Vanadium aluminium oxynitride catalysts for propane ammoxidation reaction: Effect of the V/Al ratio...

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Applied Catalysis A: General 286 (2005) 1–10

0

d

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,�

a University of Bucharest, Faculty of Chemistry, B-dul Regina Elisabeta 4-12, 030016 Bucharest, Romaniab Universite Catholique de Louvain, Unite de Catalyse et Chimie des Materiaux Divises,

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.� Deceased.

926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

oi: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.

M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–102

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.

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 0.10 0.10 160 117

VAlON0.25 0.25 0.25 153 133

VAlON0.5 0.50 0.50 130 78

VAlON0.7 0.70 0.66 122 93

VAlON0.9 0.90 0.82 143 124

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 h�1) 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 10�2N (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 DILOR-

JOBIN YVON-SPEX spectrometer, model OLYMPUS DX-

40, equipped with a He–Ne (l = 632.8 nm) laser. The

spectra were recorded in the 200–1600 cm�1 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 tempera-

ture 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 spectro-

meter was used to check the NOx formation. A carbon mass

balance of 80–90% was obtained, because of the condensa-

tion 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

M. Florea et al. / Applied Catalysis A: General 286 (2005) 1–10 3

Table 2

Nitrogen content before and after catalytic test as a function of the V/Al

ratio

Catalyst NT (%)a NK (%)b

Before test After test Before test After test

VAlON0.1 4.93 2.30 n.a.c n.a.

VAlON0.25 5.35 5.18 3.06 3.57

VAlON0.5 4.76 2.91 2.72 0.85

VAlON0.7 3.28 1.86 1.36 0.47

VAlON0.9 2.26 1.80 1.02 0.51

a Total nitrogen content determined by Grekov method.b Nitrogen content determined by Kjeldahl method.c 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 cm�1

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 cm�1 is assigned to surface

decavanadate species [HnV10O28](6�n)� [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).

940 cm�1 confirms the presence of [(VO3)n]n� metavana-

date species [13], and (iii) the band at 1050 cm�1 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 cm�1. 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 cm�1 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 character-

ized 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


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 74.1 531.0 517.1 399.8

515.7

Used 73.5 530.3 516.4 399.2

515.2 402.8

VAlON0.25

Fresh 74.2 531.2 517.5 397.5

516.3 399.9

401.8

Used 73.9 530.8 517.5 400.3

516.4 398.5

VAlON0.5

Fresh 74.3 531.2 517.4 397.1

515.5 400.0

402.6

Used 73.6 530.2 516.8 399.3

515.3 402.3

VAlON0.7

Fresh 74.4 531.2 517.5 397.4

516.0 400.3

402.1

Used 72.9 530.7 516.9 398.6

515.5 401.7

VAlON0.9

Fresh 74.45 531.1 517.3 397.4

515.5 399.8

401.8

Used 74.3 530.9 517.2 399.0

515.8 400.6

Fig. 2. XPS spectra of the N 1s region.

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 0.72

Used 0.42

VAlON0.25

Fresh 0.76

Used 0.27

VAlON0.5

Fresh 0.79

Used 0.63

VAlON0.7

Fresh 0.80

Used 0.72

VAlON0.9

Fresh 0.81

Used 0.73

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).


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 32.95 62.36 1.90 1.77 0.06

Used 2.30 33.83 64.75 2.4 1.32 0.07

VAlON0.25

Fresh 5.35 28.7 65.45 4.20 2.64 0.15

Used 5.18 31.03 61.47 4.0 3.12 0.13

VAlON0.5

Fresh 4.76 21.73 68.02 7.02 2.40 0.31

Used 2.91 28.28 60.05 7.33 2.65 0.26

VAlON0.7

Fresh 3.28 20.94 66.80 8.90 1.18 0.42

Used 1.86 23.0 65.36 8.10 1.76 0.34

VAlON0.9

Fresh 2.26 23.34 67.80 8.20 1.64 0.35

Used 1.80 32.45 62.71 5.90 1.84 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 ammoxida-

tion, 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


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

Fig. 6. Catalytic behaviour over VAlON catalysts with different V/Al molar ratio,

propane conversion; (&) selectivity to ACN; (~) yield of ACN; (*) selectivity

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)

GHSV = 16 800 ml/g h, propane:oxygen:ammonia = 1.25:3:1, 500 8C: (~)

to AcCN; (*) selectivity to COx; (&) selectivity to N2.


Fig. 7. Variation of different nitrogen component area determined by XPS

as a function of V/Al ratio: (*) N3�; (&) NHx; (~) dinitrogen.

Fig. 8. N3� species determined by differences of total nitrogen and surface

nitrogen (Kjedhal) (&); N3� species determined by XPS (~).

! 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 cm�1). Besides, the decrease of the

vanadium loading increased the quantity of isolated

vanadium species (band at 1050 cm�1) and of metavanadate

species (band at 940 cm�1) 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).

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 poly-

vanadates 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. 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 correspond-

ing 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].


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-

Fig. 11. Selectivities to ACN (&) and COx (*) as a funct

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 frame-

work 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

ion of the bulk nitrogen (a) and surface nitrogen (b).


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 Generale

des Technologies, de la Recherche et de l’Energie’’ de la

Region Wallonne (GREDECAT) and ‘‘La Communaute

Francaise de Belgique’’ for financial support.

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