J. Phys. Chem. B 1998, 102, 4865-4870
4865
Interpreting the Oxidative Catalytic Activity in Iron-Substituted Ferrierites Using in Situ Mo1 ssbauer Spectroscopy K. La´ za´ r,*,† G. Lejeune,†,‡ R. K. Ahedi,§ S. S. Shevade,§ and A. N. Kotasthane§ Institute of Isotope and Surface Chemistry, Budapest, P. O. Box 77, H-1525, Hungary, and Catalysis DiVision, National Chemical Laboratory, Pune, 411 008, India ReceiVed: October 27, 1997; In Final Form: March 18, 1998
Iron- and iron + aluminum-substituted ferrierites (Fe-FER and Fe+Al-FER) have been characterized and their properties compared. Fe-FER exhibits excellent catalytic performance in mild oxidation processes, whereas Fe+Al-FER is hardly active. To specify the source of catalytic activity, in situ Mo¨ssbauer studies were performed to distinguish the various iron-containing species. Upon treatment in hydrogen performed on Fe-FER, the temporary presence of the Fe2+ state was evidenced in tetrahedral framework sites, and removal of iron did not occur in significant amounts from these lattice positions. In contrast, similar treatment on Al+Fe-FER results in permanent Fe3+ to Fe2+ reduction and in removal of iron in considerable amounts from the framework sites. From comparison of the behavior of samples and additional analysis of data, it is suggested that dinuclear Felattice-O-Feextralattice centers are present in Fe-FER and that they have a primary role in mild oxidations.
Introduction Ferrisilicates with zeolite analogue structures have been synthesized and used as catalysts since the 1980s. They are used primarily because of their acidic character; e.g., the acidity of aluminum-containing zeolites was damped by exchanging the Al ions for Fe ions in the tetrahedral framework positions.1 Subsequently, certain ferrisilicates were tested and proved to exhibit excellent activity in oxidative dehydrogenation,2,3 in partial oxidation of stable hydrocarbons (benzene to phenol),4 and more recently, in deNOx processes.5 Detailed isotope exchange studies have confirmed that the high activity cannot be attributed to the high-dispersion iron oxide phase formed upon removal of iron from the framework, as the mechanisms of oxygen transfer were different in ferrisilicates and in iron impregnated zeolites.6 Mild oxidation on ferrisilicates of cyclohexane,7 benzene to phenol,8 and methane to methanol9 at ambient (“biomimetic”) conditions have been reported. As a means of explaining the catalytic activity, it was postulated that dinuclear centers played the primary role in analogy to centers present in the monooxygenase enzyme of methanotrophic bacteria.10 The results of quantum chemical calculations performed on dinuclear Fe-O-Fe and Al-O-Fe hydroxide centers suggest that Al-O-Fe pairs are unable to transfer oxygen, whereas Fe-O-Fe may act as an active center, and dehydroxylation promotes the process.11 Another suggestion relates to an active structure containing dioxygen (Roxygen) in Fe-O-O-Fe fragments.12 In ferrisilicates iron may be located in three positions: in substituted tetrahedral framework sites, in charge-compensating ionic form at extraframework positions, and in neutral highdispersion oxyhydroxides in the cages.1 It is usually a * Corresponding author: e-mail
[email protected]. † Institute of Isotope and Surface Chemistry. ‡ On leave from the Department of Chemistry, Universite ´ de Poitiers, Poitiers F-86022, France. § National Chemical Laboratory.
complicated matter to distinguish between the contributions of these forms in experiments. Various methods [X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultravioletvisible (UV-vis), and infrared (IR) spectroscopy, electron spin resonance (ESR), and extended X-ray absorption fine structure (EXAFS)] have been used for characterization,1,13,14 and Mo¨ssbauer spectroscopy is another tool providing information specifically related to the oxidation and coordination states of iron.15,16 The potential of the method can be extended by applying in situ conditions: coordination and removal of ligands (water, ammonia) in a vacuum17 and differences in reducibility of extraframework and framework substituted ions in hydrogen or reaction mixtures18,19 can be observed. Recently, the rapid synthesis of a novel ferrisilicate with ferrierite structure (Fe-FER) was reported,20 indicating the substitution of iron in tetrahedral lattice positions in high proportion. This new substance exhibited high activity in mild oxidation of hexane by H2O2. The aluminoferrisilicate of the same ferrierite structure (Fe+Al-FER) does not display any catalytic activity in the same reaction.20 Here, we report on detailed in situ Mo¨ssbauer characterization of the two ferrisilicates under various conditions (evacuation, redox treatments), and further results are also included on catalytic and structural characterizations. The various states of iron and their transitions are distinguished. The differences in the behavior of Fe-FER and Fe+Al-FER are correlated with the actual states of iron in the two substances. Evidence is put forward for the presence of dinuclear Felattice-O-Feextraframework pairs, and their primary catalytic role is suggested in the FeFER lattice. Experimental Section Synthesis. Two samples, Al+Fe-FER and Fe-FER, were synthesized with Si/T ratio ) 16. For the preparation of the Al+Fe-FER sample the Al/Fe ratio was 3. Initially an acidic solution was prepared containing the required amounts of
S1089-5647(97)03463-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/05/1998
4866 J. Phys. Chem. B, Vol. 102, No. 25, 1998 Al2(SO4)3‚16H2O (Loba Chemie), Fe2(SO4)3‚6H2O (Loba Chemie), water, and sulfuric acid (98% H2SO4, S. D. Fine Chemicals Ltd.); the solution gave a pH value of 1.58. In the next step this acidic solution was slowly added to a required amount of sodium silicate solution (28.0% SiO2 and 9.0% Na2O; pH ) 11.0) with stirring. Finally, highly basic pyrrolidine was mixed in the gel and stirred for 1 h, until it became homogenized. The pH value of the final bluish-green gel was 11.7 ( 0.2. The gel was autoclaved at 433 K for 48 h. The product was separated by filtration and washed with hot water. The Fe-FER sample was prepared in an analogous manner and, to promote crystallization, seed crystals of H-Al-FER precursors were added to the gel in an amount 0.3-0.4 wt % before autoclaving as described in detail elsewhere.20 As synthesized, Al+Fe-FER is white in color and the Fe-FER product was off-white. These samples were calcined at 773 K to convert them into their sodium/H+ form. This form of both the systems was converted into the ammonium form by multiple exchanges with 2 M ammonium nitrate at 363 K. The NH4+ forms were dried at 383 K and then calcined in a stepwise mode at 753 K for 6 h. Methods of Characterization. X-ray Diffraction. The crystallinity and phase purity of the Al+Fe-FER and Fe-FER samples were examined by X-ray powder diffraction (Rigaku D Max III VC) using Cu KR radiation. Elemental Analysis. The chemical compositions of Fe-FER and Al+Fe-FER were determined by X-ray fluorescence (XRF); the samples were analyzed for SiO2, Al2O3, and Fe2O3 by lithium tetraborate fusion followed by XRF wavelength dispersive analysis (Rigaku 3070). Ion-Exchange Capacity. Ion-exchange measurements were performed by titrating the H form of Al+Fe-FER and FeFER zeolites (1.0 g) in 10 g of distilled water with 0.05 N KOH up to pH ) 8 and then stirring the mixture for 1 h, washing thoroughly, and drying to obtain the K form of Al+Fe-FER and Fe-FER zeolites. Both the K forms were analyzed by atomic absorption spectroscopy (Hitachi 8000) to obtain the ionexchange capacity. Mo¨ ssbauer Measurements. Two treatments (both at 620 K for 2 h) were applied to the samples; (i) evacuation in 10-3 mBar, and (ii) reducing treatment in hydrogen. After the treatments, in situ Mo¨ssbauer spectra were obtained at room temperature (300 K), at 140 K (for measurement in a vacuum this is the lowest attainable temperature for the given experimental setup), and at liquid nitrogen temperature (77 K, in hydrogen atmosphere), as well. Spectra were recorded sequentially after the various treatments on the same sample (either Fe-FER or Al+Fe-FER). Spectra were fitted assuming a Lorentzian line shape; none of the positional parameters were constrained. Isomer shift values are related to the center of the metallic R-iron spectrum at 300 K; the estimated accuracy of the positional parameters is ( 0.03 mm/s. During evacuation/ reduction treatments nearly 7.0 wt % mass loss of adsorbed water from the samples was detected. Catalytic Studies. Catalytic oxygenation of n-hexane and phenol hydroxylation were carried out in a batch-type reactor. Oxidation of n-hexane was performed in a mixture of n-hexaneH2O2-acetonitrile at molar ratios of 1:1:7.5, respectively, at 80 °C under constant agitation for 20 h. Fe-FER catalyst was added to the mixture in an amount of 20 wt % of the hexane. Phenol hydroxylation was performed in a phenol-H2O2-water mixture at molar ratios of 1:1:5 under constant agitation at 80 °C for 20 h. Fe-FER catalyst was added to the mixture in an amount of 20 wt % of the phenol.
La´za´r et al. TABLE 1: Physicochemical Properties of Fe-FER and Al+Fe-FER Samples sample
SiO2/M2O3 unit cell parameters (Å) unit cell K/Fe gel product (XRF) a0 volume ratio b0 c0
Al-FER 60 Al+Fe-FER 60 Fe-FER 60
34 30 32
18.82 18.88 18.97
14.06 14.10 14.13
7.43 7.44 7.45
1957 1980 1997
0.93 0.75 0.81
The reaction products were analyzed by a Hewlett-Packard 5880 gas chromatograph using a methyl silicon gum capillary column of 50 m length. Both the oxidation reactions were also attempted under similar experimental conditions with the Al+Fe-FER sample. Results and Discussion Synthesis and Characterization. The X-ray diffraction patterns for Al+Fe-FER and Fe-FER are found to be in good agreement with those reported for the FER phase.21 The main features are similar and are characteristic of the ferrierite structure; however, slight differences in the relative intensities after calcination appeared at low-angle 2θ as well as between 2θ ) 22° and 28° (the corresponding diffractograms are shown in ref 20). Least-squares analysis of the data and the computed values of unit cell volumes (Table 1) from the XRD data reveal significant expansion in the unit cells of Fe-FER, indicating successful insertion of the iron ions during hydrothermal synthesis. The trivalent metal ion substitution causes similar changes in powder diffraction patterns in other zeolite frameworks including MFI,22 EU-1,23 and FAU.24 The ion-exchange capacities of the samples are presented in Table 1 and the slightly less than unity values for the K/Fe ratio clearly indicate partial loss of exchangeable sites from both samples.25 The higher value (K/Fe ) 0.81) obtained for the ion-exchange capacity of the Fe-FER sample attests that the synthesis and the ion exchange for H+ were successful; a large proportion of iron can be located at sites capable of ion exchange, i.e., in isomorphously substituted Si-O-Fe(OH)Si groups. Mo1 ssbauer Measurements on the Fe-FER Sample. A number of spectra recorded on the Fe-FER sample are displayed in Figure 1. The 77 K spectrum of the as-synthesized sample (prior to removal of the pyrrolidine template) is shown as trace a, and those on calcination and after evacuation (620 K, 2h, 10-3 mBar) are shown as traces b and c. In spectra a and b of Figure 1 the Fe3+ components exhibit relatively low quadrupole splitting, from which coordination of template and/ or water molecules can be deduced; the ligands equalize the asymmetry of the charge distribution around Fe3+ ions to a certain extent. In contrast, heating in a vacuum results in the removal of the coordinated ligands, and after this treatment the most characteristic spectrum can be recorded (Figure 1c). (The corresponding data obtained from the fits are reported in Table 2.) Further on a reduction treatment was applied to the same sample. The corresponding spectra are shown in Figure 1d-f. Two main features could be observed. First, Fe3+f Fe2+ reduction takes place to a considerable extent; ca. half of the Fe3+ is reduced to the Fe2+ state. This can be deduced from the conversion of the doublet of Fe3+ to that of Fe2+ exhibiting characteristic large isomeric shift (IS) and quadrupole splitting (QS) values (Table 2). It should be mentioned that spectrum d of Figure 1 was obtained after rapid cooling of the sample (ca. 1 h from 620 to 77 K). The second particular feature is the lack of reversibility: if one increases the temperature from 77
Interpreting Catalytic Activity in Ferrierites
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4867 TABLE 2: Mo1 ssbauer Data Obtained on Sample Fe-FER (Si/Fe ) 16)a 300 K
77 K
treatment
comp/coordb
IS
QS
RI
IS
QS
RI
as synthesized as received (calc.) evacuated, 620 K hydrogen, 620 Kc
Fe3+(Td/Oh) Fe3+(Oh) Fe3+(Oh) Fe3+(Td/Oh) Fe3+(Oh) Fe3+(Td,2) Fe3+(Oh) Fe3+(Td,1) Fe3+(Td/Oh) Fe2+(Td/Oh) Fe2+(Oh) Fe3+(Td,1) Fe3+(Td/Oh) Fe2+(Td/Oh) Fe2+(Oh) Fe3+(Td,2) Fe3+(Oh) Fe3+(Oh) Fe3+(Td,1) Fe3+(Td/Oh) Fe2+(Td/Oh) Fe2+(Oh) Fe3+(Td) Fe3+(Td/Oh) Fe3+(Oh) Fe2+(Td/Oh) Fe2+(Oh) Fe3+(Td,2) Fe3+(Td/Oh) Fe2+(Td) Fe2+(Td/Oh)
0.22 0.31 0.33 0.29 0.36 0.24 0.37 0.29 0.31 0.86 1.31
0.71 1.43 0.56 0.96 1.98 1.17 1.58 0.94 2.34 2.44
89 11 33 26 40 71 29 40 51 4 4
0.32 0.40 0.45
0.78 1.23
85 14 58
0.42
0.73
42
0.34 0.44 1.13 1.43 0.40 0.40 1.17 1.52
1.81 1.29 2.03 2.46 1.49 0.89 2.60 2.85
16 32 39 12 51 38 5 6
0.20 0.47 0.44
1.98 1.91 0.80
60 25 15 0.36 0.40 1.15 1.32 0.31 0.32 0.33 1.05 1.30
1.66 1.14 2.02 2.65 1.84 1.39 0.92 2.01 2.40
17 26 30 27 21 18 24 21 15
evacuated, 620 K (2nd) hydrogen 650 K (2nd) evacuated, 140 Kd 300 K evacuated, 620 K (3rd)
0.25
1.64
51
0.30 0.99 1.32 0.22 0.30 0.99 1.04
1.02 1.99 2.27 1.96 1.38 0.80 1.80
26 13 9 57 21 5 17
a IS, isomer shift, related to R-iron, mm/s; QS, quadrupole splitting, mm/s; RI, relative intensity, %. b The assignments for coordinations are approximate. c First set of 77 K entries were measured after rapid cooling from 620 to 77 K in hydrogen (Figure 2d). Second set were repeated measurement at 77 K, following the 300 K spectrum (Figure 2f). d Right side set: spectrum recorded at 140 K in a vacuum, following the 77 K measurement in hydrogen.
Figure 1. In situ Mo¨ssbauer spectra of Fe-FER sample (Si/Fe ) 16). Spectra were recorded after subsequent treatments performed on the same sample: (a) as synthesized, recorded at 77 K; (b) as received (after calcination, in H form, recorded at 77 K); (c) after evacuation at 620 K (recorded at 300 K in a vacuum); (d) after hydrogen treatment at 620 K (cooled immediately to 77 K and kept in H2); (e) as for (d), measured at 300 K, in H2; (f) as for (e), measured again at 77 K, in H2; (g) after repeated evacuation at 620 K (measured at 300 K); (h) after repeated hydrogen treatment at 620 K (cooled immediately to 77 K, in H2); (i) as for 1 h, measured at 140 K in a vacuum; (j) as for (i), measured at 300 K in a vacuum; (k) as for (j) after repeated evacuation at 620 K and measured in a vacuum at 300 K.
to 300 K (Figure 1e) and repeats the measurement at 77 K (Figure 1f), a distinctly different spectrum is obtained. (cf. traces d and f). Thus, for a moderate rise in measurement temperature a considerable change takes place, i.e., Fe2+ f Fe3+ oxidation. Spectra d-f were obtained on a sample kept in hydrogen atmosphere. The state of the sample reverted to a state similar to that of the starting one by repeating the evacuation at 620 K (Figure 1g). In the next series of spectra the effects of a similar reduction process are illustrated. First, reduction was repeated on the sample at 620 K in hydrogen flow; the sample was then cooled to 77 K (Figure 1h), then it was evacuated at 140 K and a spectrum was recorded at this temperature (Figure 1i); a further
spectrum was then recorded at 300 K - still in a vacuum (Figure 1j). The Fe2+ f Fe3+ oxidation is clearly demonstrated in this series, too, by raising the temperature of measurement. The difference in the shapes of spectra i and j in comparison with those of spectra e and f are due to the removal of coordinated water in the latter series (the characteristic QS values are smaller for Fe2+ and larger for Fe3+). At the end, with the repetition of the evacuation at 620 K and recording the spectrum at 300 K (Figure 1k), the starting (evacuated) state can be almost restored; this spectrum is very similar to that recorded at the start of the series of treatments (Figure 1c). It can be seen from the decomposition of spectra that a small amount of the substituted iron is probably lost from the framework, as attested by the decrease of the relative intensity of the Fe3+Td,2 component from 71% to 57% (Table 2). It should be noted here that the assignments of coordinations should be considered as approximations; they can be more or less modified. It is recalled here that a great variety of coordinations may be stabilized in silicate minerals.26 Further, it should also be mentioned that IS and QS values may strongly depend on the secondary coordination of ligands (e.g., water) around the charged iron ions in the Si-O-Fe(OH)-Si groups.27 In short, basic types of primary coordinations of iron ions were distinguished by fitting our spectra, the annotations in Tables 2 and 3 should be considered as guiding approximations. Mo1 ssbauer Measurements on the Al+Fe-FER Sample. The series of in situ spectra recorded on the Al+Fe-FER
4868 J. Phys. Chem. B, Vol. 102, No. 25, 1998
La´za´r et al.
TABLE 3: Mo1 ssbauer Data Obtained on Sample Al+Fe-FER (Si/(Al + Fe) ) 16, Al/Fe ) 3)a 300 K treatment as received (calc.) evacuated, 620 K hydrogen, 620 K evacuated, 620 K (2nd)
comp/coordb Fe3+
(Oh)
Fe3+(Oh) Fe2+(Td/Oh) Fe3+(Td,2) Fe2+(trig) Fe2+(Td/Oh) Fe3+(Td,1) Fe2+(trig) Fe2+(Td/Oh) Fe2+(Oh) Fe3+(Td,2) Fe2+(trig) Fe2+(Td/Oh)
SCHEME 1 77 K
IS
QS
RI
IS
QS
RI
0.37 0.35 0.95 0.27 0.89 1.04 0.18 0.84 1.03 1.47 0.20 0.89 1.02
1.19 0.70 2.09 1.88 0.58 2.27 1.25 0.65 2.15 2.93 1.81 0.63 1.94
62 32 5 78 13 9 33 9 49 9 35 20 45
0.47 0.46 1.29
1.17 0.66 2.76
66 27 7
0.34 0.99 1.13 1.44
1.67 0.55 2.45 2.67
24 5 30 41
IS, isomer shift, related to R-iron, mm/s; QS, quadrupole splitting, mm/s; RI, relative intensity, %. b The assignments for coordinations are approximate. a
Figure 2. In situ Mo¨ssbauer spectra of Al+Fe-FER sample [Si/(Al + Fe) ) 16, Al/Fe ) 3]. Spectra were recorded after subsequent treatments performed on the same sample: (a) as received in H form (77 K); (b) after evacuation at 620 K (recorded at 300 K in a vacuum); (c) after hydrogen treatment at 620 K (cooled immediately to 77 K and kept in H2); (d) as for (c) measured at 300 K, in H2; (e) as for (d) after a repeated evacuation at 620 K and measured in a vacuum at 300 K.
sample are shown in Figure 2. (Data obtained from the decompositions are presented in Table 3.) These spectra exhibit distinct differences compared with those of the Fe-FER sample: the coordination and oxidation states of iron are characteristically modified by the presence of Al. There are three important differences: the first important difference is that already upon the first evacuation at 620 K, Fe3+ f Fe2+ reduction has taken place to a considerable extent. As a result a new, reduced Fe2+ component appears (IS ) 0.89, QS ) 0.58 mm/s) that was not detected in any previous spectrum of the Fe-FER sample (Figure 2b). The second important difference is that the hydrogen treatment results in permanent Fe3+ f Fe2+ reduction. Variation in the measurement temperature does not influence strongly the proportions of Fe3+ and Fe2+ contributions in the spectra (Figure 2c,d). These two spectra display the temperature dependence
of quadrupole splitting, without characteristic change in the oxidation state of iron. The change of the percentage of Fe2+ contribution in the spectrum is only modest (29% and 33% RI values, respectively). A slight change in the percentages can also be attributed to the different temperature dependences of the probabilities of the recoilless transitions of iron in the various components, i.e., to the change of the so-called f factor.28 The third noteworthy feature is that only a few treatments changed the state of iron to a great extent; the starting state of the sample can be only partially restored (compare the spectra obtained after evacuation: Figure 2, traces b and e). The original T-site substitution of iron dropped from an RI value of 78% to 35%; i.e., a significant amount of iron was removed from the framework by two evacuations and a further hydrogen treatment at 620 K. Interpretation of Mo1 ssbauer Spectra. To correlate the different coordination states with the IS and QS parameters we may rely on data obtained on Fe-MFI zeolites,13,15 and also on results obtained from in situ Fe-MFI studies.18,19 The substituted tetrahedral iron in T-sites is characterized by a low IS value (IS < 0.3 mm/s) and may exhibit different QS values depending on the symmetry of coordination of ligands. For Fe3+ the greater the asymmetry, the larger the QS value. Thus, for the substituted Fe3+Td two components were separated by decomposing the spectra: (i) a component with a smaller asymmetry (Fe3+Td,1, Scheme 1), and (ii) another one with larger asymmetry in the electric field around the iron ion (Fe3+Td,2, Scheme 1), as suggested in refs 17 and 18. The extraframework Fe3+ is probably closer to the octahedral coordination; i.e., it exhibits larger IS (> 0.3 mm/s) and smaller QS (0.8 < QS < 1.2 mm/s) values (analysis of Mo¨ssbauer data for extraframework iron ions can be found, for example, in refs 29-31). It should be noted (as mentioned previously) that the tetrahedral and octahedral terms are used only as approximations. Thus, for the as-synthesized and calcined samples it may be concluded generally that the Mo¨ssbauer method provides similar results to the other methods; the synthesis proved to be successful, and most of the iron ions are located at Fe3+Td sites. The greater part of Fe3+Td sites can be assigned to single iron substitutions in Si-O-Fe(OH)-Si groups (Fe3+Td,2) with relative spectral contributions of 71% for Fe-FER and 78% for Al+Fe-FER. Indications for the Presence of Felattice-O-Feextraframework Pairs. The Fe-FER sample exhibited an Fe2+ f Fe3+ valency change after the 620 K treatment in hydrogen in a high proportion already when the measurement temperature was
Interpreting Catalytic Activity in Ferrierites SCHEME 2
increased from 77 to 300 K, and this particular feature can be observed either in hydrogen (spectra of Figure 1d-f) or in a vacuum (Figure 1h,i). It is suggested that this phenomenon can be interpreted by assuming that there are Felattice-O-Feextraframework pairs and that both of the iron ions in them are reduced temporarily to the Fe2+ state. Felattice-O-Feextraframework pairs can easily be formed in these systems since (hydrated) FeO+ may also act as a charge-compensating cation. During the treatment in hydrogen at 620 K both iron ions reduce to Fe2+ and their state may be preserved during the relatively rapid cooling to 77 K (Scheme 2). To support this assumption it is mentioned that the existence of divalent ions in T-positions has already been observed (e.g., Co)32 and the size difference of Fe3+ (64 pm) and Fe2+ (74 pm) does not introduce a large additional distortion to the lattice. It should also be mentioned that there is a likelihood that some of the iron is removed from the framework in the reducing atmosphere since even autoreduction may also occur in these zeolites, which contain transition metal ions enabling them to change their valency.33 However, this latter process is probably not dominant, since the amount of the Fe3+Td,2 component does not decrease to a large extent. The RI values showed only a 14% decrease from the starting treatment to the final one for the corresponding Fe3+Td,2 value. After the measurement temperature is raised from 77 to 300 K, the structure relaxes and transition to a more stable arrangement takes place (Scheme 2). The Mo¨ssbauer parameters of components forming the structures shown in Scheme 2 are probably different from those of the T-substituted Fe3+Td,2 one since the charge distribution is less asymmetric around these ions than in the Si-O-Fe(OH)-Si groups. Formation of a further Fe2+ component is suggested in the framework upon reduction in hydrogen. The relatively low quadrupole splitting values (QS77K ) 2.0 mm/s) and the low IS values (IS77K ) 1.1 mm/s) probably indicate a coordination approaching tetrahedral symmetry of ligands;26 thus the Fe2+Td-Oh notation is used. A very rough approximation suggests that ca. 1/4-1/3 of the iron is to be found in this reduced Fe2+Td-Oh lattice T position at 77 K shortly after the hydrogen treatment. It is also worth mentioning that nearly half the total amount of iron remains in the Fe3+ state; this component can be attributed to the Si-O-Fe(OH)-Si groups containing single iron centers, which are more difficult to reduce. (The quadrupole splitting on Fe3+Td may vary due to the coordination of some of the water formed during the reduction.) It should also be mentioned that certain data in Table 2 may be interpreted as indications of intervalence states of iron.34 Thus, in certain cases the occurrence of this interaction cannot be excluded. However, the vast majority of data can be considered as characterizing nonmixed Fe2+ and Fe3+ states. Comparison of Fe-FER with Al+Fe-FER. The in situ Mo¨ssbauer spectra obtained on the Al+Fe-FER sample ex-
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4869 hibited characteristic differences compared with those recorded on the Fe-FER sample. The very first treatment (evacuation at 620 K) resulted in a certain reduction, and after the treatment about 1/5 of the spectral area could be assigned to the Fe2+ component. In the Al+Fe-FER sample the removal of substituted Fe and Al ions was probably more expressed, as is suggested for autoreduction processes taking place in frameworks containing multivalent transition metal and aluminum ions.33 It therefore seems that the framework simultaneously containing different T ions (e.g., Al and Fe) is less stable than that containing only one type of foreign T ion (e.g., Fe) in the same Si/T ratio. Furthermore, the greater part of the reduced component (Fe2+Td) can be characterized by parameters not detected in Fe-FER spectra at all (IS) 0.89 and QS) 0.58 mm/s). These low IS and QS values are indicative of trigonal ferrous coordination state, Fe2+trig, as suggested, for example, in ref 35. Since this species is not a component of the FeFER spectra it can probably be related to the presence of AlO-Fe pairs in the Al+Fe-FER sample. Further characteristic differences can be observed by comparing the Al+Fe-FER and Fe-FER spectra recorded after the 620 K treatment in hydrogen. First, in the Al+Fe-FER a much larger amount of iron was reduced to the Fe2+ state. Second, the most characteristic difference observed is that when the measurement temperature was raised from 77 to 300 K it was not accompanied by oxidation, and the amounts of the Fe2+ components are similar (approximately 1/3 of the spectral area). The change in shape of the corresponding Al+Fe-FER spectra can primarily be attributed to the temperature dependence of IS and QS values of Fe2+ components. The spectrum obtained after the repeated evacuation proves that reduction took place in a large portion; indeed, only about 1/3 of the spectral area can be assigned to the original ferric state, and about 1/5 of the area can be assigned to Fe2+Td. Thus, it can be concluded that the Al+Fe-FER framework is less stable, and thereby the removal of the substituted T ions from the framework was easier. The appearance of a new spectral component (Fe2+trig) also suggests the presence of Al-O-Fe pairs. In addition, it is mentioned that the stochastic probability of formation of FeO-Fe pairs is considerably smaller in the Al+Fe-FER sample (at the given Al:Fe ) 3 ratio) than in the Fe-FER one. Comparison of the general behavior of the two samples indicates that the Fe-FER sample is more stable, as the T-substituted iron sites can be preserved in it in high proportion even after repeated reduction cycles. The stability of the framework can probably be partially attributed to the different behavior of Al-O-Fe and Felattice-O-Feextraframework pairs. Formation of the former, mixed oxide is accompanied by the removal of the ions substituted previously in the framework. In contrast, in the Felattice-O-Feextraframework pairs, Fe3+ f Fe2+ valency change may take place without removal of a significant amount of the T-substituted part from the framework. Therefore, formation of the Fe2+Td-Oh component during the 620 K reduction and its temporary stabilization at 77 K is suggested. In a relaxation process of the lattice at room temperature, probably the equilibrium of the redox process is shifted toward the stabilization of Fe3+ states in both iron components of the Fe-O-Fe pairs. Catalytic Studies. Two types of catalytic reaction were studied on Fe-FER catalyst: (i) oxidation of n-hexane and (ii) hydroxylation of phenol. In reaction i, the conversion was 15% and hexan-3-one was the main product with a selectivity
4870 J. Phys. Chem. B, Vol. 102, No. 25, 1998 exceeding that for hexan-1-one. In the hydroxylation of phenol, 55% conversion was observed, with a catechol to hydroquinone quotient of 1.4. In sharp contrast, the Al+Fe-FER sample did not exhibit any catalytic activity in either of the reactions. Thus, the catalytic properties of the two samples proved to be dramatically different in the oxidation reactions studied. Comparison indicates that a primary role can be attributed to Felattice-OFeextraframework pairs in the oxygen transfer since this component exhibited the ability to change the valency of iron ions without being removed from the latticesas proved by the experimental evidence presented here. Moreover, this interpretation is in full agreement with the results of the mentioned model calculations performed on the Fe-O-Fe centers.11,12 (In these calculations the possibility of reversible oxygen fixation is proposed, though it should be added that these calculations were performed solely on dinuclear centers, neglecting their actual attachment to the lattice.) Our study indicates that the zeolite framework stabilizes the dinuclear centers provided they are structural constituents in the framework. In contrast, for Al+Fe-FER samples the presence of AlO-Fe pairs prevailed and Fe-O-Fe pairs were not identified. The presence of Al-O-Fe pairs forming a mixed Fe-Al oxide phase in Al+Fe-FER cannot be ruled out. Thus, the lack of reversible charge-transfer processes in Al+Fe-FER catalyst can be attributed to the presence of a mixed Al + Fe oxide phase formation, which may be accompanied by the considerable destruction of the lattice on reduction. This is also in accordance with results of other observations claiming that the presence of Al in ferrisilicates inhibits their catalytic activity1 as well as with the results of model calculations in which the Al-O-Fe centers were found not capable for reversible oxygen transfer.11 Conclusions Two iron-substituted ferrierite zeolites with a ratio of Si/T ) 16 have been studied by in situ Mo¨ssbauer spectroscopy. Various oxidation and coordination states of iron have been identified and their transformations during evacuation and reducing treatment have been monitored. The Fe-FER exhibited higher stability in retaining Fe3+ ions in T-substituted sites. Upon treatment at 620 K in hydrogen the Fe3+Td state was mainly preserved in the Si-O-Fe(OH)Si groups containing single iron centers. In groups comprising Felattice-O-Feextraframework pairs the temporary reduction of Fe3+Td,lattice to the Fe2+ state is suggested with one Fe2+ component remaining in the framework; this was confirmed by 77 K Mo¨ssbauer spectra. Later, when the framework is stabilized at 300 K, the redox equilibrium is probably shifted toward the stabilization of the Fe3+ state in both constituent iron ions. The Al+Fe-FER sample exhibited a significantly lower stability in preserving the trivalent ions in T positions. An amount of Fe3+ was reduced to the Fe2+ state and the removal of trivalent ions from the framework was also observed upon evacuation. Formation of Al-O-Fe pairs is also suggested. The 650 K treatment in hydrogen results in further irreversible reduction and removal of Fe3+Td from the lattice. With respect to the catalytic ability in mild oxygenation processes exhibited only by the Fe-FER zeolite, it seems plausible to attribute the catalytic performance to the presence
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