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Nature of the active sites of (VO)2P2O7 in the selective oxidation of n-butane
Applied Catalysis, 48 (1989) 13-24 Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Nature of the Active Sites of (VO)2P207 in the Selective Oxidation of n-Butane Evidence from Doping Experiments GCENTI, G. GOLINELLI and F. TRIFIRO’* Department of Industrial Chemistry and Materials, V. le Risorgimento 4, 40136 Bologna (Italy) (This paper was presented at the Bicentenary Catalysis Conference in Sydney, 1-2 September 19881.
ABSTRACT The effects on the selective oxidation of n-butane of surface doping vanadyl pyrophosphate with potassium and co-feeding ammonia or sulphur dioxide are reported. The active sites of consecutive maleic anhydride oxidation to CO, can be specifically poisoned by the addition of sulphur dioxide. The presence of ammonia or doping with very limited amounts of potassium specifically inhibit the active sites of n-butane activation and oxygen insertion, but have less effect on the transformation of but-l-ene to butadiene and furan. The lowering of the selectivity to maleic anhydride parallels the reduction in the surface Brensted acidity. It is suggested that Bransted sites are responsible for the formation or stabilization of strongly held surface adsorbates and that this interaction is the key step in the mechanism of maleic anhydride synthesis. Vv sites are principally responsible for the consecutive oxidation of maleic anhydride. Their interaction with sulphur dioxide forms a probably stable V’v sulphate with inhibition of the rate of combustion of maleic anhydride.
INTRODUCTION
Remarkable advances have been made in recent years towards understanding the mechanism of the selective oxidation of n-butane to maleic anhydride [l-4], but the nature of the active sites at the molecular level is still not known. All workers agree with the identification of vanadyl pyrophosphate 1(VOM2071 as th e only active phase for this reaction, but the reason for this specificity is not clear. The presence of different valence states of vanadium (VI”, VIv and Vv) on the surface of the catalyst after interaction with hydrocarbons has been shown by several workers (refs. l-4 and references cited therein and ref. 5), but much debate remains about their function. In particular, the role of Vv in the different stages of the complex mechanism [l-4] and the identification of surface amorphous V” phases as the active sites for the reaction are disputed [ 61.
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
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Not only have different vanadium sites and/or surface species been suggested to play a role in the reaction mechanism, but also in recent years it has been proposed that activated adsorbed oxygen species are responsible for some stages of the reaction. In particular, their participation in the steps of alkane activation and furan conversion to maleic anhydride has been indicated [ 1,2,7]. Recent kinetic data support this indication [ 81. It also seems that different pathways for the selective transformation of n-butane operate simultaneously at the vanadyl pyrophosphate surface [ 71. These possible pathways involve active sites with different natures and reactivities [ 1,2,7]. An additional aspect complicates the analysis of the nature of the active sites in the selective oxidation of n-butane. Recently, it has been shown that some transient effects characterize the attainment of steady-state behaviour, even for already activated catalysts [ 91. In particular, it was reported that the rate of n-butane depletion decreases with a parallel considerable increase in the selectivity to maleic anhydride (from 0 to about 60-79% ). A similar transient effect on the activity was observed by Buchanan and Sundaresan [lo]. Parallel to the increase in the selectivity, the amount of strongly adsorbed species detected on the vanadyl pyrophosphate surface by “stopped-flow desorption” experiments also increases [ 1,9]. It was suggested [ 9,111 that strongly held adsorbates may be involved in the formation of selective oxidation sites and may be a key factor in the analysis of the peculiar oxidation properties of vanadyl pyrophosphate. The active layer of vanadyl pyrophosphate, therefore, consists of both the surface of the catalyst and the species adsorbed on it formed during the dynamic conditions of the catalytic reaction. It is very difficult to obtain reliable information on the nature of the active sites during ex situ characterizations that do not simulate the real surface situation during the selective oxidation of n-butane. In order to obtain direct evidence on the nature of the active sites involved during the steady-state reaction, we studied the effect of doping and poisoning agents added to the vanadyl pyrophosphate or co-fed with the n-butane-air mixture. In particular, the effects of ammonia, potassium and sulphur dioxide on the catalytic behaviour of vanadyl pyrophosphate in the selective transformation of n-butane to maleic anhydride are reported. EXPERIMENTAL
Preparation of vanadyl pyrophosphate V,O, was stirred into 0.6 1 of a 1:2 mixture of technical-grade benzyl and isobutyl alcohols. Orthophosphoric acid (in such an amount as to give a final phosphorus-to-vanadium atomic ratio of 1.1) dissolved in 0.11 of isobutyl alcohol was then added to form a slurry which gradually darkened on heating for several hours at the reflux temperature (393 K). Subsequently, the mixture
15
was cooled with continuous stirring and the light-green product was isolated by filtration, washed and dried overnight. Both X-ray diffraction and Fourier transform (FT) IR examinations indicated the solid to be (VOHPO,),*H,O. This vanadyl hydrogenphosphate hemihydrate was then converted at 673 K to vanadyl pyrophosphate in a flow of 1.5% n-butane in air. This already activated catalyst sample was doped with potassium. Doping was carried out by incipient wet impregnation in an anhydrous medium (waterfree ethanol) using calibrated amounts of potassium acetate. After impregnation, the catalysts were dried overnight in air at 393 K and activated in a butane-air flow at 673 K. Reference tests without potassium were made in order to verify the absence of negative effects from the impregnation procedure.
Apparatus
Catalytic tests under steady-state conditions were made in a continuous down-flow fixed-bed integral apparatus [ 121. The flow reactor was charged with 4 g of sample with particle dimensions in the range 0.125-0.250 mm. The reactor was provided with an axial thermocouple sliding inside in order to control isothermicity during catalytic runs. The absence of significant diffusional limitations (heat and mass transfer) to the rate of hydrocarbon depletion was verified experimentally by varying the feed rate at a constant W/ F ratio or testing catalysts with different particle sizes. Additional calculations (thermal balance over the catalyst granules using the Colburn analogy, determination of the Biot number) indicated the isothermicity of the whole particle. The reactor assembly was interfaced between the section for reagent mixture preparation and flow control and the section for on-line gas chromatographic analyses of the reagent composition and of the reaction products. The outlet stream from the reactor was kept at 493 K to prevent condensation of organic products that were analysed in a first gas chromatograph using a flame ionization detector. After cooling of the gas stream, oxygen, nitrogen and nitrogen oxides, carbon monoxide and carbon dioxide were analysed in a second gas chromatograph using a thermal conductivity detector. A 3-m Porapak QS column was utilized in the first chromatograph, the oven temperature being programmed to rise from 353 to 503 K at 16 K min-I. The second chromatograph was operated with a Carbosieve-II column, the oven temperature being programmed from room temperature to 503 at 32 K min-l after an initial 9 min under isothermal conditions. An additional column of dimethylsulpholane on Chromosorb P at room temperature was utilized for the separation of C, hydrocarbons. The following definitions of the yields and selectivities are utilized:
16
Yield of maleic anhydride, maleimide, butadiene or furan =
moles of the product moles of n-butane
Yield of carbon monoxide or carbon dioxide -
Selectivity to maleic anhydride= Selectivity to carbon oxides =
moles of n-butane converted to CO, moles of n-butane
moles of maleic anhydride moles of n-butane converted
moles of n-butane converted to CO, moles of n-butane converted
RESULTS
Gas-phase ammonia addition Fig. 1 shows the catalytic behaviour of vanadyl pyrophosphate in the selective oxidation of n-butane to maleic anhydride when increasing amounts of ammonia are co-fed. The presence of very low amounts of ammonia in the inlet reagent mixture strongly inhibits both the activity (n-butane conversion) and the formation of maleic anhydride. For both reaction temperatures, when the ammonia-to-n-butane ratio is higher than about 0.5-0.6 no formation of maleic anhydride is detected. Limited amounts of maleimide are formed which, however, are not sufficient to account for the considerable decrease in the formation of maleic anhydride. The formation of carbon oxides is only slightly affected by the increase in ammonia concentration and, therefore, the selectivity to products of partial oxidation (maleic anhydridefmaleimide) de-
A-A-
ammonia\n-butane
Fig. 1. Effect of NHJn-butane ratio on hydrocarbon conversion (-----) (0 ) and yields to maleic anhydride ( 0 1, maleimide ( q ), CO ( n ) and CO, ( A ). Reaction temperature: (A) 629 K; (B ) 656 K. Experimental conditions: n-butane, 1.6%; oxygen, 15.3%; 4.0 g of catalyst; total flow-rate (STP), 40 cm” min-‘.
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Similar effects were observed using but-1-ene instead of the alkane as the starting hydrocarbon. In this instance, however, the catalyst is quickly deactivated in the presence of ammonia with a rate of deactivation proportional to the amount of ammonia in the reagent mixture. Fig. 2A and B show the change in the catalytic behaviour as a function of time on-stream in but-1-ene conversion in the presence of different concentrations of ammonia in the reagent mixture. An initial transient increase in the yield to maleic anhydride is found with a consecutive decrease up to values near to zero (Fig. 2B ) . The time required for complete inhibition of maleic anhydride synthesis depends inversely on the amount of ammonia (around 200 min for 0.4% and 90 min for 0.9% of ammonia). Parallel to the increase in the yield to maleic anhydride, the formation of nitrogen is observed, the amount of which decreases with time onstream. With n-butane the formation of nitrogen was detected only at higher reaction temperatures (656 K), but it should be noted that the data for n-butane refer to steady-state behaviour and not to a transient behaviour as with but-l-ene. As previously discussed, the formation of nitrogen is an index of the oxidative conversion of ammonia and of a partial surface reduction of the catalyst. Just as for n-butane, limited amounts of maleimide were detected. However, with but-1-ene, butadiene is also formed together with smaller amounts of furan. The amounts of butadiene and furan increase as a function of time on-stream. They become the principal products of selective partial OXidation when the formation of maleic anhydride is inhibited (Fig. 2B ).
-A
2 E m 0 ‘5 z
E ,o h L
Li . .4 -
. .2-
.?
5 AI I
. 0573
l-*-_a-
I 653 613 temperature. K
I
693
Fig.3.Effectof dopingthe (VO),P,O, catalystwithK on theconversionof n-butane(A) andon the selectivityto maleicanhydride.Experimental conditionsas in Fig.1. Amountof potassium, wt.-%: (0) 0.0, ((3) 0.00041,(al) 0.11,(0) 0.98.
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Poisoning with potassium Fig. 3 shows the effect of the doping of vanadyl pyrophosphate with potassium. Even very low contamination of the surface with potassium drastically inhibits the activity of the catalyst in n-butane conversion. At the same time the selectivity to maleic anhydride decreases whereas that to carbon oxides (mainly carbon dioxide) increases. X-ray diffraction and FT-IR spectroscopy do not show any modification of the vanadyl pyrophosphate structure on addition of potassium either before or after the catalytic tests. The catalytic effect of potassium poisoning is in good agreement with the results for the ammonia additions, indicating that the addition to the vanadyl pyrophosphate of elements with basic characteristics specifically destroys the selective pathway for the synthesis of maleic anhydride. Gas-phase addition of sulphur dioxide Fig. 4 shows the effect of the presence of 0.48% of sulphur dioxide in the inlet feed on the catalytic behavior of vanadyl pyrophosphate. The presence of sulphur dioxide has only a slight effect on the conversion of n-butane, whereas it causes a significant increase in the yield of maleic anhydride. The effect is particularly evident at higher reaction temperatures and parallels the decrease in the formation of carbon dioxide. The presence of sulphur dioxide has little effect on the formation of carbon monoxide. If the catalyst after the tests with sulphur dioxide is again tested in the absence of sulphur dioxide (case C ) , an intermediate behaviour between case A (absence of sulphur dioxide) and case B (presence of sulphur dioxide) is found. In general, after sulphur dioxide treatment the catalytic behaviour slowly decreases to the initial performance after about 2-3 days of working tests. This indicates that sulphur dioxide prob-
temperature,
K
Fig. 4. Effect of co-feeding SO, on the catalytic behavior of (VO),P,07 in n-butane selective oxidation. Experimental conditions as in Fig. I. Symbols: ( 0 ) conversion, ( 0 ) yield of maleic anhydride, (A) yield of CO, (A) yield of CO,. (----): tests without SO, (case A); (p): tests in the presence of 0.48% SO, (case B); (**.**** ): tests without SO,, but after catalytic tests in the presence of 0.48% SO, (case C
).
conversion
Fig. 5. Effect of the conversion at constant reaction temperature (637 K) on yield of maleic anhydride for two vanadyl pyrophosphate catalysts in the presence or absence of SO, in the feed: (-) in the presence of 0.48% SO, (full symbols); (----) tests without SO, (empty symbols). Catalyst A: ( 0, l ) ; Catalyst B: ( 0 ,m ) .
Fig. 6. Effect of the amount of SO, in the inlet feed on the catalytic behavior of vanadyl pyrophosphate at 639 K in n-butane oxidation. Symbols and experimental conditions as in Fig. 1.
ably interacts with the catalyst surface to form a relatively stable compound which slowly decomposes in the absence of sulphur dioxide in the gas phase. Fig. 5 evidences that the effect of sulphur dioxide is mainly connected with the inhibition of the rate of the consecutive reaction of maleic anhydride to form carbon oxides. At low conversion the behaviours in the absence and presence of sulphur dioxide are quite similar. At high conversions (higher than about 80-90% for catalyst A) in the absence of sulphur dioxide the yield of maleic anhydride starts to decrease, whereas in the presence of sulphur dioxide the yield of maleic anhydride continues to increase. A further interesting aspect is shown in Fig. 5, where the behaviour of the same catalyst after longterm activation in an n-butane-air flow (about 200 h, catalyst A) or shortterm activation in an n-butane-air flow (6 h, catalyst B) are compared. Physice-chemical characterization (X-ray diffraction patterns and FT-IR spectra) does not show any appreciable difference in the two catalysts. In both instances only the vanadyl pyrophosphate phase is detected, but catalyst B shows a higher rate of oxidation of VT” to Vv. The performance of catalyst B is poorer than that of catalyst A, especially at higher conversions. As we have already pointed out [ 16-181, this effect may be attributed to the formation of a Vv surface layer on the catalyst, owing to its different redox properties. The higher surface
21
concentration of Vv sites in catalyst B enhances the rate of consecutive oxidation of maleic anhydride compared with catalyst A. In spite of the considerable difference between the two catalysts in the absence of sulphur dioxide, the catalytic behaviour in the presence of sulphur dioxide are very similar. We may conclude that sulphur dioxide causes a specific, very selective poisoning of the sites of maleic anhydride decomposition. Thermogravimetric tests with vanadyl orthophosphate indicate a continuous increase in weight (around Z3% after 1 h) in the presence of 0.3% of sulphur dioxide at temperatures in the range 600-650 K. As only sulphate species are stable at these temperatures, the formation of a V’” sulphate is very likely. Fig. 6 summarizes the effect of sulphur dioxide on the catalytic behaviour of vanadyl pyrophosphate. An increase in sulphur dioxide in the inlet reagent mixture only slightly decreases the conversion. In contrast the selectivity to maleic anhydride increases depending on the amount of sulphur dioxide, with a parallel decrease in the formation of carbon oxides. DISCUSSION
Poisoning of active sites of maleic anhydride combustion The experiments with sulphur dioxide clearly show that specific poisoning of the sites of the consecutive oxidation of maleic anhydride is possible without modification of the sites for alkane activation and selective transformation to maleic anhydride. This means that, under the flow reactor conditions, the sites of selective synthesis of maleic anhydride and those for its consecutive oxidation to CO, are different. This important aspect of the nature of the active sites is not clear in the literature. Both reactions have been generically attributed to Vv sites [l-4]. The effect of sulphur dioxide is semi-permanent. After treatment with sulphur dioxide, the catalytic performances in the absence of co-fed sulphur dioxide return to the initial levels after about 2-3 days of working tests. This is evidence that sulphur dioxide specifically interacts with the catalyst forming a surface species that only slowly decomposes. Thermogravimetric tests with a Vv phosphate indicate a weight increase of the catalyst in the presence of a sulphur dioxide-air flow at the temperatures of catalytic reaction. Sulphur species are not stable at these reaction temperatures. It is likely that V” catalyses the oxidation of sulphur dioxide to sulphur trioxide with the probably final formation of a Vn’ sulphate. The formation of this stable compound prevents or limits the vanadium redox cycle, inhibiting its participation in the catalytic reaction of maleic anhydride oxidation. The initial effect of ammonia at the higher reaction temperatures is in agreement with this suggestion. The formation of an intergrowth vanadyl pyrophosphate and orthophosphate structure or of contiguous microdomains has been proposed as giving
22
rise to the active sites for the transformation of butane to maleic anhydride [ 191. Similarly, Bergeret et al. [ 61 have proposed as active sites small entities of V” surrounded by a large number of phosphorus-containing groups on the surface of the vanadyl pyrophosphate matrix. The present data on the effect of sulphur dioxide cannot definitely exclude these hypotheses, but strongly suggest that, under flow reactor conditions, these Vv species are probably involved mainly in the consecutive combustion of maleic anhydride. The specification of flow reactor conditions is necessary. Maleic anhydride can also be formed from n-butane on vanadyl pyrophosphate under anaerobic conditions (pulse reactor). It is very likely that Vv sites are involved in this oxidation mechanism (pulse reactor). However, we have recently shown with “temporal analysis of products” (TAP ) experiments [ 1,271 that both long-lived oxygen species and lattice V5+ species are selective in maleic anhydride synthesis, but the former react faster and are more selective. We therefore tentatively suggest that under the flow reactor dynamic conditions, V” sites behave as a parasitic species responsible for the partial consecutive oxidation of maleic anhydride. Sulphur dioxide prevents their reactivity through the formation of a fairly stable surface species which allows a significant increase in catalytic performance. Possible nature of the active layer of vanadyl pyrophosphate Catalytic tests in the presence of co-fed ammonia and doping experiments with potassium indicate that elements with basic characteristics can selectively inhibit the synthesis of maleic anhydride. In particular, the comparison between n-butane and but-l-ene oxidation indicates specific poisoning of the sites of alkane activation and of oxygen insertion to form maleic anhydride, whereas the sites for conversion of olefin to butadiene and furan are only slightly affected. The residual activity to carbon oxides that remains after poisoning is probably related to unselective modes of hydrocarbon activation. Fourier-transform infrared characterization of the surface acidity of potassium-doped vanadyl pyrophosphate [20] indicate that potassium doping results in a selective inhibition of Bronsted acidity, whereas the Lewis acidity remains unchanged. In contrast, by adsorption of ammonia it is possible to detect by FT-IR spectroscopy the bands typical of chemisorbed ammonia and of ammonia cations [ 15]. Both species are stable under evacuation up to 570 K, indicating that they are due to interaction with strong Lewis and Brransted centres, respectively. The comparison between ammonia and potassium doping suggests that not only Lewis acid centres [ 1,2,7,15] but also Bronsted sites play a central role in the mechanism of oxidation of butane to maleic anhydride on vanadyl pyrophosphate. Recent data [ 211 also indicate a high concentration of surface POH groups on the most selective vanadyl pyrophosphate catalyst for oxidation of n-butane to maleic anhydride. The present data do not allow an unequivocal
23
explanation of this relationship between Brernsted acidity due to P-OH groups and oxidation activity. We may advance three possible hypotheses. P-OH groups can be involved in the stabilization of some of the reaction intermediates. This hypothesis is reasonable, but has two main criticisms. First, it does not explain the parallel decrease in the rate of butane activation. Second, it does not explain an infrared observation [ 201 that on admission of oxygen the adsorbed intermediates do not form maleic anhydride on potassium-doped catalysts. A second possible hypothesis is that P-OH groups can be involved in the stabilization of some adsorbed oxygen species, but literature data [22] indicate that this stabilization is very weak. A further hypothesis is based on the observation that surface condensed compounds are formed from C4 hydrocarbons on orthophosphoric acid-impregnated oxides [ 231. The possible role of condensed (carbonaceous) surface compounds formed on acidic oxides in oxidation reactions is known. Toluene can be selectively oxidized to toluonitrile on silica-alumina and the activity has been attributed to the organic surface layer formed during the reaction [24]. An organic layer also has been indicated as the active component in the ammoxidation of cyclohexanone on silica and silica-alumina [ 25 1. Similarly, it can be suggested that the Brcansted sites on vanadyl pyrophosphate can generate some organic surface species that can be involved in the formation of active oxygen species or can mediate their surface transport. These hypotheses regarding the nature of the active layer require much more detailed investigations. However, it should be emphasized that they may explain some previous observations in the literature. During n-butane conversion under anaerobic conditions, strongly adsorbed species are formed that do not desorb at the reaction temperature (673 K) [ 7,111. “0 pulse tests indicate that the migration of oxygen-containing species in the surface layer is faster than the oxidation of these strongly adsorbed species [ 261. During the transient behaviour up to steady-state activity, strongly adsorbed species are formed on the surface of the catalyst with a parallel increase in the selectivity [9]. All these observations are in agreement with the proposed model for the nature of the active layer. Previous evidence [ 11 that n-butane activation requires a concerted mechanism (simultaneous abstraction of two hydrogen atoms by a strong Lewis acid site and a second oxygen site) is also in agreement with the present observations. The base associated with P-OH sites or surface reactive oxygen species generated according to the models previously discussed can act as the second hydrogen-acceptor site for the concerted mechanism of butane activation [ 1,2]. In conclusion, these doping and poisoning experiments have provided useful information on the surface nature of active sites in the complex mechanism for the selective oxidation of n-butane to maleic anhydride and also allow some interesting hypotheses to be made regarding their surface nature. Much more detailed physico-chemical investigations are necessary which, however, must
24
simulate the real surface situation of vanadyl pyrophosphate during the continuous-flow oxidation of n-butane to maleic anhydride. ACKNOWLEDGEMENT
Financial support for this work was received from the Minister0 Pubblica Istruzione (National Group on the Surface Reactivity of Surfaces ), Rome, Italy.
REFERENCES
8 9 10 11 12 13 14 15 16 17 18 19 20 21
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24 25 26
G. Centi, F. Trifiro’, J.R. Ebner and V.W. Franchetti, Chem. Rev., 88 (1988) 55. G. Centi and F. Trifiro’, Chim. Ind. (Milan), 68 (1986) 74. B.K. Hodnett, Catal. Rev. Sci. Eng., 27 (1985) 373. Catal. Today, 1 (1987) 477. J. Curilla, R. Domansky and B. Wichterlova, Appl. Catal., 36 (1988) 119. G. Bergeret, M. David, J.P. Broyer and J.C. Volta, Catal. Today, 1 (1987) 37. G. Centi, F. Trifiro’, G. Busca, J.R. Ebner and J.T. Cleaves, in M.J. Philips and M. Ternan (Editors), Proceedings of the 9th International Congress on Catalysis, Chemical institute of Canada, Ottawa, 1988, Vol. 4, p. 1538. P. Schneider, G. Emig and H. Hofmann, Ind. Eng. Chem. Res., 26 (1987) 2236. G. Busca, G. Centi and F. Trifiro’, in B. Delmon and G.F. Froment (Editors), Catalyst Deactivation 1987, Elsevier, Amsterdam, 1987, p. 427. J.S. Buchanan and S. Sundaresan, Appl. Catal., 26 (1986) 211. G. Busca and G. Centi, J. Am. Chem. Sot., in press. G. Centi, G. Fornasari and F. Trifiro’, Ind. Eng. Chem., Prod. Res. Dev., 24 (1985) 32. H. Bosh and F. Janssen, Catal. Today, 2 (1988) 369. G. Centi, D. Pesheva and F. Trifiro’, Appl. Catal., 33 (1987) 343. G. Busca, G. Centi, F. Trifiro’ and V. Lorenzelli, J. Phys. Chem., 90 (1986) 1337. G. Busca, F. Cavani, G. Centi and F. Trifiro’, J. Catal., 99 (1986) 400. F. Cavani, G. Centi, F. Trifiro’ and A. Vaccari, in M. Che and G.C. Bond (Editors), Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 287. F. Cavani, G. Centi, F. Trifiro’, Appl. Catal., 15 (1985) 151. E. Bordes, Catal. Today, 1 (1987) 499, and references cited therein. G. Centi, F. Trifiro’, G. Busca, J. Ebner and J. Gleaves, J. Chem. Sot. Faraday Discuss., in press. J. Haas, C. Plog, W. Maunz, K. Mittag, K.-D. Gollmer and B. Klopries, in M.J. Philips and M. Ternan (Editors), Proceedings of the 9th International Congress on Catalysis, Vol. 4, Chemical Jnstitute of Canada, Ottawa, 1988, p. 1632. M. Che and A.J. Tenth, Adv. Catal., 32 (1983) 1. G. Ramis, G. Busca, V. Lorenzelli, P.F. Rossi, M. Bensitel, 0. Saur and J.C. Lavalley, in M.J. Philips and M. Ternan (Editors), Proceedings of the 9th International Congress on Catalysis, Vol. 4, Chemical Insitute of Canada, Ottawa, 1988, p. 1875. M. Niwa, M. Sago, H. Ando and Y. Murakami, J. Catal., 69 (1981) 69. J.N. Armor, E.J. Carlson, S. Soled, W.C. Conner, A. Laverick, B. DeRites and W. Gates, J. Catal., 70 (1981) 84. M.A. Pepera, J.L. Callahan, M.J. Desmond, E.C. Milberger, P.R. Blum and N.J. Bremer, J. Am. Chem. Sot., 107 (1985) 4883.
Printed in The Netherlands
Nature of the Active Sites of (VO)2P207 in the Selective Oxidation of n-Butane Evidence from Doping Experiments GCENTI, G. GOLINELLI and F. TRIFIRO’* Department of Industrial Chemistry and Materials, V. le Risorgimento 4, 40136 Bologna (Italy) (This paper was presented at the Bicentenary Catalysis Conference in Sydney, 1-2 September 19881.
ABSTRACT The effects on the selective oxidation of n-butane of surface doping vanadyl pyrophosphate with potassium and co-feeding ammonia or sulphur dioxide are reported. The active sites of consecutive maleic anhydride oxidation to CO, can be specifically poisoned by the addition of sulphur dioxide. The presence of ammonia or doping with very limited amounts of potassium specifically inhibit the active sites of n-butane activation and oxygen insertion, but have less effect on the transformation of but-l-ene to butadiene and furan. The lowering of the selectivity to maleic anhydride parallels the reduction in the surface Brensted acidity. It is suggested that Bransted sites are responsible for the formation or stabilization of strongly held surface adsorbates and that this interaction is the key step in the mechanism of maleic anhydride synthesis. Vv sites are principally responsible for the consecutive oxidation of maleic anhydride. Their interaction with sulphur dioxide forms a probably stable V’v sulphate with inhibition of the rate of combustion of maleic anhydride.
INTRODUCTION
Remarkable advances have been made in recent years towards understanding the mechanism of the selective oxidation of n-butane to maleic anhydride [l-4], but the nature of the active sites at the molecular level is still not known. All workers agree with the identification of vanadyl pyrophosphate 1(VOM2071 as th e only active phase for this reaction, but the reason for this specificity is not clear. The presence of different valence states of vanadium (VI”, VIv and Vv) on the surface of the catalyst after interaction with hydrocarbons has been shown by several workers (refs. l-4 and references cited therein and ref. 5), but much debate remains about their function. In particular, the role of Vv in the different stages of the complex mechanism [l-4] and the identification of surface amorphous V” phases as the active sites for the reaction are disputed [ 61.
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
14
Not only have different vanadium sites and/or surface species been suggested to play a role in the reaction mechanism, but also in recent years it has been proposed that activated adsorbed oxygen species are responsible for some stages of the reaction. In particular, their participation in the steps of alkane activation and furan conversion to maleic anhydride has been indicated [ 1,2,7]. Recent kinetic data support this indication [ 81. It also seems that different pathways for the selective transformation of n-butane operate simultaneously at the vanadyl pyrophosphate surface [ 71. These possible pathways involve active sites with different natures and reactivities [ 1,2,7]. An additional aspect complicates the analysis of the nature of the active sites in the selective oxidation of n-butane. Recently, it has been shown that some transient effects characterize the attainment of steady-state behaviour, even for already activated catalysts [ 91. In particular, it was reported that the rate of n-butane depletion decreases with a parallel considerable increase in the selectivity to maleic anhydride (from 0 to about 60-79% ). A similar transient effect on the activity was observed by Buchanan and Sundaresan [lo]. Parallel to the increase in the selectivity, the amount of strongly adsorbed species detected on the vanadyl pyrophosphate surface by “stopped-flow desorption” experiments also increases [ 1,9]. It was suggested [ 9,111 that strongly held adsorbates may be involved in the formation of selective oxidation sites and may be a key factor in the analysis of the peculiar oxidation properties of vanadyl pyrophosphate. The active layer of vanadyl pyrophosphate, therefore, consists of both the surface of the catalyst and the species adsorbed on it formed during the dynamic conditions of the catalytic reaction. It is very difficult to obtain reliable information on the nature of the active sites during ex situ characterizations that do not simulate the real surface situation during the selective oxidation of n-butane. In order to obtain direct evidence on the nature of the active sites involved during the steady-state reaction, we studied the effect of doping and poisoning agents added to the vanadyl pyrophosphate or co-fed with the n-butane-air mixture. In particular, the effects of ammonia, potassium and sulphur dioxide on the catalytic behaviour of vanadyl pyrophosphate in the selective transformation of n-butane to maleic anhydride are reported. EXPERIMENTAL
Preparation of vanadyl pyrophosphate V,O, was stirred into 0.6 1 of a 1:2 mixture of technical-grade benzyl and isobutyl alcohols. Orthophosphoric acid (in such an amount as to give a final phosphorus-to-vanadium atomic ratio of 1.1) dissolved in 0.11 of isobutyl alcohol was then added to form a slurry which gradually darkened on heating for several hours at the reflux temperature (393 K). Subsequently, the mixture
15
was cooled with continuous stirring and the light-green product was isolated by filtration, washed and dried overnight. Both X-ray diffraction and Fourier transform (FT) IR examinations indicated the solid to be (VOHPO,),*H,O. This vanadyl hydrogenphosphate hemihydrate was then converted at 673 K to vanadyl pyrophosphate in a flow of 1.5% n-butane in air. This already activated catalyst sample was doped with potassium. Doping was carried out by incipient wet impregnation in an anhydrous medium (waterfree ethanol) using calibrated amounts of potassium acetate. After impregnation, the catalysts were dried overnight in air at 393 K and activated in a butane-air flow at 673 K. Reference tests without potassium were made in order to verify the absence of negative effects from the impregnation procedure.
Apparatus
Catalytic tests under steady-state conditions were made in a continuous down-flow fixed-bed integral apparatus [ 121. The flow reactor was charged with 4 g of sample with particle dimensions in the range 0.125-0.250 mm. The reactor was provided with an axial thermocouple sliding inside in order to control isothermicity during catalytic runs. The absence of significant diffusional limitations (heat and mass transfer) to the rate of hydrocarbon depletion was verified experimentally by varying the feed rate at a constant W/ F ratio or testing catalysts with different particle sizes. Additional calculations (thermal balance over the catalyst granules using the Colburn analogy, determination of the Biot number) indicated the isothermicity of the whole particle. The reactor assembly was interfaced between the section for reagent mixture preparation and flow control and the section for on-line gas chromatographic analyses of the reagent composition and of the reaction products. The outlet stream from the reactor was kept at 493 K to prevent condensation of organic products that were analysed in a first gas chromatograph using a flame ionization detector. After cooling of the gas stream, oxygen, nitrogen and nitrogen oxides, carbon monoxide and carbon dioxide were analysed in a second gas chromatograph using a thermal conductivity detector. A 3-m Porapak QS column was utilized in the first chromatograph, the oven temperature being programmed to rise from 353 to 503 K at 16 K min-I. The second chromatograph was operated with a Carbosieve-II column, the oven temperature being programmed from room temperature to 503 at 32 K min-l after an initial 9 min under isothermal conditions. An additional column of dimethylsulpholane on Chromosorb P at room temperature was utilized for the separation of C, hydrocarbons. The following definitions of the yields and selectivities are utilized:
16
Yield of maleic anhydride, maleimide, butadiene or furan =
moles of the product moles of n-butane
Yield of carbon monoxide or carbon dioxide -
Selectivity to maleic anhydride= Selectivity to carbon oxides =
moles of n-butane converted to CO, moles of n-butane
moles of maleic anhydride moles of n-butane converted
moles of n-butane converted to CO, moles of n-butane converted
RESULTS
Gas-phase ammonia addition Fig. 1 shows the catalytic behaviour of vanadyl pyrophosphate in the selective oxidation of n-butane to maleic anhydride when increasing amounts of ammonia are co-fed. The presence of very low amounts of ammonia in the inlet reagent mixture strongly inhibits both the activity (n-butane conversion) and the formation of maleic anhydride. For both reaction temperatures, when the ammonia-to-n-butane ratio is higher than about 0.5-0.6 no formation of maleic anhydride is detected. Limited amounts of maleimide are formed which, however, are not sufficient to account for the considerable decrease in the formation of maleic anhydride. The formation of carbon oxides is only slightly affected by the increase in ammonia concentration and, therefore, the selectivity to products of partial oxidation (maleic anhydridefmaleimide) de-
A-A-
ammonia\n-butane
Fig. 1. Effect of NHJn-butane ratio on hydrocarbon conversion (-----) (0 ) and yields to maleic anhydride ( 0 1, maleimide ( q ), CO ( n ) and CO, ( A ). Reaction temperature: (A) 629 K; (B ) 656 K. Experimental conditions: n-butane, 1.6%; oxygen, 15.3%; 4.0 g of catalyst; total flow-rate (STP), 40 cm” min-‘.
18
Similar effects were observed using but-1-ene instead of the alkane as the starting hydrocarbon. In this instance, however, the catalyst is quickly deactivated in the presence of ammonia with a rate of deactivation proportional to the amount of ammonia in the reagent mixture. Fig. 2A and B show the change in the catalytic behaviour as a function of time on-stream in but-1-ene conversion in the presence of different concentrations of ammonia in the reagent mixture. An initial transient increase in the yield to maleic anhydride is found with a consecutive decrease up to values near to zero (Fig. 2B ) . The time required for complete inhibition of maleic anhydride synthesis depends inversely on the amount of ammonia (around 200 min for 0.4% and 90 min for 0.9% of ammonia). Parallel to the increase in the yield to maleic anhydride, the formation of nitrogen is observed, the amount of which decreases with time onstream. With n-butane the formation of nitrogen was detected only at higher reaction temperatures (656 K), but it should be noted that the data for n-butane refer to steady-state behaviour and not to a transient behaviour as with but-l-ene. As previously discussed, the formation of nitrogen is an index of the oxidative conversion of ammonia and of a partial surface reduction of the catalyst. Just as for n-butane, limited amounts of maleimide were detected. However, with but-1-ene, butadiene is also formed together with smaller amounts of furan. The amounts of butadiene and furan increase as a function of time on-stream. They become the principal products of selective partial OXidation when the formation of maleic anhydride is inhibited (Fig. 2B ).
-A
2 E m 0 ‘5 z
E ,o h L
Li . .4 -
. .2-
.?
5 AI I
. 0573
l-*-_a-
I 653 613 temperature. K
I
693
Fig.3.Effectof dopingthe (VO),P,O, catalystwithK on theconversionof n-butane(A) andon the selectivityto maleicanhydride.Experimental conditionsas in Fig.1. Amountof potassium, wt.-%: (0) 0.0, ((3) 0.00041,(al) 0.11,(0) 0.98.
19
Poisoning with potassium Fig. 3 shows the effect of the doping of vanadyl pyrophosphate with potassium. Even very low contamination of the surface with potassium drastically inhibits the activity of the catalyst in n-butane conversion. At the same time the selectivity to maleic anhydride decreases whereas that to carbon oxides (mainly carbon dioxide) increases. X-ray diffraction and FT-IR spectroscopy do not show any modification of the vanadyl pyrophosphate structure on addition of potassium either before or after the catalytic tests. The catalytic effect of potassium poisoning is in good agreement with the results for the ammonia additions, indicating that the addition to the vanadyl pyrophosphate of elements with basic characteristics specifically destroys the selective pathway for the synthesis of maleic anhydride. Gas-phase addition of sulphur dioxide Fig. 4 shows the effect of the presence of 0.48% of sulphur dioxide in the inlet feed on the catalytic behavior of vanadyl pyrophosphate. The presence of sulphur dioxide has only a slight effect on the conversion of n-butane, whereas it causes a significant increase in the yield of maleic anhydride. The effect is particularly evident at higher reaction temperatures and parallels the decrease in the formation of carbon dioxide. The presence of sulphur dioxide has little effect on the formation of carbon monoxide. If the catalyst after the tests with sulphur dioxide is again tested in the absence of sulphur dioxide (case C ) , an intermediate behaviour between case A (absence of sulphur dioxide) and case B (presence of sulphur dioxide) is found. In general, after sulphur dioxide treatment the catalytic behaviour slowly decreases to the initial performance after about 2-3 days of working tests. This indicates that sulphur dioxide prob-
temperature,
K
Fig. 4. Effect of co-feeding SO, on the catalytic behavior of (VO),P,07 in n-butane selective oxidation. Experimental conditions as in Fig. I. Symbols: ( 0 ) conversion, ( 0 ) yield of maleic anhydride, (A) yield of CO, (A) yield of CO,. (----): tests without SO, (case A); (p): tests in the presence of 0.48% SO, (case B); (**.**** ): tests without SO,, but after catalytic tests in the presence of 0.48% SO, (case C
).
conversion
Fig. 5. Effect of the conversion at constant reaction temperature (637 K) on yield of maleic anhydride for two vanadyl pyrophosphate catalysts in the presence or absence of SO, in the feed: (-) in the presence of 0.48% SO, (full symbols); (----) tests without SO, (empty symbols). Catalyst A: ( 0, l ) ; Catalyst B: ( 0 ,m ) .
Fig. 6. Effect of the amount of SO, in the inlet feed on the catalytic behavior of vanadyl pyrophosphate at 639 K in n-butane oxidation. Symbols and experimental conditions as in Fig. 1.
ably interacts with the catalyst surface to form a relatively stable compound which slowly decomposes in the absence of sulphur dioxide in the gas phase. Fig. 5 evidences that the effect of sulphur dioxide is mainly connected with the inhibition of the rate of the consecutive reaction of maleic anhydride to form carbon oxides. At low conversion the behaviours in the absence and presence of sulphur dioxide are quite similar. At high conversions (higher than about 80-90% for catalyst A) in the absence of sulphur dioxide the yield of maleic anhydride starts to decrease, whereas in the presence of sulphur dioxide the yield of maleic anhydride continues to increase. A further interesting aspect is shown in Fig. 5, where the behaviour of the same catalyst after longterm activation in an n-butane-air flow (about 200 h, catalyst A) or shortterm activation in an n-butane-air flow (6 h, catalyst B) are compared. Physice-chemical characterization (X-ray diffraction patterns and FT-IR spectra) does not show any appreciable difference in the two catalysts. In both instances only the vanadyl pyrophosphate phase is detected, but catalyst B shows a higher rate of oxidation of VT” to Vv. The performance of catalyst B is poorer than that of catalyst A, especially at higher conversions. As we have already pointed out [ 16-181, this effect may be attributed to the formation of a Vv surface layer on the catalyst, owing to its different redox properties. The higher surface
21
concentration of Vv sites in catalyst B enhances the rate of consecutive oxidation of maleic anhydride compared with catalyst A. In spite of the considerable difference between the two catalysts in the absence of sulphur dioxide, the catalytic behaviour in the presence of sulphur dioxide are very similar. We may conclude that sulphur dioxide causes a specific, very selective poisoning of the sites of maleic anhydride decomposition. Thermogravimetric tests with vanadyl orthophosphate indicate a continuous increase in weight (around Z3% after 1 h) in the presence of 0.3% of sulphur dioxide at temperatures in the range 600-650 K. As only sulphate species are stable at these temperatures, the formation of a V’” sulphate is very likely. Fig. 6 summarizes the effect of sulphur dioxide on the catalytic behaviour of vanadyl pyrophosphate. An increase in sulphur dioxide in the inlet reagent mixture only slightly decreases the conversion. In contrast the selectivity to maleic anhydride increases depending on the amount of sulphur dioxide, with a parallel decrease in the formation of carbon oxides. DISCUSSION
Poisoning of active sites of maleic anhydride combustion The experiments with sulphur dioxide clearly show that specific poisoning of the sites of the consecutive oxidation of maleic anhydride is possible without modification of the sites for alkane activation and selective transformation to maleic anhydride. This means that, under the flow reactor conditions, the sites of selective synthesis of maleic anhydride and those for its consecutive oxidation to CO, are different. This important aspect of the nature of the active sites is not clear in the literature. Both reactions have been generically attributed to Vv sites [l-4]. The effect of sulphur dioxide is semi-permanent. After treatment with sulphur dioxide, the catalytic performances in the absence of co-fed sulphur dioxide return to the initial levels after about 2-3 days of working tests. This is evidence that sulphur dioxide specifically interacts with the catalyst forming a surface species that only slowly decomposes. Thermogravimetric tests with a Vv phosphate indicate a weight increase of the catalyst in the presence of a sulphur dioxide-air flow at the temperatures of catalytic reaction. Sulphur species are not stable at these reaction temperatures. It is likely that V” catalyses the oxidation of sulphur dioxide to sulphur trioxide with the probably final formation of a Vn’ sulphate. The formation of this stable compound prevents or limits the vanadium redox cycle, inhibiting its participation in the catalytic reaction of maleic anhydride oxidation. The initial effect of ammonia at the higher reaction temperatures is in agreement with this suggestion. The formation of an intergrowth vanadyl pyrophosphate and orthophosphate structure or of contiguous microdomains has been proposed as giving
22
rise to the active sites for the transformation of butane to maleic anhydride [ 191. Similarly, Bergeret et al. [ 61 have proposed as active sites small entities of V” surrounded by a large number of phosphorus-containing groups on the surface of the vanadyl pyrophosphate matrix. The present data on the effect of sulphur dioxide cannot definitely exclude these hypotheses, but strongly suggest that, under flow reactor conditions, these Vv species are probably involved mainly in the consecutive combustion of maleic anhydride. The specification of flow reactor conditions is necessary. Maleic anhydride can also be formed from n-butane on vanadyl pyrophosphate under anaerobic conditions (pulse reactor). It is very likely that Vv sites are involved in this oxidation mechanism (pulse reactor). However, we have recently shown with “temporal analysis of products” (TAP ) experiments [ 1,271 that both long-lived oxygen species and lattice V5+ species are selective in maleic anhydride synthesis, but the former react faster and are more selective. We therefore tentatively suggest that under the flow reactor dynamic conditions, V” sites behave as a parasitic species responsible for the partial consecutive oxidation of maleic anhydride. Sulphur dioxide prevents their reactivity through the formation of a fairly stable surface species which allows a significant increase in catalytic performance. Possible nature of the active layer of vanadyl pyrophosphate Catalytic tests in the presence of co-fed ammonia and doping experiments with potassium indicate that elements with basic characteristics can selectively inhibit the synthesis of maleic anhydride. In particular, the comparison between n-butane and but-l-ene oxidation indicates specific poisoning of the sites of alkane activation and of oxygen insertion to form maleic anhydride, whereas the sites for conversion of olefin to butadiene and furan are only slightly affected. The residual activity to carbon oxides that remains after poisoning is probably related to unselective modes of hydrocarbon activation. Fourier-transform infrared characterization of the surface acidity of potassium-doped vanadyl pyrophosphate [20] indicate that potassium doping results in a selective inhibition of Bronsted acidity, whereas the Lewis acidity remains unchanged. In contrast, by adsorption of ammonia it is possible to detect by FT-IR spectroscopy the bands typical of chemisorbed ammonia and of ammonia cations [ 15]. Both species are stable under evacuation up to 570 K, indicating that they are due to interaction with strong Lewis and Brransted centres, respectively. The comparison between ammonia and potassium doping suggests that not only Lewis acid centres [ 1,2,7,15] but also Bronsted sites play a central role in the mechanism of oxidation of butane to maleic anhydride on vanadyl pyrophosphate. Recent data [ 211 also indicate a high concentration of surface POH groups on the most selective vanadyl pyrophosphate catalyst for oxidation of n-butane to maleic anhydride. The present data do not allow an unequivocal
23
explanation of this relationship between Brernsted acidity due to P-OH groups and oxidation activity. We may advance three possible hypotheses. P-OH groups can be involved in the stabilization of some of the reaction intermediates. This hypothesis is reasonable, but has two main criticisms. First, it does not explain the parallel decrease in the rate of butane activation. Second, it does not explain an infrared observation [ 201 that on admission of oxygen the adsorbed intermediates do not form maleic anhydride on potassium-doped catalysts. A second possible hypothesis is that P-OH groups can be involved in the stabilization of some adsorbed oxygen species, but literature data [22] indicate that this stabilization is very weak. A further hypothesis is based on the observation that surface condensed compounds are formed from C4 hydrocarbons on orthophosphoric acid-impregnated oxides [ 231. The possible role of condensed (carbonaceous) surface compounds formed on acidic oxides in oxidation reactions is known. Toluene can be selectively oxidized to toluonitrile on silica-alumina and the activity has been attributed to the organic surface layer formed during the reaction [24]. An organic layer also has been indicated as the active component in the ammoxidation of cyclohexanone on silica and silica-alumina [ 25 1. Similarly, it can be suggested that the Brcansted sites on vanadyl pyrophosphate can generate some organic surface species that can be involved in the formation of active oxygen species or can mediate their surface transport. These hypotheses regarding the nature of the active layer require much more detailed investigations. However, it should be emphasized that they may explain some previous observations in the literature. During n-butane conversion under anaerobic conditions, strongly adsorbed species are formed that do not desorb at the reaction temperature (673 K) [ 7,111. “0 pulse tests indicate that the migration of oxygen-containing species in the surface layer is faster than the oxidation of these strongly adsorbed species [ 261. During the transient behaviour up to steady-state activity, strongly adsorbed species are formed on the surface of the catalyst with a parallel increase in the selectivity [9]. All these observations are in agreement with the proposed model for the nature of the active layer. Previous evidence [ 11 that n-butane activation requires a concerted mechanism (simultaneous abstraction of two hydrogen atoms by a strong Lewis acid site and a second oxygen site) is also in agreement with the present observations. The base associated with P-OH sites or surface reactive oxygen species generated according to the models previously discussed can act as the second hydrogen-acceptor site for the concerted mechanism of butane activation [ 1,2]. In conclusion, these doping and poisoning experiments have provided useful information on the surface nature of active sites in the complex mechanism for the selective oxidation of n-butane to maleic anhydride and also allow some interesting hypotheses to be made regarding their surface nature. Much more detailed physico-chemical investigations are necessary which, however, must
24
simulate the real surface situation of vanadyl pyrophosphate during the continuous-flow oxidation of n-butane to maleic anhydride. ACKNOWLEDGEMENT
Financial support for this work was received from the Minister0 Pubblica Istruzione (National Group on the Surface Reactivity of Surfaces ), Rome, Italy.
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