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Vanadium aluminium oxynitride catalysts for propane ammoxidation reaction
Applied Catalysis A: General 286 (2005) 1–10 www.elsevier.com/locate/apcata
Vanadium aluminium oxynitride catalysts for propane ammoxidation reaction Effect of the V/Al ratio on the structure and catalytic behaviour Mihaela Florea a,b,1,*, Ricardo Prada Silvy b,1, Paul Grange b,1,ä a
University of Bucharest, Faculty of Chemistry, B-dul Regina Elisabeta 4-12, 030016 Bucharest, Romania b Universite´ Catholique de Louvain, Unite´ de Catalyse et Chimie des Mate´riaux Divise´s, Croix du Sud 2, Boite 17, 1348 Louvain-la-Neuve, Belgium Received 29 September 2004; received in revised form 15 February 2005; accepted 15 February 2005 Available online 5 April 2005
Abstract The influence of the V/Al ratio composition on the physico-chemical and catalytic properties of the vanadium aluminium oxynitride system was investigated. The samples were prepared by co-precipitation of vanadium and aluminium solutions containing different metal compositions (0.1–0.9 V/Al) at pH 5.5 and characterized by XRD, XPS, Raman and BET surface area. Catalytic activity measurements for the propane ammoxidation reaction were carried out under optimal acrylonitrile selectivity conditions. X-ray diffraction pattern indicated that all the catalysts in both oxide precursor and nitride state show amorphous character. BET surface area was higher for the sample with V/Al ratio of 0.25, before and after nitridation treatment. This sample showed optimal catalytic performances, with 50% acrylonitrile selectivity and 60% propane conversion. The optimal nitridation degree, which induces an optimal reduction degree of vanadium, would explain the maximal catalytic activity observed for the sample prepared using the V/Al ratio composition of 0.25. # 2005 Elsevier B.V. All rights reserved. Keywords: Vanadium; Ammoxidation; Acrylonitrile
1. Introduction Among the most significant examples of industrial application in heterogeneous oxidation catalysis is the production of acrylonitrile (ACN) through the propane ammoxidation process. The advantages for replacing olefins by alkanes in the current ammoxidation process are essentially the lower price of propane with respect to propylene and the risk of propylene shortage due to its increasing consumption and the increasing worldwide demand of nitriles and other derived products. Mixed metal-oxides based on VMoMeOx [1,2] and VSbMeOx [3,4] * Corresponding author. Tel.: +40 21 4103178; fax: +40 21 3159249. E-mail address: [email protected] (M. Florea). 1 Fax: +32 10 473649. ä Deceased. 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.02.032
are typical propane ammoxidation catalysts. Recently, Mo– V–Te–Nb mixed oxides have been proposed as the most active and selective catalysts in the ammoxidation of propane to acrylonitrile, giving acrylonitrile yields up to 62% [5,6]. The active components of these catalysts have been shown to be two phases called M1 and M2. The respective roles of the two phases are not yet fully understood but it has been reported that their concomitant presence is needed to obtain effective catalysts. It has recently been shown that these phases were presenting orthorhombic and hexagonal type structures, respectively and both contained the four metallic elements [6]. However, this catalytic system has not yet provided a competitive advantage in ACN productivity versus conventional propylene ammoxidation catalysts, which explains why the propane ammoxidation process has not yet been commercially scaled up.
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We have been investigating new catalytic materials for alkane activation reaction. Vanadium aluminium oxynitride (VAlON) catalysts show excellent properties for propane ammoxidation and we have shown that the catalytic properties strongly depend on the nitridation on the reaction conditions [7] and on the preparation method [8]. The results also indicated that the VAlON catalytic system has a high ACN productivity at lower propane space–time relative to VMoMeOx and VSbMeOx systems [9]. The dispersion of the vanadium site at the surface has been recognized as the key factor required for achieving high activity and, most important, high acrylonitrile selectivity for vanadium-based catalysts [10,11]. In this study, we investigate the influence of the composition of the vanadium aluminium oxynitride, namely the V/Al ratio, on the physico-chemical and catalytic properties in propane ammoxidation.
2. Experimental Vanadium aluminium oxynitride catalyst with different V/Al atomic ratios were prepared by thermal nitridation of a co-precipitated V–Al oxide precursor. The preparation of the oxide precursor is given elsewhere [8]. The nitridation of the samples was carried out in a tubular rotating reactor under a flow of pure ammonia (30 l h1) at 500 8C for 5 h. The system was subsequently cooled down to room temperature under a flow of pure nitrogen. The principle of the chemical analysis of total nitrogen content is based on the reaction of the nitrogen species from the catalyst with a strong base (KOH) at 450 8C and the formation of ammonia which is then titrated with a standard solution of sulphuric acid 102N (Grekov method). The superficial nitrogen species were quantified by the Kjeldahl method. The alkaline attack was realized using a KOH saturated solution at 100 8C. The titrated ammonia corresponds to the superficial NHx (NH and NH2) species. Bulk nitrogen does not react under these conditions. The BET surface area corresponding to oxynitride catalysts before and after reaction was evaluated using a Micromeritics Flow Sorb II 2300 apparatus. Raman spectroscopy was performed with a DILORJOBIN YVON-SPEX spectrometer, model OLYMPUS DX40, equipped with a He–Ne (l = 632.8 nm) laser. The spectra were recorded in the 200–1600 cm1 range. XRD lines were recorded using a Siemens D-5000 powder diffractometer equipped with a Ni-filtered Cu Ka ˚ ). radiation (l = 1.5418 A XPS spectra were collected with an SSX-100, model 206 Surface Science Instrument spectrometer at room temperature and under a vacuum of 1.33 mPa. Monochromatized Al Ka radiation (hn = 1486.6 eV), obtained by bombarding the Al anode with an electron gun operating at a beam current of 12 mA and an accelerating voltage of 10 kV was used. The charge correction was made considering that the C 1s signal
of contaminating carbon (C–C or C–H bonds) was centered at 284.8 eV. The C 1s, V 2p3, Al 2p, N 1s and O 1s levels were chosen for characterization, since their signals are the most intense and do not overlap. Catalytic tests were performed in a fixed bed quartz micro-reactor at atmospheric pressure and temperature of 500 8C, 0.1 g of catalyst and W/F = 8 g h/mol of C3H8. Feed composition was 1.25:3:1 of C3H8:O2:NH3. The activity results are reported after 24 h on stream. Feed and products were analysed on-line using a gas chromatograph, equipped with FID and TCD detectors and an on-line mass spectrometer was used to check the NOx formation. A carbon mass balance of 80–90% was obtained, because of the condensation of the reaction products inside of the connection line between the reactor and the gas chromatograph. To avoid the condensation of acrylonitrile, the connection line was heated at 150 8C. Propane conversion is defined as the % ratio between the mole of propane consumed per mole of propane in the feed; the ACN selectivity as mole of ACN in the product per mole of propane consumed; and the ACN yield as mole of ACN in the product per mole of propane in the feed.
3. Results 3.1. Chemical composition, textural and structural characteristics Samples with V/Al atomic ratios in the range of 0.1–0.9 were prepared by the co-precipitation method followed by nitridation in the presence of ammonia at 500 8C for 5 h. All the samples are X-ray amorphous before and after the catalytic test. Table 1 compiles the theoretical and experimental V/Al ratios and the values of the surface areas of the oxynitride powders before and after propane ammoxidation. The V/Al molar ratios in the obtained catalysts are lower than the theoretical values for the ratios higher than 0.7. The oxynitrides exhibit a specific area between 125 and 160 m2/g and depend on the V/Al ratio, as seen in Table 1. The surface area increases with the decreasing V/Al ratio, with highest surface area of 160 m2/g being observed for VAlON0.1 sample. The tested catalysts exhibit surface areas 15–25% lower than those of the corresponding fresh Table 1 V/Al ratios and surface areas of VAlON catalysts Catalyst
V/Al (theoretical)
V/Al (experimental)
Surface area before test (m2/g)
Surface area after test (m2/g)
VAlON0.1 VAlON0.25 VAlON0.5 VAlON0.7 VAlON0.9
0.10 0.25 0.50 0.70 0.90
0.10 0.25 0.50 0.66 0.82
160 153 130 122 143
117 133 78 93 124
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10 Table 2 Nitrogen content before and after catalytic test as a function of the V/Al ratio Catalyst
NT (%)a Before test
VAlON0.1 VAlON0.25 VAlON0.5 VAlON0.7 VAlON0.9 a b c
4.93 5.35 4.76 3.28 2.26
NK (%)b After test 2.30 5.18 2.91 1.86 1.80
Before test c
n.a. 3.06 2.72 1.36 1.02
After test n.a. 3.57 0.85 0.47 0.51
Total nitrogen content determined by Grekov method. Nitrogen content determined by Kjeldahl method. Not analysed.
materials. The nitrogen contents of the nitrided powders before and after the catalytic test are given in Table 2. An increase of total and Kjeldahl nitrogen content was observed with a decrease of the V/Al ratio until 0.25, below this ratio, the nitrogen content slightly diminished. The highest nitrogen content of 5.35% was obtained for VAlON0.25. For all the samples, the total nitrogen content diminished upon use in propane ammoxidation. For the used catalysts, a slight increase of the surface nitrogen content was observed only for the sample with V/Al of 0.25. 3.2. Raman spectroscopy The Raman spectroscopy technique can discriminate between different vanadium oxide species formed during the preparation of the oxide precursor. The Raman spectra of the oxide precursor with different V/Al ratios are presented in Fig. 1. The Raman results indicate that different vanadium species are formed during the co-precipitation by changing the V/Al ratio. The bands at 1050, 980 and 940 cm1 confirm that there are both isolated and polymeric vanadium oxide surface species formed as a function of the V/Al ratio. For instance: (i) the band at 980 cm1 is assigned to surface decavanadate species [HnV10O28](6n) [12], (ii) the band at
Fig. 1. Raman spectra of vanadium aluminium oxide with various V/Al ratios: 0.9 (a), 0.7 (b), 0.25 (c) and 0.1 (d).
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940 cm1 confirms the presence of [(VO3)n]n metavanadate species [13], and (iii) the band at 1050 cm1 is assigned to isolated VO4 vanadium oxide species [14] with one terminal V O bond and three Raman inactive V–O–Al bonds [15]. The highest concentration of isolated vanadium species is found for the sample with a V/Al ratio of 0.1, as indicated by the highest intensity of the Raman band at 1050 cm1. With increasing V/Al ratio, this band, assigned to isolated vanadium species, diminished and disappeared for V/Al of 0.7 and 0.9. The amount of polymeric species increased with the V/Al ratio. The Raman band corresponding to the V O terminal bond of surface decavanadate at 980 cm1 is characteristic for the V/Al ratios of 0.7 and 0.9. It is worth mentioning that in the isolated species as well in the metavanadate species (VO3)n, vanadium presents a tetrahedral coordination, while in the decavanadate species (HnV10O28), vanadium has an octahedral coordination. Thus, the vanadium has a tetrahedral coordination for the low vanadium loading and octahedral coordination for high vanadium loading. 3.3. XPS characterization The XPS technique provides information regarding the nature of the surface nitrogen species before and after catalytic test, as well as about the surface vanadium atoms. Table 3 shows the binding energies corresponding to Al 2p, O 1s, V 2p and N 1s levels. The binding energy of the O 1s level was not modified by changing the V/Al ratio nor by nitrogen insertion. Concerning the Al 2p level, an increase of binding energy was observed with increasing V/Al ratio, from 74.1 eV for VAlON0.1 to 74.45 eV for VAlON0.9. However, the binding energies of both O 1s and Al 2p levels diminished for the used catalysts, indicating that changes in the structure of the catalyst are induced by the reagents. The XPS spectra of all the fresh samples are characterized by two values of the binding energy for the V 2p3/2 level, obtained at 517.1–517.5 and 515.5–516.3 eV (see Table 3). There is controversy concerning the assignment of these peaks, as it can be seen from the different results reported in the literature [16–18]. The position of the first component agrees well with the V 2p3/2 binding energy in V2O5 and NH4VO3 (517.3 eV). Thus we assign this component to the V5+ oxidation state. Because of the wide variation in the reported binding energies for all vanadium oxidation states, and since both V3+ and V4+ are proposed in the literature, the components at 516.3–515.5 eV region cannot be readily assigned to a specific oxidation state. An approximate estimation of the atomic V5+/(V5+ + V4+) surface ratio can be made on the basis of these results. The values for the fresh and used catalysts are presented in Table 4. The reduction degree increases with the V/Al ratio and increases more upon propane ammoxidation for all
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Table 3 Binding energy (eV) for VAlON catalysts Sample
Al 2p (eV)
O 1s (eV)
V 2p3/2 (eV)
N 1s (eV)
VAlON0.1 Fresh
74.1
531.0
517.1 515.7
399.8
Used
73.5
530.3
516.4 515.2
399.2 402.8
VAlON0.25 Fresh 74.2
531.2
517.5 516.3
397.5 399.9 401.8
Used
73.9
530.8
517.5 516.4
400.3 398.5
VAlON0.5 Fresh
74.3
531.2
517.4 515.5
397.1 400.0 402.6
Used
73.6
530.2
516.8 515.3
399.3 402.3
VAlON0.7 Fresh
74.4
531.2
517.5 516.0
397.4 400.3 402.1
Used
72.9
530.7
516.9 515.5
398.6 401.7
Fig. 2. XPS spectra of the N 1s region.
VAlON0.9 Fresh
74.45
531.1
517.3 515.5
397.4 399.8 401.8
Used
74.3
530.9
517.2 515.8
399.0 400.6
for higher V/Al ratios (0.5, 0.7 and 0.9) the reduction was about 10–20%. The formation of different nitrogen species depends on the V/Al ratio. Fig. 2 presents the N 1s spectra of the fresh catalysts as a function of the V/Al ratio. The decomposition of the experimental signal led to three types of nitrogen containing species. The less energetic one, corresponding to binding energies in the range 397.1– 396.6 eV, is ascribed to nitride species (N3) [19]. The component at about 399.8–400.1 eV corresponds to NHx groups (x = 1 or 2), and the peak at about 401.8–402.8 eV is assigned to dinitrogenous species, M–NN–M (where M is V or Al) [19]. Table 5 shows the atomic composition and the V/Al surface ratio for the oxynitrides before and after the catalytic test. In all the cases, the V/Al surface ratios calculated from the XPS data are smaller than the theoretical ones. An increase of the amount of aluminium at the surface was observed for the used catalyst, and, as a consequence, the V/ Al ratio diminished upon use in propane ammoxidation. Moreover, an increase of surface nitrogen and a decrease of the surface oxygen were observed for the used catalysts, due to the substitution of oxygen by nitrogen during the catalytic test. The surface nitrogen content increased with decreasing V/Al ratio up to 0.25. For the VAlON0.1 sample, the nitrogen content diminished. A similar evolution was observed for the total nitrogen content (Table 2).
samples, due to the presence of propane in the reaction mixture. A higher reduction degree was observed for the V/ Al ratio of 0.1 and 0.25, of 42 and 65%, respectively while Table 4 Atomic V5+/(V5+ + V4+) surface ratio for fresh and used catalysts Sample
V5+/(V5+ + V4+)
VAlON0.1 Fresh Used
0.72 0.42
VAlON0.25 Fresh Used
0.76 0.27
VAlON0.5 Fresh Used
0.79 0.63
VAlON0.7 Fresh Used
0.80 0.72
VAlON0.9 Fresh Used
0.81 0.73
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
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Table 5 Atomic compositions derived from the XPS analysis for the VAlON catalysts Sample
N (% bulka)
Al (%)
O (%)
V (%)
N (%)
V/Al ratio
VAlON0.1 Fresh 4.93 Used 2.30
32.95 33.83
62.36 64.75
1.90 2.4
1.77 1.32
0.06 0.07
VAlON0.25 Fresh 5.35 Used 5.18
28.7 31.03
65.45 61.47
4.20 4.0
2.64 3.12
0.15 0.13
VAlON0.5 Fresh 4.76 Used 2.91
21.73 28.28
68.02 60.05
7.02 7.33
2.40 2.65
0.31 0.26
VAlON0.7 Fresh 3.28 Used 1.86
20.94 23.0
66.80 65.36
8.90 8.10
1.18 1.76
0.42 0.34
VAlON0.9 Fresh 2.26 Used 1.80
23.34 32.45
67.80 62.71
8.20 5.90
1.64 1.84
0.35 0.20
a
Determined by Grekov method. Fig. 4. XPS spectra of the N 1s region for VAlON0.9 for fresh and used sample.
The comparison of the V 2p spectra for fresh and used samples (Fig. 3) revealed a different participation of the vanadium species in the reaction. The component at 515.5– 516.3 eV increased upon exposure to propane ammoxidation, while the component at 517.1–517.5 eV diminished. Modifications of the used catalyst compared with the fresh catalyst were also observed in the N 1s region (Fig. 4). A general trend is the diminution of the signal corresponding to N3. Moreover, for VAlON0.9 the components at 396.6– 397.1 and 401.8–402.8 eV corresponding to N3 and to dinitrogenous species disappeared, as shown in Fig. 4. However, the surface nitrogen content increased for the used catalysts (see Table 4).
Fig. 3. XPS spectra of V 2p region for VAlON0.9 and VAlON0.25 for fresh and used samples.
3.4. Catalytic activity The catalytic tests were carried out in the optimal reaction conditions, namely with C3H8:O2:NH3 molar ratio of 1.25:3:1 and 500 8C. No propylene and HCN were detected in these reaction conditions. A carbon mass balance of 80– 90% was obtained, due to the condensation of acrylonitrile inside of the connection line between the reactor and the gas chromatograph. Fig. 5 presents the conversion of propane and the selectivity to the reaction products as a function of time on stream for VAlON0.25. Propane conversion was almost constant with time on stream, while the selectivity and yield to acrylonitrile increased in the first part of the reaction. The steady-state conditions were reached after 4 h of time on stream. After 24 h of time on stream the highest selectivity in acrylonitrile was 50% for a conversion level of propane of 60%. In parallel, the selectivity to COx decreased with time on stream leading to the idea that the total oxidation became limited in time. Propylene formation was not observed during the 24 h of reaction and no deactivation of the catalyst was noted during this period. For the catalysts with different V/Al ratios, the trends were similar to that presented in Fig. 5. The catalytic activity of the VAlON catalysts depends on the V/Al ratio as observed from the results depicted in Fig. 6, after 24 h of reaction. The optimal V/Al ratio appeared to be 0.25, which presents the highest ACN yield of 30%. In parallel, for VAlON0.25, the selectivity to COx was the lowest. The formation of COx and N2 was favoured over VAlON0.9 and
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Fig. 5. Time on stream behaviour over VAlON0.25 catalyst, GHSV = 16.8 l/g h, propane:oxygen:ammonia = 1.25:3:1: (~) propane conversion; (&) selectivity to ACN; (~) yield of ACN; (*) selectivity to AcCN; (*) selectivity to COx; (&) selectivity to N2.
VAlON0.7, with higher content of vanadium and lower content of nitrogen.
4. Discussion The study of the relation between structure and activity is essential to understand the factors controlling the reactivity and the key features of a catalyst. Particularly, for ammoxidation catalysts, the role of different vanadium species (V5+, V4+, V3+) and the role of different nitrogen species are very important and are still under debate. To answer these questions, the structural characteristics of the vanadium aluminium oxynitride catalysts as a function of the V/Al ratio will first be discussed, and then related to their catalytic performances. 4.1. Structural changes induced by different compositions in ‘‘VAlON’’ catalysts The experimentally determined V/Al molar ratios were lower than the theoretical values for the ratios higher than 0.7 (see Table 1), probably due to the poor precipitation of
vanadium with aluminium in the starting solution. This appears to be an important parameter in the preparation of the ‘‘VAlO’’ precursor. The highest surface area is observed for VAlON0.1 with the lowest content of vanadium (Table 1). By increasing the V/Al ratio, the surface area diminished, and in addition, the nitrogen content diminished as well, this suggesting that, for higher V/Al ratios, nitrogen insertion was more difficult to achieve. We observed the same tendency for the total nitrogen content as for the surface nitrogen determined by Kjeldahl method. As indicated in Table 2, the nitrogen content changed for the used catalysts. This obviously means that the nitridation, for all the V/Al ratios, does not allow an optimal activation of the catalysts and that the reduction–oxidation process during the catalytic test allows to stabilize the solid. Raman spectroscopy provides information about the surface vanadium species of the vanadium aluminium oxide precursor as a function of the V/Al ratio. The survey of the literature [20] shows the following evolution of the different vanadium oxide species with the increase of the vanadium loading: orthovanadate (VO4) ! pyrovanadate (V2O7) ! metavanadate (VO3)n ! decavanadate (V10O28)
Fig. 6. Catalytic behaviour over VAlON catalysts with different V/Al molar ratio, GHSV = 16 800 ml/g h, propane:oxygen:ammonia = 1.25:3:1, 500 8C: (~) propane conversion; (&) selectivity to ACN; (~) yield of ACN; (*) selectivity to AcCN; (*) selectivity to COx; (&) selectivity to N2.
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
! vanadium pentoxide (V2O5). There are three possible active bridging oxygen sites: terminal V O, binding surface polymeric species V–O–V, and binding vanadium to support V–O–S. The isolated vanadium oxide species have one terminal V O bond and three V–O–Al bonds, while the polymeric vanadium oxide species consist of a terminal V O bond with one V–O-support and two bridging V–O–V bonds. [21]. The spectra of the vanadium aluminium oxide precursor presented in Fig. 1 as a function of vanadium loading are in agreement with the literature data, meaning that higher vanadium contents favour the formation of decavanadate species (band at 980 cm1). Besides, the decrease of the vanadium loading increased the quantity of isolated vanadium species (band at 1050 cm1) and of metavanadate species (band at 940 cm1) formed. A correlation can be made between the nature of the vanadium oxide species and the nitrogen content. For the higher V/Al ratios, the V–O–V bonds of polymeric vanadium oxide species are predominant and at the same time the nitrogen content decreases, while for the lower V/ Al ratios there is a predominance of the V–O–Al bonds of isolated vanadium oxide species type and the nitrogen content is higher. This suggests that, when increasing concentration of the monomeric vanadium species, the degree of nitridation increases and one may suppose that the oxygen replaced by nitrogen during the nitridation process could be the one bridged to V and Al. Thus, the substitution of oxygen by nitrogen is favoured for the lower V/Al ratios, for which we found a higher surface concentration of isolated vanadium species. Also, if we take in account the type of coordination of vanadium in these different species, one can observe that the tetrahedral coordination favours the substitution of oxygen by nitrogen, while for the octahedral coordination the nitridation is more difficult. The XPS data also shows differences as a function of the V/Al ratio, as for example: (i) generation of different nitrogen species, and (ii) different reduction degree of vanadium. Fig. 7 points to a direct relation between the V/Al ratio and the type of nitrogen species as determined by XPS formed during the nitridation process. This plot shows that the formation of nitride species (N3) is favoured at high V/Al ratio while the amount of NHx species is favoured on the samples with low V/Al ratios. However, the total amount of nitrogen species increased with the decrease of the V/Al ratio. Contradictory results are obtained from the chemical analysis of the total and surface nitrogen content. If we admit that the difference between the total nitrogen content determined by Grekov method and the surface nitrogen content determined by Kjeldahl method represent the amount of N3 species from the bulk of the catalysts, one can observe that the amount of N3 species increases with decreasing V/Al ratio (Fig. 8).
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Fig. 7. Variation of different nitrogen component area determined by XPS as a function of V/Al ratio: (*) N3; (&) NHx; (~) dinitrogen.
The amount of bulk nitrogen species (N3) determined from the differences between total and surface nitrogen contents (from chemical analysis), as well as N3 determined by XPS, are plotted in Fig. 8. It is seen that the bulk nitrogen determined by chemical analysis and by XPS is the same for the VAlON0.9 sample, indicating the homogeneity of this sample. It is well known from the literature and supported by the RAMAN spectra of the VAlO samples, that when the vanadium loading increases, the possibility of formation of surface polyvanadates increases [22]. If we take into account that, in the case of VAlON catalysts these polyvanadate species are formed for the higher V/Al ratios, it indicates that the nitridation of these species seems more difficult to achieve and probably only the terminal V O species could be nitrided. When the V/Al molar ratio diminishes, the probability of formation of polyvanadate species diminishes, and the nitridation is much deeper for lower V/Al ratio. The increase of the total and surface nitrogen content with the V/ Al molar ratio is consistent with this hypothesis. The fact that, after the catalytic test, the bulk nitrogen content diminished compared with the bulk nitrogen content for the fresh samples can be explained by the migration of
Fig. 8. N3 species determined by differences of total nitrogen and surface nitrogen (Kjedhal) (&); N3 species determined by XPS (~).
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Fig. 9. Correlation between the bulk nitrogen and degree of reduction of vanadium.
the nitride species to the surface of the catalyst during the catalytic test in the presence of oxygen from the reaction mixture. This phenomenon was also observed during the oxidation of ‘‘AlGaPON’’ catalysts [23]. Moreover, in the case of the VAlON0.9 catalysts, the component corresponding to N3 disappeared after the reaction, as a consequence of the consumption of these species during the reaction. Another relevant aspect of the VAlON catalysts determined from the XPS data is the reduction degree of vanadium. The XPS spectra of the fresh catalysts present a split of the V 2p3/2 level, observed at 517.1–517.5 and 515.5–516.3 eV (Table 3). Taking into account the limitation in the assignment of these peaks, an approximate estimation of the atomic V5+/(V5+ + V4+) surface ratio can be made using the XPS technique (Table 4). For the ‘‘VAlON’’ samples, the atomic V5+/(V5+ + V4+) surface ratio decreases with increasing V/Al ratio. Moreover, by decreasing the V/ Al ratio, the amount of nitrogen incorporated during the nitridation process increases, and as consequence the reduction degree of vanadium increased. The reduction degree of vanadium as a function of bulk nitrogen is shown in Fig. 9. This dependence presented in Fig. 9 suggests that the oxidation state of vanadium depends on the quantity of nitrogen inserted by oxygen substitution. Thus, the highest degree of reduction of vanadium (0.72) corresponds to the highest content of bulk nitrogen inserted by nitridation (2.73%). However, both the degree of nitridation and the degree of reduction could be linked to the degree of polymerisation of vanadium. A diminution of atomic V5+/(V5+ + V4+) surface ratio is observed for all the used catalysts, meaning that vanadium is also reduced during the reaction (Table 5), due to the presence of propane and ammonia in the gas feed. The highest reduction occurring for the lower V/Al ratio, which could be explained by the presence of isolated vanadium species, are easier to reduce as compared with the polymeric species [24]. The progressive and general decreasing trend of the binding energies of Al 2p with the atomic nitrogen
percentage, as presented in Table 3, was explained by the higher nucleophilic character of nitrogen compared to oxygen, which reduces the positive charge around the aluminium. The role of Al3+ ions in the VAlON system is not yet well established. We believe that aluminium ions are not directly involved in the catalytic reaction. However, they could play an important role in increasing the surface dispersion of vanadium. In summary, the catalysts characterization showed the following trends as a function of the V/Al ratio: (i) the surface area increases with the decrease of the V/Al ratio, (ii) the nitrogen content increases with the decrease of V/Al ratio, (iii) the type of nitrogen species depends on the V/Al ratio, higher content of bulk nitrogen was observed for the lower V/Al ratio, (iv) higher degree of polymerisation of vanadium oxide species for the higher V/Al ratio and (v) the reduction degree is different as a consequence of the nitrogen insertion during the nitridation step. 4.2. Study of the catalytic behaviour The catalytic performances of ‘‘VAlON’’ as a function of the V/Al ratio of the catalysts are presented in Fig. 6. The increase of the V/Al ratio from 0.25 to 0.9 leads to: (i) a decrease of the selectivity to acrylonitrile, (ii) an increase of the selectivity to carbon oxides and (iii) an increase of the amount of nitrogen formed by ammonia oxidation. It must be stressed that regardless of the V/Al ratio, the conversion of propane is nearly constant. The highest yield of acrylonitrile is obtained over VAlON0.25. Particular results were obtained for the VAlON0.1 sample, with the lowest vanadium content: (i) a decrease of the selectivity to acrylonitrile is accompanied by (ii) a decrease of propane conversion, indicating that under these conditions there are not enough active sites for propane activation. As seen in Fig. 5, the steady-state conditions are achieved after 4 h on stream. An explanation of this phenomenon could be the fact that the degree of reduction of the catalyst can increase with time on stream due to the presence of propane and ammonia in the gas feed. Also, these results correlate well with the increase of the nitrogen content as a function of time on stream, until the steady state is reached. Below, we shall attempt to correlate the catalytic behaviour of the VAlON system with two important parameters, which depend on the V/Al molar ratio: (i) the reduction degree of vanadium and (ii) the quantity and type of nitrogen species. In order to establish a structure–activity relationship and to understand the factors controlling the reactivity and the key features of the catalyst, the role of different vanadium species (V5+, V4+, V3+) in propane ammoxidation mechanism is often under debate. Most of the studies attribute the activation of propane to V5+ and V4+ centers [25]. However, a recent publication stressed that V3+ may also have an effect on the enhancement of the activity of an alkane molecule [26].
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
Fig. 10. Selectivity to ACN (&), nitrogen (&) and conversion of propane (~) as a function of the V5+/(V5+ + V4+) ratio determined by XPS.
Fig. 10 depicts the catalytic results of the ‘‘VAlON’’ system as a function of the surface V5+/(V5+ + V4+) atomic ratios measured after the catalytic test. With decreasing surface V5+/(V5+ + V4+) atomic ratio, the selectivity to acrylonitrile increased and reached a maximum for VAlON0.25. It should also be mentioned that the formation of nitrogen from ammonia oxidation and the carbon oxides production from total oxidation reaction increased with the increase of the amounts of the V5+ species. These species enhance the side reaction of ammonia oxidation to N2 (see Fig. 10). From the literature, it is known that V2O5 is the active phase for the oxidation of ammonia into nitrogen [27,28]. A maximum of the catalytic activity is obtained for the V5+/(V5+ + V4+) ratio of 0.27 (VAlON0.25 sample). Thus, the presence of V5+ species in large amounts on the surface has a negative effect due to side reactions, but its role when present in lower amounts is questionable. The most efficient sites of the VAlON system for the acrylonitrile formation seems to be the V4+ species, which agrees with the recent studies of Millet and coworkers on the oxidation state of vanadium in mixed vanadium and iron antimonite oxides [29]. By changing the V/Al ratio, the degree of the nitridation is different, as it was observed from the XPS results. Since the catalytic activity of the VAlON in propane ammoxida-
9
tion depends on the V/Al ratio, and that the nitrogen content also depends on the V/Al ratio, we can admit that the quantity of nitrogen generated by nitridation has an influence on the catalytic behaviour. Indeed, the selectivity to ACN as well as the formation of COx depends on the bulk nitrogen content as well as on the surface nitrogen content, illustrated in Fig. 11. The selectivity to acrylonitrile increases with the nitrogen content, both for surface and bulk nitrogen species, indicating the role of these species in the N-insertion step to form acrylonitrile. Changes in the concentration of nitrogen species were observed upon use in propane ammoxidation, indicating the participation of nitrogen species from the VAlON framework in the N-insertion step to form acrylonitrile. Thus, it is necessary to pay a special attention to the surface restructuring phenomena during the catalytic reaction, as a central aspect for the understanding of the surface reactivity of the oxynitrides. On the vanadium aluminium oxynitrides, this phenomenon is possible as the nitridation process is a reversible one. This behaviour was observed also on aluminium phosphate oxynitride catalysts, AlPON [23], and should be confirmed on the case of VAlON. Taking into account that (i) the total nitrogen content of VAlON catalysts is modified upon use in propane ammoxidation according to XPS and chemical analysis, and (ii) the nitridation process is a reversible one, a Mars and Van Krevelen mechanism applied to nitrogen could be considered. Deeper investigations of this phenomenon are the subject of others studies performed over VAlON catalysts and will be published elsewhere [30]. As indicated by the above results, propylene formation as an intermediate was not observed for all the series of VAlON systems. Moreover, by performing propylene ammoxidation on these catalysts a conversion of 100% was reached. This observation, together with the fact that the VAlON system shows a high acrylonitrile selectivity at very low space–time value, suggests a different reaction mechanism as compared with conventional metallic oxide ammoxidation systems. In the case of VAlON, the catalytic behaviour can be explained by a combination of different simultaneous effects: (i) a better efficiency in the activation of the C–H bond of the
Fig. 11. Selectivities to ACN (&) and COx (*) as a function of the bulk nitrogen (a) and surface nitrogen (b).
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M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
alkane due to a balanced redox capacity of vanadium species, and (ii) a high nitrogen content which enhances the nitrogen insertion rate for the acrylonitrile formation.
5. Conclusions The results presented above show that the catalytic performances depend on the V/Al ratio, with an optimal selectivity to acrylonitrile obtained for V/Al of 0.25. These results could be explained by an optimal degree of polymerisation of vanadium species from oxide precursor, which induces an optimal degree of nitridation and, further, an optimal reduction degree of vanadium. The prior nitridation of the V–Al oxide precursor with the generation of the nitrogen species is beneficial for ammoxidation of propane, indicating the positive role of these species in this reaction. The results obtained in this reaction established that these new compounds are important catalytic materials, presenting tuneable properties by variation of both the V/Al and N/O ratios.
Acknowledgements The authors would like to thank the ‘‘Direction Ge´ ne´ rale des Technologies, de la Recherche et de l’E´ nergie’’ de la Re´ gion Wallonne (GREDECAT) and ‘‘La Communaute´ Franc¸aise de Belgique’’ for financial support.
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[4] G. Centi, S. Perathoner, F. Trifiro`, Appl. Catal. A 157 (1997) 143. [5] M. Aouine, J.L. Dubois, J.M.M. Millet, Chem. Commun. (2001) 1180. [6] R.K. Grasselli, D.J. Buttrey, P. Desanto, J.D. Burrington, C.G. Lugmair, A.F. Volpe, T. Weingand, Catal. Today 91–92 (2004) 251. [7] M. Florea, R. Prada Silvy, P. Grange, Appl. Catal. 255 (2) (2003) 289. [8] N. Blangenoise, M. Florea, R. Prada Silvy, P. Grange, S.P. Chenaking, J.M. Bastin, N. Kruse, B.P. Barbero, L. Cadus, Appl. Catal. 263 (2) (2004) 163. [9] R. Prada Silvy, M. Florea, N. Blangenois, P. Grange, AIChE 49 (8) (2003) 2228. [10] A. Andersson, S.L.T. Andersson, G. Centi, R.K. Grasselli, M. Sanati, F. Trifiro, Appl. Catal. A 113 (1994) 43. [11] G. Centi, P. Mazzoli, Catal. Today 28 (1996) 351. [12] H. Kno¨ zinger, H. Krietenbrik, H.D. Muller, W. Schultz, in: G.C. Bond, P.B. Wells (Eds.), Proceedings of the 6th International Congress on Catalysis, vol. 1, The Chemical Society, London, 1976, p. 183. [13] G.C. Bond, Appl. Catal. 71 (1991) 1. [14] G.T. Went, L.J. Leu, A.T. Bell, J. Catal. 134 (1992) 479. [15] J.M. Kanervo, M.E. Harlin, A.O.I. Krause, M.A. Ban˜ ares, Catal. Today 78 (2003) 171. [16] J.M. Kanervo, M.E. Harlin, A.O.I. Krause, M.A. Ban˜ øares, Catal. Today 78 (2003) 171. [17] X. Gao, P. Ruiz, Q. Xin, X. Guo, B. Delmon, J. Catal. 148 (1994) 56. [18] R.J. Colton, A.M. Guzman, J.W. Rabalais, J. Appl. Phys. 49 (1978) 409. [19] J.P. Nogier, N. Jammul, M.J. Delamar, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 279. [20] J.B. Peri, J. Phys. Chem. 69 (1965) 231. [21] G.G. Cortez, M.A. Ban˜ ares, J. Catal. 209 (2002) 197. [22] B.M. Weckhuysen, D.E. Keller, Catal. Today 78 (2003) 25. [23] M.A. Centeno, P. Grange, J. Phys. Chem. B 103 (1999) 34. [24] M. Bosch, B.J. Kip, J.G. van Ommen, P.J. Gellings, J. Chem. Soc., Faraday Trans. 80 (1994) 2479. [25] B. Grzybowska-Swierkosz, Appl. Catal. A 157 (1997) 409. [26] F. Cavani, S. Ligi, T. Monti, F. Pierelli, F. Trifiro`, S. Albonetti, G. Mazzoni, Catal. Today 61 (2000) 203. [27] Y. Li, J. Armor, Appl. Catal. B 13 (1997) 131. [28] A. Andersson, S.L.T. Andersson, G. Centi, R.K. Grasselli, M. Sanati, F. Trifiro, New Frontiers Catal. (1992) 691. [29] D.L. Nguyen, Y.B. Taarit, J.M.M. Millet, Catal. Lett. 90 (2003) 65. [30] M. Olea, M. Florea, I. Sack, R. Prada Silvy, E.M. Gaigneaux, G.B. Marin, P. Grange, J. Catal., in press.
Vanadium aluminium oxynitride catalysts for propane ammoxidation reaction Effect of the V/Al ratio on the structure and catalytic behaviour Mihaela Florea a,b,1,*, Ricardo Prada Silvy b,1, Paul Grange b,1,ä a
University of Bucharest, Faculty of Chemistry, B-dul Regina Elisabeta 4-12, 030016 Bucharest, Romania b Universite´ Catholique de Louvain, Unite´ de Catalyse et Chimie des Mate´riaux Divise´s, Croix du Sud 2, Boite 17, 1348 Louvain-la-Neuve, Belgium Received 29 September 2004; received in revised form 15 February 2005; accepted 15 February 2005 Available online 5 April 2005
Abstract The influence of the V/Al ratio composition on the physico-chemical and catalytic properties of the vanadium aluminium oxynitride system was investigated. The samples were prepared by co-precipitation of vanadium and aluminium solutions containing different metal compositions (0.1–0.9 V/Al) at pH 5.5 and characterized by XRD, XPS, Raman and BET surface area. Catalytic activity measurements for the propane ammoxidation reaction were carried out under optimal acrylonitrile selectivity conditions. X-ray diffraction pattern indicated that all the catalysts in both oxide precursor and nitride state show amorphous character. BET surface area was higher for the sample with V/Al ratio of 0.25, before and after nitridation treatment. This sample showed optimal catalytic performances, with 50% acrylonitrile selectivity and 60% propane conversion. The optimal nitridation degree, which induces an optimal reduction degree of vanadium, would explain the maximal catalytic activity observed for the sample prepared using the V/Al ratio composition of 0.25. # 2005 Elsevier B.V. All rights reserved. Keywords: Vanadium; Ammoxidation; Acrylonitrile
1. Introduction Among the most significant examples of industrial application in heterogeneous oxidation catalysis is the production of acrylonitrile (ACN) through the propane ammoxidation process. The advantages for replacing olefins by alkanes in the current ammoxidation process are essentially the lower price of propane with respect to propylene and the risk of propylene shortage due to its increasing consumption and the increasing worldwide demand of nitriles and other derived products. Mixed metal-oxides based on VMoMeOx [1,2] and VSbMeOx [3,4] * Corresponding author. Tel.: +40 21 4103178; fax: +40 21 3159249. E-mail address: [email protected] (M. Florea). 1 Fax: +32 10 473649. ä Deceased. 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.02.032
are typical propane ammoxidation catalysts. Recently, Mo– V–Te–Nb mixed oxides have been proposed as the most active and selective catalysts in the ammoxidation of propane to acrylonitrile, giving acrylonitrile yields up to 62% [5,6]. The active components of these catalysts have been shown to be two phases called M1 and M2. The respective roles of the two phases are not yet fully understood but it has been reported that their concomitant presence is needed to obtain effective catalysts. It has recently been shown that these phases were presenting orthorhombic and hexagonal type structures, respectively and both contained the four metallic elements [6]. However, this catalytic system has not yet provided a competitive advantage in ACN productivity versus conventional propylene ammoxidation catalysts, which explains why the propane ammoxidation process has not yet been commercially scaled up.
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We have been investigating new catalytic materials for alkane activation reaction. Vanadium aluminium oxynitride (VAlON) catalysts show excellent properties for propane ammoxidation and we have shown that the catalytic properties strongly depend on the nitridation on the reaction conditions [7] and on the preparation method [8]. The results also indicated that the VAlON catalytic system has a high ACN productivity at lower propane space–time relative to VMoMeOx and VSbMeOx systems [9]. The dispersion of the vanadium site at the surface has been recognized as the key factor required for achieving high activity and, most important, high acrylonitrile selectivity for vanadium-based catalysts [10,11]. In this study, we investigate the influence of the composition of the vanadium aluminium oxynitride, namely the V/Al ratio, on the physico-chemical and catalytic properties in propane ammoxidation.
2. Experimental Vanadium aluminium oxynitride catalyst with different V/Al atomic ratios were prepared by thermal nitridation of a co-precipitated V–Al oxide precursor. The preparation of the oxide precursor is given elsewhere [8]. The nitridation of the samples was carried out in a tubular rotating reactor under a flow of pure ammonia (30 l h1) at 500 8C for 5 h. The system was subsequently cooled down to room temperature under a flow of pure nitrogen. The principle of the chemical analysis of total nitrogen content is based on the reaction of the nitrogen species from the catalyst with a strong base (KOH) at 450 8C and the formation of ammonia which is then titrated with a standard solution of sulphuric acid 102N (Grekov method). The superficial nitrogen species were quantified by the Kjeldahl method. The alkaline attack was realized using a KOH saturated solution at 100 8C. The titrated ammonia corresponds to the superficial NHx (NH and NH2) species. Bulk nitrogen does not react under these conditions. The BET surface area corresponding to oxynitride catalysts before and after reaction was evaluated using a Micromeritics Flow Sorb II 2300 apparatus. Raman spectroscopy was performed with a DILORJOBIN YVON-SPEX spectrometer, model OLYMPUS DX40, equipped with a He–Ne (l = 632.8 nm) laser. The spectra were recorded in the 200–1600 cm1 range. XRD lines were recorded using a Siemens D-5000 powder diffractometer equipped with a Ni-filtered Cu Ka ˚ ). radiation (l = 1.5418 A XPS spectra were collected with an SSX-100, model 206 Surface Science Instrument spectrometer at room temperature and under a vacuum of 1.33 mPa. Monochromatized Al Ka radiation (hn = 1486.6 eV), obtained by bombarding the Al anode with an electron gun operating at a beam current of 12 mA and an accelerating voltage of 10 kV was used. The charge correction was made considering that the C 1s signal
of contaminating carbon (C–C or C–H bonds) was centered at 284.8 eV. The C 1s, V 2p3, Al 2p, N 1s and O 1s levels were chosen for characterization, since their signals are the most intense and do not overlap. Catalytic tests were performed in a fixed bed quartz micro-reactor at atmospheric pressure and temperature of 500 8C, 0.1 g of catalyst and W/F = 8 g h/mol of C3H8. Feed composition was 1.25:3:1 of C3H8:O2:NH3. The activity results are reported after 24 h on stream. Feed and products were analysed on-line using a gas chromatograph, equipped with FID and TCD detectors and an on-line mass spectrometer was used to check the NOx formation. A carbon mass balance of 80–90% was obtained, because of the condensation of the reaction products inside of the connection line between the reactor and the gas chromatograph. To avoid the condensation of acrylonitrile, the connection line was heated at 150 8C. Propane conversion is defined as the % ratio between the mole of propane consumed per mole of propane in the feed; the ACN selectivity as mole of ACN in the product per mole of propane consumed; and the ACN yield as mole of ACN in the product per mole of propane in the feed.
3. Results 3.1. Chemical composition, textural and structural characteristics Samples with V/Al atomic ratios in the range of 0.1–0.9 were prepared by the co-precipitation method followed by nitridation in the presence of ammonia at 500 8C for 5 h. All the samples are X-ray amorphous before and after the catalytic test. Table 1 compiles the theoretical and experimental V/Al ratios and the values of the surface areas of the oxynitride powders before and after propane ammoxidation. The V/Al molar ratios in the obtained catalysts are lower than the theoretical values for the ratios higher than 0.7. The oxynitrides exhibit a specific area between 125 and 160 m2/g and depend on the V/Al ratio, as seen in Table 1. The surface area increases with the decreasing V/Al ratio, with highest surface area of 160 m2/g being observed for VAlON0.1 sample. The tested catalysts exhibit surface areas 15–25% lower than those of the corresponding fresh Table 1 V/Al ratios and surface areas of VAlON catalysts Catalyst
V/Al (theoretical)
V/Al (experimental)
Surface area before test (m2/g)
Surface area after test (m2/g)
VAlON0.1 VAlON0.25 VAlON0.5 VAlON0.7 VAlON0.9
0.10 0.25 0.50 0.70 0.90
0.10 0.25 0.50 0.66 0.82
160 153 130 122 143
117 133 78 93 124
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10 Table 2 Nitrogen content before and after catalytic test as a function of the V/Al ratio Catalyst
NT (%)a Before test
VAlON0.1 VAlON0.25 VAlON0.5 VAlON0.7 VAlON0.9 a b c
4.93 5.35 4.76 3.28 2.26
NK (%)b After test 2.30 5.18 2.91 1.86 1.80
Before test c
n.a. 3.06 2.72 1.36 1.02
After test n.a. 3.57 0.85 0.47 0.51
Total nitrogen content determined by Grekov method. Nitrogen content determined by Kjeldahl method. Not analysed.
materials. The nitrogen contents of the nitrided powders before and after the catalytic test are given in Table 2. An increase of total and Kjeldahl nitrogen content was observed with a decrease of the V/Al ratio until 0.25, below this ratio, the nitrogen content slightly diminished. The highest nitrogen content of 5.35% was obtained for VAlON0.25. For all the samples, the total nitrogen content diminished upon use in propane ammoxidation. For the used catalysts, a slight increase of the surface nitrogen content was observed only for the sample with V/Al of 0.25. 3.2. Raman spectroscopy The Raman spectroscopy technique can discriminate between different vanadium oxide species formed during the preparation of the oxide precursor. The Raman spectra of the oxide precursor with different V/Al ratios are presented in Fig. 1. The Raman results indicate that different vanadium species are formed during the co-precipitation by changing the V/Al ratio. The bands at 1050, 980 and 940 cm1 confirm that there are both isolated and polymeric vanadium oxide surface species formed as a function of the V/Al ratio. For instance: (i) the band at 980 cm1 is assigned to surface decavanadate species [HnV10O28](6n) [12], (ii) the band at
Fig. 1. Raman spectra of vanadium aluminium oxide with various V/Al ratios: 0.9 (a), 0.7 (b), 0.25 (c) and 0.1 (d).
3
940 cm1 confirms the presence of [(VO3)n]n metavanadate species [13], and (iii) the band at 1050 cm1 is assigned to isolated VO4 vanadium oxide species [14] with one terminal V O bond and three Raman inactive V–O–Al bonds [15]. The highest concentration of isolated vanadium species is found for the sample with a V/Al ratio of 0.1, as indicated by the highest intensity of the Raman band at 1050 cm1. With increasing V/Al ratio, this band, assigned to isolated vanadium species, diminished and disappeared for V/Al of 0.7 and 0.9. The amount of polymeric species increased with the V/Al ratio. The Raman band corresponding to the V O terminal bond of surface decavanadate at 980 cm1 is characteristic for the V/Al ratios of 0.7 and 0.9. It is worth mentioning that in the isolated species as well in the metavanadate species (VO3)n, vanadium presents a tetrahedral coordination, while in the decavanadate species (HnV10O28), vanadium has an octahedral coordination. Thus, the vanadium has a tetrahedral coordination for the low vanadium loading and octahedral coordination for high vanadium loading. 3.3. XPS characterization The XPS technique provides information regarding the nature of the surface nitrogen species before and after catalytic test, as well as about the surface vanadium atoms. Table 3 shows the binding energies corresponding to Al 2p, O 1s, V 2p and N 1s levels. The binding energy of the O 1s level was not modified by changing the V/Al ratio nor by nitrogen insertion. Concerning the Al 2p level, an increase of binding energy was observed with increasing V/Al ratio, from 74.1 eV for VAlON0.1 to 74.45 eV for VAlON0.9. However, the binding energies of both O 1s and Al 2p levels diminished for the used catalysts, indicating that changes in the structure of the catalyst are induced by the reagents. The XPS spectra of all the fresh samples are characterized by two values of the binding energy for the V 2p3/2 level, obtained at 517.1–517.5 and 515.5–516.3 eV (see Table 3). There is controversy concerning the assignment of these peaks, as it can be seen from the different results reported in the literature [16–18]. The position of the first component agrees well with the V 2p3/2 binding energy in V2O5 and NH4VO3 (517.3 eV). Thus we assign this component to the V5+ oxidation state. Because of the wide variation in the reported binding energies for all vanadium oxidation states, and since both V3+ and V4+ are proposed in the literature, the components at 516.3–515.5 eV region cannot be readily assigned to a specific oxidation state. An approximate estimation of the atomic V5+/(V5+ + V4+) surface ratio can be made on the basis of these results. The values for the fresh and used catalysts are presented in Table 4. The reduction degree increases with the V/Al ratio and increases more upon propane ammoxidation for all
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M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
Table 3 Binding energy (eV) for VAlON catalysts Sample
Al 2p (eV)
O 1s (eV)
V 2p3/2 (eV)
N 1s (eV)
VAlON0.1 Fresh
74.1
531.0
517.1 515.7
399.8
Used
73.5
530.3
516.4 515.2
399.2 402.8
VAlON0.25 Fresh 74.2
531.2
517.5 516.3
397.5 399.9 401.8
Used
73.9
530.8
517.5 516.4
400.3 398.5
VAlON0.5 Fresh
74.3
531.2
517.4 515.5
397.1 400.0 402.6
Used
73.6
530.2
516.8 515.3
399.3 402.3
VAlON0.7 Fresh
74.4
531.2
517.5 516.0
397.4 400.3 402.1
Used
72.9
530.7
516.9 515.5
398.6 401.7
Fig. 2. XPS spectra of the N 1s region.
VAlON0.9 Fresh
74.45
531.1
517.3 515.5
397.4 399.8 401.8
Used
74.3
530.9
517.2 515.8
399.0 400.6
for higher V/Al ratios (0.5, 0.7 and 0.9) the reduction was about 10–20%. The formation of different nitrogen species depends on the V/Al ratio. Fig. 2 presents the N 1s spectra of the fresh catalysts as a function of the V/Al ratio. The decomposition of the experimental signal led to three types of nitrogen containing species. The less energetic one, corresponding to binding energies in the range 397.1– 396.6 eV, is ascribed to nitride species (N3) [19]. The component at about 399.8–400.1 eV corresponds to NHx groups (x = 1 or 2), and the peak at about 401.8–402.8 eV is assigned to dinitrogenous species, M–NN–M (where M is V or Al) [19]. Table 5 shows the atomic composition and the V/Al surface ratio for the oxynitrides before and after the catalytic test. In all the cases, the V/Al surface ratios calculated from the XPS data are smaller than the theoretical ones. An increase of the amount of aluminium at the surface was observed for the used catalyst, and, as a consequence, the V/ Al ratio diminished upon use in propane ammoxidation. Moreover, an increase of surface nitrogen and a decrease of the surface oxygen were observed for the used catalysts, due to the substitution of oxygen by nitrogen during the catalytic test. The surface nitrogen content increased with decreasing V/Al ratio up to 0.25. For the VAlON0.1 sample, the nitrogen content diminished. A similar evolution was observed for the total nitrogen content (Table 2).
samples, due to the presence of propane in the reaction mixture. A higher reduction degree was observed for the V/ Al ratio of 0.1 and 0.25, of 42 and 65%, respectively while Table 4 Atomic V5+/(V5+ + V4+) surface ratio for fresh and used catalysts Sample
V5+/(V5+ + V4+)
VAlON0.1 Fresh Used
0.72 0.42
VAlON0.25 Fresh Used
0.76 0.27
VAlON0.5 Fresh Used
0.79 0.63
VAlON0.7 Fresh Used
0.80 0.72
VAlON0.9 Fresh Used
0.81 0.73
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
5
Table 5 Atomic compositions derived from the XPS analysis for the VAlON catalysts Sample
N (% bulka)
Al (%)
O (%)
V (%)
N (%)
V/Al ratio
VAlON0.1 Fresh 4.93 Used 2.30
32.95 33.83
62.36 64.75
1.90 2.4
1.77 1.32
0.06 0.07
VAlON0.25 Fresh 5.35 Used 5.18
28.7 31.03
65.45 61.47
4.20 4.0
2.64 3.12
0.15 0.13
VAlON0.5 Fresh 4.76 Used 2.91
21.73 28.28
68.02 60.05
7.02 7.33
2.40 2.65
0.31 0.26
VAlON0.7 Fresh 3.28 Used 1.86
20.94 23.0
66.80 65.36
8.90 8.10
1.18 1.76
0.42 0.34
VAlON0.9 Fresh 2.26 Used 1.80
23.34 32.45
67.80 62.71
8.20 5.90
1.64 1.84
0.35 0.20
a
Determined by Grekov method. Fig. 4. XPS spectra of the N 1s region for VAlON0.9 for fresh and used sample.
The comparison of the V 2p spectra for fresh and used samples (Fig. 3) revealed a different participation of the vanadium species in the reaction. The component at 515.5– 516.3 eV increased upon exposure to propane ammoxidation, while the component at 517.1–517.5 eV diminished. Modifications of the used catalyst compared with the fresh catalyst were also observed in the N 1s region (Fig. 4). A general trend is the diminution of the signal corresponding to N3. Moreover, for VAlON0.9 the components at 396.6– 397.1 and 401.8–402.8 eV corresponding to N3 and to dinitrogenous species disappeared, as shown in Fig. 4. However, the surface nitrogen content increased for the used catalysts (see Table 4).
Fig. 3. XPS spectra of V 2p region for VAlON0.9 and VAlON0.25 for fresh and used samples.
3.4. Catalytic activity The catalytic tests were carried out in the optimal reaction conditions, namely with C3H8:O2:NH3 molar ratio of 1.25:3:1 and 500 8C. No propylene and HCN were detected in these reaction conditions. A carbon mass balance of 80– 90% was obtained, due to the condensation of acrylonitrile inside of the connection line between the reactor and the gas chromatograph. Fig. 5 presents the conversion of propane and the selectivity to the reaction products as a function of time on stream for VAlON0.25. Propane conversion was almost constant with time on stream, while the selectivity and yield to acrylonitrile increased in the first part of the reaction. The steady-state conditions were reached after 4 h of time on stream. After 24 h of time on stream the highest selectivity in acrylonitrile was 50% for a conversion level of propane of 60%. In parallel, the selectivity to COx decreased with time on stream leading to the idea that the total oxidation became limited in time. Propylene formation was not observed during the 24 h of reaction and no deactivation of the catalyst was noted during this period. For the catalysts with different V/Al ratios, the trends were similar to that presented in Fig. 5. The catalytic activity of the VAlON catalysts depends on the V/Al ratio as observed from the results depicted in Fig. 6, after 24 h of reaction. The optimal V/Al ratio appeared to be 0.25, which presents the highest ACN yield of 30%. In parallel, for VAlON0.25, the selectivity to COx was the lowest. The formation of COx and N2 was favoured over VAlON0.9 and
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Fig. 5. Time on stream behaviour over VAlON0.25 catalyst, GHSV = 16.8 l/g h, propane:oxygen:ammonia = 1.25:3:1: (~) propane conversion; (&) selectivity to ACN; (~) yield of ACN; (*) selectivity to AcCN; (*) selectivity to COx; (&) selectivity to N2.
VAlON0.7, with higher content of vanadium and lower content of nitrogen.
4. Discussion The study of the relation between structure and activity is essential to understand the factors controlling the reactivity and the key features of a catalyst. Particularly, for ammoxidation catalysts, the role of different vanadium species (V5+, V4+, V3+) and the role of different nitrogen species are very important and are still under debate. To answer these questions, the structural characteristics of the vanadium aluminium oxynitride catalysts as a function of the V/Al ratio will first be discussed, and then related to their catalytic performances. 4.1. Structural changes induced by different compositions in ‘‘VAlON’’ catalysts The experimentally determined V/Al molar ratios were lower than the theoretical values for the ratios higher than 0.7 (see Table 1), probably due to the poor precipitation of
vanadium with aluminium in the starting solution. This appears to be an important parameter in the preparation of the ‘‘VAlO’’ precursor. The highest surface area is observed for VAlON0.1 with the lowest content of vanadium (Table 1). By increasing the V/Al ratio, the surface area diminished, and in addition, the nitrogen content diminished as well, this suggesting that, for higher V/Al ratios, nitrogen insertion was more difficult to achieve. We observed the same tendency for the total nitrogen content as for the surface nitrogen determined by Kjeldahl method. As indicated in Table 2, the nitrogen content changed for the used catalysts. This obviously means that the nitridation, for all the V/Al ratios, does not allow an optimal activation of the catalysts and that the reduction–oxidation process during the catalytic test allows to stabilize the solid. Raman spectroscopy provides information about the surface vanadium species of the vanadium aluminium oxide precursor as a function of the V/Al ratio. The survey of the literature [20] shows the following evolution of the different vanadium oxide species with the increase of the vanadium loading: orthovanadate (VO4) ! pyrovanadate (V2O7) ! metavanadate (VO3)n ! decavanadate (V10O28)
Fig. 6. Catalytic behaviour over VAlON catalysts with different V/Al molar ratio, GHSV = 16 800 ml/g h, propane:oxygen:ammonia = 1.25:3:1, 500 8C: (~) propane conversion; (&) selectivity to ACN; (~) yield of ACN; (*) selectivity to AcCN; (*) selectivity to COx; (&) selectivity to N2.
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! vanadium pentoxide (V2O5). There are three possible active bridging oxygen sites: terminal V O, binding surface polymeric species V–O–V, and binding vanadium to support V–O–S. The isolated vanadium oxide species have one terminal V O bond and three V–O–Al bonds, while the polymeric vanadium oxide species consist of a terminal V O bond with one V–O-support and two bridging V–O–V bonds. [21]. The spectra of the vanadium aluminium oxide precursor presented in Fig. 1 as a function of vanadium loading are in agreement with the literature data, meaning that higher vanadium contents favour the formation of decavanadate species (band at 980 cm1). Besides, the decrease of the vanadium loading increased the quantity of isolated vanadium species (band at 1050 cm1) and of metavanadate species (band at 940 cm1) formed. A correlation can be made between the nature of the vanadium oxide species and the nitrogen content. For the higher V/Al ratios, the V–O–V bonds of polymeric vanadium oxide species are predominant and at the same time the nitrogen content decreases, while for the lower V/ Al ratios there is a predominance of the V–O–Al bonds of isolated vanadium oxide species type and the nitrogen content is higher. This suggests that, when increasing concentration of the monomeric vanadium species, the degree of nitridation increases and one may suppose that the oxygen replaced by nitrogen during the nitridation process could be the one bridged to V and Al. Thus, the substitution of oxygen by nitrogen is favoured for the lower V/Al ratios, for which we found a higher surface concentration of isolated vanadium species. Also, if we take in account the type of coordination of vanadium in these different species, one can observe that the tetrahedral coordination favours the substitution of oxygen by nitrogen, while for the octahedral coordination the nitridation is more difficult. The XPS data also shows differences as a function of the V/Al ratio, as for example: (i) generation of different nitrogen species, and (ii) different reduction degree of vanadium. Fig. 7 points to a direct relation between the V/Al ratio and the type of nitrogen species as determined by XPS formed during the nitridation process. This plot shows that the formation of nitride species (N3) is favoured at high V/Al ratio while the amount of NHx species is favoured on the samples with low V/Al ratios. However, the total amount of nitrogen species increased with the decrease of the V/Al ratio. Contradictory results are obtained from the chemical analysis of the total and surface nitrogen content. If we admit that the difference between the total nitrogen content determined by Grekov method and the surface nitrogen content determined by Kjeldahl method represent the amount of N3 species from the bulk of the catalysts, one can observe that the amount of N3 species increases with decreasing V/Al ratio (Fig. 8).
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Fig. 7. Variation of different nitrogen component area determined by XPS as a function of V/Al ratio: (*) N3; (&) NHx; (~) dinitrogen.
The amount of bulk nitrogen species (N3) determined from the differences between total and surface nitrogen contents (from chemical analysis), as well as N3 determined by XPS, are plotted in Fig. 8. It is seen that the bulk nitrogen determined by chemical analysis and by XPS is the same for the VAlON0.9 sample, indicating the homogeneity of this sample. It is well known from the literature and supported by the RAMAN spectra of the VAlO samples, that when the vanadium loading increases, the possibility of formation of surface polyvanadates increases [22]. If we take into account that, in the case of VAlON catalysts these polyvanadate species are formed for the higher V/Al ratios, it indicates that the nitridation of these species seems more difficult to achieve and probably only the terminal V O species could be nitrided. When the V/Al molar ratio diminishes, the probability of formation of polyvanadate species diminishes, and the nitridation is much deeper for lower V/Al ratio. The increase of the total and surface nitrogen content with the V/ Al molar ratio is consistent with this hypothesis. The fact that, after the catalytic test, the bulk nitrogen content diminished compared with the bulk nitrogen content for the fresh samples can be explained by the migration of
Fig. 8. N3 species determined by differences of total nitrogen and surface nitrogen (Kjedhal) (&); N3 species determined by XPS (~).
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Fig. 9. Correlation between the bulk nitrogen and degree of reduction of vanadium.
the nitride species to the surface of the catalyst during the catalytic test in the presence of oxygen from the reaction mixture. This phenomenon was also observed during the oxidation of ‘‘AlGaPON’’ catalysts [23]. Moreover, in the case of the VAlON0.9 catalysts, the component corresponding to N3 disappeared after the reaction, as a consequence of the consumption of these species during the reaction. Another relevant aspect of the VAlON catalysts determined from the XPS data is the reduction degree of vanadium. The XPS spectra of the fresh catalysts present a split of the V 2p3/2 level, observed at 517.1–517.5 and 515.5–516.3 eV (Table 3). Taking into account the limitation in the assignment of these peaks, an approximate estimation of the atomic V5+/(V5+ + V4+) surface ratio can be made using the XPS technique (Table 4). For the ‘‘VAlON’’ samples, the atomic V5+/(V5+ + V4+) surface ratio decreases with increasing V/Al ratio. Moreover, by decreasing the V/ Al ratio, the amount of nitrogen incorporated during the nitridation process increases, and as consequence the reduction degree of vanadium increased. The reduction degree of vanadium as a function of bulk nitrogen is shown in Fig. 9. This dependence presented in Fig. 9 suggests that the oxidation state of vanadium depends on the quantity of nitrogen inserted by oxygen substitution. Thus, the highest degree of reduction of vanadium (0.72) corresponds to the highest content of bulk nitrogen inserted by nitridation (2.73%). However, both the degree of nitridation and the degree of reduction could be linked to the degree of polymerisation of vanadium. A diminution of atomic V5+/(V5+ + V4+) surface ratio is observed for all the used catalysts, meaning that vanadium is also reduced during the reaction (Table 5), due to the presence of propane and ammonia in the gas feed. The highest reduction occurring for the lower V/Al ratio, which could be explained by the presence of isolated vanadium species, are easier to reduce as compared with the polymeric species [24]. The progressive and general decreasing trend of the binding energies of Al 2p with the atomic nitrogen
percentage, as presented in Table 3, was explained by the higher nucleophilic character of nitrogen compared to oxygen, which reduces the positive charge around the aluminium. The role of Al3+ ions in the VAlON system is not yet well established. We believe that aluminium ions are not directly involved in the catalytic reaction. However, they could play an important role in increasing the surface dispersion of vanadium. In summary, the catalysts characterization showed the following trends as a function of the V/Al ratio: (i) the surface area increases with the decrease of the V/Al ratio, (ii) the nitrogen content increases with the decrease of V/Al ratio, (iii) the type of nitrogen species depends on the V/Al ratio, higher content of bulk nitrogen was observed for the lower V/Al ratio, (iv) higher degree of polymerisation of vanadium oxide species for the higher V/Al ratio and (v) the reduction degree is different as a consequence of the nitrogen insertion during the nitridation step. 4.2. Study of the catalytic behaviour The catalytic performances of ‘‘VAlON’’ as a function of the V/Al ratio of the catalysts are presented in Fig. 6. The increase of the V/Al ratio from 0.25 to 0.9 leads to: (i) a decrease of the selectivity to acrylonitrile, (ii) an increase of the selectivity to carbon oxides and (iii) an increase of the amount of nitrogen formed by ammonia oxidation. It must be stressed that regardless of the V/Al ratio, the conversion of propane is nearly constant. The highest yield of acrylonitrile is obtained over VAlON0.25. Particular results were obtained for the VAlON0.1 sample, with the lowest vanadium content: (i) a decrease of the selectivity to acrylonitrile is accompanied by (ii) a decrease of propane conversion, indicating that under these conditions there are not enough active sites for propane activation. As seen in Fig. 5, the steady-state conditions are achieved after 4 h on stream. An explanation of this phenomenon could be the fact that the degree of reduction of the catalyst can increase with time on stream due to the presence of propane and ammonia in the gas feed. Also, these results correlate well with the increase of the nitrogen content as a function of time on stream, until the steady state is reached. Below, we shall attempt to correlate the catalytic behaviour of the VAlON system with two important parameters, which depend on the V/Al molar ratio: (i) the reduction degree of vanadium and (ii) the quantity and type of nitrogen species. In order to establish a structure–activity relationship and to understand the factors controlling the reactivity and the key features of the catalyst, the role of different vanadium species (V5+, V4+, V3+) in propane ammoxidation mechanism is often under debate. Most of the studies attribute the activation of propane to V5+ and V4+ centers [25]. However, a recent publication stressed that V3+ may also have an effect on the enhancement of the activity of an alkane molecule [26].
M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10
Fig. 10. Selectivity to ACN (&), nitrogen (&) and conversion of propane (~) as a function of the V5+/(V5+ + V4+) ratio determined by XPS.
Fig. 10 depicts the catalytic results of the ‘‘VAlON’’ system as a function of the surface V5+/(V5+ + V4+) atomic ratios measured after the catalytic test. With decreasing surface V5+/(V5+ + V4+) atomic ratio, the selectivity to acrylonitrile increased and reached a maximum for VAlON0.25. It should also be mentioned that the formation of nitrogen from ammonia oxidation and the carbon oxides production from total oxidation reaction increased with the increase of the amounts of the V5+ species. These species enhance the side reaction of ammonia oxidation to N2 (see Fig. 10). From the literature, it is known that V2O5 is the active phase for the oxidation of ammonia into nitrogen [27,28]. A maximum of the catalytic activity is obtained for the V5+/(V5+ + V4+) ratio of 0.27 (VAlON0.25 sample). Thus, the presence of V5+ species in large amounts on the surface has a negative effect due to side reactions, but its role when present in lower amounts is questionable. The most efficient sites of the VAlON system for the acrylonitrile formation seems to be the V4+ species, which agrees with the recent studies of Millet and coworkers on the oxidation state of vanadium in mixed vanadium and iron antimonite oxides [29]. By changing the V/Al ratio, the degree of the nitridation is different, as it was observed from the XPS results. Since the catalytic activity of the VAlON in propane ammoxida-
9
tion depends on the V/Al ratio, and that the nitrogen content also depends on the V/Al ratio, we can admit that the quantity of nitrogen generated by nitridation has an influence on the catalytic behaviour. Indeed, the selectivity to ACN as well as the formation of COx depends on the bulk nitrogen content as well as on the surface nitrogen content, illustrated in Fig. 11. The selectivity to acrylonitrile increases with the nitrogen content, both for surface and bulk nitrogen species, indicating the role of these species in the N-insertion step to form acrylonitrile. Changes in the concentration of nitrogen species were observed upon use in propane ammoxidation, indicating the participation of nitrogen species from the VAlON framework in the N-insertion step to form acrylonitrile. Thus, it is necessary to pay a special attention to the surface restructuring phenomena during the catalytic reaction, as a central aspect for the understanding of the surface reactivity of the oxynitrides. On the vanadium aluminium oxynitrides, this phenomenon is possible as the nitridation process is a reversible one. This behaviour was observed also on aluminium phosphate oxynitride catalysts, AlPON [23], and should be confirmed on the case of VAlON. Taking into account that (i) the total nitrogen content of VAlON catalysts is modified upon use in propane ammoxidation according to XPS and chemical analysis, and (ii) the nitridation process is a reversible one, a Mars and Van Krevelen mechanism applied to nitrogen could be considered. Deeper investigations of this phenomenon are the subject of others studies performed over VAlON catalysts and will be published elsewhere [30]. As indicated by the above results, propylene formation as an intermediate was not observed for all the series of VAlON systems. Moreover, by performing propylene ammoxidation on these catalysts a conversion of 100% was reached. This observation, together with the fact that the VAlON system shows a high acrylonitrile selectivity at very low space–time value, suggests a different reaction mechanism as compared with conventional metallic oxide ammoxidation systems. In the case of VAlON, the catalytic behaviour can be explained by a combination of different simultaneous effects: (i) a better efficiency in the activation of the C–H bond of the
Fig. 11. Selectivities to ACN (&) and COx (*) as a function of the bulk nitrogen (a) and surface nitrogen (b).
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alkane due to a balanced redox capacity of vanadium species, and (ii) a high nitrogen content which enhances the nitrogen insertion rate for the acrylonitrile formation.
5. Conclusions The results presented above show that the catalytic performances depend on the V/Al ratio, with an optimal selectivity to acrylonitrile obtained for V/Al of 0.25. These results could be explained by an optimal degree of polymerisation of vanadium species from oxide precursor, which induces an optimal degree of nitridation and, further, an optimal reduction degree of vanadium. The prior nitridation of the V–Al oxide precursor with the generation of the nitrogen species is beneficial for ammoxidation of propane, indicating the positive role of these species in this reaction. The results obtained in this reaction established that these new compounds are important catalytic materials, presenting tuneable properties by variation of both the V/Al and N/O ratios.
Acknowledgements The authors would like to thank the ‘‘Direction Ge´ ne´ rale des Technologies, de la Recherche et de l’E´ nergie’’ de la Re´ gion Wallonne (GREDECAT) and ‘‘La Communaute´ Franc¸aise de Belgique’’ for financial support.
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