l.W. Hightower. W.N. Delgass. E. Iglesia and A.T. BelI (Eds.)11th International Congress on Catalysis - 40th AnniversaryStudies in Surface Science and Catalysis. Vol. IOI1996 Elsevier Science B.V.

Role ofwater on the properties ofFePO catalysts used for theoxidative dehydrogenation ofisobutyric acid.

J.M.M. Milleta, M. Forissierb, D. Rouziesa, P. Bonneta and J.C. Vedrinea

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a Institut de Recherches sur la Catalyse, CNRS, conventionne avec l'UniversiteClaude Bernard, Lyon I, 2 avenue A. Einstein, F-69626 Villeurbanne Cedex(France).b Laboratoire de Genie des Procedes Catalytiques, CPE Lyon, 43 Bvd du 11novembre 1918, PB 2077, F-69616 Villeurbanne Cedex (France).

A study of the oxidative dehydrogenation of isobutyric acid (IBA) intomethacrylic acid (MAA) has been carried out on different FePO catalysts inorder to define the effects of water added in the feed, both on the selectivity andthe activity of these catalysts. The experimental data show that water has twomain effects; the first one is to avoid the formation of coke at the surface of thecatalysts and the second one is to stabilize the hydroxylated forms of the activephases at their surface. The temperature of the maximum of efficiency of thecatalytic reaction coincides with that of the beginning of the dehydroxylation ofthe active phases. At this temperature the mobility in the solid of both theprotons and electrons is enhanced. The water has thus a role in maintainingthe catalyst in a metastable state where a steady state is obtained between thehydrogenation/reduction of the solid (dehydrogenation of the IBA molecules)and its combined dehydroxylation-oxidation as schematized :

Fe3+3(P030H)3(P04) + xAIB -7 Fe3+3_xFe2+x(P030H)3+X(P04h_x + xMAAFe3+3_xFe2+x(P030H)3+x(P04h_x + x/4 02 -7 Fe3+3(P030H)3(P04) + x/2H20

Such role of water may be applied to other catalytic materials used for oxidativedehydrogenation, specially heteropolyanions-based catalysts.

1. INmODUCTION

The activity and selectivity of FePO catalysts used in the oxidativedehydrogenation of isobutyric acid (IBA) int(Ymethacrylic acid (MAA) arehighly dependent upon the amount of water added to the feed [1,2] which mayreach 12 water molecules per mole of IBA [3]. To our knowledge such a featurehas never been reported for other catalytic systems.

In the course of a general study of active and selective FePO based catalystsfor this reaction, we were confronted with the problem of understanding therole of water which in this case may be considered as a co-catalyst since formost of the FePO catalysts the presence of water was necessary in order to have

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any detectable activity. In this study we have particularly focused our attentionon the effect of water on the catalytic properties of three different FePOcatalysts with the specific objective of understanding the possible roles ofwater.

The selected catalysts were (i) an industrial catalyst prepared using acommon method reported in almost all the patent literature [3-4] andcontaining besides Fe and P relatively large amount of Cs, (ii) anhydroxyphosphate, Fe3(P04h<OH)2 which was the most active and selectivephase of this type for the reaction under consideration [5-6] (iii) an acidicpyrophosphate, Fe2(P030H)P207 which has recently been shown to be a veryactive and selective catalyst [7]. Itmay be noted that a fourth type of catalystprepared with a new method as proposed by Ai et al. [8] corresponded to theindustrial type of catalysts and contained after testing the same phases, exceptthe CsFeP207 phase which was substituted in some instance by NH4FeP207.

2. EXPERIMENTAL

2.1 Catalyst precursors preparationAmong the numerous FePO catalysts prepared three of them have been

selected for comparison in this study. First, an industrial type catalyst wasprepared according to patent indications [3] by a precipitation method fromaqueous solution of iron and cesium nitrates and phosphoric acid. The solidobtained was fired in air at 723 K for 5 h. The X-ray diffraction analysis of thesolid showed only one phase with a tridymite type structure. Second, a catalystwas obtained by hydrothermal synthesis using vivianite Fe3(P04)2.8H20 [9].The hydrothermal synthesis was conducted at 473 K for 120 hours. The sampleobtained was filtered, washed with water and dried at 373 K overnight. The X­ray diffraction analysis of the solid showed only one phase identified asFe3(P04h(OH)2. Third, an amorphous catalyst was prepared by reactingvivianite and H4P207 in acetone under reflux at 348 K [7]. The reactant PlFeratio was equal to 4.5 and the duration of the reaction was 15 h. Aftercompletion of the reaction, the solid was filtered, washed with acetone anddried at 313 K in air. All the catalysts were transformed under the catalysisconditions in the course of activation processes which lasted approximatelyone hour for the first two catalysts and 25 h for the last one. The results of thecharacterization of the catalysts after testing are given in the results section.

2.2 Catalysts characterization and testingCatalyst precursors and final catalysts were analyzed by a broad range of

techniques including : powder X-ray diffrac.tion (XRD), N2 surface areameasurement (BET method), Mossbauer spectroscopy, thermogravimetricanalysis with simultaneous measurement of evolved gases.

Catalyst precursors were used in powder form and evaluated for theoxidative dehydrogenation of IBA in a standard laboratory microreactor [10].Before comparing their catalytic properties, the precursors (50 mg) weretransformed in the reactor under identical conditions (02/IBA/H20/N 2 :4.26/5.86/72.9/19.6 kPa; total flow rate 1 cm3s- 1; reaction temperature 658 K).The catalytic properties were compared under the same experimental

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conditions with a conversion ranging from 15 % to 20 %. The selectivity inMAA, on these catalysts, increased slightly with conversion (except for veryhigh conversion: >95%) [1,11]. Consequently the catalytic properties presentedin this paper were not as high as what could have been published or patentedsince conversion was voluntarely limited in order to obtain more reliable datato compare the effect of water. Propene (PRO), acetone (ACE) and C02 were theby-products formed with MAA. The techniques, the apparatus and the gaschromatographic separation conditions have been described elsewhere [10].

To study the influence of water, the oxygen and IBA partial pressures werekept constant at 4.26 and 5.86 kPa respectively and the water partial pressurevaried from 0 to 72.9 kPa. For transient kinetics experiments, the water flowwas stopped, whereas the other partial pressures were kept constant, toobserve the change with time of the partial pressure of the products. To achievea better comparison of the effect of water partial pressure on the catalyticproperties of the catalysts a new term R has been defined as the ratio of the rateof MAA formation to the maximum rate of MAA formation obtained under theconditions where the effect of water partial pressure has been examined. Foreach catalyst, R has been plotted as a function of the H20/IBA ratio.

3.RFSULTS

3.1 Characterization of the final catalystsUnder the conditions of catalytic reaction, all the synthetized precursors

underwent a total transformation with the crystallization of new phases (Table1). The phases formed have been identified and quantitatively evaluated usingXRD and Mossbauer spectroscopy. The industrial catalyst was transformedinto a mixture of three phases CsFeP207, aFe3(P207)2 and FeP04. Dependingupon the conditions of catalysis Fe2P207 has been observed in place ofFeP04. Ithas been shown that CsFeP207, Fe2P207 and FeP04 were not active phases [12]and that the catalytic activity of the industrial catalyst was due to the presenceof the aFe3(P207)2 phase [13]. Fe3(P04)2(OH)2 was oxidized and transformedinto a phase which belonged to the solid solution Fe3+4-xFe2+3x(P04}J(OHh­3x03x with O~Sl which extended from Fe3+4(P04)s(OH)3 to pFe3+Fe2+(P04)O [6].The amorphous phase corresponding to the third catalyst precursor wastransformed into the phase Fe2(P030H)P207. A study of this phase aftercatalytic reaction has shown that it was superficially reduced [7].

3.2 Thermogravimetric analysis of the catalystsThe solids were transformed in the conditions of catalysis which

corresponded to high water partial pressure and thermogravimetric analysesof the samples were performed after catalytic testing (Figure 1). The analysis ofthe catalysts showed that they were loosing weight in two more or less welldefined steps. The industrial catalyst showed two weight losses between 653 and773 K and 773 and 1023 K, Fe3(P04MOH~ between 623 and 853 K and 853 and 973K, Fe2(P030H)P207 between 573 and 723 K and 723 and 973 K. The massspectroscopic analyses performed simultaneously allowed to attribute the firstmass loss to the departure of water and the second to that of water,CO and C02.

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The first loss should thus correspond to the dehydroxylation of the compoundsand the second to the decomposition of adsorbed hydrocarbon molecules.

Table 1- Bulk characterization results on the final catalysts.precursors final catalysts ref.

phases relative weight %

TG('To) DTG('To!min)

-0.3 - 0.005

-0.5 - 0.010

-0.7

-0.5 - 0.01

b- 0.03-1.5

-2.5 -0.05

- 0.07

-1.5 C- 0.04

-3.0-0.08

-4.5 - 0.12

- 0.16

573 673 773 873 973 T (K)

Figure 1- Thermogravimetric and differential thermogravimetric curves of thecatalysts; (a) the industrial catalyst (b) Fea(P04)2(OH>2(c)Fe2(POaOH)P207.

For Fe2(POaOH)P207 the first water loss corresponded to 0.49 mol of water permol of compound which is in agreement with the theoretical value (0.50). ForFea(P04h(OHh, the first water loss corresponded to 0.74 mol per mol which isless than the expected value of 1.07. This may be.explained by the fact that thedehydration of the hydroxyphosphates generally takes place in several steps ata higher temperature [14]. For the industrial catalyst all the phases presentwere not able to be hydrated except aFea(P207h which may be hydrated to formFea(POaOH)4. In this case the first weight loss which occured in the samerange of temperatures as that corresponding to the dehydration ofFea(POaOH)4 [15], was not important and corresponded to only 0.05 mol permol of aFea(P207h. This indicated that the latter phase should be hydratedonly superficially_ Considering the first water loss as the loss of hydroxylation

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water the formula FeS(P207h.92(POSOH)o.08 has been calculated for the activephase of the industrial catalyst. This result may be related to those obtainedearlier by Y. Barbaux et a1. [16] from XPS analysis of the industrial catalystproposing that hydrogenophosphate like species were present at the surface ofthe solid. The presence of the hydroxylated form of cxFeS(P20 7)2 instead of anhydrogenophosphate may explain as well their results.

3.3 Catalytic properties of the catalystsThe results obtained for the three catalysts in the oxidative dehydrogenation

reaction of IBA are compared in Table 2. The product selectivities werecomparable for the three catalysts with a slightly higher selectivity to MAAwith Fe2(POsOH)P207 and slightly lower with the hydroxyphosphate. Iftheindustrial catalyst and FeS(P04)2(OHh exhibited the same activity,Fe2(POsOH)P207 appeared to be more than four times more active.

Table 2- Catalytic data of the catalysts at 658 K, in the oxidative dehydrogenationof isobutyric acid; PRO: propene, ACE: acetone, MAA: metpacrylic acid.

catalyst

indusrial catalyst 6.4FeS(P04)2(OH)2 1.6Fe2(POsOH)P207 2.0

262

9 19 70 16 2207 21 66 17 2208 16 74 20 960

3.4 Effect ofwater on the catalytic properties of the catalystsThe evolution of the ratio (R) of the rate of MAA formation to the maximum

rate of MAA formation as a function of the H20lIBA ratio as plotted in Figure 2shows that the water partial pressure had a significant effect on the activity ofall catalysts, especially at low partial pressure where the activities weresignificantly lower. Unlike the industrial catalyst and the hydroxyphosphatecatalyst whose activity decreased until no products were detected when therewas no water in the feed, the Fe2(POsOH)P207 catalyst retained a significantlevel of activity even in the absence of water. At the same time the selectivities toMAA decreased and those of acetone and C02 increased with decreasing H20partial pressure in a similar manner for all catalysts. At high partial pressureof H20, a decrease in the rate of formation of MAA was observed for theFe2(POsOH)P207 catalyst. The catalytic activity of Fe2(P030H)P207 reported infigure 2, was measured in the absence of water after 40 hours in steady stateconditions. In order to study the stability of the catalysts under reactionconditions without water, the performances of the catalysts were analyzed as afunction oftime. Itcan be seen in Figure 3 that-the ratio of the rate of formationof MAA to the initial rate decreased with time for all the catalysts. The activity ofthe industrial catalyst was getting nul rapidly whereas that of Fes(P04)2(OH)2and Fe2(P030H)P207 decreased much more slowly. For these two last catalystsit may be observed that the activity decreased rapidly at the beginning and thenmuch more slowly. When the water feed was cut off the change in selectivitiesobserved for the three catalysts was even more pronounced than when the waterpartial pressure was only decreased; the selectivity in MAA decreased, those of

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1.2

1.0

0.8

R0.6

0.4

0.2

0.0a 5 10 15 PH2o'PIBA

Figure 2· Comparison of the ratio (R) of the rate of MAA formation to themaximum rate as a function of time after stopping of the water feed (72.9 kPa)for the catalysts:( )the industrial catalyst,( )Fe3(P04h(OHh,( e) Fe2(P030H)P207

1.0

0.8

0.6

R0.4

0.2

0.0o 20 40 60 time (h)

Figure 3· Variations of the ratio of the rate of formation of MAA to the initialrate as a function of time after stopping of the water feed (72.9 kPa) forsamples: ( ) the ind~strial catalyst, ( ) Fe3(P04MOHh, (e) Fe2(P030H)P207.

acetone and C02 increased whereas that in propene did not vary appreciably.For Fe2(P030H)P207 the selectivity to MAA after 68 hours dropped from 74 to 59% while the rate of MAA formation dropped to 40 % of the initial activity(Figure 4). Since after 68 hours this catalyst retained a significant level ofactivity, the water partial pressure was reestablished (Figure 4). When thewater partial pressure was reestablished the catalyst displayed its initial MAAselectivity (Figure 5) but did not recover its initial activity, the rate offormationof MAA reaching only to 65 % of the initial rate.

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2

a1.0

O.S 1

0.6

R

0.4

0.2

0.00 20 40 60 SO

time (h)

Figure 4 - a Variation of the ratio of the rate of formation of MAAto the initialrate as a function of time on Fe2(P030H)P207 with and without added water; (1)cut off of the water feed (72.9 kPa) (2) reestablishment of the water feed (72.9kPa). - b Variation of the selectivities as a function of time A : before stoppingthe water feed B : after 65 hours without water, C : after 70 h (68 h withoutwater and 2 h after the reestablishment of the water feed).

4. DISCUSSION

The results of the characterization of the solids suggest that the active phaseof the industrial catalyst cxFe3(P207)2 and Fe2(P030H)P207 contained at theirsurface the same species, namely Fe3+, Fe2+ cations and (P030H)2- groups.Moreover the study of the structure of these phases showed the same type ofclusters of face-sharing ferric and ferrous octahedra. This may explain whythe new catalyst and the industrial catalysts exhibited similar selectivities forall the products. The fact that the new catalyst was slightly more selective inMAA and much more active than the industrial catalysts can easily beexplained by the fact that the industrial catalyst contained, besides the activeand selective phase, other phases with lower activities and selectivities. It canbe observed that Fe3(P04)2(OH)2 presents the same local arrangements offerric and ferrous ions than the two acidic phosphates. It has the sameFe3+/Fe2+ redox couple but different acidlbase sites since 02-/0H- have beensubstituted by (P04)3-/(P030H)2- for the oxido-reduction process which has beenshown to involve a dehydroxylation/hydroxylatiun process. From catalytic datait seems that this system was active and selective for the formation of MAAbutdid not reach the efficiency of the acidic phosphates system.

The oxidative dehydrogenation of IBA into methacrylic acid was relativelyexothermic and it is certain that water vapor has an effect with respect to itsproperties as a heat transfer agent. However, if this role has to be considered athigh conversion level it cannot explain the effect detected when the conversionnever exceeded 20% under isothermal conditions. Water is also known toprevent the formation of coke at the surface of catalysts in oxidative

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dehydrogenation reactions and it is obvious that in our case this role isimportant. The direct observation of the catalysts after catalytic reactionshowed that the less water was added to the gas feed, the darker was the graycolor of the recovered solids. Furthermore XPS data showed an increase of theintensity of the carbon peak for the catalyst between before and after catalytictesting, confirming the formation of coke. However water is a product of thereaction and the addition of large amounts of water should have a negativeeffect on the rate and not only at very high partial pressure like it is slightlyobserved on the industrial catalyst. From the data obtained in thecharacterization of the catalysts, we proposed that water plays a role instabilizing the hydroxylated form of the active phases which are in fact the"real active phases". One of its major role would be to displace the followingdehydroxylation equilibria to the left: 2(POsOH)2- ~ (P207)4-+H20 forFe2(POsOH)P207 and aFes(POSOH)4 and 2(OH)- ~ 02-+H20 for FeS(P04n(OH)2.This second role justifies the large quantities of water needed. Thedehydroxylations of these phases occur in the same range of temperature asthe catalytic reaction. This reaction in the case of the active phase of theindustrial catalyst could be written for the dehydrogenation of MAA (a) and forthe reoxidation of the catalyst (b) as :(a) FeS+s(POsOH)s(P04) + xAIB~FeS+a_xFe2+x(POaOH)s+x(P04h-x+ xMAA(b) FeS+S_xFe2+x(POaOH)s+x(P04)1-x~FeS+a(POsOH)s(P04)1-x(P207)X/2+xl2H20(b) FeS+a(POaOH)a(P04h_x(P207)X/2 + xl4 02 ~ Fea+a(POaOH)a(P04)These two steps may be concerted in one. The water stabilized the hydroxylatedphase which was also the oxidized one. The observation that when no waterwas added to the feed the Fea(P04)2(OH)2 and Fe2(POsOH)P207 catalysts werenot rapidly and completely deactivated like the industrial catalyst was butrather underwent a slow deactivation could be explained by the fact that theformer catalysts were hydroxylated in the bulk while the active phase of theindustrial catalyst was hydroxylated only on the surface. This feature shouldallow a better stabilization of the (POaOH) groups which are necessary for thereaction on the surface of the catalysts.

The two main roles of water are to prevent the formation of coke and thedehydration of the catalyst, and account for the deactivation observed with thethree catalysts when the water feed was stopped. In the case ofFe2(POaOH)P207 a difference may be made between the irreversibledeactivation due to the deposition of coke on the solid and the reversibledeactivation due to the reversible dehydroxylation of the solid (Figures 4 and 5)which caused a loss of activity but not selectivity because the active sites werereduced but not modified. The hydroxylation state appeared to be essential andplayed a role not only at the surface of the catalysts but also in the bulk. Thereduction of the catalyst involves the removaCby the solid of two hydrogenatoms of IBA molecules. The reoxidation of the catalyst is preceeded bydehydroxylation of the solid before the reoxidation step by gaseous oxygen. Ifthe catalytic site differs from the reoxidation site, which is the case most of thetime for partial oxidation catalysts, one may suggest that the rate of the MAAformation could depend upon the rate of proton diffusion from the reaction siteto the reoxidation site. At the temperature of catalysis the phases should be in ametastable "pre-reactionnal state" corresponding to a state where the protons

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mobility in the bulk was very high due to the existence of a tunneling process[17]. Such a state was demonstrated to exist and was characterized for alkalineearth metal hydroxides [18]. Since the loss of the first water molecules, theacidic phosphates or the hydroxyphosphates are becoming extrinsic protonicsemi-conductors, doped with proton lacks «P04)3- for acidic phosphates and0 2- for hydroxyphosphates) and consequently of type n. This implies that thecharge carriers will mainly be conduction electrons which will participate inthe ionosorption of oxygen. One may thus understand the importance of theprotons mobility in the solids which may become the rate limiting step of thedehydrogenation reaction.

For acetone and CO2 formation, oxidation of the organic molecule with thedeparture of a 0 2- ion from the solid yields an oxygen vacancy. The reoxidationof the catalytic site could differ from that in the case of MAA formation and onemay suggest that the rate of reaction will depend on oxygen ion diffusion ratefrom the reoxidation site to the catalytic site. Itwill thus depend much less onthe proton diffusion which explains why in the absence of water the decreasein selectivity in MAA occured to the benefit of those in acetone and C02. Theformation of propene has been shown to correspond to a simple decomposition(acid-base reaction) as discussed before [19,20].

Itis interesting to note that heteropolyacids like H3+xPMo12-xVx040 havealso been shown to be active and selective catalysts for the same reaction [21].Comparing the activity of these catalysts in the reaction of oxidativedehydrogenation of IBA and the oxidation of methacrolein (MAL), Misono etal. [22] reached the conclusion that in the case of the first reaction themigration of the protons in the bulk takes part in the reoxidation process of thecatalysts whereas in the second the slow diffusion of oxygen through the solidwas used for MAL oxidation. It can be noticed that for the oxidativedehydrogenation of IBA, the temperature ranges of catalytic reaction and ofdehydration for these compounds (573-623 K) are identical although beingdifferent of those for iron phosphates catalysts. The same roles of water may inthis case be postulated although lower amount of water is needed (c.a. 2 H20per mol. of IBA).

5. CONCLUSION

This study showed that all iron phosphate based catalysts need largequantities of water in order to be active and selective. It also shows that thewater had two main effects. The first one was to avoid the formation of coke atthe surface of the catalysts as it is the case most of the time in selectiveoxidation or oxidative dehydrogenation reactions. However the data obtainedshowed that water must also intervene directlyin the catalytic reaction. Wepropose that water plays a role in stabilizing the hydroxylated form of the activephases which is also the oxized phase of the redox couple.

The temperature of the maximum of efficiency of the catalytic reaction doesnot coincide systematically with that of the beginning of the dehydroxylation ofthe active phases by chance. It is at this temperature that the mobility in thesolid of both the protons and electrons was enhanced due to the partial loss ofwater and the solid exhibits semi-conductor properties. The water has thus a

1020

role in maintaining the catalyst in a metastable state where a steady state isobtained between the hydrogenation/reduction of the solid (dehydrogenation ofthe IBA molecules) and its combined dehydroxylation-oxidation. Such role ofwater may be applied to other catalytic system used for oxidativedehydrogenation, specially heteropolyanions-based catalytic materials.

Finally concerning the electron mobility, one may notice that in order to getefficient catalysts, it should be high in the whole bulk of the solids. We haveseen that the active phases studied contained clusters of two or three faceshearing Fe octahedra. In a previous study of hydroxyphosphates with clustersof different size, we had also observed that the electron mobility between twoadjacent iron cations should not be too high to get efficient catalysts [6]. Thiscontradiction may perhaps explain the difficulties sometimes encountered inrelating total conductivity and catalytic properties. It may thus be interesting todifferentiate both types of electron mobility in oxysalt based catalysts whichhave both an effect on the catalytic properties.

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