Valency and coordination of iron in FeAIPO molecular sieves: an in situ Mössbauer study

I. Kiricsi, G. P~iI-Borb61y, J.B. Nagy, H.G. Karge (Editors) Porous Materials in Environmentally Friendly Processes Studies in Surface Science and Catalysis, Vol. 125 9 1999 Elsevier Science B.V. All rights reserved.

213

Valency and coordination of iron in FeAIPO molecular sieves: an in situ MSssbauer study* K. I_~.~ra and J. Cejka b a Institute of Isotope and Surface Chemistry, Chemical Research Center, Budapest P.O.B. 77, H-1525, Hungary b j. Heyrovsky Institute of Physical Chemistry, Dolejskova 3, 182 23 Prague 8, Czech Republic

Changes in the valency and coordination states of iron in FeAIPO-5 and FeAIPO-11 during reduction-oxidation treatments were followed by extracting isomer shift, quadrupole splitting and relative absorption area data from in situ M6ssbauer spectra. Facile Fe3+ ~ Fe 2+ reduction was detected, and the Fe2+ formed was stabilized in a low symmetry coordination state. The bonding strength, characterized by the Debye temperature (| was considerably smaller for Fe2+ than for Fe3+ (eD(Fe2+) = 170 K, OO(Fe3+ ) = 500 K). 1. INTRODUCTION

Aluminophosphate based molecular sieves represent interesting materials from both structural and catalytic point of view [1]. Incorporation of other three- and particularly two-valent cations may significantly change the stability of the framework, and can induce acidity which may play a role in acid catalyzed reactions. Moreover, upon substituting a transient metal ion (e.g. Fe) the change in the valency can be utilized in redox reactions, as well. A good example of the latter is FeAIPO-11 which exhibited a superior performance in hydroxylation of phenol with H202 [2]. Simple iron phosphates also promote mild oxidation. In phosphates, iron exhibits octahedral coordination prior to and after oxidation as well [3]. In contrast to phosphates the large-pore AIPO molecular sieves are assumed to retain their original structure without losing oxygen from their framework, and the framework substituted iron ions are supposed to be sited in tetrahedral positions. MSssbauer spectroscopy is a unique technique for obtaining information simultaneously on both the oxidation and coordination states of iron. (EPR * This work was financiaiiy supported by the research grants obtained from the Hungarian National Science Research Fund (OTKA, project T021131) and that from the Grant Agency of the Academy of Sciences of Czech Republic (A4040707)

214

spectroscopy can also be used but the method is primarily restricted to detection of paramagnetic Fe3+ ions [4].) Indeed, several studies have been performed on various FeAIPO-s by MSssbauer spectroscopy, e.g. [5-8]. These studies, performed "ex situ", were mainly devoted to structural characterization, to investigate the efficiency of the synthesis and the effects of calcination. The actual states of iron during catalytic processes are preferably studied by in situ M6ssbauer measurements performed under reduction-oxidation treatments. Moreover, to complement the "regular" MOssbauer parameters, viz. the isomer shift (IS), and quadrupole splitting (QS), it is also worth extracting the relative recoilless resonant absorption area (RAA) data from the spectra. From this latter parameter the value of Debye temperature (eD) can be determined and conclusions can be drawn on the bonding strength of iron [9]. In the present contribution in situ MOssbauer studies performed on FeAIPO-5 and FeAIPO-11 under reduction-oxidation treatments are reported. Changes of the coordination and oxidation states of iron are monitored and the value of ~ is estimated. Some assumption is also given in regard to catalytic performance in oxygenation reactions.

2. EXPERIMENTAL 2.1. Synthesis To synthesize FeAIPO-5 and FeAIPO-11 sieves 12.25 g of pseudoboehmite (Catapal B, Vista) was added to a mixture of 19.85 g H3PO4 (85 %) and 50 g water while vigorously stirring. The gel formed was stirred for at least 2 h at ambient temperature. Afterwards, tetraethylamrnonium hydroxide (in 20 % aqueous solution) to produce FeAIPO-5, or, di-n-propylamine (8.65 g in 3.5 g water)in order to form FeAIPO-11, as templates were added, while stirring further for another 2 h. Then an appropriate amount of iron nitrate solution was added to the mixture, again followed by stirring (for 2.5 h). In the next step the gel was transferred to a teflon lined autoclave (90 ml) and heated (at 470 K for FeAIPO-5, or at 430 K for FeAIPO-11) under autogeneous pressure for 18 h. After the synthesis the solid phase was separated by filtration; washed with deionized water, and dried at 350 K overnight. 2.2. Analysis and characterization Good crystallinity and phase purity of FeAIPO samples were confirmed by XRD (Siemens D5005) using Cu-K~ radiation with a Ni filter. The iron content in the samples was determined by ICP-OES (induced coupled plasma - optical emission spectroscopy). In situ MOssbauer spectra were recorded at 77 and 300 K in a sequence of treatments (evacuation, reduction in hydrogen, oxidation in air). FeAIPO-5 was treated at 620 K, the less stable FeAIPO-11 at 530 K. The samples showed a 10-12 % weight loss due to removal of adsorbed water during the treatments.

215

3. RESULTS

A

3.1. Synthesis XRD patterns for both FeAIPO-5 and FeAIPO-11 present welldeveloped structures corresponding to those reported earlier [7] (Fig. 1). In both cases the iron content was similar: 0.8 wt % for FeAIPO-5 and 0.75 wt % for FeAIPO-11. (This iron content corresponds to a substitution of ca. each 43th AI3+ ion for iron in the framework, presuming even distribution of iron.)

~ (/} t"

B

t-

0

10

20

30

40

50

angle

Figure 1. XRD diffractograms of synthesized samples A: FeAIPO-5, B: FeAIPO-11

3.2. MSssbauer measurements: isomer shift (IS) and quadrupole splitting (QS) The structure of APO-5 is stable allowing various treatments to be carried out at 620 K on FeAIPO-5. The in situ spectra recorded after sequence of steps of treatments are shown in Fig. 2, the extracted MSssbauer data are collected in Table 1. When interpreting the data, it could be borne in mind that the sample contains iron both in Fe3+ and Fe2+ states at the outset (Fig. 2a). Thus, it seems that Fe2+ ions can also be stabilized and accommodated in the framework in spite of the long storage in air. The second significant feature is that the major part of Fe3+ can be converted to Fe 2+ upon a mere evacuation (Fig. 2b). It is important to stress that the IS and QS data obtained for Fe2+ (in particular the low QS values) cannot be attributed to octahedral coordination, they probably characterize an environment of Fe2+ closer to tetrahedral or another arrangement of distorted symmetry [10]. It should be noted that the subsequent reducing treatment in hydrogen does not significantly change the previous proportions of Fe2+ and Fe3+ (Fig. 2c), although slight changes can be observed in the spectra recorded at 300 K. As IS and QS parameters attest, Fe2+ still remains in a state characterized by distorted symmetry. In contrast, in spectra recorded at 77 K the largest portion of Fe2+ exhibits large QS values, characteristic for coordinations closer to octahedral. The strong change of QS

216

300 K

9" - . . ~

77 K

,~~--'i"

b

Figure 2. In situ M6ssbauer spectra of FeAIPO-5 sample recorded after sequence of treatments; a: as synthesized, b: after evacuation at 620 K, c: after H2 treatment at 620 K, d: temperature of measurement increased from 77 K to 300 K, e: repeated evacuation at 620 K, f: after storage in air for 11 days

e

f

-4-'2 !

1

6

,

1

4-2 i

I

J

6

|

l

4 'F

VELOCITY ( r a m / s )

depends exclusively on the measuring temperature - as is demonstrated by the repetition of measurement in a 77 K --> 300 K ~ 77 K cycle resulting in the similar shape of corresponding spectra (cf. Fig. 2c and 2d). The starting state of the FeAIPO-5 sample can be restored by evacuation (Fig. 2e is similar to Fig. 2b), and after storage in air for 11 days the spectrum of Fig. 2f can be recorded which is rather similar to that of the starting one (Fig. 2a). The structure of APO-11 is less stable than that of APO-5, thus FeAIPO-11 was treated at 530 K. The spectra obtained were similar to those shown in Fig. 2 for FeAIPO-5, thus they are not displayed. The data extracted from the fits are reported in Table 1. FeAIPO-11 exhibited similar features to those found for FeAIPO-5. Expressed reduction is attained by the hydrogen treatment, and the Fe2+ component exhibiting low QS appears in this sample, too. In addition, the reversibility in the change of QS can also be demonstrated in the 77 K ~ 300 K ~ 77 K cycle of measuring temperatures. A further similarity to FeAIPO-5 is that the starting state of the sample can be restored by storage in air.

3.3. Resonant absorption area (fA) and Debye temperature (OD) values The analysis of relative resonant absorption area data (RAA)is less usual when interpreting M6ssbauer spectra. This parameter is proportional to the resonant

217

Table 1 Data extracted from 300 K and 77 K in situ MOssbauer spectra (IS: isomer shift, relative to s-iron, ram/s; QS: quadrupole splitting, mm/s; RI: relative intensity, %; RAA: relative absorption area; fA(300/77)" ratio of relative absorptions, i.e. RAA(300K)/RAA(77K)) Temperature

300 K

Treat.

QS

Comp.a IS

77 K RI

" as rec.

Fe3+ Fe 2+

620 K evac.

0.36 0.94

RAA

QS

RI

RAA

fA(300/77)

FeAIPO-5 sample 0.73 2.16

78 22

Fe(n+) 0.69 Fe 2+ 1 . 0 1 Fe2+ 0.99

0.15 1.05 1.73

7 41 52

620 K H2

Fe3+ Fe 2+ Fe 2+

0.24 0.82 1.12

0.70 1.36 1.32

17 14 69

0.797

300 K H2

Fe3+ Fe(n+) 0.49 Fe 2+ 0.77 Fe 2+ 0.89

0.33 1.57 2.08

64 15 21

0.800

620 K evac.

Fe(n+) 0.56 Fe 2+ 0.94 Fe 2+ 0.96

0.14 1.27 1.85

18 42 40

300 K air

Fe3+ Fe 2+

0.70 2.27

64 36

0.40 0.97

IS

0.25 1.17 1.19

0.80 2.10 2.54

10 25 65

1.82

0.44

0.22

0.72

10

1.95

0.41

1.09

1.80

6

1.19

2.49

84

FeAIPO-11 sample as rec.

Fe3+ Fe 2+

0.37 0.98

0.67 2.08

75 25

0.834 0.285

0.42 0.83 1.21 2.37

61 39

0.897 0.93 0.583 0.49

530 K H2

Fe3+ Fe 2+ Fe2+

0.32 1.03 1.07

0.68 45 1.31 28 1.79 27

0.907

0.36 1.19 1.19

0.64 2.26 2.60

13 56 31

2.49

0.36

300 K H2

Fe3+ Fe 2+

0.44 1.18

0.66 2.45

27 73

2.35

0.39

300 K air

Fe3+ Fe 2+

0.37 1.06

0.63 43 2.01 57

a Fe(n+) component: characterizing probably intervalence states of iron, (see e.g. in [10])

2
218

absorption (fA), which contains information primarily on the bonding state of the resonant nucleus. The RAA values can be extracted from the spectra by relating the number of "missing" counts (originating from the resonant nuclei forming the absorption spectrum) to the counts in the base line (collected from "pquanta penetrating the sample without recoilless events). By determining the temperature dependence of fA the value of the Debye temperature (eD) can be determined; the larger the value of e D the stronger the bond. For accurate determination of e o, the values of fA should be determined in a wide temperature range and the corresponding curve should be fitted for e D [9]. For a first, rough approximation however, comparison of RAA values obtained at 300 K and 77 K may also be accepted [11]. In this respect there are two further important observations. First, the _..(RAA300K)/RAA(77K) ratios are small in the reduced samples (with dominance of the Fe ~'+ state), a rough approximation provides a eD(Fe2+) value of ca. 170 K for FeAIPO-5 and an even smaller value for FeAIPO-11. The second observation is that the low e D is characteristic only for Fe 2+. In the case of Fe3+, a markedly larger value ( OD(Fe3+) ca. 500 K) can be obtained from the corresponding decomposition and comparison of the first 300 K and 77 K spectra of the FeAIPO-11. It should be mentioned that the latter OD(Fe3+) value is in the range characteristic of strong ionic bonds, whereas values similar to the previous one (eD(Fe2+) of ca. 170 K) can be observed in complex molecules of iron (e.g. in Fe-phthalocyanine [11]).

4. DISCUSSION 4.1. Location of iron ions The first point which should be clarified for interpreting the spectra is to provide information on the location of iron, i.e. whether it occupies preferably framework substituted lattice positions, or, whether it is located in extra-framework positions inside the pores. First, it is pointed out that the two synthesized FeAIPO-5 and FeAIPO-11 contain both Fe 2+ and Fe3+ in their as received state. Thus, it is suggested that at least two positions are occupied by iron ions. However, from further data analysis, the dominance of framework positions is suggested. At the starting state a large value was found for O D(Fe3+) (ca 500 K) indicating strong ionic bonds. The large QD(Fe3+) probably originates from ions incorporated into the primary lattice. (Since the primary structure is principally neutral no charge compensating extraframework ions are necessary, thus QD Fe3+) for small neutral clusters in the pores would be a small value.) Further on, the(low IS and QS values obtained in the 300 K spectra for the major part of Fe 2+ after the reducing treatment reflect a distorted environment, close to tetrahedral coordination. (Fe2+ ions in extra-framework positions could occupy enough space to fill an octahedral coordination sphere.) For example, the Fe2+ state characterized by the pair of low IS and QS values does not appear in various iron phosphate catalysts - large IS and QS values are always found for Fe 2+ in the 300 K spectra indicating octahedral symmetry [3]. Thus, the major part

219

of iron is probably incorporated into lattice sites substituting AI in the primary structure. 4.2. Facile Fe 3+ <-->Fe 2+ valency changes and stability of Fe 2+ state in FeAIPO-s As for the catalytic oxidation, it is important to recognize that the Fe3+ --> Fe2+ reduction is facile and the Fe2+ that is formed stabilizes easily. For comparison it might be noted that in ferrisilicates (Fe-MFI and Fe-FER) the Fe2+ state cannot be stabilized for framework substituted ions under the same experimental conditions. The Fe2+ ion is present only temporarily and is reoxidiz,ed soon after the reduction; the rise in the measuring temperature from 77 K to 300 K is sufficient for the Fe2+ --> Fe3+ oxidation of framework substituted ions [11]. In contrast, as was demonstrated here for FeAIPO-s, the Fe 2+ oxidation state is stable and is not affected by varying the temperature of measurement (although the QS value varies strongly with the temperature). 4.3. Bonding strength and different eD-S for Fe 3+ and Fe 2+ The marked difference in the estimated values of eD(Fe3+) (500 K), and OD(Fe2+) (170 K) indicates that the bonding states of Fe3+ and Fez+ are distinctly different. The ferric ion exhibits a strong ionic bond, whereas the ferrous state is characterized by a considerably softer bond. The low value of eD(Fe2+) and the expressed temperature dependence QS(Fe2+) are more characteristic for covalent bonds. As a means of explanation, one might also consider either removal of oxygen or attachment of hydrogen to oxygen in the close vicinity of iron ions taking place during reducing treatment. Thus, an active role is proposed for the neighborhood of iron in selective oxidation processes, or in hydroxylation - mentioned in the Introduction [2]. As a possible path the reaction may proceed on centers in the vicinity of iron by starting the partial disruption of the -AI-O-P-O-Fe(3+)-O-P- chain to -AI-O-P(OH) . Fe(2+)-O-P- for transferring oxygen. The vacancy may be filled up again, and the cycle repeated.

5. CONCLUSIONS FeAIPO-5 and FeAIPO-11 (ca. 0.8 wt % Fe) were studied under reductionoxidation conditions to reveal changes in the coordination and the oxidation state of iron taking place during mild oxidation processes. Facile reduction of Fe3+ to Fe2+ is detected (in comparison, for example, with MFI and FER ferrisilicates); after reduction, the Fe 2+ state prevails and is stabilized in a coordination state of low symmetry. Based on the Debye temperature data considerably smaller bonding strength was estimated for Fe 2+ (e D(Fe2+) = 170 K) than for Fe3+ (eD(Fe3+) = 500 K). The starting state of the FeAIPO-s can, in fact, be restored by exposure to air at room temperature. In catalytic oxidation processes, removal of oxygen from the vicinity of iron with simultaneous Fe3+ -+ Fe 2+ reduction might be proposed; or, in hydroxylation, -P(OH) groups formed adjacent to iron may play a role by transferring oxygen.

220

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