Redistribution of iron ions in porous ferrisilicates during redox treatments

Fe-FER

This sample contained only silicon and iron, i.e. aluminum was not present in it. Thus, charge compensation of tetrahedral Fe³⁺_FW sites can be provided with extra-framework iron ions and with Brønsted acidic H⁺ protons. Mössbauer spectra were collected on Fe-FER samples with Si/Fe=16 ratio. The first step, the hydrothermal synthesis resulted primarily in the inclusion of Fe³⁺ ions into tetrahedral FW sites. In the course of the further treatments a part of iron was relocated to EFW sites, as in situ Mössbauer spectra attest (Fig. 2 and Table 1).

Fig. 2:

Series of in situ Mössbauer spectra recorded in sequence after various treatments. (Spectra were recorded at ambient temperature, except spectrum d, 77 K) FW Fe³⁺contributions are marked in blue, EFW Fe³⁺ in brown and EFW Fe²⁺ in green.

First it should be noticed that the evacuation at 620 K after calcination results in the removal of adsorbed water from the vicinity of the FW substituted iron and Brønsted acidic sites form with H⁺ charge compensation (Fig. 2b). The distorted tetrahedral symmetry results in appearance of large quadrupole splitting for the FW iron ions (Δ=1.97 mm/s). The reducing treatment in hydrogen at 620 K results in FW => EFW removal for significant part of iron, 51 % of spectral area can be assigned to Fe²⁺. These ions are probably located at EFW sites (due to the larger ionic radii they cannot be accomodated in FW sites any more). The proportion of Brønsted acidic Fe³⁺ FW sites (Δ=1.81 mm/s) decreased to 16 %. The larger portion of Fe³⁺ component exhibit smaller quadrupole splitting (Δ=1.29 mm/s). This can be attributed to the formation of Fe³⁺_FW-O-Fe²⁺_EFW pairs (Fig. 2c). Fe²⁺ ions are larger, they distort the symmetry around the FW Fe³⁺ ions in lesser extent than H⁺ ions do. The final evacuation restores the H⁺ charge compensation for 39 % of FW Fe³⁺ iron (Δ=2.00). The other main Fe³⁺ component can be assigned to iron located in Fe³⁺_FW-O-Fe³⁺_EFWpairs (Fig. 2d). The presented series of spectra illustrates the redistribution of iron ions during treatments. Upon the reducing treatment in hydrogen a significant part of iron was removed from the FW sites EFW ones, and Fe_FW-O-Fe_EFW pairs were formed. It is worth noticing that reversible redox Fe³⁺⇔Fe²⁺ transition has taken place only on the extra-framework part of Fe_FW-O-Fe_EFW dimers, the FW Fe³⁺ has not changed its valency.

The Fe-FER sample was evaluated in catalytic processes as well, in oxidation of n-hexane and in hydroxylation of phenol both with hydrogen peroxide in liquid phase. The catalyst was acitve in both reactions promoting the transfer of oxygen by forming hexan-3-one and hydroquinone [17].

(Al+Fe)-FER

The Al:Fe ratio in this sample was 3:1, i.e. the probability of formation of Fe_FW-O-Fe_EFW pairs diminishes, since even the whole amount of iron is not sufficient to provide charge compensation for framework Al³⁺ sites. In situ Mössbauer spectra recorded on this sample after various treatments are shown in Fig. 3.

Fig. 3:

In situ Mössbauer spectra of (Al+Fe)-FER sample recorded after series of various treatments. (Spectra were recorded at ambient temperature, except spectrum c, recorded at 77 K) FW Fe³⁺contributions are marked in blue, EFW Fe³⁺ in brown and EFW Fe²⁺ in green.

The shape of spectra recorded after treatments are characteristically different from those of the Fe-FER shown in Fig. 2. The EFW Fe³⁺ is practically absent, except the first spectrum of the calcined component. The first in situ treatment, evacuation of the sample at 620 K C results in the appearance of the Fe_FWcomponent, with Brønsted acidic H⁺ charge compensation, similarly as was the case with Fe-FER. The subsequent reduction converts ¾ of iron into EFW Fe²⁺ the remainder ¼ is still present as FW Fe³⁺ (Δ=1.67 mm/s). Remarkable difference in comparison to spectra of the previous Fe-FER sample is the complete lack of the Fe³⁺_FW/EFW component assigned to the Fe_FW-O-Fe_EFW pairs. This lack can be attributed to the dominance of Al ions in those tetahedral sites which require charge compensation by EFW ions (Al:Fe=3:1). The final evacuation does not result in further changes in the distribution of iron among FW and EFW sites. Further data for decomposition of spectra are provided in [17].

The (Al+Fe)-FER sample was tested in the same reactions under the same conditions (oxidation of hexane and phenol) as Fe-FER in the preivious case. The sample did not exhibit any catalytic activity for oxidation. Thus, the comparison of Fe-FER and (Al+Fe)-FER samples shows that the catalytic activity for oxygen transfer in liquid phase oxidations can be attributed to presence of Fe_FW-O-Fe_EFW pairs, where the Fe²⁺⇔Fe³⁺ process may proceed on the extra-framework iron component. The aluminum containing counterparts, the Al_FW-O-Fe_EFW pairs are probably not active in this oxidation.

It might be added that similar redistribution of iron ions were observed on three types of MCM-22 samples, with possible formation of Fe_FW-O-Fe_EFW pairs (Fe³⁺_FW with Δ=1.53 mm/s) [24]. These Fe-MCM-22 samples were not tested in catalytic processes.

Changes in coordination of iron in mesoporous MCM-41

Channel diameters in mesoporous substances are usually in the 3–7 nm range. These values are larger with c.a. one order of magnitute as compared to microporous zeolites. Walls of channels and pores in mesoporous substances are partly amorphous, crystallinity is less expressed in them than in microporous zeolites [25]. Similarly to zeolites not only silicon, but aluminum, and other transitional metal ions can also be incorporated into them, and structural rearrangements may also proceed in the neighborhood of these ions, as well. This feature can clearly be observed on mesoporous ferrisilicates, and can be followed by Mössbauer spectroscopy, too. As an illustration a series of in situ Mössbauer spectra recorded on Fe-MCM-41 ferrisilicate exposed to a sequence of treatments at different conditions is presented and interpreted below (Fig. 4).

Fig. 4:

In situ Mössbauer spectra of Fe-MCM-41 in a series of subsequent treatments. Contrubutions of Fe³⁺in octahedral coordination are marked in orange, Fe³⁺ in tetrahedral in blue, Fe²⁺ in green. Most of spectra were recorded at ambient temperature, except spectra c (620 K), d (530 K), g and h (both 570 K).

The first calcination was performed at 750 K (Fig. 4a). The subsequent evacuation at 670 K results in a Fe³⁺=> Fe²⁺ autoreduction in a considerable extent, c.a. 1/3 of iron is reduced to Fe²⁺. The most significant part of the spectrum (44 %) can be attributed to Fe³⁺ located in tetrahedral coordination. However, this siting is not characteristic for crystalline FW sites as was the case for microporous Fe-FER and Fe-MFI. Namely, in the subsequent spectrum recorded in a stream of hydrogen at 620 K almost all the iron is present in Fe²⁺ oxidation state (except 8 %, Fig. 4c). This is in strong contrast with the properties of microporous Fe-FER and Fe-MFI samples where the FW Fe³⁺ maintained its ferric state at this condition. This clearly shows the flexibility of the mesoporous structure, i.e. that the reducing hydrogen have access to almost all the ferric iron ions present in this substance previously. The following evacuation does not influence the dominance of ferrous state (Fig. 4d). A repeated 10 h long calcination at 670 K has certain influence on the structure, the proportion of the tetrahedrally coordinated Fe³⁺ component increases to 60 %. The 7^th spectrum of this series was recorded in a stream of CO at 570 K. Tetrahedrally coordinated Fe³⁺ is present in 18 %, the other speces are in ferrous state. Thus the extent of the involved proportion in the Fe³⁺ => Fe²⁺ reduction depends on the reducing component, too, (in hydrogen only 8 % retained the Fe³⁺ state, Fig. 4c). The fourth, final evacuation shows a restructured system. The contribution of the tetrahedral Fe³⁺component reaches almost ½ part of the spectral area, the remaining other ½ part is shared between Fe²⁺ species. Thus, this series of spectra clearly illustrates the high flexibility of the MCM-41 mesoporous structure, the openness of iron ions for changes both in their coordination and oxidation states. Further details of interpretation of spectra can be found in [20].

It is worth mentioning that these Fe-MCM-41 catalysts have been tested in CO oxidation. They did not exhibit catalytic activity if they were pretreated at temperatures only below 620 K. In contrast, they exhibit good activity in the CO oxidation already at ca. 350 K, provided they were exposed to reducing pretreatment at 770 K or at higher temperatures [26]. This demonstrates that he original amorphous structure is not sufficient to exert the catalytic effect yet. In order to provide the appropriate performance further rearrangements and stabilizations are necessary. The presented series of spectra attests that the Fe-MCM-41 is able for a variety of structural changes, and a catalytically efficient one can be formed, however, the active structure may also be subject to profound changes.

Fe-MFI with FW and EFW iron in N₂O decomposition at different temperatures

Presence and participation of various iron species in decomposition of a greenhouse gas, N₂O, has been the objective of several studies (recently e.g. [27]). Most of studies ascribe the catalytic activity to Fe²⁺ species,[8], [9], other studies suggest the assignment of the catalytic activity to dinuclear EFW centers similar to those shown in Fig. 1. [28], [29]. In our related study an Fe-MFI sample was synthesized into which both FW and EFW iron components were introduced intentionally, thus formation of Fe_FW-O-Fe_EFW pairs was facilitated. First FW ⁵⁷Fe³⁺ was inserted in a hydrothermal synthesis (Si/⁵⁷Fe ~200), then EFW Fe²⁺ was added by impregnation from ethanolic solution of FeSO₄. The preparation was successful, doublets for both the Fe³⁺ and Fe²⁺ components were present after an evacuation at 670 K (Fig. 5a). To check whether the Fe²⁺ component can be stabilized in N₂O atmosphere at higher temperatures an in situ spectrum was recorded in a stream of N₂O at 520 K. Surprisingly, the Fe²⁺ component was still present in this strong oxidizing atmosphere at this elevated temperature (Fig. 5b). The spectrum recorded after the treatment (not shown) is very similar to that shown in Fig. 5a. Thus, it can be concluded that the structure and proportions between Fe²⁺ and Fe³⁺ oxidation states have not been changed in this Fe-MFI up to temperature of 670 K (the temperature of evacuation).

Fig. 5:

Mössbauer spectra of (Fe²⁺/Fe³⁺) Fe-MFI. (a) After hydrothermal insertion of FW ⁵⁷Fe³⁺, subsequent impregnation with FeSO₄ and evacuation. Measured at 77 K. (b) In situ measurement in N₂O at 520 K. (c) and (d): After temperature programmed reaction test, both spectra measured at 77 K. (c) Kept in N₂O during cooling, (d) after subsequent removal of N₂O with evacuation. Fe³⁺ shown in brown, Fe²⁺ in green. Contribution from oligomeric iron is omitted for clarity.

Activation of iron species is described in most of communications to develop catalytic activity in the N₂O decomposition process. Usually a high temperature treatment is proposed in inert atmosphere. To comply with this condition our Fe-MFI sample was heated to 770 K in He stream and was conditioned for 2 h. After cooling the catalytic activity was tested in a temperature programmed reaction (10 % N₂O in He, 5 K/min ramp). The Fe-MFI catalyst was active in the 570–750 K temperature range, as shown in Fig. 6. Two local maxima appear, the first around 600 K, the second, with lesser intensity at 690–730 K.

Fig. 6:

Activity of the Fe-MFI catalyst in the temperature programmed decomposition of N₂O [19].

Mössbauer spectra were also colleced on the spent catalyst. First the catalyst was simply cooled in N₂O and measured at 77 K (Fig. 5c). Broad relaxing component and magnetically ordered sextet are displayed. These components can be attributed to various Fe_xO_y oligomers, which may be accommodated in the channels of MFI structure as illustrated in the upper right corner of Fig. 1b. The remaining significant part in the center of spectrum with 20 % contribution is a doublet which can clearly be attributed to Fe³⁺ ions (δ_77K=0.41 mm/s, and Δ_77K=0.86 mm/s, marked with brown in Fig. 5c). To check whether there exist possibility for Fe³⁺<=>Fe²⁺ reversal on the spent catalyst an evacuation was performed at 670 K. The obtained spectrum is shown in Fig. 5d. Surprisingly, the removal of sorbed N₂O resulted in the appearance of an Fe²⁺ component as well. Beside the relaxing and magnetically split oligomeric Fe³⁺ components, doublets of Fe²⁺ (δ_77K=0.89 mm/s, and Δ_77K=3.38 mm/s), and Fe³⁺ (δ_77K=0.38 mm/s, and Δ_77K=1.50 mm/s) can clearly be distingiushed with roughly similar share of contributions 13 % and 18 %, respectively. Thus, the spent catalyst, which was heated up to 860 K in the test of temperature programmed reduction was still able to exhibit the feature of Fe³⁺ ⇔ Fe²⁺ reversal. It is worth to recall that Fig. 5 exhibits two local maxima in the N₂O decomposition process. The first maximum can probably be related to catalytic centers with dominant presence of Fe³⁺_FW-O-Fe²⁺_EFW pairs as can be deduced from Mössbauer spectra shown in the upper part of Fig. 5. The second activity peak in the decomposition may probably be related a partly damaged catalyst structure, as formation of Fe_xO_yoligomers can be detected. However, the ability for Fe³⁺ ⇔ Fe²⁺ reversal is still present on similar proportions of iron ions. Thus, presence of Fe³⁺_FW-O-Fe²⁺_EFW pairs on this restructured catalyst cannot be excluded either. Further details on the studies of N₂O decomposition with the (Fe²⁺/Fe³⁺) Fe-MFI catalyst are described in [19], [30].