- Email: [email protected]
On the nature of the active sites of the VPO catalysts for n-butane oxidation to maleic anhydride
Catalysis Today, 16 (1993) 39-49 Elsevier Science Publishers B.V., Amsterdam
39
On the Nature of the Active Sites of the VP0 Catalysts for n-Butane
Oxidation
to Maleic Anhydride
Y. Zhang, R.P.A. Sneeden. and J.C. Volta Institut de Recherches sur la Catalyse, CNRS, 2 Avenue A. Einstein, 69626, Villeurbanne Cedex, France
Three fVO)2P2O7 catalys$t differing by their cyftal morphology and the P NMR are compared for superficial presence of V entities detected by butane oxidation to maleic anhydride. The study shows that Butane Oxidation is a Structure Sensitive Reaction. Formation of Maleic Anhydride occurs on the basal (100) (VO)2P2O7 face with the participation of Vs’ species. Best catalytic results correspond to a suitable V5+/p+ ratio in this face. Lateral (VOJ2P2O7 faces located in the [ 1001 direction cannot reoxidize locally @’ into V + sites so that reaction is stopped at the step of olefinic C4.
INTRODUCTION
Considering the scientific literature, it appears that there are many discrepancies concerning the nature of the active surface of VP0 catalysts for the mild oxidation of n-Butane to Maleic Anhydride. Most proposals are based on conclusions which have been drawn from connexions established between the evolution of catalytic results and physicochemical data obtained after the catalytic test (1). Another feature of this system is the high heterogeneity in the composition of the catalytic mass which is always observed between the different pellets of the catalytic bed of the integral reactor, at the level of one single pellet or through the powder of the catalytic bed of a differential reactor. As a consequence, the nature of the active phase and the identification of the active sites are subject of much discussion. The different groups working in this field agree on the fact that vanadium pyrophosphate fVO)sPaO7 (v4’) is the essential phase for this system. Indeed, the average oxidation state of vanadium (4.1) measured on the best working catalysts after the catalytic test and their characterization by XRD. are consistent with the main participation of pyrophosphate (oxidation state : 4.0)(l). A model for the oxidation of butane was even proposed on the (100) face of this phase (2). However, in agreement with the 4.1 oxidation state, the participation, to some extent, ofVOP04 (v5’) - like structures in some steps of the reaction pathway has to be considered. The difficulty in identifying the active sites stems from the fact that VP0 catalysts are composed of both well crystallized and amorphous phases whose participation to the mechanism of butane oxidation can be discussed. A recent publication brought the proof of the catalytic importance of the amorphous phase (3). The relative proportion of these different phases appears to depend strongly on the 0920-5861/93/$6.00
0
1993 Elsevier Science Publishers B.V. All rights reserved.
preparation and on the conditions of activation of the catalysts (1). We have recently developped a new method of preparation of VP0 catalysts: it is possible to control the proportions of the different VP0 phases, their repartition and thus to control the catalytic properties during the preparation of the VO(HPO& 0.5 II20 precursor by introducing a Vs’ salt (4). Some improvement can also be obtained by changing the initial HsPOa/VzOs ratio (initial P/V ratio), by introducing a doping agent (l), or even by changing the V2O5 morphology (5). All these informations let US to think that the catalytic properties of VP0 catalysts for butane oxidation to maleic anhydride strongly depended on the Vr+/Vs+ dispersion and that it was important to control better their local structure. As a consequence, we developed, in our laboratory, physicochemical techniques which analyze both the short and the long range order of these materials like RED ofX-Rays (6), 31P and % NMR (4,7,8) and l&man Spectroscopy (8). This latter technique was used to study the evolution of VP0 catalysts under the conditions of reaction using an In Situ cell built at the IRC (8). This communication deals with a contribution to the knowledge of the real nature of the vanadyl pyrophosphate catalyst and on the role of the V”’ species. Three different (VO)aP207 samples were prepared from the same VO(HPO& 0.5 Hz0 precursor by changing the temperature and the atmosphere of calcination. They were compared for the oxidation of n-butane to maleic anhydride. Catalytic differences are discussed in terms of the evolution of the physicochemis~ of the materials as studied by X-Ray diffraction and 31P NMR before and after the catalytic test. EXPERIMENTAL The VOHPO~),
0.5 l&O precursor was prepared in organic medium according to
the classical procedure as published by the EXXONgroup (9). Its identification was done by X-Ray analysis. Three different “(VO)2P207” catalysts were then prepared with this precursor by changing the gaseous atmosphere and the temperature of calcination. Conditions of preparation are summarized in Table 1. Catalyst 1 corresponds to a VP0 catalyst activated in the catalytic atmosphere (1.2% Butane/air). Catalyst 2 was obtained at the same temperature 1440°C) as for Catalyst 1, but in an Argon atmosphere (with less than 0.5 ppm 02). These conditions were chosen in order to avoid the formation of the eventual V5+ entities which were supposed to be formed for Catalyst 1. Catalyst 3 was prepared in the same atmosphere as for Catalyst 2 but at a higher temperature (880°C). Our purpose was to get the same material as in catalyst 2 but with a different grain-morphology and crystal faces repartition. The morphology of the catalyst grains was examined by Scanning Electron Microscopy using a S800 HITAsH1microscope. Their porosity was measured by N2 adsorption at 77.4 K on an adsorption-desorption apparatus built at the IRC.X-Ray diffraction patterns of the catalysts were recorded with a SIEMENS diffractometer Using Cu rC, radiation. The 31P NMR spectra were recorded with a BRUKERMSL-300 spectrometer operating at 121.4 MHz for phosphorus. They were obtained under
41
Table 1. Conditionsof preparationof three (VO)sPsO7catalystsstarting from the same VO(HOP04).0.5Hz0 precursor.
Catalyst
CondiEons Atmosphere
of calcinatfon
Temp.,'C!
T~JTE,~
SBET. m2/S
1
1.2% butane/air
440
25
22.5
2
Argon
440
25
7.7
3
Argon
880
10
2.0
MAS conditions by use of a double bearing probehead. A single pulse sequence was used in all cases and the delays were chosen in order to allow the obtention of quantitative spectra (typically the pulse width was 2 microseconds (lo’) and the delay was 10 to 100 seconds 1.The number of scans was 10 to 100. Spectra were refered towards external HsP04 (85%) and compared with pure reference phases (8). The oxidation of butane was performed on a test working under differential conditions with low amount of catalyst (350-600 mg) and a feedstock composition: QHlo/02/He = 1.2/ 16.4/82.4. Flow rate was 20 cm3/min. Maleic anhydride, CO and CO2 were the main detected products with acetic and acrylic acids, ethane, ethylene and C4 (1-butene. 2-butene cis and trans and butadiene) as traces. Carbon balance was very good (98%). Detection of reactants and products was performed on line using three chromatographs : a FID detector for analysis of oxygenates and hydrocarbons on Porapack Q (2m, 2OO”C), a FID detector for analysis of hydrocarbons on n-Octane/Porasil C (4m, 60°C). a TCD detector for analysis of CO, CO2 and 02 (4m, 100°C). Helium was the carrier gas. RESULTS X-Flay spectra are characteristic of (VO)sPsO7 for the three catalysts (Figure l)(lO). They show a difference of crystallinity between the three catalysts. Relative intensities of the main (VO)sPsO7 lines are presented in Table 2. The most striking feature is the variable relative intensity and profile of the (200) line. We will discuss this point later on. No crystalline VOPO4 phases are detected.
SEM examination of the three catalysts (FQure 2) shows a good similarity between Catalyst 1 and 2 both prepared at 44o’C, in the catalytic atmosphere for Catalyst 1 and under pure argon for Catalyst 2. Both catalysts look like the classical EXXON precursor with a rose-like aggregation of plates (9) with a high development of the basal ( 100) crystal plane. A difference of porosity is observed between both catalysts with a mesoporosity observed for Catalyst 1 (20-120 A) and a microporosity for Catalyst 2 (20 A) (Figure 3). This difference is consistent with the BET area measurements (22.5 m2/g for Catalyst 1 and 7.7 m2/g for Catalyst 2). In contrast, Catalyst 3 presents a more sintered aspect without any porosity, a low BET area (2.0 m2/g) and a different morphology with a higher development of the side crystal faces. The presence of Vs’ entities is detected by 31P NMR on Catalyst 1 and 2. but not
42 Table 2.
Relative intensities of the main (VO)sPsO7X-ray lines.
Catalyst
1200) 63.2
(024)
(0321
100.0
59.5
26.3
43.7
100.0
61.2
4.3
100.0
27.2
14.2
1
1020) 15.8
2 3
on Catalyst 3 (Figure 4 ). Taking into account our previous examination on pure VOP04 crystalline phases (8). it is diftkult. from the obtained spectra, to identify any particular VOPO4 structure in Catalysts 1 and 2. It can be thus inferred that the V5’ entities are highly dispersed on the &rO)2P207 matrix. with a higher dispersion on Catalyst 1 than on Catalyst 2. as evidenced by a better resolution in the second spectrum. The three catalysts were compared for butane oxidation in the 300-45O’C temperature range. For Catalyst 2, two successive series of measurements were performed by increasing the temperature after an intermediate cooling to room temperature. This procedure was followed in order to observe any eventual modification of the catalytic performances and of the surface composition for this catalyst between the two series of measurements under the catalytic atmosphere. For catalysts 1 and 3 no modifications of this type were observed. ZW
35
30
25
20
15
25
20
15
10
catalyst 2 0~4 ox .I‘
*
35
30
10 2w
Fig. 1.
X-Ray
spectra of the three (vO)sPsO7catalysts.as prepared.
Fig.
2 SEM examination of the three (VO)zP207 catalysts. as prepared.
Figure 5 compares the specific activity of Catalysts 1 and 2 (mol.g-’ .s-l) for butane oxidation ( lSt and Znd temperature increase). The lower activity observed for Catalyst 2 (1st ) is changed to an activity comparable to that of Catalyst 1 for Catalyst 2 (Znd), suggesting a modification of the characteristics of Catalyst 2 between the two series
44 -21i4
dVp/dRx 0.1
catalyst
10
20
50
120
3
R&
Porosity diagrams of the three (VO)zPz07 catalysts, as prepared.
Fig. 3
60
40
20
0
-20
-40
-60
-80 -100
ppm
Fig. 4 31P NMR of the three (VO)zPz07 catalysts, as prepared.
of measurements. This is confirmed in Figure 6 which shows selectivities for the transformation of butane to maleic anhydride, CO and COs for Catalysts 1 and 2, respectively: it is noteworthy that, during the second tests under the butane-air atmosphere, CataIyst 2 displays the same selectivities as for CataIyst 1. It can be inferred from this evolution of the catalytic results, that Catalyst 2 changes its R (EIl0l.g’l.S’lf x 109
1 500 -
300
350
400
450
T(“C)
Fig. 5 Comparison of specific activity for butane oxidation between Catalyst 1 and
Catalyst 2 (m=350 mg).
45
loo- S(%)
loo-S(%)
SOMA
60.
40.
2o -
40.
/,5
c2+c2= 300
350
20-
::2
400
450
Catalyst 1
,
500 ‘U”C)
300
350
400
Catalyst 2
450
500
T(“C)
Fig. 6 Comparison of selectivities spectra between Catalyst 1 and Catalyst 2 (m=350 mgl (see Fig. 5). surface structure or surface composition and approaches that of Catalyst 1. Note also that, for both cataIysts, selectivity to ethane and ethylene is very low. Figure 7 compares the evolution of the X-Ray spectra in the 20-31” (20) domain of Catalyst 2, Catalyst 2( lst) and Catalyst 2(2”d) . The theoretical position of all the VP0 phases is given (8): (VO)zP2O7 : (200) at 23.00”. (201) at 23.61’, (211) at 25.43’. (212) at 26.99”. (024) at 28.45”, (2 13) at 29.64, (032) at 29.94” and (115) at 30.70’. cqI : (101) at 25.00”. (111) at 29.14’ and (200) at 29.75”. )3: (111) at 22.40”. (200) at 22.80’. (002) at 25.54’. (201) at 26.17”. (102) at 28.07’, (020) at 29.09”, (211) at 30.02”. y : (004) at 2 1.37’, (221) at 22.65”, (040) at 23.16”. (230) at 25.40”. (213) at 25.89’, (223) at 27.69’, (105) at 28.80”, (311) at 29.16” (312) at 30.74”. 6 : (111) at 22.08”. (012) at 24.16”. (020) at 28.55, (021) at 30.26’. This shows how it will be difficult to detect the presence of low concentrations of VOP04 phases against the (VO)sPsO7 background. It appears on Figure 7. that the position of the lines at 23.0”, 28.45” and 29.94’ corresponding to the main (VO)sPsO7 lines (200) (024) and (032) are displaced towards the low angles for the three spectra which should correspond to an increase of the d spacing of the (VO)sPsO7 structure. It can also be observed that the relative intensities for the same angular positions are maintained in the three spectra. In fact, the main feature is the evolution of the profile of the line at around 23”. the width of which diminishes from the spectrum of Catalyst 2 to that of Catalyst 2( lst) and Catalyst 2(2nd). It is diilicult to give a clear expkuration of this evolution but we consider that it could be the result of an improvement in the organization of the (VO)3PsO7 structure (note the progressive
46
Catalyst
Catalyst
Catalyst 31
30
29
28
27
26
25
24
23
22
21
#
B(")
Modification of the X-ray spectra for Catalyst 2. ii) in the 20-3 1’ (2e) for Catalyst 2 after two temperature increase, iii) under reactional conditions, as compared to the VP0 phases indexation. Fig. 7
disappearance of (201) (VO)aPzO7 line) which could be associated with a modification of the dispersion of the V5+ entities. The fact that only the profile of the (200) (VO)aPaO7 line is perturbed could explain that, in this case, the Vs’ species should principally affect the corresponding (100) basal plane of (VO)sPsOr. This conclusion is supported by the evolution of the 31P NMR spectra for the corresponding catalysts ( Figure 8). There is a complete redistribution of the V5+ entities from Catalyst 2 to Catalyst 2(lst) with an increase of the size of the V5+ domains from Catalyst 2 to Catalyst 2( lst) (see the increase of the signal/noise ratio I and with a subsequent diminution of the V5+/@ local interaction. For Catalyst 2(2nd). there is a redispersion of the V5+ entities and an increase of the Vs+@+ interaction so that after the 2nd treatment it approaches the same interaction as for Catalyst 2 but with a different environment. (31P NMR spectrum of Catalyst 2 (2nd) is different from that of Catalyst 2). The result of this evolution should be that
Go 160 so
i
* -is0 . -50 - -100
ppm
-9.4
I
-48.7 23.9II ,
300 200 100
0
-100 -200 -3wJ -400
ppm
Hg. 8 Modifkation of the ‘I P NMR spectra for Catalyst 2 aftter two temperature increase under reactional conditions. Note: * rotation bands. the number of the Vs’ sites interacting locally with p sites should change from one catalyst to the other and that there should be a specific ratio for the best catalyst corresponding to small V5+ domains in weak interaction with fVO)aP207 . Catalyst 3 presents a very low specific activity (moI.g*‘.s-‘) for butane oxidation as compared to Catalyst 1 ( Figure 9 ). The distribution of the products formed is quite different (Figure 10): the selectivity to MA is lower, olefmic C4 products (1-butene, 2-butene cis, trans and butadiene) are observed at low temperature, in a temperature domain where CO2 is not detected. No modifications of the X-Ray diffraction spectrum and of the 3‘P NMR spectrum (see Figures 1 and 3) are observed for Catalyst 3 after the catalytic test and we can conclude, in contrast to Catalyst 2, that no vs’ entities have been generated on this catalyst by the reaction. As a consequence. the specific distribution of the reaction products observed for Catalyst 3 is due to the particular structure of this (VO)2P207 catalyst, the presence of only v4’ sites without any Vs+. Recall that, in this case, the (VO)aPaO7 crystals present a more smtered aspect with a comparatively lower development of the basal (100) face and a higher development of the side faces in the d direction.
48
Comparison of specific activity for butane oxidation between Catalyst 1 and Catalyst 3 (m=600 mg).
Fig. 9.
CONCLUSIONS
Comparison of the three “lVO)zP~07” catalysts gives important info~a~ons on the active sites of the VP0 catalysts for n-butane oxidation to maleic anhydride. If (VO)2P2O7 is the principal phase for this reaction, the present study confirms the participation of V5’ entities detected by 3‘P NMR. Superficial @+/V5+ distribution is determined by the atmosphere in which the catalysts is prepared. This was evidenced both by the catalytic and the physico~hemic~ evolution of CataIyst 2 from the argon atmosphere of calcination of the precursor to the atmosphere of catalysis. Best catalytic results correspond to a limited number of V5’ sites forming small domains with a strong interaction with the (VO)zPaO7 matrix. From the evolution of the X- Ray diffraction spectra of Catalyst 2. it can be postulated that these domains affect principally the basal (100) crystal face. We previously observed that the ti+/V5+ distribution depended also on the morphology of the precursor which could be determined by the morphology of the starting V2O5 (5). The oxidation of butane to maleic anhydride appears as another example of Structure Sensitive Reaction (11). The oxidation should occur on the basal (100) face as it was previously proposed (2). but with a participation of a suitable number of V5* entities. The local superikial V5’/p distribution in this face shouId control the cataIytic results. Side faces of (VO)2PzO7 located in the [loo] direction don’t appear to possess the local structure which should orient the reaction towards the final formation of maleic anhydride. It appears that there is some difficulty of local reoxidation to V5’ sites in these faces. In this case, reaction stops at the level of olefinic C4 as it was demon&r&ted from the study on Catalyst 3.
100 S(%) 80 -
80-
MA
1
60-
60. \
40-
= 40-
MA *-‘-\
I \ *
c4=
CO2
350
400
450
500
T(“C) Catalyst 1
300
350
400
450
T(“C) Catalyst 3
pig. 10. Comparison of selectivities spectra between Catalyst 1 and Catalyst 3 (m=600 mg) [see Fig. 9). ACKNOWLEDGMENTS Authors
are indebted
interpretation
to Dr. F. Lefebvre for 31P NTvlR experiments
of the corresponding
and for the
results. They thank Dr. J.C. Vedrine for fruitful1
discussions. REFERENCES 1. Centi, G., Triiho, F., Ebner, J.R. and Franchetti, V., Chew Rev., 88. 55. (1988). 2. Ziolkowski. J., Bordes, E. and Cow-tine, P., J. CataL. 122. 126. (1990). 3. Morishige. H., Tamaki, J., Miura. N. andyamazoe. N.. Chem Lett.. 1513, (1990). 4. Harrouch. Batis, N., Batis. H., Ghorbel, A., Vedrine. J.C. and Volta, J.C.. J. CataL 128, 248, (1991). 5. Guilhaume, N., Roullet, M., Pajonk. G. and Volta, J.C.. Proc. of the IIIrd European Oxidation Congress in Heterogeneous Catalysis, Louvain-la-Neuve, 199 1. 6. Bergeret. G., Broyer, J.P., M. Gallezot. P.. Hecquet, G. and Volta, J.C., J.C.S.. Chem Comma. 825, (1986). 7. David, M., Lefebvre. F. and Volta, J.C., 1 lth Iberoamericano Symposium on Catalysis. Guanajuato. 1988, IMP Edit, p.365. 8. Ben Abdelouahab. F., Olier, R., Guilhaume, N., Lefebvre. F. and Volta, J.C., ._I.CataL 134. 151 (1992). 9. Johnson, J.W., Johnston, D.C., Jacobson, A.J. and Brody. J.F.. J. Amer. Chem Sot., 106, 8123, (1984). 10. Linde, S.A.. Gorbunova, Yu. E.. Lavrov, A.V. and Kuznetsov. V.G., BokL Akao!. Nauk, SSSR, 245, 584, (1979). 11. Volta, J.C., Portefaix, J.L., AppL CataL, 18, 1, (1985).
39
On the Nature of the Active Sites of the VP0 Catalysts for n-Butane
Oxidation
to Maleic Anhydride
Y. Zhang, R.P.A. Sneeden. and J.C. Volta Institut de Recherches sur la Catalyse, CNRS, 2 Avenue A. Einstein, 69626, Villeurbanne Cedex, France
Three fVO)2P2O7 catalys$t differing by their cyftal morphology and the P NMR are compared for superficial presence of V entities detected by butane oxidation to maleic anhydride. The study shows that Butane Oxidation is a Structure Sensitive Reaction. Formation of Maleic Anhydride occurs on the basal (100) (VO)2P2O7 face with the participation of Vs’ species. Best catalytic results correspond to a suitable V5+/p+ ratio in this face. Lateral (VOJ2P2O7 faces located in the [ 1001 direction cannot reoxidize locally @’ into V + sites so that reaction is stopped at the step of olefinic C4.
INTRODUCTION
Considering the scientific literature, it appears that there are many discrepancies concerning the nature of the active surface of VP0 catalysts for the mild oxidation of n-Butane to Maleic Anhydride. Most proposals are based on conclusions which have been drawn from connexions established between the evolution of catalytic results and physicochemical data obtained after the catalytic test (1). Another feature of this system is the high heterogeneity in the composition of the catalytic mass which is always observed between the different pellets of the catalytic bed of the integral reactor, at the level of one single pellet or through the powder of the catalytic bed of a differential reactor. As a consequence, the nature of the active phase and the identification of the active sites are subject of much discussion. The different groups working in this field agree on the fact that vanadium pyrophosphate fVO)sPaO7 (v4’) is the essential phase for this system. Indeed, the average oxidation state of vanadium (4.1) measured on the best working catalysts after the catalytic test and their characterization by XRD. are consistent with the main participation of pyrophosphate (oxidation state : 4.0)(l). A model for the oxidation of butane was even proposed on the (100) face of this phase (2). However, in agreement with the 4.1 oxidation state, the participation, to some extent, ofVOP04 (v5’) - like structures in some steps of the reaction pathway has to be considered. The difficulty in identifying the active sites stems from the fact that VP0 catalysts are composed of both well crystallized and amorphous phases whose participation to the mechanism of butane oxidation can be discussed. A recent publication brought the proof of the catalytic importance of the amorphous phase (3). The relative proportion of these different phases appears to depend strongly on the 0920-5861/93/$6.00
0
1993 Elsevier Science Publishers B.V. All rights reserved.
preparation and on the conditions of activation of the catalysts (1). We have recently developped a new method of preparation of VP0 catalysts: it is possible to control the proportions of the different VP0 phases, their repartition and thus to control the catalytic properties during the preparation of the VO(HPO& 0.5 II20 precursor by introducing a Vs’ salt (4). Some improvement can also be obtained by changing the initial HsPOa/VzOs ratio (initial P/V ratio), by introducing a doping agent (l), or even by changing the V2O5 morphology (5). All these informations let US to think that the catalytic properties of VP0 catalysts for butane oxidation to maleic anhydride strongly depended on the Vr+/Vs+ dispersion and that it was important to control better their local structure. As a consequence, we developed, in our laboratory, physicochemical techniques which analyze both the short and the long range order of these materials like RED ofX-Rays (6), 31P and % NMR (4,7,8) and l&man Spectroscopy (8). This latter technique was used to study the evolution of VP0 catalysts under the conditions of reaction using an In Situ cell built at the IRC (8). This communication deals with a contribution to the knowledge of the real nature of the vanadyl pyrophosphate catalyst and on the role of the V”’ species. Three different (VO)aP207 samples were prepared from the same VO(HPO& 0.5 Hz0 precursor by changing the temperature and the atmosphere of calcination. They were compared for the oxidation of n-butane to maleic anhydride. Catalytic differences are discussed in terms of the evolution of the physicochemis~ of the materials as studied by X-Ray diffraction and 31P NMR before and after the catalytic test. EXPERIMENTAL The VOHPO~),
0.5 l&O precursor was prepared in organic medium according to
the classical procedure as published by the EXXONgroup (9). Its identification was done by X-Ray analysis. Three different “(VO)2P207” catalysts were then prepared with this precursor by changing the gaseous atmosphere and the temperature of calcination. Conditions of preparation are summarized in Table 1. Catalyst 1 corresponds to a VP0 catalyst activated in the catalytic atmosphere (1.2% Butane/air). Catalyst 2 was obtained at the same temperature 1440°C) as for Catalyst 1, but in an Argon atmosphere (with less than 0.5 ppm 02). These conditions were chosen in order to avoid the formation of the eventual V5+ entities which were supposed to be formed for Catalyst 1. Catalyst 3 was prepared in the same atmosphere as for Catalyst 2 but at a higher temperature (880°C). Our purpose was to get the same material as in catalyst 2 but with a different grain-morphology and crystal faces repartition. The morphology of the catalyst grains was examined by Scanning Electron Microscopy using a S800 HITAsH1microscope. Their porosity was measured by N2 adsorption at 77.4 K on an adsorption-desorption apparatus built at the IRC.X-Ray diffraction patterns of the catalysts were recorded with a SIEMENS diffractometer Using Cu rC, radiation. The 31P NMR spectra were recorded with a BRUKERMSL-300 spectrometer operating at 121.4 MHz for phosphorus. They were obtained under
41
Table 1. Conditionsof preparationof three (VO)sPsO7catalystsstarting from the same VO(HOP04).0.5Hz0 precursor.
Catalyst
CondiEons Atmosphere
of calcinatfon
Temp.,'C!
T~JTE,~
SBET. m2/S
1
1.2% butane/air
440
25
22.5
2
Argon
440
25
7.7
3
Argon
880
10
2.0
MAS conditions by use of a double bearing probehead. A single pulse sequence was used in all cases and the delays were chosen in order to allow the obtention of quantitative spectra (typically the pulse width was 2 microseconds (lo’) and the delay was 10 to 100 seconds 1.The number of scans was 10 to 100. Spectra were refered towards external HsP04 (85%) and compared with pure reference phases (8). The oxidation of butane was performed on a test working under differential conditions with low amount of catalyst (350-600 mg) and a feedstock composition: QHlo/02/He = 1.2/ 16.4/82.4. Flow rate was 20 cm3/min. Maleic anhydride, CO and CO2 were the main detected products with acetic and acrylic acids, ethane, ethylene and C4 (1-butene. 2-butene cis and trans and butadiene) as traces. Carbon balance was very good (98%). Detection of reactants and products was performed on line using three chromatographs : a FID detector for analysis of oxygenates and hydrocarbons on Porapack Q (2m, 2OO”C), a FID detector for analysis of hydrocarbons on n-Octane/Porasil C (4m, 60°C). a TCD detector for analysis of CO, CO2 and 02 (4m, 100°C). Helium was the carrier gas. RESULTS X-Flay spectra are characteristic of (VO)sPsO7 for the three catalysts (Figure l)(lO). They show a difference of crystallinity between the three catalysts. Relative intensities of the main (VO)sPsO7 lines are presented in Table 2. The most striking feature is the variable relative intensity and profile of the (200) line. We will discuss this point later on. No crystalline VOPO4 phases are detected.
SEM examination of the three catalysts (FQure 2) shows a good similarity between Catalyst 1 and 2 both prepared at 44o’C, in the catalytic atmosphere for Catalyst 1 and under pure argon for Catalyst 2. Both catalysts look like the classical EXXON precursor with a rose-like aggregation of plates (9) with a high development of the basal ( 100) crystal plane. A difference of porosity is observed between both catalysts with a mesoporosity observed for Catalyst 1 (20-120 A) and a microporosity for Catalyst 2 (20 A) (Figure 3). This difference is consistent with the BET area measurements (22.5 m2/g for Catalyst 1 and 7.7 m2/g for Catalyst 2). In contrast, Catalyst 3 presents a more sintered aspect without any porosity, a low BET area (2.0 m2/g) and a different morphology with a higher development of the side crystal faces. The presence of Vs’ entities is detected by 31P NMR on Catalyst 1 and 2. but not
42 Table 2.
Relative intensities of the main (VO)sPsO7X-ray lines.
Catalyst
1200) 63.2
(024)
(0321
100.0
59.5
26.3
43.7
100.0
61.2
4.3
100.0
27.2
14.2
1
1020) 15.8
2 3
on Catalyst 3 (Figure 4 ). Taking into account our previous examination on pure VOP04 crystalline phases (8). it is diftkult. from the obtained spectra, to identify any particular VOPO4 structure in Catalysts 1 and 2. It can be thus inferred that the V5’ entities are highly dispersed on the &rO)2P207 matrix. with a higher dispersion on Catalyst 1 than on Catalyst 2. as evidenced by a better resolution in the second spectrum. The three catalysts were compared for butane oxidation in the 300-45O’C temperature range. For Catalyst 2, two successive series of measurements were performed by increasing the temperature after an intermediate cooling to room temperature. This procedure was followed in order to observe any eventual modification of the catalytic performances and of the surface composition for this catalyst between the two series of measurements under the catalytic atmosphere. For catalysts 1 and 3 no modifications of this type were observed. ZW
35
30
25
20
15
25
20
15
10
catalyst 2 0~4 ox .I‘
*
35
30
10 2w
Fig. 1.
X-Ray
spectra of the three (vO)sPsO7catalysts.as prepared.
Fig.
2 SEM examination of the three (VO)zP207 catalysts. as prepared.
Figure 5 compares the specific activity of Catalysts 1 and 2 (mol.g-’ .s-l) for butane oxidation ( lSt and Znd temperature increase). The lower activity observed for Catalyst 2 (1st ) is changed to an activity comparable to that of Catalyst 1 for Catalyst 2 (Znd), suggesting a modification of the characteristics of Catalyst 2 between the two series
44 -21i4
dVp/dRx 0.1
catalyst
10
20
50
120
3
R&
Porosity diagrams of the three (VO)zPz07 catalysts, as prepared.
Fig. 3
60
40
20
0
-20
-40
-60
-80 -100
ppm
Fig. 4 31P NMR of the three (VO)zPz07 catalysts, as prepared.
of measurements. This is confirmed in Figure 6 which shows selectivities for the transformation of butane to maleic anhydride, CO and COs for Catalysts 1 and 2, respectively: it is noteworthy that, during the second tests under the butane-air atmosphere, CataIyst 2 displays the same selectivities as for CataIyst 1. It can be inferred from this evolution of the catalytic results, that Catalyst 2 changes its R (EIl0l.g’l.S’lf x 109
1 500 -
300
350
400
450
T(“C)
Fig. 5 Comparison of specific activity for butane oxidation between Catalyst 1 and
Catalyst 2 (m=350 mg).
45
loo- S(%)
loo-S(%)
SOMA
60.
40.
2o -
40.
/,5
c2+c2= 300
350
20-
::2
400
450
Catalyst 1
,
500 ‘U”C)
300
350
400
Catalyst 2
450
500
T(“C)
Fig. 6 Comparison of selectivities spectra between Catalyst 1 and Catalyst 2 (m=350 mgl (see Fig. 5). surface structure or surface composition and approaches that of Catalyst 1. Note also that, for both cataIysts, selectivity to ethane and ethylene is very low. Figure 7 compares the evolution of the X-Ray spectra in the 20-31” (20) domain of Catalyst 2, Catalyst 2( lst) and Catalyst 2(2”d) . The theoretical position of all the VP0 phases is given (8): (VO)zP2O7 : (200) at 23.00”. (201) at 23.61’, (211) at 25.43’. (212) at 26.99”. (024) at 28.45”, (2 13) at 29.64, (032) at 29.94” and (115) at 30.70’. cqI : (101) at 25.00”. (111) at 29.14’ and (200) at 29.75”. )3: (111) at 22.40”. (200) at 22.80’. (002) at 25.54’. (201) at 26.17”. (102) at 28.07’, (020) at 29.09”, (211) at 30.02”. y : (004) at 2 1.37’, (221) at 22.65”, (040) at 23.16”. (230) at 25.40”. (213) at 25.89’, (223) at 27.69’, (105) at 28.80”, (311) at 29.16” (312) at 30.74”. 6 : (111) at 22.08”. (012) at 24.16”. (020) at 28.55, (021) at 30.26’. This shows how it will be difficult to detect the presence of low concentrations of VOP04 phases against the (VO)sPsO7 background. It appears on Figure 7. that the position of the lines at 23.0”, 28.45” and 29.94’ corresponding to the main (VO)sPsO7 lines (200) (024) and (032) are displaced towards the low angles for the three spectra which should correspond to an increase of the d spacing of the (VO)sPsO7 structure. It can also be observed that the relative intensities for the same angular positions are maintained in the three spectra. In fact, the main feature is the evolution of the profile of the line at around 23”. the width of which diminishes from the spectrum of Catalyst 2 to that of Catalyst 2( lst) and Catalyst 2(2nd). It is diilicult to give a clear expkuration of this evolution but we consider that it could be the result of an improvement in the organization of the (VO)3PsO7 structure (note the progressive
46
Catalyst
Catalyst
Catalyst 31
30
29
28
27
26
25
24
23
22
21
#
B(")
Modification of the X-ray spectra for Catalyst 2. ii) in the 20-3 1’ (2e) for Catalyst 2 after two temperature increase, iii) under reactional conditions, as compared to the VP0 phases indexation. Fig. 7
disappearance of (201) (VO)aPzO7 line) which could be associated with a modification of the dispersion of the V5+ entities. The fact that only the profile of the (200) (VO)aPaO7 line is perturbed could explain that, in this case, the Vs’ species should principally affect the corresponding (100) basal plane of (VO)sPsOr. This conclusion is supported by the evolution of the 31P NMR spectra for the corresponding catalysts ( Figure 8). There is a complete redistribution of the V5+ entities from Catalyst 2 to Catalyst 2(lst) with an increase of the size of the V5+ domains from Catalyst 2 to Catalyst 2( lst) (see the increase of the signal/noise ratio I and with a subsequent diminution of the V5+/@ local interaction. For Catalyst 2(2nd). there is a redispersion of the V5+ entities and an increase of the Vs+@+ interaction so that after the 2nd treatment it approaches the same interaction as for Catalyst 2 but with a different environment. (31P NMR spectrum of Catalyst 2 (2nd) is different from that of Catalyst 2). The result of this evolution should be that
Go 160 so
i
* -is0 . -50 - -100
ppm
-9.4
I
-48.7 23.9II ,
300 200 100
0
-100 -200 -3wJ -400
ppm
Hg. 8 Modifkation of the ‘I P NMR spectra for Catalyst 2 aftter two temperature increase under reactional conditions. Note: * rotation bands. the number of the Vs’ sites interacting locally with p sites should change from one catalyst to the other and that there should be a specific ratio for the best catalyst corresponding to small V5+ domains in weak interaction with fVO)aP207 . Catalyst 3 presents a very low specific activity (moI.g*‘.s-‘) for butane oxidation as compared to Catalyst 1 ( Figure 9 ). The distribution of the products formed is quite different (Figure 10): the selectivity to MA is lower, olefmic C4 products (1-butene, 2-butene cis, trans and butadiene) are observed at low temperature, in a temperature domain where CO2 is not detected. No modifications of the X-Ray diffraction spectrum and of the 3‘P NMR spectrum (see Figures 1 and 3) are observed for Catalyst 3 after the catalytic test and we can conclude, in contrast to Catalyst 2, that no vs’ entities have been generated on this catalyst by the reaction. As a consequence. the specific distribution of the reaction products observed for Catalyst 3 is due to the particular structure of this (VO)2P207 catalyst, the presence of only v4’ sites without any Vs+. Recall that, in this case, the (VO)aPaO7 crystals present a more smtered aspect with a comparatively lower development of the basal (100) face and a higher development of the side faces in the d direction.
48
Comparison of specific activity for butane oxidation between Catalyst 1 and Catalyst 3 (m=600 mg).
Fig. 9.
CONCLUSIONS
Comparison of the three “lVO)zP~07” catalysts gives important info~a~ons on the active sites of the VP0 catalysts for n-butane oxidation to maleic anhydride. If (VO)2P2O7 is the principal phase for this reaction, the present study confirms the participation of V5’ entities detected by 3‘P NMR. Superficial @+/V5+ distribution is determined by the atmosphere in which the catalysts is prepared. This was evidenced both by the catalytic and the physico~hemic~ evolution of CataIyst 2 from the argon atmosphere of calcination of the precursor to the atmosphere of catalysis. Best catalytic results correspond to a limited number of V5’ sites forming small domains with a strong interaction with the (VO)zPaO7 matrix. From the evolution of the X- Ray diffraction spectra of Catalyst 2. it can be postulated that these domains affect principally the basal (100) crystal face. We previously observed that the ti+/V5+ distribution depended also on the morphology of the precursor which could be determined by the morphology of the starting V2O5 (5). The oxidation of butane to maleic anhydride appears as another example of Structure Sensitive Reaction (11). The oxidation should occur on the basal (100) face as it was previously proposed (2). but with a participation of a suitable number of V5* entities. The local superikial V5’/p distribution in this face shouId control the cataIytic results. Side faces of (VO)2PzO7 located in the [loo] direction don’t appear to possess the local structure which should orient the reaction towards the final formation of maleic anhydride. It appears that there is some difficulty of local reoxidation to V5’ sites in these faces. In this case, reaction stops at the level of olefinic C4 as it was demon&r&ted from the study on Catalyst 3.
100 S(%) 80 -
80-
MA
1
60-
60. \
40-
= 40-
MA *-‘-\
I \ *
c4=
CO2
350
400
450
500
T(“C) Catalyst 1
300
350
400
450
T(“C) Catalyst 3
pig. 10. Comparison of selectivities spectra between Catalyst 1 and Catalyst 3 (m=600 mg) [see Fig. 9). ACKNOWLEDGMENTS Authors
are indebted
interpretation
to Dr. F. Lefebvre for 31P NTvlR experiments
of the corresponding
and for the
results. They thank Dr. J.C. Vedrine for fruitful1
discussions. REFERENCES 1. Centi, G., Triiho, F., Ebner, J.R. and Franchetti, V., Chew Rev., 88. 55. (1988). 2. Ziolkowski. J., Bordes, E. and Cow-tine, P., J. CataL. 122. 126. (1990). 3. Morishige. H., Tamaki, J., Miura. N. andyamazoe. N.. Chem Lett.. 1513, (1990). 4. Harrouch. Batis, N., Batis. H., Ghorbel, A., Vedrine. J.C. and Volta, J.C.. J. CataL 128, 248, (1991). 5. Guilhaume, N., Roullet, M., Pajonk. G. and Volta, J.C.. Proc. of the IIIrd European Oxidation Congress in Heterogeneous Catalysis, Louvain-la-Neuve, 199 1. 6. Bergeret. G., Broyer, J.P., M. Gallezot. P.. Hecquet, G. and Volta, J.C., J.C.S.. Chem Comma. 825, (1986). 7. David, M., Lefebvre. F. and Volta, J.C., 1 lth Iberoamericano Symposium on Catalysis. Guanajuato. 1988, IMP Edit, p.365. 8. Ben Abdelouahab. F., Olier, R., Guilhaume, N., Lefebvre. F. and Volta, J.C., ._I.CataL 134. 151 (1992). 9. Johnson, J.W., Johnston, D.C., Jacobson, A.J. and Brody. J.F.. J. Amer. Chem Sot., 106, 8123, (1984). 10. Linde, S.A.. Gorbunova, Yu. E.. Lavrov, A.V. and Kuznetsov. V.G., BokL Akao!. Nauk, SSSR, 245, 584, (1979). 11. Volta, J.C., Portefaix, J.L., AppL CataL, 18, 1, (1985).