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Cesium promotion of iron phosphate catalyst and influence of steam on the oxidative dehydrogenation of isobutyric acid to methacrylic acid
V. Cortes Corbcran and S. Vic Bcllon (Editors), New Developrnenls i n Selectwe O x r d d o n 0 1994 Elscvier Science B.V. All rights reserved.
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Cesium promotion of iron phosphate catalyst and influence of steam on the oxidative dehydrogenation of isobutyric acid to methacrylic acid. J. Belkoucha, 8. Taouka, L. Monceauxa, E. Bordesa, P. Courtinea and G. Hecquetb aDepartement de Genie Chimique, Universite de Technologie de CompiPgne, B.P. 649,60206 CompiPgne Cedex, France, bElf-Atochem, Tour Aurore, 92080 Paris La Defense 2 Cedex, France. When cesium is added to iron phosphates in the oxidative dehydrogenation of isobutyric acid to methacrylic acid, both catalytic and non catalytic reactivities of the catalyst are modified, with or without steam. It is shown that both cesium and steam are responsible for regulation of the redox mechanism, and for stabilization of the catalysts by decreasing coke formation.
1- INTRODUCTION
As shown extensively by Millet et al., several iron phosphates are catalysts for the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) [l-31. They found a new phase called Fe3(P207)2, that they claim to be the active and selective one, responsible for the good performance of P/Fe = 1/1 catalysts. FegPg031, which is obtained by oxidation of pure Fe2P207, would be mainly composed of this new phase Fe3+2Fe2+(P207)2and of a-Fe2Og in a 2/1 molar ratio. The latter has been detected only by Mossbauer spectroscopy and must be amorphous because no other experimental evidence exists to suggest that it is crystalline. Dekiouk et al., studied the kinetics and investigated the composition of bulk and surface of catalysts containing an excess of phosphorus and silica. They emphasized the role of acid phosphates which should be present on the surface in presence of steam [4]. However the best catalytic properties are claimed for catalysts containing cesium (or other alkaline metal) and an excess of phosphorus (P/Fe > 1/1) [5].To understand the roles of Cs, P and steam, we have, first, examined their effects during the preparation of P/Fe = 1/1 catalyst, and second, studied the reactivity of samples with and without steam, in oxidizing and reducing conditions. Finally, the behavior of pure FeP04 and Fe2P207 has been compared with Cs-Fe-P-0, in catalytic and non catalytic conditions.
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2- EXPERIMENTAL 2-1. Preparation of catalysts and characterization. FeP04 (quartz) was synthesized from an aqueous solution containing a stoichiometric mixture of NH4H2P04 and Fe(N03)3.9H20 heated to dryness, and the residue was dried at 393 K for 24 hrs. After grinding, calcination was performed at 573 K for 12 hrs and then at 723 K for two days. By the same method CsxFeP~+yO, catalysts were prepared by adding to the preceding mixture x CsN03 and y NH4H2P04 before evaporation and calcination. Typically x = 0.15 and y = 0.24 were used [5,6]. CsFeP207 was prepared as above with stoichiometric amounts. The dried solid was calcined first at 593 K and then at 823 K after grinding. Fe2P207 was prepared by reduction of FeP04(Q) in wet H2/N2 = 10/90 (psteam= 24 mmHg) at 723 K for two days. Its reoxidation in air at the same temperature yielded a black brown compound identified as FegPg031 based on weight gain in thermal analysis (TGA). X-Ray diffraction (XRD), electron microscopies (SEM, TEM) and infrared spectroscopy were used extensively to characterize the compounds at different stages of preparation, and before and after catalytic tests. 2-2. Catalytic and non catalytic reactivities The catalytic reaction was studied at 688 K in an integral stainless steel microreactor (30 cm3) fed with 5 mol.% of IBA in N2, 0 2 and steam (02/IBA = 0.75, H20/IBA = 12), at contact time z = 0.7 sec. The analysis of effluents was made with two on-line gas chromatographs [6]. Samples examined were pure phases Fe2P207, FegPgO31, CsFeP207, the catalyst CsxFeP~+yOz, and two mixtures of CsFeP207/Fe2P207 (MM0.15 and MM0.35 for 0.15/1 and 0.35/1 molar ratios respectively) obtained by mechanical grinding. The behavior of these solids in oxidizing and reducing conditions was examined by use of thermal analysers (TGA/DTA, heating rate 5 K/min) in dry or wet atmosphere of 0 2 / N 2 = 20/80 and H2/N2 = 10/90 (feed rate 12 l/hr) respectively. 3- RESULTS
3-1. Characterization of CsxFePl+yOZ(x = 0.15, y = 0.24).
Although the conditions of preparation and calcination at 723 K of FeP04 and of CsxFePl+yO, are the same, pure FeP04 is well crystallized (quartz-type structure), unlike Cs,FeP~+yOzwhich exhibits only two lines of FeP04 tridymitetype [6]. The state of crystallization is improved after calcination at 823 K, and peaks of FeP04(T) are unambiguously identified. After 7 hrs at 923 K, FeP04(Q) (high temperature) and CsFeP207 are formed. By TGA/DTA of CsxFePl+yOzin air a small loss of weight (0.4 %) occurs near 863 K and an exothermic signal at 983 K (no weight change), which is reversible with decreasing temperature, is assigned to the transformation of FeP04 quartz from low to high temperature forms. The small loss is due to water as evidenced
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by following the peak 18 on the mass spectrometer coupled to TPD experiments in vacuum at 853 K. It could be related to the presence of iron pyrophosphate acid, HFeP207, which releases water near 873 K and is transformed into Fe(P03)3 and Feq(P207)3 [8]. The actual presence of HFeP207 could not be easily checked because of its low amount (y-x = 0.09) which hinders its detection by XRD, but the 0.4 YO loss is consistent with this hypothesis. The formula of CsxFeP~+yO, in air and below 873 K could be therefore written as (CsFeP207)x(HFeP207)y-x(FePO~)~-y. Because it crystallizes as FeP04(T), the tridymite structure of which is very loose and open, it could also be considered as a solid solution of ortho and pyrophosphate of iron, hydrogen and cesium: CsxHy-xFe(P207)y(P04)~-y, or CsxHy-x(FeP~07)y(FeP04)~-y [6]. It must be recalled indeed that FeP04(T) is stable only when dopants like A1 (replacing partly Fe) or alkaline cations, or even phosphorus are present. In the case of CsXFeP1+,0,, the tridymite structure would be stabilized not only by protons, which are necessarily present to balance the negative charges, but also by cesium cations occupying the large cavities. 3-2. Catalytic reactivity
The conversion CIBAand selectivity SMAAin MAA at 488 K and 2 = 0.7 sec are plotted against time in Fig. 1 for Fe2P207, Fe#@31, and for mechanical mixtures MM0.15 and MM0.35. The main by-products are carbon oxides, acetone, propylene and a little acetic acid. CsFeP207 was found nearly inactive. At the end of the catalytic experiments the phases inside the catalysts were identified by XRD (Table 1). Table 1 Catalytic results and identification of phases by XRD after 24 hrs. Catalysts
a: XRD after 13 days,
Phases after catalytic test (24 hrs)
IBA conv. SAMA mol.% mol.%
Yield mol.%
performance at 24 hrs.
After a transitory state which lasts from 3 to 7 hrs according to the sample, a plateau is reached which corresponds to the steady state. Fe~P207and FesPsOsl behave differently at first: CIBAand SMAAincrease for Fe2P207 while they decrease for FesP8031. After stabilization they decrease slightly with time (Fig. 1).Used catalysts in each case showed both FesP8031 and Fe2P207. The mechanical mixtures
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4 7
t
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10
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25
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30
.
TIME [ h n )
TIME (hm)
3
* *
c
10
20
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Figure 1. Conversion of IBA (a) and selectivity in MAA (b) vs time at 688 K for Fe2Pz07, FegP8031, MM0.15 and MM0.35 (CsFeP207/Fe2P207 = 0.15 and 0.35 ).
A
2 :II
CONVERSIOS
+-+--+-+-+
f
2
4
6
8
10
D
12 TlME(days
Figure 2. Conversion of IBA and selectivity in MAA and by-products vs time for Cs,FeP~+yO,(x = 0.15, y = 0.24) (688 K, z = 0.7 sec).
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MM are more stable and their catalytic properties are better than those of Fe2P207, at first and at the steady state (Fig. 1, Table 1).FegPg031 is also detected, together with CsFeP207 and Fe2P207 which are initially present in the fresh catalyst. The catalytic properties of Cs,FePl+yO, were examined during two weeks (Fig. 2). After 17 hrs of running, conversion and SMAA were 83 and 76 mol.% respectively. After 6 days, during which the steam feed stopped unexpectedly for a while, conversion and selectivity were stabilized ca. 76 mol.%. As already mentionned, the fresh sample of Cs,FePi+yO,is poorly crystallized in the FeP04(T) solid solution, but after 13 days of working the crystallinity is improved, and the three phases already identified in MM samples are also observed (Table 1). Moreover the catalyst is extensively coked as seen each time the catalyst works without steam in the feed. The comparison between these catalysts shows that the best performance is obtained with Cs,FeP~+yO,which is only slightly better than MM0.35 of similar stoichometry. At the steady state oxidized FegP~O31and reduced Fe2P207 forms of Fe-P-0 (P/Fe = 1/1) are present in all catalysts, including pure phases, whatever the initial compound is. These phases can be therefore considered as the redox partners, to which is added CsFeP207 when Cs and an excess of P are present. Although the latter is not active by itself, a synergetic effect is observed since catalytic properties of Fe-P-0 are enhanced. 3-3. Non catalytic reactivity
TGA/DTA of the oxidation of Fe2P2O7 in 02/N2 and of the reduction in H2/N2 of the products obtained were performed in dry or wet atmosphere. In dry air, pure Fe2P207 is oxidized in two steps, at 723 K in FegPgOgl which is stable up to 893 K, and further in FeP04(Q) up to 1073 K (Fig. 3). The reduction of FeP04(Q) (or of tridymite) yields directly Fe2P207, and runs faster in wet than in dry H2/N2 [6]. TGA were also conducted in isothermal conditions (723 K). In dry air the oxidation of Fe2P207 stops at FegPg031 and the reduction of the latter in dry H2/N2 gives back Fe2P207. Several redox cycles of this kind can be performed. Here, the rate of each reaction decreases in wet atmosphere and therefore steam hinders these reactions (Fig. 4). In the same conditions Cs,FePI 0, behaves somewhat differently. Initially the +y. sample is the FeP04(T) solid solution containing Cs. First, its reduction in dry H2/N2 yields Fe2P207 (Fig. 4B), which is reoxidized (in dry 02/N2) directly in FeP04(T) while CsFeP207 is present. Let us recall that starting with pure Fe2P207 would lead to FegP8031 (Fig. 4A). This step of reduction is also by far faster than when starting with pure FeP04(Q). Several redox cycles can be reproducibly made between FeP04(T) and Fe2P207 (in presence of Cs). Second, in wet atmosphere, the oxidation of FezP207 stops at Fe8PgO31, which gives back Fe2P207 by reduction. Similarly, several redox cycles can be performed between FegP8031 and Fe2P207 (in presence of Cs and H20). Oxidation and reduction run slower than in dry atmosphere. To summarize, and by comparison with pure phases, the presence of CsFeP207 can be correlated with the changes observed after the first reduction, which are (i), reoxidation up to FeP04(T) instead of FegP8031 in dry atmosphere, and, (ii), reoxi-
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Figure 3. Thermal analyses (DTA/TGA) of the oxidation of Fe2P207 in air.
B
J.
0
15
30
45
60
timeimin)
Figure 4. Reduction (B) and reoxidation (A) of Fe-P-0 (P/Fe = 1/11 and Cs-Fe-P-0 (P/Fe = 1.24) in dry or wet atmospheres. Cs, wet; (4 without Cs, dry; (.el without Cs, wet. Legend: (4Cs, dry; (4
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dation up to FegPg031 in wet atmosphere only, with higher rates than for pure phases but lower rates than in dry atmosphere. Another point is that the solids obtained in wet atmosphere are easily identified because of their better crystallinity. CsFePzO7 is therefore detected beside FegPgO3i or Fe2P207 according to the oxidizing or the reducing atmosphere used. 4- DISCUSSION
It has been already shown that Fe-P-0 catalysts work by means of a redox mechanism involving lattice oxygens of the solid [ 3 ] . All the experiments performed in this study show that Fe2P207 and FegPg031, the latter being obtained by reoxidation in the operating catalytic or non catalytic conditions provided steam is present, are the two stable partners of the redox system. Let us notice that the remarkable reproducibility of redox cycles is not consistent with the eventuality of amorphous iron oxide beside Fe3(P207)2. The presence of Cs as CsFeP207 stabilizes Fe3+, and increases the rates of both oxidation of Fe2P207 and reduction (by H2) of FegP8031. In turn, the activity is increased compared with non Cs-containing catalysts. The reason why an excess of phosphorus is used in the most efficient catalysts would be therefore related to the necessary formation of CsFeP207 inside the initial tridyrnite structure of fresh catalyst, while keeping the right stoichiometry P/Fe = 1/1 for FeP04 and redox partners. The role of steam is multiple. The catalytic phases obtained either by reduction or by oxidation in the presence of steam are in better state of crystallization and become less reactive as shown for the oxidation of Fe2P207 (Fig. 3 ) . Consequently the first action of steam is to modify the size and the texture of the crystallites, and to moderate the reactivity of CsxFeP~+yO, by limiting the oxidation of Fe2P207 present to FegPg031 (instead of FeP04). The second action of steam is to moderate both reactions, as shown for pure phases as well as for CsxFeP~+yO,,resulting in making the rates of oxidation and reduction closer to each other. Therefore the combined action of Cs and of steam can be understood as a regulation of the redox Fe2P207/Fe$@31 involved in the catalytic reaction, and of the amount of Fe3+ active sites. A comparison with the oxidative dehydrogenation of butene to butadiene which is made on iron oxides is fruitful because several points are common: the large amount of steam (H20/HC = 10-12), the addition of alkaline cations which enhance activity and selectivity, and the coking of catalysts especially in low amount of steam. In the case of iron oxides the role of steam has long been presented as a thermal diluent, an oxidant or a coke remover. Recently a study of the surface in operating conditions has evidenced another role which would be to convert the catalytic surface to hydroxyl forms such as Fe-OH-Fe (hydroxylation of an oxygen linked to two Fe) and 0 - F e O H - 0 (hydroxylation of a surface Fe), more adapted because more basic. These specie would be able to initiate the dehydrogenation by the ahydrogen abstraction as a proton from butene [9,10]. Steam would also limit the adsorption and reaction of butene owing to competitive adsorption of molecular water. However IBA (MAA) is not butene (butadiene) and the last proposition is
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no more valid since, on the contrary, steam is useful in helping MAA to desorb, as with heteropolyoxometallates active and selective for IBA-MAA [ l l ] . Surface acid ortho and/or pyrophosphates would be the sites responsible for this function, as proposed formerly [2]. Another difference is that, although alkaline cations (which increase 0 2 - or OH- basicity) are useful to maintain the active Fe3+ state, potassium ferrite KFe02 has been claimed to be the active phase [9,10], whereas CsFeP207 itself is not active and yields mainly acetone from IBA [2,6]. In a similar way, we propose that neighboring iron cations wearing surface hydroxyls could act as active sites allowing dehydrogenation of IBA, the selectivity in MAA being controlled by the action of steam on acid phosphates. REFERENCES J.M.M. Millet, J.C. Vedrine and G. Hecquet, in G. Centi and F. Trifiro Eds., New Developments in Selective Oxidation, Stud. Surf. Sci. Catal., 55 (1990) 833. 2. J.M.M. Millet and J.C. Vedrine, Appl. Catal., 76 (1991) 209-219. 3. C. Virely, M. Forissier, J.M.M. Millet and J.C. Vedrine, J. Molec. Catal., 71 1992) 199-213. 4. M. Dekiouk, N. Boisdron, S. Pietrzyk, Y. Barbaux and J. Grimblot, Appl. Catal., A, 90 (1992) 51-60 ;ibid., 61-72. 5. D. Chelliah, Ashland Oil Inc. (USA), FR. 2 498 475 (1982). 6. J. Belkouch, ThPse, CompiPgne (1991). 7. P. R6my and A. Boull6, C. R. Acad. Sci. Paris, 253 (23) (1961) 2699. 8. F. D'Yvoire, Bull. SOC.Chim. Fr., 6 (1962) 1224-1246. 9. B.J. Liaw, D.S. Cheng and B.L. Yang, J. Catal., 118 (1989) 312-326. 10. B.L. Yang, D.S. Cheng and S.B. Lee, Appl. Catal., 70 (1991) 161-173. 11. M.J. Bartoli, L. Monceaux, E. Bordes, G. Hecquet and P. Courtine, in P. Ruiz and B. Delmon Eds., New Developments in Selective Oxidation, Stud. Surf. Sci. Catal., 72 (1992) 84-92. 1.
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J. Vedrine (IRC, Villeurbanne, France): Your results complement those of Millet et al., concerning the role of Cs in the stabilization of tridymite FeP04 by adding steam and studying chemical mixtures. Could it be possible that mechanical mixtures between Fe2P207 and Cs20 give the same result, or at least affect acidic features. You have also made a correlation between oxidative dehydrogenations of butene on iron oxide catalyst and of of isobutyric acid to methacrylic acid, stating that the first step involves H abstraction on basic sites (OH- type). However butene is a basic compound while IBA is obviously acidic. How can you imagine the first step on the same basic OH- site. E. Bordes (UTC, CompiPgne, France): We are not presently able to assign the increase of performance obtained by addition of Cs either to CsFeP207 as a phase, or to Cs itself as a cation. In the latter case Cs would indeed affect acidity since it reinforces OH- or 0 2 - basicity, and so similar results could be obtained with a mixture Fe2P207/Cs20. To answer the second question, the abstraction of H(-C-C=O) (IBA) or H(-C-C=C) (butene) can be done by similar OH-, whatever the acidity or basicity of the molecule as a whole which affects mainly further steps. Hydroxyls such as OH(-Fe-0-Fe) displayed on e.g., Fe2O3, are indeed slightly different from OH(-Fe-0-P) on e.g. FeP04.
G . Emig (Institut fur Technische Chemie, Erlangen, Germany): When we used heteopolyacid catalysts for your reaction we saw also a very marked effect by the addition of water. We observed an optimal IBA/H20 ratio of 12. If one applies higher ratios a negative effect on IBA conversion is observed, because of competitional adsorption on the catalyst surface. Why do you use H2O/IBA = 12 and what happens if you go to lower values? Such a high amount of water could certainly not be beneficial for a commercialisation of your process! E. Bordes (UTC, CompiPgne, France): The H20/IBA ratio of 12 is indeed an economic handicap for the process. However a decrease of the amount of steam leads to a strong decrease in catalytic properties due partly to extensive coking of the catalyst. Stabilization of the best performance is observed for H2O/IBA = 12-15.
J. M. Millet (IRC, Villeurbanne, France): We have proposed some years ago that cesium plays a role in the stabilisation of a tridymite phase with P/Fe ratio greater than one, and that CsFeP207 itself has no catalytic role. You propose in this work that it does have a role. Since CsFeP207 has been shown to be inactive and since it is present in the catalyst in too a small amount to postulate a support effect, how do you explain the synergy effect that you observed with mechanical mixture of CsFeP207 and Fe~P207? Furthermore, your results (MM0.15 and MM0.35) seem to show that this synergy effect is quite independant of the amount of CsFeP207. E. Bordes (UTC, CompiPgne, France): CsFeP207 is indeed poorly active but we do observe a promoting effect when it is present beside active phases. Its amount is not low because Cs,FeP~+yO, (x = 0.15, y = 0.24) corresponds
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approximately to CsFeP207/Fe2P207 = 0.35/1 (molar ratio). Catalytic results are higher with mechanical mixtures MM than with pure phases (Fe2P207 or FegP8031) which means that a synergetic effect exists, even though the contacts are not optimized. Moreover MM0.35 behaves similarly to C~0.15FeP1.240~. Other ratios should be prepared to be sure that the optimum ratio lays between 0.15 and 0.35, or closer to 0.35, as we suppose.
M. Sinev (I. of Chemical Physics, Moscow, Russia): You explain the influence of water by competitive adsorption between water and hydrocarbon. But your data show (Fig. 4) that the influence of water is stronger on reoxidation than on reduction. So, 1)What is the origin of the influence of water on reoxidation of catalysts? 2) Which of the two processes (reduction or reoxidation) is responsible for the influence of water on steady-state catalytic process?
E. Bordes (UTC, Compiegne, France): All data are not presented here. A
kinetic study performed on pure phases has shown that (6), comparing oxidation and reduction without and with steam, the activation energy of oxidation Eox is slightly modified (20 and 23.5 kcal.mo1-1 respectively), whereas for reduction ERed (9 kcal.mo1-1) is multiplied by 2 or 3 according to the temperature range (similar values of preexponential factors). The adsorption of H20 on oxidized sites can be proposed as the moderating factor of reduction step. When Cs is present the main effect of steam is to limit the reoxidation of Fe2P207 to FegPg031.
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Cesium promotion of iron phosphate catalyst and influence of steam on the oxidative dehydrogenation of isobutyric acid to methacrylic acid. J. Belkoucha, 8. Taouka, L. Monceauxa, E. Bordesa, P. Courtinea and G. Hecquetb aDepartement de Genie Chimique, Universite de Technologie de CompiPgne, B.P. 649,60206 CompiPgne Cedex, France, bElf-Atochem, Tour Aurore, 92080 Paris La Defense 2 Cedex, France. When cesium is added to iron phosphates in the oxidative dehydrogenation of isobutyric acid to methacrylic acid, both catalytic and non catalytic reactivities of the catalyst are modified, with or without steam. It is shown that both cesium and steam are responsible for regulation of the redox mechanism, and for stabilization of the catalysts by decreasing coke formation.
1- INTRODUCTION
As shown extensively by Millet et al., several iron phosphates are catalysts for the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) [l-31. They found a new phase called Fe3(P207)2, that they claim to be the active and selective one, responsible for the good performance of P/Fe = 1/1 catalysts. FegPg031, which is obtained by oxidation of pure Fe2P207, would be mainly composed of this new phase Fe3+2Fe2+(P207)2and of a-Fe2Og in a 2/1 molar ratio. The latter has been detected only by Mossbauer spectroscopy and must be amorphous because no other experimental evidence exists to suggest that it is crystalline. Dekiouk et al., studied the kinetics and investigated the composition of bulk and surface of catalysts containing an excess of phosphorus and silica. They emphasized the role of acid phosphates which should be present on the surface in presence of steam [4]. However the best catalytic properties are claimed for catalysts containing cesium (or other alkaline metal) and an excess of phosphorus (P/Fe > 1/1) [5].To understand the roles of Cs, P and steam, we have, first, examined their effects during the preparation of P/Fe = 1/1 catalyst, and second, studied the reactivity of samples with and without steam, in oxidizing and reducing conditions. Finally, the behavior of pure FeP04 and Fe2P207 has been compared with Cs-Fe-P-0, in catalytic and non catalytic conditions.
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2- EXPERIMENTAL 2-1. Preparation of catalysts and characterization. FeP04 (quartz) was synthesized from an aqueous solution containing a stoichiometric mixture of NH4H2P04 and Fe(N03)3.9H20 heated to dryness, and the residue was dried at 393 K for 24 hrs. After grinding, calcination was performed at 573 K for 12 hrs and then at 723 K for two days. By the same method CsxFeP~+yO, catalysts were prepared by adding to the preceding mixture x CsN03 and y NH4H2P04 before evaporation and calcination. Typically x = 0.15 and y = 0.24 were used [5,6]. CsFeP207 was prepared as above with stoichiometric amounts. The dried solid was calcined first at 593 K and then at 823 K after grinding. Fe2P207 was prepared by reduction of FeP04(Q) in wet H2/N2 = 10/90 (psteam= 24 mmHg) at 723 K for two days. Its reoxidation in air at the same temperature yielded a black brown compound identified as FegPg031 based on weight gain in thermal analysis (TGA). X-Ray diffraction (XRD), electron microscopies (SEM, TEM) and infrared spectroscopy were used extensively to characterize the compounds at different stages of preparation, and before and after catalytic tests. 2-2. Catalytic and non catalytic reactivities The catalytic reaction was studied at 688 K in an integral stainless steel microreactor (30 cm3) fed with 5 mol.% of IBA in N2, 0 2 and steam (02/IBA = 0.75, H20/IBA = 12), at contact time z = 0.7 sec. The analysis of effluents was made with two on-line gas chromatographs [6]. Samples examined were pure phases Fe2P207, FegPgO31, CsFeP207, the catalyst CsxFeP~+yOz, and two mixtures of CsFeP207/Fe2P207 (MM0.15 and MM0.35 for 0.15/1 and 0.35/1 molar ratios respectively) obtained by mechanical grinding. The behavior of these solids in oxidizing and reducing conditions was examined by use of thermal analysers (TGA/DTA, heating rate 5 K/min) in dry or wet atmosphere of 0 2 / N 2 = 20/80 and H2/N2 = 10/90 (feed rate 12 l/hr) respectively. 3- RESULTS
3-1. Characterization of CsxFePl+yOZ(x = 0.15, y = 0.24).
Although the conditions of preparation and calcination at 723 K of FeP04 and of CsxFePl+yO, are the same, pure FeP04 is well crystallized (quartz-type structure), unlike Cs,FeP~+yOzwhich exhibits only two lines of FeP04 tridymitetype [6]. The state of crystallization is improved after calcination at 823 K, and peaks of FeP04(T) are unambiguously identified. After 7 hrs at 923 K, FeP04(Q) (high temperature) and CsFeP207 are formed. By TGA/DTA of CsxFePl+yOzin air a small loss of weight (0.4 %) occurs near 863 K and an exothermic signal at 983 K (no weight change), which is reversible with decreasing temperature, is assigned to the transformation of FeP04 quartz from low to high temperature forms. The small loss is due to water as evidenced
82 1
by following the peak 18 on the mass spectrometer coupled to TPD experiments in vacuum at 853 K. It could be related to the presence of iron pyrophosphate acid, HFeP207, which releases water near 873 K and is transformed into Fe(P03)3 and Feq(P207)3 [8]. The actual presence of HFeP207 could not be easily checked because of its low amount (y-x = 0.09) which hinders its detection by XRD, but the 0.4 YO loss is consistent with this hypothesis. The formula of CsxFeP~+yO, in air and below 873 K could be therefore written as (CsFeP207)x(HFeP207)y-x(FePO~)~-y. Because it crystallizes as FeP04(T), the tridymite structure of which is very loose and open, it could also be considered as a solid solution of ortho and pyrophosphate of iron, hydrogen and cesium: CsxHy-xFe(P207)y(P04)~-y, or CsxHy-x(FeP~07)y(FeP04)~-y [6]. It must be recalled indeed that FeP04(T) is stable only when dopants like A1 (replacing partly Fe) or alkaline cations, or even phosphorus are present. In the case of CsXFeP1+,0,, the tridymite structure would be stabilized not only by protons, which are necessarily present to balance the negative charges, but also by cesium cations occupying the large cavities. 3-2. Catalytic reactivity
The conversion CIBAand selectivity SMAAin MAA at 488 K and 2 = 0.7 sec are plotted against time in Fig. 1 for Fe2P207, Fe#@31, and for mechanical mixtures MM0.15 and MM0.35. The main by-products are carbon oxides, acetone, propylene and a little acetic acid. CsFeP207 was found nearly inactive. At the end of the catalytic experiments the phases inside the catalysts were identified by XRD (Table 1). Table 1 Catalytic results and identification of phases by XRD after 24 hrs. Catalysts
a: XRD after 13 days,
Phases after catalytic test (24 hrs)
IBA conv. SAMA mol.% mol.%
Yield mol.%
performance at 24 hrs.
After a transitory state which lasts from 3 to 7 hrs according to the sample, a plateau is reached which corresponds to the steady state. Fe~P207and FesPsOsl behave differently at first: CIBAand SMAAincrease for Fe2P207 while they decrease for FesP8031. After stabilization they decrease slightly with time (Fig. 1).Used catalysts in each case showed both FesP8031 and Fe2P207. The mechanical mixtures
822
OI\\*
.-. .
t
. .-.
4 7
t
5
0
10
I5
25
20
30
.
TIME [ h n )
TIME (hm)
3
* *
c
10
20
30
Figure 1. Conversion of IBA (a) and selectivity in MAA (b) vs time at 688 K for Fe2Pz07, FegP8031, MM0.15 and MM0.35 (CsFeP207/Fe2P207 = 0.15 and 0.35 ).
A
2 :II
CONVERSIOS
+-+--+-+-+
f
2
4
6
8
10
D
12 TlME(days
Figure 2. Conversion of IBA and selectivity in MAA and by-products vs time for Cs,FeP~+yO,(x = 0.15, y = 0.24) (688 K, z = 0.7 sec).
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MM are more stable and their catalytic properties are better than those of Fe2P207, at first and at the steady state (Fig. 1, Table 1).FegPg031 is also detected, together with CsFeP207 and Fe2P207 which are initially present in the fresh catalyst. The catalytic properties of Cs,FePl+yO, were examined during two weeks (Fig. 2). After 17 hrs of running, conversion and SMAA were 83 and 76 mol.% respectively. After 6 days, during which the steam feed stopped unexpectedly for a while, conversion and selectivity were stabilized ca. 76 mol.%. As already mentionned, the fresh sample of Cs,FePi+yO,is poorly crystallized in the FeP04(T) solid solution, but after 13 days of working the crystallinity is improved, and the three phases already identified in MM samples are also observed (Table 1). Moreover the catalyst is extensively coked as seen each time the catalyst works without steam in the feed. The comparison between these catalysts shows that the best performance is obtained with Cs,FeP~+yO,which is only slightly better than MM0.35 of similar stoichometry. At the steady state oxidized FegP~O31and reduced Fe2P207 forms of Fe-P-0 (P/Fe = 1/1) are present in all catalysts, including pure phases, whatever the initial compound is. These phases can be therefore considered as the redox partners, to which is added CsFeP207 when Cs and an excess of P are present. Although the latter is not active by itself, a synergetic effect is observed since catalytic properties of Fe-P-0 are enhanced. 3-3. Non catalytic reactivity
TGA/DTA of the oxidation of Fe2P2O7 in 02/N2 and of the reduction in H2/N2 of the products obtained were performed in dry or wet atmosphere. In dry air, pure Fe2P207 is oxidized in two steps, at 723 K in FegPgOgl which is stable up to 893 K, and further in FeP04(Q) up to 1073 K (Fig. 3). The reduction of FeP04(Q) (or of tridymite) yields directly Fe2P207, and runs faster in wet than in dry H2/N2 [6]. TGA were also conducted in isothermal conditions (723 K). In dry air the oxidation of Fe2P207 stops at FegPg031 and the reduction of the latter in dry H2/N2 gives back Fe2P207. Several redox cycles of this kind can be performed. Here, the rate of each reaction decreases in wet atmosphere and therefore steam hinders these reactions (Fig. 4). In the same conditions Cs,FePI 0, behaves somewhat differently. Initially the +y. sample is the FeP04(T) solid solution containing Cs. First, its reduction in dry H2/N2 yields Fe2P207 (Fig. 4B), which is reoxidized (in dry 02/N2) directly in FeP04(T) while CsFeP207 is present. Let us recall that starting with pure Fe2P207 would lead to FegP8031 (Fig. 4A). This step of reduction is also by far faster than when starting with pure FeP04(Q). Several redox cycles can be reproducibly made between FeP04(T) and Fe2P207 (in presence of Cs). Second, in wet atmosphere, the oxidation of FezP207 stops at Fe8PgO31, which gives back Fe2P207 by reduction. Similarly, several redox cycles can be performed between FegP8031 and Fe2P207 (in presence of Cs and H20). Oxidation and reduction run slower than in dry atmosphere. To summarize, and by comparison with pure phases, the presence of CsFeP207 can be correlated with the changes observed after the first reduction, which are (i), reoxidation up to FeP04(T) instead of FegP8031 in dry atmosphere, and, (ii), reoxi-
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Figure 3. Thermal analyses (DTA/TGA) of the oxidation of Fe2P207 in air.
B
J.
0
15
30
45
60
timeimin)
Figure 4. Reduction (B) and reoxidation (A) of Fe-P-0 (P/Fe = 1/11 and Cs-Fe-P-0 (P/Fe = 1.24) in dry or wet atmospheres. Cs, wet; (4 without Cs, dry; (.el without Cs, wet. Legend: (4Cs, dry; (4
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dation up to FegPg031 in wet atmosphere only, with higher rates than for pure phases but lower rates than in dry atmosphere. Another point is that the solids obtained in wet atmosphere are easily identified because of their better crystallinity. CsFePzO7 is therefore detected beside FegPgO3i or Fe2P207 according to the oxidizing or the reducing atmosphere used. 4- DISCUSSION
It has been already shown that Fe-P-0 catalysts work by means of a redox mechanism involving lattice oxygens of the solid [ 3 ] . All the experiments performed in this study show that Fe2P207 and FegPg031, the latter being obtained by reoxidation in the operating catalytic or non catalytic conditions provided steam is present, are the two stable partners of the redox system. Let us notice that the remarkable reproducibility of redox cycles is not consistent with the eventuality of amorphous iron oxide beside Fe3(P207)2. The presence of Cs as CsFeP207 stabilizes Fe3+, and increases the rates of both oxidation of Fe2P207 and reduction (by H2) of FegP8031. In turn, the activity is increased compared with non Cs-containing catalysts. The reason why an excess of phosphorus is used in the most efficient catalysts would be therefore related to the necessary formation of CsFeP207 inside the initial tridyrnite structure of fresh catalyst, while keeping the right stoichiometry P/Fe = 1/1 for FeP04 and redox partners. The role of steam is multiple. The catalytic phases obtained either by reduction or by oxidation in the presence of steam are in better state of crystallization and become less reactive as shown for the oxidation of Fe2P207 (Fig. 3 ) . Consequently the first action of steam is to modify the size and the texture of the crystallites, and to moderate the reactivity of CsxFeP~+yO, by limiting the oxidation of Fe2P207 present to FegPg031 (instead of FeP04). The second action of steam is to moderate both reactions, as shown for pure phases as well as for CsxFeP~+yO,,resulting in making the rates of oxidation and reduction closer to each other. Therefore the combined action of Cs and of steam can be understood as a regulation of the redox Fe2P207/Fe$@31 involved in the catalytic reaction, and of the amount of Fe3+ active sites. A comparison with the oxidative dehydrogenation of butene to butadiene which is made on iron oxides is fruitful because several points are common: the large amount of steam (H20/HC = 10-12), the addition of alkaline cations which enhance activity and selectivity, and the coking of catalysts especially in low amount of steam. In the case of iron oxides the role of steam has long been presented as a thermal diluent, an oxidant or a coke remover. Recently a study of the surface in operating conditions has evidenced another role which would be to convert the catalytic surface to hydroxyl forms such as Fe-OH-Fe (hydroxylation of an oxygen linked to two Fe) and 0 - F e O H - 0 (hydroxylation of a surface Fe), more adapted because more basic. These specie would be able to initiate the dehydrogenation by the ahydrogen abstraction as a proton from butene [9,10]. Steam would also limit the adsorption and reaction of butene owing to competitive adsorption of molecular water. However IBA (MAA) is not butene (butadiene) and the last proposition is
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no more valid since, on the contrary, steam is useful in helping MAA to desorb, as with heteropolyoxometallates active and selective for IBA-MAA [ l l ] . Surface acid ortho and/or pyrophosphates would be the sites responsible for this function, as proposed formerly [2]. Another difference is that, although alkaline cations (which increase 0 2 - or OH- basicity) are useful to maintain the active Fe3+ state, potassium ferrite KFe02 has been claimed to be the active phase [9,10], whereas CsFeP207 itself is not active and yields mainly acetone from IBA [2,6]. In a similar way, we propose that neighboring iron cations wearing surface hydroxyls could act as active sites allowing dehydrogenation of IBA, the selectivity in MAA being controlled by the action of steam on acid phosphates. REFERENCES J.M.M. Millet, J.C. Vedrine and G. Hecquet, in G. Centi and F. Trifiro Eds., New Developments in Selective Oxidation, Stud. Surf. Sci. Catal., 55 (1990) 833. 2. J.M.M. Millet and J.C. Vedrine, Appl. Catal., 76 (1991) 209-219. 3. C. Virely, M. Forissier, J.M.M. Millet and J.C. Vedrine, J. Molec. Catal., 71 1992) 199-213. 4. M. Dekiouk, N. Boisdron, S. Pietrzyk, Y. Barbaux and J. Grimblot, Appl. Catal., A, 90 (1992) 51-60 ;ibid., 61-72. 5. D. Chelliah, Ashland Oil Inc. (USA), FR. 2 498 475 (1982). 6. J. Belkouch, ThPse, CompiPgne (1991). 7. P. R6my and A. Boull6, C. R. Acad. Sci. Paris, 253 (23) (1961) 2699. 8. F. D'Yvoire, Bull. SOC.Chim. Fr., 6 (1962) 1224-1246. 9. B.J. Liaw, D.S. Cheng and B.L. Yang, J. Catal., 118 (1989) 312-326. 10. B.L. Yang, D.S. Cheng and S.B. Lee, Appl. Catal., 70 (1991) 161-173. 11. M.J. Bartoli, L. Monceaux, E. Bordes, G. Hecquet and P. Courtine, in P. Ruiz and B. Delmon Eds., New Developments in Selective Oxidation, Stud. Surf. Sci. Catal., 72 (1992) 84-92. 1.
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J. Vedrine (IRC, Villeurbanne, France): Your results complement those of Millet et al., concerning the role of Cs in the stabilization of tridymite FeP04 by adding steam and studying chemical mixtures. Could it be possible that mechanical mixtures between Fe2P207 and Cs20 give the same result, or at least affect acidic features. You have also made a correlation between oxidative dehydrogenations of butene on iron oxide catalyst and of of isobutyric acid to methacrylic acid, stating that the first step involves H abstraction on basic sites (OH- type). However butene is a basic compound while IBA is obviously acidic. How can you imagine the first step on the same basic OH- site. E. Bordes (UTC, CompiPgne, France): We are not presently able to assign the increase of performance obtained by addition of Cs either to CsFeP207 as a phase, or to Cs itself as a cation. In the latter case Cs would indeed affect acidity since it reinforces OH- or 0 2 - basicity, and so similar results could be obtained with a mixture Fe2P207/Cs20. To answer the second question, the abstraction of H(-C-C=O) (IBA) or H(-C-C=C) (butene) can be done by similar OH-, whatever the acidity or basicity of the molecule as a whole which affects mainly further steps. Hydroxyls such as OH(-Fe-0-Fe) displayed on e.g., Fe2O3, are indeed slightly different from OH(-Fe-0-P) on e.g. FeP04.
G . Emig (Institut fur Technische Chemie, Erlangen, Germany): When we used heteopolyacid catalysts for your reaction we saw also a very marked effect by the addition of water. We observed an optimal IBA/H20 ratio of 12. If one applies higher ratios a negative effect on IBA conversion is observed, because of competitional adsorption on the catalyst surface. Why do you use H2O/IBA = 12 and what happens if you go to lower values? Such a high amount of water could certainly not be beneficial for a commercialisation of your process! E. Bordes (UTC, CompiPgne, France): The H20/IBA ratio of 12 is indeed an economic handicap for the process. However a decrease of the amount of steam leads to a strong decrease in catalytic properties due partly to extensive coking of the catalyst. Stabilization of the best performance is observed for H2O/IBA = 12-15.
J. M. Millet (IRC, Villeurbanne, France): We have proposed some years ago that cesium plays a role in the stabilisation of a tridymite phase with P/Fe ratio greater than one, and that CsFeP207 itself has no catalytic role. You propose in this work that it does have a role. Since CsFeP207 has been shown to be inactive and since it is present in the catalyst in too a small amount to postulate a support effect, how do you explain the synergy effect that you observed with mechanical mixture of CsFeP207 and Fe~P207? Furthermore, your results (MM0.15 and MM0.35) seem to show that this synergy effect is quite independant of the amount of CsFeP207. E. Bordes (UTC, CompiPgne, France): CsFeP207 is indeed poorly active but we do observe a promoting effect when it is present beside active phases. Its amount is not low because Cs,FeP~+yO, (x = 0.15, y = 0.24) corresponds
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approximately to CsFeP207/Fe2P207 = 0.35/1 (molar ratio). Catalytic results are higher with mechanical mixtures MM than with pure phases (Fe2P207 or FegP8031) which means that a synergetic effect exists, even though the contacts are not optimized. Moreover MM0.35 behaves similarly to C~0.15FeP1.240~. Other ratios should be prepared to be sure that the optimum ratio lays between 0.15 and 0.35, or closer to 0.35, as we suppose.
M. Sinev (I. of Chemical Physics, Moscow, Russia): You explain the influence of water by competitive adsorption between water and hydrocarbon. But your data show (Fig. 4) that the influence of water is stronger on reoxidation than on reduction. So, 1)What is the origin of the influence of water on reoxidation of catalysts? 2) Which of the two processes (reduction or reoxidation) is responsible for the influence of water on steady-state catalytic process?
E. Bordes (UTC, Compiegne, France): All data are not presented here. A
kinetic study performed on pure phases has shown that (6), comparing oxidation and reduction without and with steam, the activation energy of oxidation Eox is slightly modified (20 and 23.5 kcal.mo1-1 respectively), whereas for reduction ERed (9 kcal.mo1-1) is multiplied by 2 or 3 according to the temperature range (similar values of preexponential factors). The adsorption of H20 on oxidized sites can be proposed as the moderating factor of reduction step. When Cs is present the main effect of steam is to limit the reoxidation of Fe2P207 to FegPg031.