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Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid
Applied Catalysis A: General 200 (2000) 89–101
Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid Part I. An experimental study on isobutane oxidation catalysts Martin Langpape, Jean-Marc M. Millet∗ Institut de Recherches sur la Catalyse, CNRS, associé a l’Université Claude-Bernard, Lyon I, 2 Avenue A. Einstein, F-69626 Villeurbanne Cedex, France Received 1 October 1999; received in revised form 1 December 1999; accepted 1 December 1999
Abstract Keggin-type molybdophosphoric heteropolyacid with protons partially substituted by iron cations in a bulk form (Fe0.85 H0.45 PMo12 O40 ) or supported on the cesium salt (Cs2 Fe0.2 H0.4 PMo12 O40 ) have been synthesized and characterized by different techniques like the Mössbauer spectroscopy and the electron spin resonance (ESR). The effect of iron on the redox and catalytic properties for the oxidation of isobutane into methacrylic acid (MAA) has also been studied. Iron has been shown to have a different effect whether acts as a counter-cation in the bulk acid or in the acid supported on the cesium salt. In the first case, it increases both the selectivity in methacrylic acid and methacrolein (MA) and the activity of the acid phase whereas in the second case, it increases only the selectivity. This difference has been explained by the existence of an electron transfer between iron and molybdenum occurring only in the bulk acid. This electron transfer was related to a combined hydration-oxidation mechanism which promotes the reducibility of the solid and consequently its catalytic activity. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Isobutane oxidation; Iron counter-ions; Heteropolymolybdates; Electron spin resonance (ESR); Iron aqua complexes; Mössbauer spectroscopy
1. Introduction Twelve-heteropolymolybdates with alkaline and transition metals substituting partially or totally the protons have recently been shown to be efficient catalysts for the oxidation of alkanes [1–6]. These compounds with the general formula Cs2.5 Mx y+ H0.5−xy PMo12 O40 with M=Mn, Fe, Co, Ni, Cu are ionic solids with discrete cations and anions. The anions, known as Keggin units (KU), are composed ∗ Corresponding author. E-mail address: [email protected] (J.-M.M. Millet).
of a central phosphorus atom bonded to four oxygens forming a tetrahedron. This tetrahedron is connected to 12 molybdenum–oxygen octahedra arranged in four groups of three sharing edges (Mo3 O13 ). Protons in the acid H3 PMo12 O40 can be substituted by other monovalent cations like K, Cs or NH4 . The preparation and firing at temperature superior or equal to the catalytic reaction temperature, of these last compounds led systematically to a mixture of two phases [7,8]. These two phases correspond to the hydrated acid and the pure cesium salt with the acid phase on top of it. For compounds having less than two cesium ions per Keggin unit, the acid was present in large
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amounts and could be detected by X-ray diffraction. For compounds having more than two cesium per Keggin unit, the precipitation rates vary leading to very small cesium salt particles. In this second case the acid phase also coats the particles, but is no longer detectable by X-ray diffraction although it can be observed by XPS or Raman spectroscopy [8,9]. When transition metal cations are added as counter-cations in the pure acid phase, they give a solid solution presenting the structural characteristics of the pure acid phase [9]. Added in the same time as cesium cations, they preferentially replace the protons in the supported acid phase and not the cesium atoms in the salt. Although in both cases they do not appear to have an important effect on the structural characteristics of the acid other than a small reduction of the hydration extent, they have an important effect on the catalytic properties of the solids in the partial oxidation of alkanes. This important effect has first been demonstrated by Mizuno et al. for various transition metals [1,2] and confirmed by us for iron and copper [10]. In this previous study, we confirmed the results of previous investigations establishing that the main factor controlling the activity of such catalysts was their reducibility. The protonic acidity which is affected by the transition metal cations substitution, appeared also to be an important factor but influencing more the selectivities towards methacrolein and methacrylic acid than the reactivity [4,5,10]. The aim of the present study was to get a better understanding of the influence of iron counter-cations on the reducibility and consequently the catalytic properties of 12-heteropolymolybdates in isobutane oxidation. Taking into account the results obtained on the characterization of cesium substituted heteropolycompounds, we undertook to investigate both bulk and supported iron substituted molybdophosphoric acid. We have thus prepared and studied two compounds corresponding, respectively, to the acid Fe0.85 H0.45 PMo12 O40 and to the acid supported on the cesium salt Cs2 Fe0.2 H0.4 PMo12 O40 . The stoichiometry of the supported acid phase corresponded to that giving the best catalytic performances [10] whereas that of the bulk acid phase has been chosen to present a H/Fe as close as possible to that of the supported acid phase. The catalysts have been characterized before and after and catalytic test of oxidation of isobutane and after reduction by hydrogen
to see if the Fe3+ /Fe2+ redox couple intervened in the oxido-reduction process.
2. Experimental The compound corresponding to the bulk acid and containing iron, Fe0.85 H0.45 PMo12 O40 (Fe0.85 H0.45 ) was prepared by crystallization from a concentrated solution containing the pure acid H3 PMo12 O40 and iron nitrate in a 1:1 stoichiometry. The pure acid was prepared as described previously [11]. The crystals obtained were dried in air, heated at 2.5 K min−1 to 623 K under an air flow and left for 5 h at this temperature. The compound corresponding to the acid phase supported on the cesium salt, Cs2 H1 PMo12 O40 (Cs2 H1 ), was synthesized by adding dropwise, at 323 K and in the desired stoichiometry, an aqueous solution of Cs2 CO3 (0.08 M) to an aqueous solution of H3 PMo12 O40 (0.06 M). After evaporating the water at reduced pressure at 323 K, the precipitate was calcined as described above. The same compound containing iron, Cs2 Fe0.2 H0.4 PMo12 O40 (Cs2 Fe0.2 ) was synthesized and heat treated in the same way, adding an aqueous solution of Cs2 CO3 (0.08 M) and iron nitrate (0.08 M). In order to characterize the Cs2 Fe0.2 compound by Mössbauer spectroscopy a sample was prepared using almost pure 57 Fe nitrate (98%). Chemical compositions have been determined by atomic emission using induced plasma technique or an air–acetylene flame. The presence of the Keggin unit has been controlled by infrared spectroscopy with KBr pellets on a Brucker Vector 22 FTIR spectrometer and the structure by X-ray diffraction using a Siemens D500 diffractometer and Cu K␣ radiation [7]. Reduction and re-oxidation of the catalysts have been studied using a Setaram TGA-DTA 92 thermobalance. The catalysts (30–40 mg) were first heated to 623 K and cooled down to 613 K under nitrogen (total flow rate 2.4 dm3 h−1 ). After 0.5 h the nitrogen flow was shifted to a H2 /N2 mixture (1:4.88) and the weight variation recorded. To study their re-oxidation, the solids were heated under nitrogen at 613 K and after 0.5 h the nitrogen flow was shifted to a O2 /N2 mixture (1:2.33). The specific surface areas were determined by the BET method using nitrogen adsorption at 77 K. They have been measured before and after catalytic test but the values reported in Table 3 and used for
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the calculations of the intrensic rates were those obtained after catalytic test as a small sinterring process was shown to occur in the conditions of catalytic test [12]. Electron spin resonance (ESR) spectra were recorded at 296 and 77 K using a Varian E9 spectrometer operating in the X-band mode. DPPH was used as a standard for g-values determinations using a dual cavity. The Mössbauer spectra were recorded at 298 K, using a 2 GBq 57 Co/Rh source and a conventional constant acceleration spectrometer, operated in triangular mode. The isomer shifts (δ) were given with respect to ␣-Fe and calculated, as the quadrupolar splittings (∆) and the line width (W), with a precision of about 0.02 mm s−1 . The validity of the computed fits was judged on the basis of both χ 2 values and convergences of the fitting processes. Isobutane oxidation into methacrylic acid (MAA) and methacrolein (MA) was carried out with a continuous-flow system at atmospheric pressure at 613 K. The reactor made of a stainless steel tube (diameter 5 mm) allowed the test of 40–100 mg of sample. The feed rates of oxygen, isobutane, nitrogen the diluant gas and helium the internal reference standard, were fixed to have a composition mixture corresponding to 33.4/17.2/10.1/40.5 kPa and a total flow rate of 6 cm3 s−1 . The effluent gas from the reactor were analyzed by GC as previously described [12]. Acetic acid (AcA), CO and CO2 were the by-products observed. The catalysts, with and without iron, have been tested at the same conversion level in order to compare their catalytic properties. All the catalysts were tested for 48 h. They were recovered after a rapid quenching from the reaction temperature to room temperature with a concomitant shift of the reaction gas flow to the same nitrogen flow.
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3. Results 3.1. Characterization of the catalysts before catalytic test or reduction The chemical compositions of the prepared solids were in good accordance with the theoretical stoichiometry (Table 1). The X-ray diffraction patterns of the compounds corresponded as expected only to the triclinic structure of the hydrated acid with 13 H2 O for Fe0.85 H0.45 and to the cubic structure of the cesium salt for Cs2 Fe0.2 . It should be recalled that the presence of the acid phase doped with iron on the cesium salt for these compounds cannot be evidenced by X-ray diffraction but could be observed using electron microscopy with EDX analyses or Raman spectroscopy [7]. The hydration extent of the solid corresponded to that of the acid since the cesium salt was almost not hydrated. It was thus possible to calculate the extend of hydration of the acid phase supported and expressed it as a number of water molecules per Keggin unit in the acid phase. It can be observed that the hydration extent of the acid slightly increased with the iron substitution in the bulk acid whereas it decreased when the acid was supported. In this last case it was already lower without iron (Table 1). The TG curves of the Cs2 H2 and Cs2 Fe0.2 compounds were comparable but those of the pure and iron containing bulk acid differed as shown in Fig. 1. We observed in this case that the crystal water loss ended at higher temperature for the iron containing compound (523 instead of 425 K). In the pure acid the intermediate stabilization observed at about eight water molecules per KU was slightly observed in the Fe0.85 H0.45 compound which exhibited a stronger stabilization at about five water molecules. The Mössbauer spectra of the
Table 1 Chemical and thermogravimetric analyses data. (KU: Keggin unit) Compound
H3 Fe0.85 H0.45 Cs2 H1 Cs2 Fe0.20
Atomic ratio
Hydration water loss
12 Cs/Mo
12 Fe/Mo
Mo/P
/KU
/KU of acid
– – 2.0 1.8
– 0.85 – 0.21
11.7 11.4 11.7 11.8
12.8 14.2 2 1.5
12.8 14.2 7 5
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Fig. 1. Thermogravimetric water loss profiles of the pure (a) and iron containing bulk acid Fe0.85 H0.45 (b).
Fe0.85 H0.45 and Cs2 Fe0.2 compounds are presented in Fig. 2 and the parameters calculated from their fits in Table 2. They both have been fitted with one doublet corresponding to ferric cations. They presented similar isomer shifts but different quadrupolar splittings. The quadrupolar splitting of the doublet in the spectrum of the supported acid was relatively large, indicating for the iron cations a more distorted environment in the acid supported on the salt than in the bulk acid. The ESR spectra of Fe0.85 H0.45 have been recorded at 295 and 77 K. They both presented four signals (Fig. 3). The two more intense signals were respectively observed at g=1.90 and g=4.20. The first signal which was very large (1H=1550 G) has been ascribed to Fe3+ with an octahedral co-ordination; this Fe3+ site was presumably in magnetic interaction since it did not follow the Curie law. The second
Table 2 Mössbauer parameters calculated from the spectrum of the compounds Fe0.85 H0.45 and Cs2 Fe0.2 recorded at 295 K before catalytic test or reduction. δ: isomer shift (given with respect to α-Fe) 1: qadrupolar splitting
Fig. 2. Experimental Mössbauer spectra recorded at 295 K of the Fe0.85 H0.45 (a) and Cs2 Fe0.2 (b) compounds. Solid lines are derived from least-square fits.
Compound
Site
δ (mm.s−1 )
1 (mm.s−1 )
Fe0.85 H0.45 Cs2 Fe0.2
Fe3+ Fe3+
0.37 0.41
0.50 0.85
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Fig. 3. The ESR spectra recorded at 77 K of the Fe0.85 H0.45 (a) and Cs2 Fe0.2 (b) compounds.
signal has been ascribed to Fe3+ with a D2h (λ=1/3) orthorhombic symmetry and should correspond to iron counter-cations occupying relatively distorted octahedral sites [13,14]. The two less intense signals have been ascribed to Mo5+ species. The first one was characterized by g1⊥ = 1.96 and g1k = 1.84 and the second very small by a g2⊥ = 1.94 and g2k = 1.88. On the basis of the published data, the first signal could be attributed to Mo5+ in axial symmetry similar to those reported in slightly reduced heteropolycompounds and corresponding to cations with a neighboring oxygen vacancy [15,16]. The second signal which was very
small could be attributed to Mo5+ species also with an axial symmetry, similar in this case to those reported for heteropolycompounds supported on silica [17] or Mo5+ in the [Mo6 O19 H2 ]2− isopolyanion [18]. This second signal has been related to a partial destruction of the Keggin units [16]. The ESR spectrum of the Cs2 Fe0.2 compound at 77 K showed only one signal corresponding to Fe3+ characterized by gxx =4.83, gyy =4.43 and gzz =3.63. The symmetry of the site that corresponded to iron counter-cations should thus be more distorted than in Fe0.85 H0.45 which was in agreement with the Mössbauer spectroscopic data.
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Table 3 Catalytic properties of the catalysts with and without iron Compound
Conv. %
Selectivities (%) CO
CO2
AcA
MA
MAA
Rate of iBu conversion (10−9 mol s−1 m−2 )
H3 Fe0.85 H0.45
5 4
50 22
32 31
7 9
7 27
3 9
26 70
Cs2 H1 Cs2 Fe0.2
7 6.8
33 23
28 23
11 9
14 17
12 24
41 42
3.2. Catalytic properties of the catalysts In order to study the effect of iron on the catalytic properties of the solids in the oxidation of isobutane, we have first compared in the same conditions and at iso-conversion the samples of the acid as a bulk or supported on the cesium salt with and without iron (Table 3). It can be observed that in the case of the pure acid phase, the substitution of iron affected positively both the activity and the selectivities in MAA and MA, whereas, in the case of the supported acid, iron has a positive effect only on the selectivity in MAA and MA but no effect on the activity. In both the cases, the increase in MA and MAA selectivity occurred at the expense of that in CO, CO2 and acetic acid. It could be noted that these increases occurred a little more to the benefit of MAA rather than of MA on the supported acid samples.
as before catalytic test and two new doublets corresponding to a ferric and a ferrous doublet. The relative contents of these two sites were respectively equal to 11 and 2%. Contrarily to Fe0.85 H0.45 the relative intensity of the ferrous doublet which was very small
3.3. Characterization of the catalysts after test The X-ray diffraction spectra of the Fe0.85 H0.45 and Cs2 Fe0.2 compounds after catalytic test were comparable to those obtained before test. The two compounds have been characterized by Mössbauer spectroscopy after catalytic reaction (Fig. 4, Table 4). The Fe0.85 H0.45 spectrum showed the presence of the same ferric doublet as before catalytic test and a new doublet characteristic of ferrous ions in smaller amount. We have analyzed the same compound periodically and have observed that the relative amount of Fe2+ increased with time up to 26% after 420 h (Fig. 5). Iron cations were thus reduced in air at room temperature. The spectrum of the Cs2 Fe0.2 compound after catalytic test showed the same ferric doublet
Fig. 4. Experimental Mössbauer spectra recorded at 295 K of the Fe0.85 H0.45 (a) and Cs2 Fe0.2 (b) compounds after catalytic test. Solid lines are derived from least-square fits.
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Table 4 Mössbauer parameters calculated from the spectrum of the compounds Fe0.85 H0.45 and Cs2 Fe0.2 recorded at 295 K after catalytic test. δ: isomer shift (given with respect to α-Fe) 1: quadrupolar splitting Compound
Site
δ (mm.s−1 )
1 (mm.s−1 )
Relative intensities (%)
Fe0.85 H0.45
Fe3+ Fe2+ Fe3+ Fe3+ Fe2+
0.36 1.23 0.34 0.50 1.22
0.50 2.92 0.97 0.63 3.19
76 24 86 12 2
Cs2 Fe0.2
did not increase with time. The ESR spectrum of the Fe0.85 H0.45 compound after catalytic test and Mössbauer spectroscopic study showed no Mo5+ , whereas, that of the Cs2 Fe0.2 compound showed relatively well resolved signal of Mo5+ characterized by g=1.960, g=1.846 and corresponding to that observed in the Fe0.85 H0.45 compound before catalytic test (Fig. 6). Small h.f.s. lines of 95,97 Mo (I=5/2) were better resolved in this spectrum. They allowed to extract the A tensor components A=0.45 G and A=105 G.
Fig. 5. The evolution of the relative amount in Fe2+ in Fe0.85 H0.45 as a function of the time at room temperature in ambient air after catalytic test.
3.4. Characterization of the catalysts after reduction In order to study how iron intervened in the reduction process of the heteropolymolybdic acid, the Fe0.85 H0.45 and Cs2 Fe0.2 compounds were reduced by hydrogen in a thermobalance as described in
Fig. 6. The ESR spectrum recorded at 77 K of the Cs2 Fe0.2 compound after catalytic test.
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Fig. 7. Experimental Mössbauer spectra recorded at 295 K of the compounds reduced at 613 K by H2 and kept up at 295 K in ambient air respectively for various time Fe0.85 H0.45 kept for 40 h (a), Fe0.85 H0.45 kept for 545 h (b) and Cs2 Fe0.2 kept up for 55 h (c). Solid lines are derived from least-square fits.
the experimental section. The reduction extent of the compounds were monitored and were respectively equal to 2 and 0.33 e− per KU. After reduction the samples have been studied by Mössbauer spectroscopy. The spectrum of Fe0.85 H0.45 after reduction (40 h) is presented in Fig. 7a and the calculated
parameters summarized in Table 5. The spectrum of Fe0.85 H0.45 displayed the same two doublets as observed after catalytic test. Similarly, we observed an increase of the relative intensity of the ferrous doublet with time in ambient air that reached 85% after 550 h (Fig. 7b). We have in the same manner studied the compound Cs2 Fe0.2 after reduction. The Mössbauer spectrum of the compound after reduction has been fitted with two ferric doublets with parameters identical to that observed for the ferric doublets after the catalytic test. After 240 h in ambient air the spectrum remained unchanged with only one ferrous doublet of low intensity and parameters similar to those obtained after catalytic test (Fig. 7c). The study of the compounds after reduction by the ESR showed the presence of Mo5+ only in the Cs2 Fe0.2 compound similarly to that of the compounds after catalytic test. These results were in agreement with those obtained after catalytic test and clearly showed that the catalysts were reduced in the conditions of catalytic test. The study was further focused on the characterization of Fe0.85 H0.45 after reduction. With that objective, the influence of the initial reduction extent on the amount of Fe2+ observed after reduction was studied. Samples of the Fe0.85 H0.45 compound have been reduced with different reduction extent in the thermobalance and analyzed at room temperature in the same manner as previously described. We have observed that for reduction extents inferior to 1 e− /Fe, the reduction of iron was proportional to the initial reduction extent of molybdenum (Fig. 8). Furthermore, at the end Mo5+ was absent in the solid as shown by the ESR. At low reduction rate, the charge transfer was thus total between iron and molybdenum. We then studied the influence of air moisture on the electron transfer. A Fe0.85 H0.45 sample has been reduced as before and maintained after reduction at room temperature under dry air for 70 h. It was then placed in ambient air and analyzed by Mössbauer spectroscopy. The evolution of the Fe2+ content as a function of time is presented in Fig. 9. It can be seen that the iron reduction started only when the sample was in ambient air. The reoxidation of the compound after the electron transfer has been studied by TGA. For that a sample of reduced Fe0.85 H0.45 was maintained under air at room temperature until the iron would be reduced. It was then heated under deoxygenated nitrogen in the thermobalance up to 613 K and after
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Table 5 Mössbauer parameters calculated from the spectrum of the compounds Fe0.85 H0.45 and Cs2 Fe0.2 recorded at 295 K after reduction respectively to 2 and 0.33 e-/KU. δ: isomer shift (given with respect to α-Fe) 1: quadrupolar splitting Compound Fe0.85 H0.45
Time after reduction (h) 40 195 265 325 545
Cs2 Fe0.2
240
Site
δ (mm.s−1 )
1 (mm.s−1 )
Relative intensities (%)
Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe3+ Fe2+
0.38 1.33 0.40 1.25 0.43 1.25 0.41 1.25 0.37 1.22 0.39 0.53 1.27
0.42 2.17 0.42 2.75 0.43 2.82 0.41 2.84 0.37 2.90 0.80 0.57 2.94
76 24 76 24 76 24 76 24 76 24 87 11 2
approximately 30 min at this temperature the nitrogen flow was shifted to an air flow (Fig. 10). The same experiment has been performed on the pure acid treated with the same protocol. During heating under nitrogen, we observed between 70 and 220 K a weight loss corresponding to the dehydration of the compounds. It can be observed that the reduced samples were not completely rehydrated, since the weight losses corresponded to only 9.6 and 11.8 H2 O per KU, respectively, for the pure and iron doped acid instead of 12.8 and 14.2 H2 O per KU measured for the fresh samples. It was interesting to note that the same stabilization at
about five water molecules per KU (5.1) was observed for the doped acid sample. After the introduction of air no weight gain was observed for the iron containing acid whereas the pure acid showed a weight gain corresponding well to the reduction extent of the solid (Fig. 10). The characterization of the Fe0.85 H0.45 compound at the end of the experiment by the Mössbauer spectroscopy and the ESR showed that it contained only Fe3+ and Mo6+ and was thus fully reoxidized.
Fig. 8. Variation of the total amount of Fe2+ formed in Fe0.85 H0.45 after reduction as a function of the initial reduction extent of the solid.
Fig. 9. The evolution of the relative amount of Fe2+ in Fe0.85 H0.45 as a function of the time at room temperature in ambient air after reduction and maintain in dry air for 70 h.
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Fig. 10. Thermogravimetric and analysis of the pure acid (a) and the acid containing iron (b) after reduction and electron transfer. The programmed heating procedure is superposed to the thermogravimetric curves. An enlargement of the figure corresponding to the time when nitrogen has been replaced by air is shown with the corresponding DTA curves (HT: heat flow).
4. Discussion The characterization of the iron counter-cations in the bulk acid (Fe0.85 H0.45 ) and in the acid supported on the cesium salt (Cs2 Fe0.2 ) evidenced differences in their environments. Mössbauer spectroscopy showed that the cations were ferric with probably an octahedral co-ordination strongly distorted in the supported acid but relatively regular in the bulk acid. The ESR data supported these results pointing out by the way a small reduction of the molybdenum cations and the destruction of some of the heteropolyanions for the bulk acid sample. We evidenced by DTG the stabilization of the bulk acid with about five water molecules which could be explained by the presence as counter-cations of Fe(H2 O)6 complexes localized between Keggin units (Fig. 11). The relatively symmetrical environment of the iron cations in such complexes and the mobility of these complexes could account respectively for the small quadrupolar splitting observed and the larger line width observed in Mössbauer spectra. The presence of such aqua complexes is not unusual and has soon been demonstrated for vanadyl ions in VO(H2 O)5 PMo12 O40 [19]. Upon heating above 533 K the aqua complex decomposed
and the co-ordination sphere of the Fe3+ ions reconstructed. The lack of crystallographic data on the structure of the hydrated acid does not allow us to propose a scheme for its new co-ordination but it should necessarily imply the outer oxygen atoms of the neighboring heteropolyanions and certainly several oxygen atoms of the same one. The environment of the iron cations in the supported acid was different. A strong interaction between the acid and the salt should exist with a reduction of the
Fig. 11. Schematic representation of the iron counter-cations in the bulk acid phase.
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inter-anionic space in the acid which did not allow the stabilization of the iron aqua complexes. The low total hydration extend of the acid observed in this case tends to confirm this proposition. we have shown that the environment of the iron cations was in this case very distorted and could be more comparable to that in the dehydrated bulk acid. The catalytic properties of the samples have been evaluated and compared to those of the same type of solid but containing no iron. First we can see that whether it contained iron or not, the supported acid was always more selective in MA and MAA than the bulk acid. This increase could be related to an increase of the acid properties of the supported acid due to its modified hydration ability resulting from its strong interaction with the cesium salt support as previously reported [20]. In the bulk acid the substitution of protons by Fe3+ has, for the same reason, the same effect on the MA and MAA selectivities but, in this case, it also strongly increased the activity. In the supported acid the substitution of protons by Fe3+ has almost no effect on the activity (rate of isobutane transformation) but it increased the selectivities in MA and MAA. This result has already been obtained by Mizuno et al. and we confirmed it well [2]. The increase in selectivities had previously been related to the reduction of the acidity with the substitution of protons by iron [10]. The acidity of the pure supported heteropolyacid should not be too high to avoid the partial decomposition of the methacrolein and methacrylic acid that are formed. In this case a lowering of the acidity by iron substitution has been shown to have a positive effect on the selectivity to MA and MAA [10]. In the same paper we studied the reducibility of the catalysts and we showed that it influenced greatly the activity. We observed that the substitution of iron had no effect on the reducibility of the catalysts which explained why the activity remained unchanged for the Cs2 Fe0.2 sample, on the other hand, when protons were substituted by copper cations, the reducibility was increased as well as the activity and that the Cu2+ /Cu+ redox couple was playing a role in the oxido-reduction of the solids [10]. The characterization of the iron counter-cations in the compounds after catalytic test and after reduction, clearly showed that they were strongly reduced in the bulk acid but almost, not in the supported one. The Fe3+ /Fe2+ redox couple should intervene
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in the oxido-reduction of the bulk acid but not of the acid supported on the cesium salt. These results were in agreement with those previously obtained and explained the positive effect of iron on the activity of the bulk acid. The characterization by the Mössbauer spectroscopy of Fe0.85 H0.45 after reduction or catalytic test, did not show the reduction of the iron cations immediately but that it was taking place slowly at room temperature. Such reduction in air at 298 K can only be explained by a charge transfer between the reduced molybdenum and iron cations: Fe3+ +Mo5+ →Fe2+ +Mo6+ . This conclusion is also supported by the ESR data which showed that the reduction of iron was coupled with the oxidation of the molybdenum cations. It is interesting to note that the reduction by H2 of the acid supported on the cesium salt and containing copper has been shown to proceed via a concerted mechanism with a similar electron transfer between the copper and molybdenum cations [10]: Cu2+ + 2Mo6+ + O2− + H2 → Cu+ + Mo5+ + Mo6+ + H2 O Cu+ + Mo5+ + Mo6+ → Cu2+ + 2Mo5+ The study of the Fe0.85 H0.45 samples with different reduction extend showed that the charge transfer was total when the reduction rate of the compounds was less than 1 e− per KU (Fig. 9). This allows to calculate from the results of the characterization after catalytic test, what should be the reduction extend of the catalysts in the conditions of catalytic test. We can see that 24% of the iron cations were reduced which corresponds to 0.2 e− per KU or 1.7% of Mo5+ . This reduction extent is quite small and would correspond, considering that a Keggin unit is preferentially reduced to two electrons, to the reduction of only 10% of the Keggin units which should be related to a reduction limited to the surface, between one and two Keggin unit layers. The study of the reduced solid kept in dry air for 70 h clearly indicated that the charge transfer between iron and molybdenum cations was related to the rehydration of the compound. The thermogravimetric analyses of the samples after reduction showed that the compounds recovered easily an hydration extent close to that before reduction and that the hydrated iron complexes were again formed in the Fe0.85 H0.45
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Fig. 12. Schematic representation of the charge transfer mechanism (a) and of the oxidation mechanism of the reduced heteropolycompound after the charge transfer (b).
compound. The Mössbauer parameters of the ferric cations after reduction were the same as before and no new iron species were detected. Furthermore, the parameters of the ferric doublets in the reduced samples did not vary tremendously with the reducing extent (Table 5). These results clearly indicated that the rehydration of the iron cations was rapid and conditioned the charge transfer (Fig. 12a). This explained why the iron in the supported acid which cannot be hydrated could not be reduced. In this last case, iron cations were quasi not reduced (only 2% of Fe2+ ) and the reduction of the supported acid involved mainly the reduction of the molybdenum cations as confirmed by the ESR. The new ferric doublet identified in the Mössbauer spectrum of the supported acid could correspond to iron cations in close proximity of a reduced molybdenum. Experiments are presently conducted in order to confirm this interpretation. We have shown that the aqua complexes in Fe0.85 H0.45 decomposed at 533 K which was lower than the reaction temperature (623 K). However, in the condi-
tions of catalytic test, water formed from the total oxidation of isobutane was present in the gas phase and both hydrated and dehydrated iron species should be in equilibrium at the surface of the heteropolycompound. When the catalysts were recovered, the shift of the gas feed for a nitrogen flow at 623 K should have brought the dehydration of the catalysts, explaining why the rehydration with the charge transfer was observed at room temperature. The re-oxidation experiments performed after the reduction and the rehydration showed that the Fe0.85 H0.45 compound, contrarily to the pure acid was undergoing an oxidation upon heating under pure nitrogen. The same type of mechanism has already been described on hydrated iron phosphates which can be oxidized upon heating under nitrogen or vacuum [20,21]. The mechanism proposed for this auto-oxidation reaction was based upon the reversible decomposition of crystal water bounded to the iron cations to form hydroxyl groups: 2 (Fe2+ –[H2 O])→2 ([Fe3+ –OH])+H2 [21,22]. It can be proposed that the
M. Langpape, J.-M.M. Millet / Applied Catalysis A: General 200 (2000) 89–101
same type of reactions occurred in the heteropolycompounds allowing its re-oxidation (Fig. 12b). After a final rehydration of the iron cation, the catalytic site would be restored for a new reaction. The presence of the charge transfer between the iron counter-cation and molybdenum in the Keggin unit and the possible oxido-reduction of the iron cations, which have both been demonstrated here, makes iron directly involved in the oxido-reduction process of the Keggin unit and explain it has an effect on the catalytic reaction rate. The oxido-reduction process of iron based upon a dehydration/hydroxylation reaction may directly be involved in the oxidation of the isobutane molecules.
5. Conclusion The results obtained in this study confirm that the catalytic activity in oxidation reactions of molybdophosphoric heteropolycoumpounds depended on their reducibility and show that the positive effect on the catalytic activity of a transition metal counter-cation like iron is related to a charge transfer between the molybdenum cations in the heteropolyanion and the counter-cation. In the case of iron, this electron transfer is taking place when the solid is hydrated with the formation of an iron aqua complex. Such iron species can be formed and maintained in equilibrium with dehydrated species at the surface of the catalysts in the conditions of catalytic test, since water is always present in the gas phase due to the unselective total oxidation of isobutane. Similar electron transfer should take place in the case of vanadyl counter-cations which also form aqua complexes when present as counter-cations and have a positive effect on the catalytic activity of the catalysts in oxidation reaction like the oxidative dehydrogenation of isobutyric acid [19]. When the acid is supported on the cesium salt Cs3 PMo12 O40 , it is in strong interaction with the support. As a consequence it is less hydrated,
101
the iron aqua complexes are not formed and neither the charge transfer nor the oxido-reduction of iron take place. References [1] N. Mizuno, M. Tateishi, M. Iwamoto, J. Chem. Soc., Chem. Commun. (1994) 1411. [2] N. Mizuno, M. Tateishi, M. Iwamoto, J. Catal. 163 (1996) 87. [3] N. Mizuno, M. Tateishi, T. Hirose, M. Iwamoto, Chem. Lett. 5 (1993) 2137. [4] N. Mizuno, M. Tateishi, T. Hirose, M. Iwamoto, J. Mol. Catal. 88 (1994) L125. [5] N. Mizuno, M. Tateishi, M. Iwantoto, Appl. Catal. A 128 (1995) L165. [6] F. Cavani, E. Etienne, M. Favaro, A. Galli, F. Trifiro, G. Hecquet, Catal. Lett. 32 (1995) 215. [7] J.B. Black, N.J. Clayden, P.L. Gai, J.D. Scott, E.M. Serwicka, J.B. Goodenough, J. Catal. 106 (1987) 1. [8] M. Langpape, J.-M.M. Millet, U. Oskan, M. Boudeulle, J. Catal. 181 (1999) 80. [9] I.N. Staroverova, M. Kutyrev, A. Stakheev, Kinetika i Kataliz 33 (1) (1992) 127. [10] M. Langpape, J.-M.M. Millet, U. Oskan, P. Delichère, J. Catal. 181 (1999) 148. [11] G.A. Tsigdinos, Ind. Eng. Chem., Prod. Res. Develop. 13 (1974) 267. [12] M. Langpape, PhD Thesis, Lyon, 1997. [13] D. Olivier, J.C. Vedrine, M. Pezerat, J. Solid State Chem. 20 (1977) 267. [14] J.M. Gaité, S.S. Hafner, J. Chem. Phys. 80 (1984) 274. [15] E.M. Serwicka, J.B. Black, J.B. Goodenough, J. Catal. 106 (1987) 23. [16] R. Fricke, G. Öhlmann, J. Chem. Soc., Faraday Trans. 82 (1986) 263. [17] C. Louis, M. Che, J. Phys. Chem. 91 (1987) 2875. [18] S. Albonetti, F. Cavani, M. Gazzano, M.Yu. Kutyrev, F. Trifiro, F.C. Aissi, A. Aboukais, M. Guelton, J. Catal. 146 (2) (1994) 491. [19] R. Bayer, M. Marchal, F.X. Liu, A. Tézé, G. Hervé, J. Mol. Catal. 110 (1996) 65. [20] K. Bruckman, J. Haber, E.M. Servicka, Faraday Discuss. Chem. Soc. 87 (1989) 173. [21] D. Hanzel, W. Meisel, D. Hanzel, P. Gütlich, Solid State Commun. 76 (1990) 307. [22] D. Rouzies, J. Varloud, J.-M.M. Millet, J. Chem. Soc., Faraday Trans. 90 (1994) 3335.
Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid Part I. An experimental study on isobutane oxidation catalysts Martin Langpape, Jean-Marc M. Millet∗ Institut de Recherches sur la Catalyse, CNRS, associé a l’Université Claude-Bernard, Lyon I, 2 Avenue A. Einstein, F-69626 Villeurbanne Cedex, France Received 1 October 1999; received in revised form 1 December 1999; accepted 1 December 1999
Abstract Keggin-type molybdophosphoric heteropolyacid with protons partially substituted by iron cations in a bulk form (Fe0.85 H0.45 PMo12 O40 ) or supported on the cesium salt (Cs2 Fe0.2 H0.4 PMo12 O40 ) have been synthesized and characterized by different techniques like the Mössbauer spectroscopy and the electron spin resonance (ESR). The effect of iron on the redox and catalytic properties for the oxidation of isobutane into methacrylic acid (MAA) has also been studied. Iron has been shown to have a different effect whether acts as a counter-cation in the bulk acid or in the acid supported on the cesium salt. In the first case, it increases both the selectivity in methacrylic acid and methacrolein (MA) and the activity of the acid phase whereas in the second case, it increases only the selectivity. This difference has been explained by the existence of an electron transfer between iron and molybdenum occurring only in the bulk acid. This electron transfer was related to a combined hydration-oxidation mechanism which promotes the reducibility of the solid and consequently its catalytic activity. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Isobutane oxidation; Iron counter-ions; Heteropolymolybdates; Electron spin resonance (ESR); Iron aqua complexes; Mössbauer spectroscopy
1. Introduction Twelve-heteropolymolybdates with alkaline and transition metals substituting partially or totally the protons have recently been shown to be efficient catalysts for the oxidation of alkanes [1–6]. These compounds with the general formula Cs2.5 Mx y+ H0.5−xy PMo12 O40 with M=Mn, Fe, Co, Ni, Cu are ionic solids with discrete cations and anions. The anions, known as Keggin units (KU), are composed ∗ Corresponding author. E-mail address: [email protected] (J.-M.M. Millet).
of a central phosphorus atom bonded to four oxygens forming a tetrahedron. This tetrahedron is connected to 12 molybdenum–oxygen octahedra arranged in four groups of three sharing edges (Mo3 O13 ). Protons in the acid H3 PMo12 O40 can be substituted by other monovalent cations like K, Cs or NH4 . The preparation and firing at temperature superior or equal to the catalytic reaction temperature, of these last compounds led systematically to a mixture of two phases [7,8]. These two phases correspond to the hydrated acid and the pure cesium salt with the acid phase on top of it. For compounds having less than two cesium ions per Keggin unit, the acid was present in large
0926-860X/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 6 3 7 - 2
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amounts and could be detected by X-ray diffraction. For compounds having more than two cesium per Keggin unit, the precipitation rates vary leading to very small cesium salt particles. In this second case the acid phase also coats the particles, but is no longer detectable by X-ray diffraction although it can be observed by XPS or Raman spectroscopy [8,9]. When transition metal cations are added as counter-cations in the pure acid phase, they give a solid solution presenting the structural characteristics of the pure acid phase [9]. Added in the same time as cesium cations, they preferentially replace the protons in the supported acid phase and not the cesium atoms in the salt. Although in both cases they do not appear to have an important effect on the structural characteristics of the acid other than a small reduction of the hydration extent, they have an important effect on the catalytic properties of the solids in the partial oxidation of alkanes. This important effect has first been demonstrated by Mizuno et al. for various transition metals [1,2] and confirmed by us for iron and copper [10]. In this previous study, we confirmed the results of previous investigations establishing that the main factor controlling the activity of such catalysts was their reducibility. The protonic acidity which is affected by the transition metal cations substitution, appeared also to be an important factor but influencing more the selectivities towards methacrolein and methacrylic acid than the reactivity [4,5,10]. The aim of the present study was to get a better understanding of the influence of iron counter-cations on the reducibility and consequently the catalytic properties of 12-heteropolymolybdates in isobutane oxidation. Taking into account the results obtained on the characterization of cesium substituted heteropolycompounds, we undertook to investigate both bulk and supported iron substituted molybdophosphoric acid. We have thus prepared and studied two compounds corresponding, respectively, to the acid Fe0.85 H0.45 PMo12 O40 and to the acid supported on the cesium salt Cs2 Fe0.2 H0.4 PMo12 O40 . The stoichiometry of the supported acid phase corresponded to that giving the best catalytic performances [10] whereas that of the bulk acid phase has been chosen to present a H/Fe as close as possible to that of the supported acid phase. The catalysts have been characterized before and after and catalytic test of oxidation of isobutane and after reduction by hydrogen
to see if the Fe3+ /Fe2+ redox couple intervened in the oxido-reduction process.
2. Experimental The compound corresponding to the bulk acid and containing iron, Fe0.85 H0.45 PMo12 O40 (Fe0.85 H0.45 ) was prepared by crystallization from a concentrated solution containing the pure acid H3 PMo12 O40 and iron nitrate in a 1:1 stoichiometry. The pure acid was prepared as described previously [11]. The crystals obtained were dried in air, heated at 2.5 K min−1 to 623 K under an air flow and left for 5 h at this temperature. The compound corresponding to the acid phase supported on the cesium salt, Cs2 H1 PMo12 O40 (Cs2 H1 ), was synthesized by adding dropwise, at 323 K and in the desired stoichiometry, an aqueous solution of Cs2 CO3 (0.08 M) to an aqueous solution of H3 PMo12 O40 (0.06 M). After evaporating the water at reduced pressure at 323 K, the precipitate was calcined as described above. The same compound containing iron, Cs2 Fe0.2 H0.4 PMo12 O40 (Cs2 Fe0.2 ) was synthesized and heat treated in the same way, adding an aqueous solution of Cs2 CO3 (0.08 M) and iron nitrate (0.08 M). In order to characterize the Cs2 Fe0.2 compound by Mössbauer spectroscopy a sample was prepared using almost pure 57 Fe nitrate (98%). Chemical compositions have been determined by atomic emission using induced plasma technique or an air–acetylene flame. The presence of the Keggin unit has been controlled by infrared spectroscopy with KBr pellets on a Brucker Vector 22 FTIR spectrometer and the structure by X-ray diffraction using a Siemens D500 diffractometer and Cu K␣ radiation [7]. Reduction and re-oxidation of the catalysts have been studied using a Setaram TGA-DTA 92 thermobalance. The catalysts (30–40 mg) were first heated to 623 K and cooled down to 613 K under nitrogen (total flow rate 2.4 dm3 h−1 ). After 0.5 h the nitrogen flow was shifted to a H2 /N2 mixture (1:4.88) and the weight variation recorded. To study their re-oxidation, the solids were heated under nitrogen at 613 K and after 0.5 h the nitrogen flow was shifted to a O2 /N2 mixture (1:2.33). The specific surface areas were determined by the BET method using nitrogen adsorption at 77 K. They have been measured before and after catalytic test but the values reported in Table 3 and used for
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the calculations of the intrensic rates were those obtained after catalytic test as a small sinterring process was shown to occur in the conditions of catalytic test [12]. Electron spin resonance (ESR) spectra were recorded at 296 and 77 K using a Varian E9 spectrometer operating in the X-band mode. DPPH was used as a standard for g-values determinations using a dual cavity. The Mössbauer spectra were recorded at 298 K, using a 2 GBq 57 Co/Rh source and a conventional constant acceleration spectrometer, operated in triangular mode. The isomer shifts (δ) were given with respect to ␣-Fe and calculated, as the quadrupolar splittings (∆) and the line width (W), with a precision of about 0.02 mm s−1 . The validity of the computed fits was judged on the basis of both χ 2 values and convergences of the fitting processes. Isobutane oxidation into methacrylic acid (MAA) and methacrolein (MA) was carried out with a continuous-flow system at atmospheric pressure at 613 K. The reactor made of a stainless steel tube (diameter 5 mm) allowed the test of 40–100 mg of sample. The feed rates of oxygen, isobutane, nitrogen the diluant gas and helium the internal reference standard, were fixed to have a composition mixture corresponding to 33.4/17.2/10.1/40.5 kPa and a total flow rate of 6 cm3 s−1 . The effluent gas from the reactor were analyzed by GC as previously described [12]. Acetic acid (AcA), CO and CO2 were the by-products observed. The catalysts, with and without iron, have been tested at the same conversion level in order to compare their catalytic properties. All the catalysts were tested for 48 h. They were recovered after a rapid quenching from the reaction temperature to room temperature with a concomitant shift of the reaction gas flow to the same nitrogen flow.
91
3. Results 3.1. Characterization of the catalysts before catalytic test or reduction The chemical compositions of the prepared solids were in good accordance with the theoretical stoichiometry (Table 1). The X-ray diffraction patterns of the compounds corresponded as expected only to the triclinic structure of the hydrated acid with 13 H2 O for Fe0.85 H0.45 and to the cubic structure of the cesium salt for Cs2 Fe0.2 . It should be recalled that the presence of the acid phase doped with iron on the cesium salt for these compounds cannot be evidenced by X-ray diffraction but could be observed using electron microscopy with EDX analyses or Raman spectroscopy [7]. The hydration extent of the solid corresponded to that of the acid since the cesium salt was almost not hydrated. It was thus possible to calculate the extend of hydration of the acid phase supported and expressed it as a number of water molecules per Keggin unit in the acid phase. It can be observed that the hydration extent of the acid slightly increased with the iron substitution in the bulk acid whereas it decreased when the acid was supported. In this last case it was already lower without iron (Table 1). The TG curves of the Cs2 H2 and Cs2 Fe0.2 compounds were comparable but those of the pure and iron containing bulk acid differed as shown in Fig. 1. We observed in this case that the crystal water loss ended at higher temperature for the iron containing compound (523 instead of 425 K). In the pure acid the intermediate stabilization observed at about eight water molecules per KU was slightly observed in the Fe0.85 H0.45 compound which exhibited a stronger stabilization at about five water molecules. The Mössbauer spectra of the
Table 1 Chemical and thermogravimetric analyses data. (KU: Keggin unit) Compound
H3 Fe0.85 H0.45 Cs2 H1 Cs2 Fe0.20
Atomic ratio
Hydration water loss
12 Cs/Mo
12 Fe/Mo
Mo/P
/KU
/KU of acid
– – 2.0 1.8
– 0.85 – 0.21
11.7 11.4 11.7 11.8
12.8 14.2 2 1.5
12.8 14.2 7 5
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Fig. 1. Thermogravimetric water loss profiles of the pure (a) and iron containing bulk acid Fe0.85 H0.45 (b).
Fe0.85 H0.45 and Cs2 Fe0.2 compounds are presented in Fig. 2 and the parameters calculated from their fits in Table 2. They both have been fitted with one doublet corresponding to ferric cations. They presented similar isomer shifts but different quadrupolar splittings. The quadrupolar splitting of the doublet in the spectrum of the supported acid was relatively large, indicating for the iron cations a more distorted environment in the acid supported on the salt than in the bulk acid. The ESR spectra of Fe0.85 H0.45 have been recorded at 295 and 77 K. They both presented four signals (Fig. 3). The two more intense signals were respectively observed at g=1.90 and g=4.20. The first signal which was very large (1H=1550 G) has been ascribed to Fe3+ with an octahedral co-ordination; this Fe3+ site was presumably in magnetic interaction since it did not follow the Curie law. The second
Table 2 Mössbauer parameters calculated from the spectrum of the compounds Fe0.85 H0.45 and Cs2 Fe0.2 recorded at 295 K before catalytic test or reduction. δ: isomer shift (given with respect to α-Fe) 1: qadrupolar splitting
Fig. 2. Experimental Mössbauer spectra recorded at 295 K of the Fe0.85 H0.45 (a) and Cs2 Fe0.2 (b) compounds. Solid lines are derived from least-square fits.
Compound
Site
δ (mm.s−1 )
1 (mm.s−1 )
Fe0.85 H0.45 Cs2 Fe0.2
Fe3+ Fe3+
0.37 0.41
0.50 0.85
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Fig. 3. The ESR spectra recorded at 77 K of the Fe0.85 H0.45 (a) and Cs2 Fe0.2 (b) compounds.
signal has been ascribed to Fe3+ with a D2h (λ=1/3) orthorhombic symmetry and should correspond to iron counter-cations occupying relatively distorted octahedral sites [13,14]. The two less intense signals have been ascribed to Mo5+ species. The first one was characterized by g1⊥ = 1.96 and g1k = 1.84 and the second very small by a g2⊥ = 1.94 and g2k = 1.88. On the basis of the published data, the first signal could be attributed to Mo5+ in axial symmetry similar to those reported in slightly reduced heteropolycompounds and corresponding to cations with a neighboring oxygen vacancy [15,16]. The second signal which was very
small could be attributed to Mo5+ species also with an axial symmetry, similar in this case to those reported for heteropolycompounds supported on silica [17] or Mo5+ in the [Mo6 O19 H2 ]2− isopolyanion [18]. This second signal has been related to a partial destruction of the Keggin units [16]. The ESR spectrum of the Cs2 Fe0.2 compound at 77 K showed only one signal corresponding to Fe3+ characterized by gxx =4.83, gyy =4.43 and gzz =3.63. The symmetry of the site that corresponded to iron counter-cations should thus be more distorted than in Fe0.85 H0.45 which was in agreement with the Mössbauer spectroscopic data.
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Table 3 Catalytic properties of the catalysts with and without iron Compound
Conv. %
Selectivities (%) CO
CO2
AcA
MA
MAA
Rate of iBu conversion (10−9 mol s−1 m−2 )
H3 Fe0.85 H0.45
5 4
50 22
32 31
7 9
7 27
3 9
26 70
Cs2 H1 Cs2 Fe0.2
7 6.8
33 23
28 23
11 9
14 17
12 24
41 42
3.2. Catalytic properties of the catalysts In order to study the effect of iron on the catalytic properties of the solids in the oxidation of isobutane, we have first compared in the same conditions and at iso-conversion the samples of the acid as a bulk or supported on the cesium salt with and without iron (Table 3). It can be observed that in the case of the pure acid phase, the substitution of iron affected positively both the activity and the selectivities in MAA and MA, whereas, in the case of the supported acid, iron has a positive effect only on the selectivity in MAA and MA but no effect on the activity. In both the cases, the increase in MA and MAA selectivity occurred at the expense of that in CO, CO2 and acetic acid. It could be noted that these increases occurred a little more to the benefit of MAA rather than of MA on the supported acid samples.
as before catalytic test and two new doublets corresponding to a ferric and a ferrous doublet. The relative contents of these two sites were respectively equal to 11 and 2%. Contrarily to Fe0.85 H0.45 the relative intensity of the ferrous doublet which was very small
3.3. Characterization of the catalysts after test The X-ray diffraction spectra of the Fe0.85 H0.45 and Cs2 Fe0.2 compounds after catalytic test were comparable to those obtained before test. The two compounds have been characterized by Mössbauer spectroscopy after catalytic reaction (Fig. 4, Table 4). The Fe0.85 H0.45 spectrum showed the presence of the same ferric doublet as before catalytic test and a new doublet characteristic of ferrous ions in smaller amount. We have analyzed the same compound periodically and have observed that the relative amount of Fe2+ increased with time up to 26% after 420 h (Fig. 5). Iron cations were thus reduced in air at room temperature. The spectrum of the Cs2 Fe0.2 compound after catalytic test showed the same ferric doublet
Fig. 4. Experimental Mössbauer spectra recorded at 295 K of the Fe0.85 H0.45 (a) and Cs2 Fe0.2 (b) compounds after catalytic test. Solid lines are derived from least-square fits.
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Table 4 Mössbauer parameters calculated from the spectrum of the compounds Fe0.85 H0.45 and Cs2 Fe0.2 recorded at 295 K after catalytic test. δ: isomer shift (given with respect to α-Fe) 1: quadrupolar splitting Compound
Site
δ (mm.s−1 )
1 (mm.s−1 )
Relative intensities (%)
Fe0.85 H0.45
Fe3+ Fe2+ Fe3+ Fe3+ Fe2+
0.36 1.23 0.34 0.50 1.22
0.50 2.92 0.97 0.63 3.19
76 24 86 12 2
Cs2 Fe0.2
did not increase with time. The ESR spectrum of the Fe0.85 H0.45 compound after catalytic test and Mössbauer spectroscopic study showed no Mo5+ , whereas, that of the Cs2 Fe0.2 compound showed relatively well resolved signal of Mo5+ characterized by g=1.960, g=1.846 and corresponding to that observed in the Fe0.85 H0.45 compound before catalytic test (Fig. 6). Small h.f.s. lines of 95,97 Mo (I=5/2) were better resolved in this spectrum. They allowed to extract the A tensor components A=0.45 G and A=105 G.
Fig. 5. The evolution of the relative amount in Fe2+ in Fe0.85 H0.45 as a function of the time at room temperature in ambient air after catalytic test.
3.4. Characterization of the catalysts after reduction In order to study how iron intervened in the reduction process of the heteropolymolybdic acid, the Fe0.85 H0.45 and Cs2 Fe0.2 compounds were reduced by hydrogen in a thermobalance as described in
Fig. 6. The ESR spectrum recorded at 77 K of the Cs2 Fe0.2 compound after catalytic test.
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Fig. 7. Experimental Mössbauer spectra recorded at 295 K of the compounds reduced at 613 K by H2 and kept up at 295 K in ambient air respectively for various time Fe0.85 H0.45 kept for 40 h (a), Fe0.85 H0.45 kept for 545 h (b) and Cs2 Fe0.2 kept up for 55 h (c). Solid lines are derived from least-square fits.
the experimental section. The reduction extent of the compounds were monitored and were respectively equal to 2 and 0.33 e− per KU. After reduction the samples have been studied by Mössbauer spectroscopy. The spectrum of Fe0.85 H0.45 after reduction (40 h) is presented in Fig. 7a and the calculated
parameters summarized in Table 5. The spectrum of Fe0.85 H0.45 displayed the same two doublets as observed after catalytic test. Similarly, we observed an increase of the relative intensity of the ferrous doublet with time in ambient air that reached 85% after 550 h (Fig. 7b). We have in the same manner studied the compound Cs2 Fe0.2 after reduction. The Mössbauer spectrum of the compound after reduction has been fitted with two ferric doublets with parameters identical to that observed for the ferric doublets after the catalytic test. After 240 h in ambient air the spectrum remained unchanged with only one ferrous doublet of low intensity and parameters similar to those obtained after catalytic test (Fig. 7c). The study of the compounds after reduction by the ESR showed the presence of Mo5+ only in the Cs2 Fe0.2 compound similarly to that of the compounds after catalytic test. These results were in agreement with those obtained after catalytic test and clearly showed that the catalysts were reduced in the conditions of catalytic test. The study was further focused on the characterization of Fe0.85 H0.45 after reduction. With that objective, the influence of the initial reduction extent on the amount of Fe2+ observed after reduction was studied. Samples of the Fe0.85 H0.45 compound have been reduced with different reduction extent in the thermobalance and analyzed at room temperature in the same manner as previously described. We have observed that for reduction extents inferior to 1 e− /Fe, the reduction of iron was proportional to the initial reduction extent of molybdenum (Fig. 8). Furthermore, at the end Mo5+ was absent in the solid as shown by the ESR. At low reduction rate, the charge transfer was thus total between iron and molybdenum. We then studied the influence of air moisture on the electron transfer. A Fe0.85 H0.45 sample has been reduced as before and maintained after reduction at room temperature under dry air for 70 h. It was then placed in ambient air and analyzed by Mössbauer spectroscopy. The evolution of the Fe2+ content as a function of time is presented in Fig. 9. It can be seen that the iron reduction started only when the sample was in ambient air. The reoxidation of the compound after the electron transfer has been studied by TGA. For that a sample of reduced Fe0.85 H0.45 was maintained under air at room temperature until the iron would be reduced. It was then heated under deoxygenated nitrogen in the thermobalance up to 613 K and after
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Table 5 Mössbauer parameters calculated from the spectrum of the compounds Fe0.85 H0.45 and Cs2 Fe0.2 recorded at 295 K after reduction respectively to 2 and 0.33 e-/KU. δ: isomer shift (given with respect to α-Fe) 1: quadrupolar splitting Compound Fe0.85 H0.45
Time after reduction (h) 40 195 265 325 545
Cs2 Fe0.2
240
Site
δ (mm.s−1 )
1 (mm.s−1 )
Relative intensities (%)
Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe3+ Fe2+
0.38 1.33 0.40 1.25 0.43 1.25 0.41 1.25 0.37 1.22 0.39 0.53 1.27
0.42 2.17 0.42 2.75 0.43 2.82 0.41 2.84 0.37 2.90 0.80 0.57 2.94
76 24 76 24 76 24 76 24 76 24 87 11 2
approximately 30 min at this temperature the nitrogen flow was shifted to an air flow (Fig. 10). The same experiment has been performed on the pure acid treated with the same protocol. During heating under nitrogen, we observed between 70 and 220 K a weight loss corresponding to the dehydration of the compounds. It can be observed that the reduced samples were not completely rehydrated, since the weight losses corresponded to only 9.6 and 11.8 H2 O per KU, respectively, for the pure and iron doped acid instead of 12.8 and 14.2 H2 O per KU measured for the fresh samples. It was interesting to note that the same stabilization at
about five water molecules per KU (5.1) was observed for the doped acid sample. After the introduction of air no weight gain was observed for the iron containing acid whereas the pure acid showed a weight gain corresponding well to the reduction extent of the solid (Fig. 10). The characterization of the Fe0.85 H0.45 compound at the end of the experiment by the Mössbauer spectroscopy and the ESR showed that it contained only Fe3+ and Mo6+ and was thus fully reoxidized.
Fig. 8. Variation of the total amount of Fe2+ formed in Fe0.85 H0.45 after reduction as a function of the initial reduction extent of the solid.
Fig. 9. The evolution of the relative amount of Fe2+ in Fe0.85 H0.45 as a function of the time at room temperature in ambient air after reduction and maintain in dry air for 70 h.
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Fig. 10. Thermogravimetric and analysis of the pure acid (a) and the acid containing iron (b) after reduction and electron transfer. The programmed heating procedure is superposed to the thermogravimetric curves. An enlargement of the figure corresponding to the time when nitrogen has been replaced by air is shown with the corresponding DTA curves (HT: heat flow).
4. Discussion The characterization of the iron counter-cations in the bulk acid (Fe0.85 H0.45 ) and in the acid supported on the cesium salt (Cs2 Fe0.2 ) evidenced differences in their environments. Mössbauer spectroscopy showed that the cations were ferric with probably an octahedral co-ordination strongly distorted in the supported acid but relatively regular in the bulk acid. The ESR data supported these results pointing out by the way a small reduction of the molybdenum cations and the destruction of some of the heteropolyanions for the bulk acid sample. We evidenced by DTG the stabilization of the bulk acid with about five water molecules which could be explained by the presence as counter-cations of Fe(H2 O)6 complexes localized between Keggin units (Fig. 11). The relatively symmetrical environment of the iron cations in such complexes and the mobility of these complexes could account respectively for the small quadrupolar splitting observed and the larger line width observed in Mössbauer spectra. The presence of such aqua complexes is not unusual and has soon been demonstrated for vanadyl ions in VO(H2 O)5 PMo12 O40 [19]. Upon heating above 533 K the aqua complex decomposed
and the co-ordination sphere of the Fe3+ ions reconstructed. The lack of crystallographic data on the structure of the hydrated acid does not allow us to propose a scheme for its new co-ordination but it should necessarily imply the outer oxygen atoms of the neighboring heteropolyanions and certainly several oxygen atoms of the same one. The environment of the iron cations in the supported acid was different. A strong interaction between the acid and the salt should exist with a reduction of the
Fig. 11. Schematic representation of the iron counter-cations in the bulk acid phase.
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inter-anionic space in the acid which did not allow the stabilization of the iron aqua complexes. The low total hydration extend of the acid observed in this case tends to confirm this proposition. we have shown that the environment of the iron cations was in this case very distorted and could be more comparable to that in the dehydrated bulk acid. The catalytic properties of the samples have been evaluated and compared to those of the same type of solid but containing no iron. First we can see that whether it contained iron or not, the supported acid was always more selective in MA and MAA than the bulk acid. This increase could be related to an increase of the acid properties of the supported acid due to its modified hydration ability resulting from its strong interaction with the cesium salt support as previously reported [20]. In the bulk acid the substitution of protons by Fe3+ has, for the same reason, the same effect on the MA and MAA selectivities but, in this case, it also strongly increased the activity. In the supported acid the substitution of protons by Fe3+ has almost no effect on the activity (rate of isobutane transformation) but it increased the selectivities in MA and MAA. This result has already been obtained by Mizuno et al. and we confirmed it well [2]. The increase in selectivities had previously been related to the reduction of the acidity with the substitution of protons by iron [10]. The acidity of the pure supported heteropolyacid should not be too high to avoid the partial decomposition of the methacrolein and methacrylic acid that are formed. In this case a lowering of the acidity by iron substitution has been shown to have a positive effect on the selectivity to MA and MAA [10]. In the same paper we studied the reducibility of the catalysts and we showed that it influenced greatly the activity. We observed that the substitution of iron had no effect on the reducibility of the catalysts which explained why the activity remained unchanged for the Cs2 Fe0.2 sample, on the other hand, when protons were substituted by copper cations, the reducibility was increased as well as the activity and that the Cu2+ /Cu+ redox couple was playing a role in the oxido-reduction of the solids [10]. The characterization of the iron counter-cations in the compounds after catalytic test and after reduction, clearly showed that they were strongly reduced in the bulk acid but almost, not in the supported one. The Fe3+ /Fe2+ redox couple should intervene
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in the oxido-reduction of the bulk acid but not of the acid supported on the cesium salt. These results were in agreement with those previously obtained and explained the positive effect of iron on the activity of the bulk acid. The characterization by the Mössbauer spectroscopy of Fe0.85 H0.45 after reduction or catalytic test, did not show the reduction of the iron cations immediately but that it was taking place slowly at room temperature. Such reduction in air at 298 K can only be explained by a charge transfer between the reduced molybdenum and iron cations: Fe3+ +Mo5+ →Fe2+ +Mo6+ . This conclusion is also supported by the ESR data which showed that the reduction of iron was coupled with the oxidation of the molybdenum cations. It is interesting to note that the reduction by H2 of the acid supported on the cesium salt and containing copper has been shown to proceed via a concerted mechanism with a similar electron transfer between the copper and molybdenum cations [10]: Cu2+ + 2Mo6+ + O2− + H2 → Cu+ + Mo5+ + Mo6+ + H2 O Cu+ + Mo5+ + Mo6+ → Cu2+ + 2Mo5+ The study of the Fe0.85 H0.45 samples with different reduction extend showed that the charge transfer was total when the reduction rate of the compounds was less than 1 e− per KU (Fig. 9). This allows to calculate from the results of the characterization after catalytic test, what should be the reduction extend of the catalysts in the conditions of catalytic test. We can see that 24% of the iron cations were reduced which corresponds to 0.2 e− per KU or 1.7% of Mo5+ . This reduction extent is quite small and would correspond, considering that a Keggin unit is preferentially reduced to two electrons, to the reduction of only 10% of the Keggin units which should be related to a reduction limited to the surface, between one and two Keggin unit layers. The study of the reduced solid kept in dry air for 70 h clearly indicated that the charge transfer between iron and molybdenum cations was related to the rehydration of the compound. The thermogravimetric analyses of the samples after reduction showed that the compounds recovered easily an hydration extent close to that before reduction and that the hydrated iron complexes were again formed in the Fe0.85 H0.45
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Fig. 12. Schematic representation of the charge transfer mechanism (a) and of the oxidation mechanism of the reduced heteropolycompound after the charge transfer (b).
compound. The Mössbauer parameters of the ferric cations after reduction were the same as before and no new iron species were detected. Furthermore, the parameters of the ferric doublets in the reduced samples did not vary tremendously with the reducing extent (Table 5). These results clearly indicated that the rehydration of the iron cations was rapid and conditioned the charge transfer (Fig. 12a). This explained why the iron in the supported acid which cannot be hydrated could not be reduced. In this last case, iron cations were quasi not reduced (only 2% of Fe2+ ) and the reduction of the supported acid involved mainly the reduction of the molybdenum cations as confirmed by the ESR. The new ferric doublet identified in the Mössbauer spectrum of the supported acid could correspond to iron cations in close proximity of a reduced molybdenum. Experiments are presently conducted in order to confirm this interpretation. We have shown that the aqua complexes in Fe0.85 H0.45 decomposed at 533 K which was lower than the reaction temperature (623 K). However, in the condi-
tions of catalytic test, water formed from the total oxidation of isobutane was present in the gas phase and both hydrated and dehydrated iron species should be in equilibrium at the surface of the heteropolycompound. When the catalysts were recovered, the shift of the gas feed for a nitrogen flow at 623 K should have brought the dehydration of the catalysts, explaining why the rehydration with the charge transfer was observed at room temperature. The re-oxidation experiments performed after the reduction and the rehydration showed that the Fe0.85 H0.45 compound, contrarily to the pure acid was undergoing an oxidation upon heating under pure nitrogen. The same type of mechanism has already been described on hydrated iron phosphates which can be oxidized upon heating under nitrogen or vacuum [20,21]. The mechanism proposed for this auto-oxidation reaction was based upon the reversible decomposition of crystal water bounded to the iron cations to form hydroxyl groups: 2 (Fe2+ –[H2 O])→2 ([Fe3+ –OH])+H2 [21,22]. It can be proposed that the
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same type of reactions occurred in the heteropolycompounds allowing its re-oxidation (Fig. 12b). After a final rehydration of the iron cation, the catalytic site would be restored for a new reaction. The presence of the charge transfer between the iron counter-cation and molybdenum in the Keggin unit and the possible oxido-reduction of the iron cations, which have both been demonstrated here, makes iron directly involved in the oxido-reduction process of the Keggin unit and explain it has an effect on the catalytic reaction rate. The oxido-reduction process of iron based upon a dehydration/hydroxylation reaction may directly be involved in the oxidation of the isobutane molecules.
5. Conclusion The results obtained in this study confirm that the catalytic activity in oxidation reactions of molybdophosphoric heteropolycoumpounds depended on their reducibility and show that the positive effect on the catalytic activity of a transition metal counter-cation like iron is related to a charge transfer between the molybdenum cations in the heteropolyanion and the counter-cation. In the case of iron, this electron transfer is taking place when the solid is hydrated with the formation of an iron aqua complex. Such iron species can be formed and maintained in equilibrium with dehydrated species at the surface of the catalysts in the conditions of catalytic test, since water is always present in the gas phase due to the unselective total oxidation of isobutane. Similar electron transfer should take place in the case of vanadyl counter-cations which also form aqua complexes when present as counter-cations and have a positive effect on the catalytic activity of the catalysts in oxidation reaction like the oxidative dehydrogenation of isobutyric acid [19]. When the acid is supported on the cesium salt Cs3 PMo12 O40 , it is in strong interaction with the support. As a consequence it is less hydrated,
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