Fe3+ is about 10% smaller than in octahedral 57Fe3+[15,211. The investigation of the spectroscopic properties of Fe3+ during the course of the synthesis showed that the appearance of the characteristic ED-EPR and ENDOR spectra of j7FeSOD1 correlates well with the occurance of the Sodalite phase in the products as determined from the X-ray diffraction results. This provides additional evidence for framework substitution. After 2 hr of synthesis (at 150%) the product consists of 10% and 90% zeolite A, yet the ENDOR spectrum is dominated by the 57FeSOD signal, showing, nontheless, the presence of non framework Fe3+, which amounts to roughly 20% of the ED-EPR signal. This extra framework Fe3+ was not found in detectable amounts in the final product, indicating that it has been either incorporated into the T sites of newly formed FeSOD during the next two hours of synthesis or that is has been washed away with the filtrate. While zeolite A constitutes of the majority of the product after 2 hr of synthesis, Fe3+ associated with it does not show a unique ENDOR signal and it is accounted for only through a broad and minor contribution. Considering the similarity in the structures of zeolite A and Sodalite it is rather suprising that 57Fe signals similar to those observed in 57FeSOD are not detected from T sites in zeolite A. This implies that Fe3+ is not incorporated into T sites of zeolite A under our synthesis conditions. This suggestion is supported by the rejection of Ga by the A framework as shown by recent NMR measurements22. If the latter is indeed the case, it has important implications regarding the synthesis mechanism. The large increase in the 57FeSOD signal after 4 hr synthesis (pure Sodalite) indicates that Fe3+ which was not in T sites in A wm nevertheless incorporated into T sites of Sodalite. This implies that the transformation form the A structure into the Sodalite structure during synthesis goes through a stage of desolution rather than through a solid state transformation. To substantiate the above suggestion further systematic ENDOR, ESEEM and ED-EPR measurements combined with quantitative chemical analysis of the Fe3+ distribution among the m o u s phases during the synthesis process are required.

CONCLUSIONS At low substitution levels iron can be exclusively incorporated into framework T sites of Sodalite. It exhibit an EPR signal at 9=2 and has relatively small ZFS parameters, indicating a small deviation from tetrahedral symmetry. These conditions allow the observation of a characteristic of 57Fe ENDOR spectrum from which a hyperfine coupling of 128.6)MHz was obtained. The unique spectroscopic chararacteristics of Fe3+ in Sodalite T sites make it a simple model to which Fe T sites in other zeolites can be compared. Furthermore, it can be used as a probe for the investigation of the synthesis mechanism.

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