On the active role of water during partial oxidation of n-butane to maleic anhydride over (VO)2P2O7 catalysts

Applied Catalysis A: General 172 (1998) 49±58

On the active role of water during partial oxidation of n-butane to maleic anhydride over (VO)2P2O7 catalysts H.W. Zanthoffa,*, M. Sananes-Schultza, S.A. Buchholza, U. Rodemerckb, B. Kubiasb, M. Baernsb a Lehrstuhl fuÈr Technische Chemie, Ruhr-UniversitaÈt Bochum, D-44780 Bochum, Germany Institut fuÈr Angewandte Chemie, Berlin-Adlershof e.V., Rudower Chaussee 5, D-12484 Berlin, Germany

b

Received 21 October 1997; received in revised form 2 December 1997; accepted 11 March 1998

Abstract The role of water in the reaction of n-butane to maleic anhydride over vanadyl pyrophosphate prepared in aqueous solution was studied under vacuum conditions by transient experiments in the temporal-analysis-of-products reactor. Under the existing low surface coverage conditions, two main effects of water on the reaction could be identi®ed. First, water prevents the non-reactive irreversible adsorption of butane on the V±P±O catalyst and it enhances the desorption of the reaction products. Secondly, it dissociatively adsorbs on V±P-oxide, breaking V±O±V, V±O±P or P±O±P bonds and forming V±OH and/or P±OH groups. This structure-rupturing process leads to an easier reoxidation behaviour, necessary to keep the catalyst in a reactive state to form maleic anhydride. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Butane oxidation; Maleic anhydride; Water; Vanadyl pyrophosphates; Reaction mechanism

1. Introduction The partial oxidation of n-butane to maleic anhydride (MA) over vanadyl pyrophosphate is the only commercialised application of catalytic gas-phase oxidative conversion of alkanes so far. Since the introduction of this reaction by Bergman and Frisch [1] in 1966, more than 1000 patents and papers have been dedicated to this subject, dealing with all kinds of problems, but mainly catalyst preparation, structure± reactivity relationships and mechanistic studies. The

*Corresponding author. Tel.: +49 234 7004518; fax: +49 234 7094115; e-mail: [email protected] 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00092-1

reaction was found to occur via several intermediates (butenes, butadiene, crotonaldehyde, furan, crotonlactone, etc.) [2,3] which, however, can only be observed under special reaction conditions which enable their desorption from the catalyst surface, i.e. reduced pressure and/or oxygen de®ciency in the gas phase [4]. Gas-phase oxygen itself does not directly participate in the reaction pathway from butane to maleic anhydride [5]. Its main role is to reoxidise the catalyst to the appropriate V4‡/V5‡ ratio which enables the formation of MA [6,7]. However, there are certain indications that electrophilic short-lived oxygen species which are formed during the reoxidation sequence contribute to the undesired formation of COx [5,8] before they are incorporated into the catalyst lattice.

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H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

Besides these results, only very little is known about the details of the in¯uence of the interaction of the reaction products with the catalyst on the reaction pathways although their importance has to be identi®ed. Especially water seems to affect catalyst properties and reaction performance as has been reported in earlier studies [9±13]. Several explanations were proposed according to different reaction conditions applied. A bene®cial effect of water during catalyst preparation was reported by Wrobleski et al. [9]. The pretreatment of their catalyst precursor in a stream of water in N2 for 5 h resulted in a catalytic solid that led to MA yields of 60.5% at 82% conversion, which is quite near to commercial results. An explanation for the effect of water, however, was not reported. On the other hand a detrimental effect of steam on the catalytic performance was recently found by Contractor et al. [10] if regenerating a working catalyst in the presence of steam, after being run in the redox mode in a riser reactor. Water also directly affects the reaction of butane to maleic anhydride. Arnold and Sundaresan [11] observed a short-time reversible blocking effect that leads to a decrease in activity, accompanied by a simultaneous increase in selectivity towards MA, acetic and acrylic acid using a butane/air mixture with 0±14% steam over an -VOPO4/(VO)2P2O7 catalyst. Contractor et al. [13] also observed this short-time effect leading to an increase in MA selectivity (butane/ oxygen/inertˆ10:5:85 plus 11% steam). However, the decrease in butane conversion did not occur if no oxygen was cofed in the riser/regenerator reactor applied. They concluded that water inhibits the activation of gas-phase oxygen by blocking of the respective sites, while the utilisation of lattice oxygen is not affected. In recent work of several of the present authors using a well-crystallised (VO)2P2O7 catalyst [14] and a reaction gas with a high amount of steam (20%), however, a decrease in butane conversion and MA selectivity was observed, which was attributed to an interaction of water with the catalyst being reversible on a minute time scale. A second, long range effect, which is irreversible and takes several days to manifest itself completely [11], plays a signi®cant role in the development of the solid structure, in particular the evolution of the catalyst surface area. Additionally, it was speculated that water alters the

P/V ratio in the surface layer [11]. Recently, this speculation was proven by XPS and ISS measurements [14]. Water also affects the surface of V±P±O catalysts by converting Lewis-acidic sites into Brùnsted acidic ones as was found by IR spectroscopic investigations on the adsorption of water and pyridine on anhydrous vanadyl pyrophosphate by Puttock and Rochester [12]. In the present paper we report on the in¯uence of water in n-butane oxidation to maleic anhydride and in the oxidation behaviour of a well-crystallised aqueously prepared (VO)2P2O7 catalyst using transient experiments under vacuum conditions in the temporal-analysis-of-products reactor (TAP). Under these conditions the different inhibiting effects of water which have been observed in the earlier works due to the presence of large amounts of steam might be negligible due to the low surface coverage realised by the vacuum conditions. 2. Experimental 2.1. Catalyst preparation and characterisation The (VO)2P2O7 catalyst applied in this work was prepared from an aqueous solution: the catalyst precursor VOHPO4
0.5H2O was obtained by evaporating a solution of V2O5 and oxalic acid in diluted H3PO4 at 423 K. This precursor was pressed into tablets which were crushed and then sieved. The grain fraction from 0.3 to 0.5 mm was calcined for 4 h at 753 K in a stream of pure nitrogen. The BET surface area of the calcined catalyst amounted to 4.01.0 m2 gÿ1 (one-point BET method) and an average vanadium valence state of ‡4.01 determined by potentiometric titration (see [6]). Treating the (VO)2P2O7 in a ¯ow of a 1.5% butane-in-air mixture (2 h at 753 K) and adjusting butane conversion to above 99% resulted in a ``quasiequilibrated'' catalyst [15]. This catalyst showed an activity and selectivity very similar to that of a catalyst after several hundred hours on stream (so-called ``equilibrated'' catalyst [4]). The BET surface area of the quasi-equilibrated catalyst amounted to 5.80.4 m2 gÿ1 and an average vanadium valence state of ‡4.02. The only crystalline phase in both types of catalysts detected by XRD and FTIR was (VO)2P2O7 (JCPDS No. 34-1381).

H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

2.2. Vacuum transient experiments in the TAP reactor The in¯uence of water on the reaction and oxidation processes was investigated using the TAP reactor. A detailed description of the equipment and its application to oxidation catalysis, especially butane oxidation, is given elsewhere [5,6,16]. Therefore, only the typical experimental modes used in this work are presented here. 0.1±0.2 g of catalyst were used in the experiments. The catalyst was placed between two layers of quartz to ®x it in the isothermal zone of the micro reactor. To measure the reaction temperature a thermocouple was placed in the centre of the catalyst bed. The reactor was heated in vacuum (approx. 10ÿ4 mbar) to TBˆ750 K to remove all loosely bound species from the catalyst surface before the subsequent experiments. Both experimental modes of the TAP reactor system, scan mode and pulse modes, were used. Fragmentation patterns of the pure substances (gases and liquids with vapour pressures >20 kPa) were obtained by feeding substance/inert mixtures to the quartz ®lled reactor and recording the mass spectra with the QMS. The solid substance MA was put into the empty reactor which was then heated to obtain a suf®ciently high vapour pressure to record a mass spectrum. The reaction products were identi®ed and their concentrations were measured by the following molecular ions: Ne (m/eˆ20) as internal standard, CO2 (m/eˆ44), butane (m/eˆ58), butenes (m/eˆ56), butadiene (m/eˆ54), furan (m/eˆ68), and MA (m/eˆ 98). The signals recorded for the respective AMU values were deconvoluted accounting for the different contribution of fragments of substances (see also [6]). The degree of butane conversion was calculated from the areas of the butane peaks in the presence of the catalyst compared to those obtained in a reactor ®lled with quartz only. Due to irreversible adsorption of nbutane during the transient experiments mass balances are not complete in the experiments without steam. Additionally, the sensitivity factor (signal area/ amount of substance molecules) of MA could only be estimated on the basis of other oxygen containing hydrocarbon probe molecules because the vapour pressure of MA is too low to allow the investigation of the fragmentation pattern as a gas. In case of the experiments with 18 O2 and H18 2 O the oxygenated products were identi®ed according to their 16 O, 18 O content by the following molecular ions: O2

51

(m/eˆ32, 34, 36), CO (m/eˆ28, 30), CO2 (m/eˆ44, 46, 48), furan (m/eˆ68, 70) and MA (m/eˆ98, 100, 102, 104). From the proportions of the molecular ions related to the 18 O=16 O distribution the relative amounts were calculated for each oxygen containing product assuming an identical sensitivity and fragmentation for all isotopic isomers. For example, the relative content of MA containing three 18 O atoms is equal to the intensity of the molecular ion with m/eˆ104 in relation to the sum of the intensities of the ions with m/eˆ98, 100, 102 and 104. 3. Results 3.1. Vacuum pre-treatment of the catalyst Prior to determining the in¯uence of water on the reaction and oxidation of (VO)2P2O7 in the TAP reactor, the catalysts were tested in vacuum for changes due to reduction. The catalysts were heated up to TBˆ920 K linearly with time. With increasing temperature water and CO2 are desorbed from the catalysts. Fig. 1(a) shows the thermal desorption of water from a calcined and a quasi-equilibrated catalyst. Water is desorbed throughout the whole temperature range investigated. H2O desorption maxima appeared at 400 and 850 K. The amount of water desorbed at temperatures below 750 K is four times higher for the calcined sample, while a similar amount is observed for the desorption maximum at 850 K. It should be mentioned that the high temperature desorption peak does not occur on pure V‡5±P±O phases [17]. Only minor amounts of CO2 were observed (cf. Fig. 1(b)). CO2 desorption starts at TBˆ400 K and increased with increasing temperature running through a maximum at 750 K for the calcined and quasiequilibrated catalyst. The amount of CO2 was three times higher for the catalyst pretreated with the reaction mixture (n-butane/air) indicating that some carbon residues due to reaction of butane with the catalyst remained on the surface. No detectable oxygen desorption could be observed during these experiments. 3.2. Interaction of water with the V±P±O catalyst In order to study the reversible interaction of water with the (VO)2P2O7 catalyst, a quasi-continuous ¯ow

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H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

Fig. 1. Temperature-programmed desorption of (a) water (amuˆ18) and (b) CO2 (amuˆ44) from a calcined and equilibrated (VO)2P2O7 catalyst under vacuum conditions (heating rate: 20 K/ min; mcatˆ0.202 g).

experiment was performed. In fast subsequent pulses of water over the catalyst (
tˆ0.2 s) the surface was saturated to a steady state level at different temperatures (cf. Fig. 2). Then the pulsing was stopped and the desorption behaviour of water was followed over time. The average residence time of H2O amounts to about 5±10 s decreasing with increasing temperature as can be derived from the inlay ®gure. However, small amounts of water are desorbed even 100 s after stopping the H2O pulses for all temperatures. 3.3. Oxidation behaviour of the catalyst Both the calcined and equilibrated (VO)2P2O7 catalysts were almost not oxidisable under the vacuum conditions applied; however, a small uptake of oxygen

could be observed. Fig. 3(a) shows the oxygen consumption on the equilibrated catalyst when pulsing a 18 O2 /Ne mixture (1:1) over the catalyst. Only for the very ®rst pulses complete oxygen conversion was observed. With an increasing number of pulses the oxygen conversion decreased, approaching zero after a total of about 1.6
1019 molecules of oxygen had been pulsed. A slight increase in oxygen conversion was observed after dosing 1
1019 molecules of oxygen and keeping the catalyst overnight in vacuum. This increase might originate from either desorption of oxygen into the vacuum or, more likely, oxygen diffusion into the bulk of the catalyst leaving vacant sites on the surface which can be easily ®lled up by the subsequent pulses of oxygen. The total uptake of oxygen amounted to 14.3 mmol oxygen atoms. This corresponds to an oxidation of 3.9 hypothetical monolayers of V4‡ centres considering that the (1 0 0) surface of (VO)2P2O7 contains 8.4 mmol/m2 V4‡ [18]. In addition to the oxygen uptake a small oxygen exchange was observed (cf. Fig. 3(b)) for the ®rst 1018 molecules of oxygen pulsed. Precalcination of the quasi-equilibrated catalyst at a higher temperature of 933 K in the vacuum, however, resulted in a strong decrease in the oxygen uptake not exceeding 0.5 hypothetical monolayers of vanadium V4‡ centres. In the presence of water (O2/H2O/Neˆ1:0.25:4.3) a larger oxygen uptake was observed (cf. Fig. 4). No oxygen response could be detected at the reactor outlet for the ®rst 1018 molecules of oxygen pulsed. Then, again, a decrease in oxygen uptake occurred. After 4
1018 molecules of oxygen introduced the conversion level approached the amount obtained in the absence of water. The additional uptake of oxygen due to the presence of water amounted to 1.4 mmol oxygen atoms which correspond to about 0.38 hypothetical (VO)2P2O7 monolayers. Similar experiments were performed using the calcined catalyst. Again, the oxygen uptake of the catalyst was higher in the presence of water. 3.4. Influence of water on the butane oxidation reaction On pulsing a butane/Ne mixture over the quasiequilibrated (VO)2P2O7 catalyst (pretreated with 1000 O2/Ne pulses) at TBˆ712 K maleic anhydride, CO, CO2 as well as the intermediates butene, butadiene and

H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

53

Fig. 2. Reversible interaction of H2O (amuˆ18) with an equilibrated (VO)2P2O7 catalyst (mcatˆ0.200 g; TBˆ398 K;
t between H2O pulses: 0.2 s). The inlay represents the desorption part only for various temperatures.

Fig. 3. (a) Oxygen conversion (XO2 ˆoxygen uptake) over an equilibrated (VO)2P2O7 catalyst during sequential pulsing of an O2/ Ne mixture (TBˆ745 K; 18 O2/Neˆ1:1; mcatˆ0.152 g; pulse size: 2
1015 molecules/pulse). Symbols refer to different charges of catalysts. (b) Isotopic ratio 16 O18 O=18 O2 at the reactor outlet during pulsing of an 18 O2 /Ne mixture over an equilibrated (VO)2P2O7 catalyst (TBˆ745 K; 18 O2/Neˆ1:1; mcatˆ0.152 g; pulse size: 2
1015 molecules/pulse).

furan could be observed as products. Table 1 shows the conversion of n-butane and the yields of the reaction products. On the fresh catalyst a high butane conversion of 38.5% was obtained, decreasing with increasing number of pulses over the catalyst due to subsequent reduction of the surface. After 1.44
1019 molecules of n-butane had been pulsed the conversion decreased to 32.4%. Most of the activity loss could be recovered after additional reoxidation by 1000 pulses of O2/Ne. The yields of the selective products on the freshly oxidised catalyst amounted to Y(MA‡ furane‡butene‡butadiene)ˆ4.9% decreasing to 2.9% with increasing reduction degree of the catalyst. Carbon oxides were formed in the same amounts (CO2: 1%; CO: 2%), again, decreasing with increasing number of butane pulses. Carbon mass balances of <20% indicated that a large amount of butane was irreversibly adsorbed or converted to strongly bonded products onto the catalyst surface under the reaction conditions applied. The existence of strongly bound carbon species was con®rmed by subsequent oxidation of the butane treated catalyst in a continuous ¯ow of O2/Ne (1:1) at 823 K. Carbon monoxide and carbon dioxide were observed at the reactor outlet in the ®rst minutes on-stream, decreasing with increasing reaction time.

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H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

Fig. 4. Oxygen conversion (XO2 ˆ oxygen uptake) over an equilibrated (VO)2P2O7 catalyst during subsequent pulsing of an O2/Ne and O2/ H2O/Ne mixture (TBˆ734 K; 18 O2 /Neˆ1:4.5; 18 O2 /H2O/Neˆ1:0.25:4.3; mcatˆ0.152 g; pulse size: 1.4
1015 molecules/pulse).

Table 1 Conversion of n-butane and product yields when pulsing an n-butane/Ne mixture over an equilibrated (VO)2P2O7 catalyst pretreated with 1000 O2/Ne pulses (TBˆ712 K; mcatˆ0.101 g; n-C4H10/Neˆ1:1; pulse sizeˆ1.2
1016 molecules/pulse) Amount of n-C4H10 pulsed (molecules)

Xn-C4 H10 (%)

Ð 8.58
1017 1.36
1019 1.44
1019 After reoxidation 7.68
1017

Yields (%) MA

Furane

Butenes

Butadiene

CO2

CO

38.45 34.88 32.20 32.36

1.58 1.10 0.68 0.62

0.73 0.40 0.16 0.15

0.87 0.82 0.87 0.81

1.76 1.55 1.36 1.30

0.95 0.61 0.30 0.23

1.97 0.98 0.24 0.15

38.31 34.74

1.13 0.77

0.52 0.34

0.92 0.90

1.58 1.25

0.79 0.51

1.38 0.73

In the presence of water the conversion of butane was much lower. On the freshly oxidised catalyst a value of XC4 H10 ˆ 13:3% only was obtained strongly decreasing with increasing catalyst reduction due to subsequent feeding of butane. The product yields, however, were 20±50% higher compared to the respective ones in the absence of water (cf. Table 2). The carbon mass balances now exhibited acceptable values (>90%) indicating that no or only small amounts of butane were irreversibly adsorbed. A similar qualitative behaviour was obtained when pretreating the quasi-equilibrated catalyst in situ under vacuum conditions with an n-butane/O2/Ne (1:4:6) mixture (28 min; VLj0.4 ml/min). However, the conversions obtained in the subsequent n-butane/Ne and n-butane/H2O/Ne pulses were much lower (XC4 H10 ˆ

14.8% and 6.8%, respectively) than in the case of the catalyst pretreated with 1000 O2/Ne pulses. 3.5. Butadiene oxidation in the presence of H218 O In addition to the transient pulse experiments described above the effect of water on the oxidation of the intermediate of the n-butane oxidation, 1,3butadiene, was investigated using isotopically labelled water H18 2 O. The calcined catalyst was treated in vacuum at TBˆ723 K with a quasi-continuous ¯ow of H18 2 O vapour (Ppreˆ1.8 kPa, valve A, multipulses with high pulse frequency). Simultaneously, a quasicontinuous ¯ow of butadiene/16 O2 mixture (C4 H6 =16 O2 ˆ1:3.3) was fed by valve B (multipulses with high pulse frequency).

H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

55

Table 2 Conversion of n-butane and product yields when pulsing an n-butane/H2O/Ne mixture over an equilibrated (VO)2P2O7 catalyst pretreated with 1000 O2/Ne pulses (TBˆ712 K; mcatˆ0.101 g; n-C4H10/H2O/Neˆ1:0.25:1.5; pulse sizeˆ1.5
1016 molecules/pulse) Amount of n-C4H10 pulsed (molecules)

Xn-C4 H10 (%)

Ð 9.84
1017 1.26
1019 1.32
1019 After reoxidation 6.31
1017

Yields (%) MA

Furane

Butenes

Butadiene

CO2

CO

13.30 5.57 2.00 1.96

2.28 1.49 0.91 0.86

0.92 0.43 0.18 0.18

1.18 1.23 1.23 1.29

2.80 2.60 2.14 2.03

1.21 0.68 0.35 0.31

2.54 0.80 0.24 0.04

8.86 5.54

1.96 1.18

0.67 0.36

1.23 1.19

2.66 2.06

0.99 0.57

1.88 0.87

At time-on-streamˆ0 min only products containing O were detected in the mass spectrum (MA m/eˆ98, furan m/eˆ68). After 1
1018 molecules of H18 2 O were fed peaks at m/eˆ100 (MA containing one 18 O atom) 16

and m/eˆ70 (furan containing 18 O) occurred and after dosing about 4
1019 molecules of H18 2 O signi®cant peaks at m/eˆ102 and 104 (MA containing two or three 18 O atoms) were detected, too (cf. Fig. 5(a)). The

18 Fig. 5. Oxidation of butadiene in the presence of H18 O content in the reaction products (a) MA, (b) furan, 2 O on calcined catalyst: relative and (c) CO2 with time-on-stream, TBˆ723 K, mcatˆ0.2 g, butadiene/O2/H2 18 O ˆ1:3.3:0.5.

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H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

18

O in the maleic anhydride exhibited a statistical distribution. A similar increasing content of 18 O was also observed for furan, CO2 (cf. Fig. 5(b) and (c)), CO and crotonlactone (it was not possible to quantify these data). 18 O originated from H18 2 O is inserted into the intermediate furan more rapidly than into the ®nal product MA. After 9.5
1019 molecules of H18 2 O were fed to the catalyst (5 h on-stream) the relative 18 O content amounted to 62% in furan, while it was 41% in MA, and 44% in CO2. Similar behaviour of 18 O occurrence were obtained for the oxidation of butane‡16 O2 and for the anaerobic oxidation of butane, butadiene and 2,5-dihydrofuran in the presence of H18 2 O. 4. Discussion The freshly calcined and quasi-equilibrated (VO)2P2O7 catalysts desorb water under the vacuum conditions applied at temperatures above 300 K. Two distinct maxima at 400 and 850 K were observed; smaller amounts of water desorbed in the temperature range between these maxima. This indicates that water is adsorbed on the catalyst in various kinds. This observation is consistent with earlier studies of water desorption from V±P±O catalysts by Takita et al. [19]. They conducted temperature-programmed water desorption studies of V2O5 and different V5‡ and V4‡ phosphates under atmospheric conditions. On all catalysts investigated they observed the desorption peak at about 400 K. This low temperature peak can be ascribed to physisorbed water on the catalyst surface. Takita et al. [19] further observed that (VO)2P2O7 impregnated with phosphoric acid exhibited an increase in the water desorption between 450 and 573 K. Therefore, the smaller amount of water desorption observed in the present work in the range between 400 and 700 K can be attributed to water adsorbed/reacted with phosphate groups. The water desorption peak at the high temperature of >700 K appeared only on vanadylpyrophosphate phases under atmospheric conditions mainly consisting of V4‡ [19]. Therefore, it might be assumed that these water molecules originating from OH-groups (e.g. V±OH‡HO±P !V±O±P‡H2O) are most probably coordinated to V4‡ sites. However, under the vacuum conditions applied in the present work also V2O5 exhibited a

small water desorption peak at about 800 K. This, however, can be ascribed to V4‡ sites in the solid used which amounted to about 5% of the total vanadium content. The strong interaction of water with the vanadyl pyrophosphate catalyst is also recognisable from the quasi-continuous ¯ow experiments described in Section 3.2. Although the water adsorption is reversible its residence time distribution on the surface is very broad resulting in an average surface residence time being much larger (5 s at 550 K) compared to that of butane (<0.01 s); at temperatures below 450 K  increased resulting in high values for water outside the accessible time window of the TAP equipment whereas it remained low for butane (t0.5 s at 550 K). The observation that oxygen from water molecules was found in the reaction products of butadiene and butane oxidation further indicates that the adsorption of water under reaction conditions from 600 to 800 K occurs at least partly by a dissociative process with the rupture of a H±O bond in the H2O molecule. This is supported by H2O/D2O/pyridine adsorption FTIR studies of Puttock and Rochester [12] who showed that adsorbed water converts Lewis acid sites on (VO)2P2O7 into Brùnsted acid sites which were involved in hydrogen-bond interactions, probably with adjacent surface phosphate groups. 18 O originating from water molecules was found in all oxygen containing reaction products in the oxidation of butane, butadiene, or dihydrofuran, in the 18 O content in the presence of H18 2 O. The relative products is zero at the beginning of the reaction increasing with time-on-stream. This indicates that water does not directly participate in the oxygen insertion step but delivers 18 O via an exchange process with the catalyst matrix prior to the insertion reaction. Probably, H18 2 O is ®rst adsorbed on P±OH groups and on coordinatively unsaturated VIV Lewis centres followed by rupturing of P±O±P, V±O±P or V±O±V bridging bonds and the formation of 18 OH and 16 OH groups. Due to the reversibility of these processes 18 O atoms can remain in the catalyst matrix after desorption of a H16 2 O molecule which can be incorporated in the oxidation products according to the Mars van Krevelen reaction mechanism observed in our earlier work [5]. The formation of irreversible adsorbed C-residues from n-butane under the pulse vacuum conditions of

H.W. Zanthoff et al. / Applied Catalysis A: General 172 (1998) 49±58

low surface coverage (cf. Tables 1 and 2) most probably occurs at strong adsorption sites, i.e. Lewis acid sites. These sites are blocked in the presence of water by competitive adsorption of the water molecules (the adsorption of water is reversible). Therefore, the ``non-reactive'' conversion drops, i.e. more butane is observed at the reactor exit. This blockage can also be understood in terms of temporary change in the surface acidity [20]. This leads to a modi®cation of the reaction pathways resulting in an enhanced desorption of maleic anhydride intermediates and their consecutive reaction products and a reduction of the amount of strongly bound residues which are converted to COx in the presence of oxygen. In the presence of large amounts of water in the educt gas under atmospheric conditions, this effect of water is superimposed by additionally blocking most of the V4‡ Lewis acid sites necessary for reactive adsorption of butane and/or its intermediate products [21]. This, in consequence, results in a decrease in conversion [13,14]. A similar effect can be obtained on exchanging the H in the Brùnsted sites to potassium ions [22]. These ions prevent the condensation of the V±OH/P±OH groups thus decreasing the amount of conversion by blocking of active sites. However, based on the present work the direct participation of the Brùnsted acid sites V±18 OH and/or P±18 OH formed by adsorbed water cannot be totally ruled out. A further effect of water is the enhancement of the reoxidation capability as can be derived from Fig. 4. The easier reoxidation in the presence of water is consistent with the above model that water dissociatively, reversibly adsorbs on the catalyst surface. This behaviour might be explained by the fact that due to, at least, partly dissociative water adsorption the V±O±P groups are converted to a V±OH and a HO±P group. The latter one is mobile as has been shown recently [14] and may lead to the formation of surface defect areas which enables easier oxidation of subsurface vanadium sites. Such defect areas have recently been observed on exposure of the catalyst to steam applying high resolution electron microscopy [23]. An enhanced reoxidation due to removal of surface residuals can be excluded under the conditions applied in the present work since a catalyst calcined in N2, which was not in contact with butane before, exhibited a similar behaviour. (The total uptake of oxygen by this fresh catalyst specimen, however, was lower, due to

57

the higher vanadium oxidation state of the calcined catalyst.) In the presence of butane a further bene®cial effect of water on the reoxidation behaviour might be to prevent the formation of large amounts of carbonaceous surface species which may restrain the reoxidation. The inhibitive behaviour of such surface residues has already been shown by Centi et al. [24] also using the TAP reactor device. From the results of the present work and those reported in literature it might be concluded that water adsorbs on Lewis acid V4‡ and/or V5‡ sites and on P± OH sites on the pyrophosphate surface. A dissociation with participation of V±O±P and P±O±P bondings leads to the formation of P±OH and V±OH groups. Due to these processes phosphate groups can migrate and enrich at the surface. This explains the reversible increase in the surface P/V ratio of (VO)2P2O7 upon interaction with water vapour as has been early suggested by Arnold and Sundaresan [11] and was recently experimentally manifested by XPS and ISS measurements [14]. Furthermore, the proposed model explains the increase of reoxidation capability of the (VO)2P2O7 in the presence of water. Due to the dissociation of the P±O±V bonds the pyrophosphate structure is opened forming defect areas and oxygen gets access to further vanadium centres to be oxidised. In the presence of butane it helps in avoiding the formation of strongly bound species which decrease the reaction and reoxidation rates. Acknowledgements The presented work was ®nancially supported by the Bundesministerium fuÈr Erziehung, Wissenschaft, Forschung und Technologie (contract no. 03D 0024 C1).

References [1] R.L. Bergman, N.W. Frisch, US Patent 3 293 268 (1966). [2] S. SzakaÂcs, H. Wolf, G. Mink, I. Bertoti, N. WuÈstneck, B. LuÈcke, H. Seeboth, Catal. Today 1 (1987) 27. [3] B. Kubias, U. Rodemerck, H.W. Zanthoff, M. Meisel, Catal. Today 32(1-4) (1996) 243. [4] F. Cavani, F. Trifiro, in: Catalysis, vol. 11, Royal Society of Chemistry, Cambridge, 1994, p. 246. [5] U. Rodemerck, B. Kubias, H.W. Zanthoff, M. Baerns, Appl. Catal. A 153 (1997) 203.

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[6] U. Rodemerck, B. Kubias, H.W. Zanthoff, M. Baerns, Appl. Catal. A 153 (1997) 217. [7] G.W. Coulston, S.R. Bare, H. Kung, K. Birkeland, G.K. Bethke, R. Harlow, N. Herron, P.L. Lee, Science 275 (1997) 191. [8] J.R. Ebner, J.T. Gleaves, in: A.E. Martell, D.T. Sawyer (Eds.), Oxygen Complexes and Oxygen Activation by Transition Metals, Plenum Press, New York, 1988, p. 273. [9] J.T. Wrobleski, C.R. Graham, R.A. Keppel, H. Raffelson, J.W. Edwards, Eur. Pat. Appl. EP 151 192 (1985). [10] R.M. Contractor, H.S. Horowitz, G.M. Sisler, E. Bordes, Proceedings of the 14th North American Meeting Catal. Soc., Snowbird, Utah, USA, 11±16 June 1995, T-256. [11] E.W. Arnold, S. Sundaresan, Appl. Catal. 41 (1988) 225. [12] S.J. Puttock, C.H. Rochester, J. Chem. Soc., Faraday Trans. 1 82 (1986) 2773. [13] R.M. Contractor, H.S. Horowitz, G.M. Sisler, E. Bordes, Catal. Today 37 (1997) 51. [14] B. Kubias, F. Richter, H. Papp, A. Krepel, A. Kretschmer; in: R.K. Grasselli, S.T. Oyama, A.M. Gaffney, J.E. Lyons (Eds.),

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

Proceedings of the Third World Congress on Oxidation Catalysis, Elsevier, Amsterdam, 1997, p. 461. B. Kubias, M. Meisel, G.U. Wolf, U. Rodemerck, Stud. Surf. Sci. Catal. 82 (1994) 195. J.T. Gleaves, J.R. Ebner, T.C. Kuechler, Catal. Rev.-Sci. Eng. 30 (1988) 49. N. Duvauchelle, H.W. Zanthoff, E. Bordes, B. Kubias, to be published. A. Satsuma, Y. Tanaka, A. Hattori, Y.J. Murakami, J. Chem. Soc., Chem. Commun. (1994) 1073. Y. Takita, T. Sakai, Y. Mizuhara, T. Ishihara, Chem. Lett. (1992) 415. G. Busca, V. Lorenzelli, G. Oliveri, G. Ramis, Stud. Surf. Sci. Catal. 82 (1994) 253. E. Bordes, Catal. Today 16 (1993) 27. G. Centi, G. Golinelli, F. Trifiro, Appl. Catal. 48 (1989) 13. P.L. Gai, K. Kourtakis, D.R. Coulston, G.C. Sonnichsen, J. Phys. Chem. B 101 (1997) 9916. G. Centi, F. Trifiro, G. Busca, J. Ebner, J. Gleaves, Faraday Discuss. Chem. Soc. 87 (1989) 215.