ORIGINAL PAPER

Ammoxidation of ethylene to acetonitrile over chromium or cobaltalumina catalysts prepared by sol–gel method

F. Ayari Æ M. Mhamdi Æ G. Delahay ÆA. Ghorbel

Received: 19 May 2008 / Accepted: 12 November 2008 / Published online: 4 December 2008

� Springer Science+Business Media, LLC 2008

Abstract A series of Cr/Al2O3 and Co/Al2O3 catalysts were

tested in the selective ammoxidation of ethylene to acetoni-

trile. Catalysts were prepared either by sol–gel method or by

impregnation with chromium or cobalt acetylacetonate salts.

Physicochemical properties of catalysts were accomplished

by several techniques such as chemical analysis, physisorp-

tion of N2, X-ray diffraction (XRD), 27Al MAS NMR, UV–

Visible diffuse reflectance (DRS) and Raman spectroscopy

and temperature programmed reduction of H2 (H2–TPR).

Textural analysis reveals that mesoporous materials with

pronounced surface areas were obtained using sol–gel pro-

cedure while impregnation of the support produces a moderate

decrease of its surface area and pore volume. XRD analysis

confirms the presence of highly dispersed metal species which

reside essentially on the surface and measure less than 4 nm.

Furthermore, 27Al MAS NMR shows that for xerogels, part of

metal species occupies sites on/in A12O3 in close vicinity of

octahedral 27Al. This, apparently, is not the case for aerogels.

For Cr/Al2O3 catalysts, isolated Cr6?, mono and polychro-

mate species were identified using DRS, Raman Spectroscopy

and H2–TPR which seem to play a key role in the ammoxi-

dation of ethylene. Furthermore, for cobalt doped catalysts,

CoAl2O4 was identified as active phase on the basis of DRS

and H2–TPR results. From the supercritical drying, it results

generally better catalysts than catalysts calcined by ordinary

procedure which leads to inactive agglomerated Co3O4 and

CoO–Al2O3 phase.

Keywords Cobalt � Chromium � Alumina � Sol–gel �Ammoxidation � Acetonitrile

Abbreviation

ATB Aluminum-tri-sec-butoxide

SB Sec-butanol

TCD Thermal conductivity detector

1 Introduction

Sol–gel processing designates a type of highly divided

solid materials synthesis procedure, performed at low

temperature (typically T \ 100 �C) in a homogeneous

solution that contains both the metal and support precur-

sors. In general, sol–gel can allows the obtention of

materials such as glasses and ceramics with a metastable

character and very attractive physical and chemical prop-

erties not achievable by other means of low temperature

chemical synthesis [1]. Sol–gel solids have characteristics

which make them good materials for applications in many

catalytic reactions such as selective oxidation [2], selective

reduction [3], polymerization [4] and nitroxidation of ole-

fins [5], paraffins [6] and aromatics [7–10]. Catalytic

reactions can be of the acid–base or of the redox type. In

the second case redox catalytic sites are due to transition

metal atoms (Cu, Fe, Mo, V, Cr, Co) able to change their

valence state [11].

It is well known that alumina constitutes a family of

materials whose importance in the field of heterogeneous

catalysis is enormous and constantly growing. In fact,

F. Ayari (&) � M. Mhamdi � A. Ghorbel

Laboratoire de Chimie des Materiaux et Catalyse, Departement

de Chimie, Faculte des Sciences de Tunis, Campus Universitaire

Tunis ElManar, 2092 Tunis, Tunisie

e-mail: lcmc_fa@yahoo.fr

G. Delahay

Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM2-

ENSCM-UM1, Eq. ‘‘Materiaux Avances pour la Catalyse et la

Sante’’ ENSCM (MACS - Site la Galera), 8, rue Ecole Normale,

34296 Montpellier Cedex 5, France

123

J Sol-Gel Sci Technol (2009) 49:170–179

DOI 10.1007/s10971-008-1860-7

Al2O3 has been extensively used as support in many cat-

alyst formulations, mainly due to their low cost, particular

texture, good thermal stability, and the important interac-

tion that it exhibits with deposited transition metals [12].

Cr/Al2O3 catalysts are active in the catalytic dehydro-

genation of alkanes. The main interest was devoted to the

dehydrogenation of propane and isobutane. In fact, propene

is used like precursor for the synthesis of high-purity

polypropylene, whereas, isobutene is needed for the syn-

thesis of some gasoline additives (like MTBE and ETBE)

[13–15]. Co/Al2O3 catalysts have important application in

Fischer-Tropsch synthesis [16–18] which is a promising

process on the conversion from natural gas and coal to oil

products, and exhibited in particular appreciable capacity

for the selective catalytic reduction (SCR) of NO with

propane or propene in the presence of oxygen [19–21].

Some works [19, 20, 22, 23] reveal that an optimum of NOx

SCR activity was observed in the case of Co/Al2O3 cata-

lysts prepared by sol-gel method.

Selective ammoxidation of light hydrocarbons (C2) to

acetonitrile is of a prime practical signification as it could

substitute the source for synthesis of a valuable nitrile cur-

rently produced from hazardous, high cost, and less-selective

processes. First, Li and Armor [24, 25], lately Wichterlova

and co-workers [26], and recently Ghorbel and co-workers

[27] reported that ethane or ethylene can be efficiently con-

verted to acetonitrile in the presence of ammonia and oxygen

over cobalt exchanged zeolite catalysts.

During the course of this work, a series of Cr/Al2O3 and

Co/Al2O3 catalysts were prepared using either the sol–gel

or impregnation methods, and then tested in the ammoxi-

dation of ethylene to acetonitrile. Impregnation method

was performed using aerogel support in the order to syn-

thesise reproducible, homogeneous and thermally resistant

materials [28]. The texture of catalysts was studied by

physical adsorption of N2 at liquid nitrogen temperature to

determine total surface areas and porosity. XRD and 27Al

MAS NMR were used to monitor the structure of catalysts,

while DRS, Raman and H2–TPR were used to elucidate the

metal coordination and the oxidation state. Catalytic

behavior of solids in the ammoxidation reaction is then

reported. This includes selectivity and activity results

versus the reaction temperature, the nature of the metal

incorporated, and the method of catalyst preparation.

2 Experimental

2.1 Catalysts preparation

Sol–gel derived materials were prepared as follow: ATB

(Acros 97%) was dissolved at room temperature in SB

(Acros 99%) for 1 hour. An appropriate amount of

chromium or cobalt metal, issued from chromium (III)

acetylacetonate (Acros 97%) or cobalt (II) acetylacetonate

(Merk 98%) respectively, was then added to the alcoholic

solution of aluminum alkoxide and the stirring was con-

tinued for a period of 1 h. A controlled amount of water

was added dropwise into the solution for hydrolysis. The

system gelled in a few minutes, resulting in a consistent

and coloured gel depending on the nature of the metal.

The molar ratios of reactants were SB/ATB = 1/0.1, H2O/

ATB = 0.15/0.1, Cr/ATB = 0.007/0.1, and Co/ATB =

0.006/0.1. Drying procedure is done either in supercritical

conditions of solvent or by evaporating under normal

conditions. In the first way, the gel was placed inside an

autoclave (Prolabo) that was sealed and heated to 275 �C,

resulting in pressure rise at 41 bar. The sample was

maintained for 0.5 h under supercritical conditions and the

aerogel (Ae) was obtained after solvent evacuation. Fur-

thermore, xerogel (Xe) is obtained after SB evaporation in

oven at 120 �C for 24 h. Aerogel and xerogel samples were

calcined in oxygen at 500 �C for 3 h (1.8 L/h, 2 �C/mn).

Wherein, this group of catalysts was referred to us by

AlAeMSG and AlXeMSG, where Al stands to alumina, M

to chromium or cobalt, and SG to sol–gel. For impregna-

tion, alumina aerogel was prepared according to the same

procedure described above but no metal salts were added to

the alcoholic solution of ATB before hydrolysis. The molar

ratios of reactants were kept the same mentioned above.

Before impregnation, a calcination of alumina aerogel

support was done in oxygen stream for 3 h at 500 �C.

Impregnation were performed by mixing alumina aerogel

support with chromium (III) or cobalt (II) acetylacetonate

in a minimum amount of SB for 0.5 h. Cr/ATB and

Co/ATB molar ratios were 0.007/0.1 and 0.006/0.1,

respectively, which correspond to 5wt% of metal. The

mixture was then dried at 120 �C for 24 h, heated in

helium atmosphere for 12 h at 500 �C (1.8 L/h, 2 �C/mn)

and finally calcined in oxygen stream for 3 h at 500 �C.

Impregnated catalysts and alumina support were labelled

as AlAeMI and AlAe500, respectively. CrO3–Al2O3 and

Cr2O3–Al2O3 catalysts were prepared by a simple

mechanical mixture of CrO3 (Merck 99%) and Cr2O3

(Aldrich 99.9%) with commercial grade alumina to serve

as a reference in catalysis.

2.2 Catalysts characterization

The analytical metal content of catalysts was determined

by atomic absorption spectroscopy using a Solar S4, PYE

UNICAM type spectrometer. Materials were dissolved in

HF, HCl and HNO3 at room temperature for 24 h and then

heated by mild heating. The remaining residue was dis-

solved in a minimum amount of HNO3 and made up to

standard volume. SBET and porosity were determined from

J Sol-Gel Sci Technol (2009) 49:170–179 171

123

adsorption–desorption isotherms measured at 77 K in an

automatic Micromeritics Asap 2000 Analyser. Before the

measurement, the samples were outgassed at 200 �C during

7 h. BET model was used to calculate the specific surface

area. The total pore volume was calculated by means of the

total amount of adsorbed gas at P/P� = 0.98. Furthermore,

desorption isotherms were used to calculate pore size dis-

tribution by applying the BJH method. XRD patterns of

catalysts were collected on a X’Pert Pro PANalytical dif-

fractometer with CuKa radiation, k = 1.54060 A at 40 kV

and 40 mA in the range 2h of 20–70�. The diffractometer

was operated at 1.0� diverging and 0.1� receiving slits at a

scan rate of 0.05�/mn. 27Al MAS NMR spectra were

recorded at 78.20609 MHz on a Bruker WB spectrometer

using AlClO3 � 6H2O as reference. An overall 4096 free

induction decays were accumulated. The excitation pulse

and recycle time were 6 ls and 0.063 s, respectively. DRS

spectra were recorded at room temperature in the range

200–900 nm on a Perkin Elmer Lambda 45 spectropho-

tometer equipped with a DR attachment. A Certified

Reflectance Standard (Labsphere) was used as the refer-

ence material for all catalysts and DR spectra were

converted to the Kubelka-Munk function in order to con-

vert reflectance data into pseudo-absorbance. Raman

measurements were carried out on a confocal Thermo

Scientific DXR Raman Microscopy system using the visi-

ble line at 532 nm. The maximum incident power at the

sample was approximately 10 mW. TPR in H2/Ar (3%,

temperature ramped from ambient to 1000 �C at 15�C/min)

was performed with 0.04 g of catalyst using a Micromer-

itics Autochem 2910 Analyser, in a Pyrex U-tube reactor

and with an on line TCD. The TCD, connected to a PC

computer, allowed studying the reduction profiles of H2

with the temperature. Catalytic experiments were per-

formed under atmospheric pressure using down flow

tubular glasses reactor. The temperature of the reactor was

controlled by a proportional band temperature controller,

using a Chromel–Alumel sensing thermocouple that was

directly attached to the outside of the reactor at the mid-

point of the catalyst bed. Measurements of the catalytic

activity of the various catalysts were performed at 450–

500 �C using an exact amount (0.1 g) of catalyst and a

reaction stream consisted of a mixture of 10 vol.% ethylene

(Air Liquid 99.995%), 10 vol.% ammonia (Air Liquid

99.96%), and 10 vol.% oxygen (Air Liquid 99.995%).The

total flow rate was then maintained at 0.1 L/mn by

appropriate adjustment of the remaining helium (Air

Liquid 99.998%) flow rate. The analysis of the outlet flow

was recorded on line by two chromatographic units, one

operated with a flame ionization detector while the other

was equipped with a TCD.

The conversion, selectivity and activity are defined as

follow:

Conversion of C2H4 : X ¼P

i yini

yEnE þP

i yini

Selectivity of product Pi Acetonitrileð Þ : Si ¼yiniPi yini

Activity of product Pi Acetonitrileð Þ : Ai ¼na � 190:97

mcatalyst

where yi and yE are the mole fractions of product Pi and

C2H4, respectively; ni and nE are the number of carbon

atoms in each molecule of product Pi and C2H4, respec-

tively, and na and mcatalyst are the mole number of produced

acetonitrile and catalyst weight, respectively. All the terms

were evaluated for the exit stream.

3 Results and discussion

3.1 Chemical analysis

The metal compositions of the different catalysts are pre-

sented in Table 1. As expected, the metal content is higher

for samples prepared by sol–gel method. Catalysts issued

from impregnation exhibit a low chromium or cobalt

retention since preparation steps result in a loss of metal.

The sublimation of metal acetylacetonate can explains both

the low Cr and Co content.

3.2 BET measurements

Nitrogen adsorption results indicate that adsorption–

desorption isotherms can be classified as type IV according

to the BDDT classification [29], and show additionally a

limited N2 uptake at low relative pressure, indicating the

presence of a mesoporous structure. The shape of the hys-

teresis as indicated by DeBoer classification [30] is similar to

the type ‘A’ hysteresis loop attributed to solids having

cylindrical pores with constant cross section. A pronounced

mesoporosity with a relatively mono-modal pore size dis-

tribution is observed for all samples, excepting AlXeCoSG

catalyst which has a bi-modal pore size distribution and point

out the presence of a second mesopore with a size close to the

Table 1 Metal content of catalysts

Catalysts Cr (wt%) Co (wt%)

AlAeCrSG 3.5 –

AlXeCrSG 3.9 –

AlAeCrI 2.1 –

AlAeCoSG – 4.2

AlXeCoSG – 3.9

AlAeCoI – 2.9

172 J Sol-Gel Sci Technol (2009) 49:170–179

123

macropore range. Alumina support exhibits the same type of

isotherm with a type ‘A’ hysteresis loop, indicating the

presence of the same network structure.

Table 2 summarises textural properties of synthesised

materials. It can be seen that SBET and Vp of support

(AlAe500) decrease after impregnation. On the other hand,

there is no noticeable change in the pore size distribution of

AlAe500 sample after impregnation, but Dp exhibits a

slight decrease after chromium doping providing the evi-

dence that some active phase is dispersed into the

intraparticle porosity of support [31]. Sol–gel derived cat-

alysts exhibit the highest SBET when compared to those of

impregnated catalysts. In fact, SBET values are in the range

of 346–430 m2/g while the higher are attributed to samples

which are dried in ordinary conditions. It can be also shown

that Vp and pore size of impregnated catalysts do not

undergo a significant change when compared to those of

sol–gel derived catalysts since the alumina support used is

also sol–gel derived.

Difference in textural properties for impregnated and

sol–gel derived catalysts can be explained as follow: the

SBET surface area of parent support undergoes a little

decrease after impregnation indicating an uniform spread-

ing of metal residues on the surface of alumina which leads

to pore blockage. The metal ions which most probably

exist at the pore entrance reduce the empty space available

for the adsorption of N2 [32, 33]. For sol–gel derived cat-

alysts, the direct inclusion of metal species to the solvated

alumina matrix during the sol–gel synthesis allowed the

formation of nanoscale primary particles which are con-

nected to form aggregates during gelation. After

calcination, these primary particles are bound together

strongly to form a solid network, and a large intraparticle

space with uniform nanoscale pore is formed and the larger

surface areas are expected [34].

3.3 XRD

Figure 1 shows the XRD patterns of Cr/Al2O3 and Co/

Al2O3 catalysts. The patterns are ascribed to an amorphous

phase in which the metal ions are dispersed at atomic level.

XRD patterns of aerogels show diffraction peaks at 2hvalues around 37, 46, and 67� which are attributed to cubic

c-Al2O3 [35]. No chromium or cobalt oxide peaks were

detected due to the high dispersion and their overlap with

those for c-Al2O3. This result suggests that metal species

measured less than 4 nm and reside essentially on the

surface. The crystallographic structure of prepared cata-

lysts is independent of the nature of metal but it largely

depends on drying method. It seems that drying in normal

conditions results in poorly crystallite phase but super-

critical drying reveals a nanosized crystallite state.

3.4 27Al MAS NMR

27Al MAS NMR spectra of Cr/Al2O3 and Co/Al2O3 cata-

lysts are shown in Figs. 2 and 3, respectively. As expected

from Fig. 2b and 3b, AlXeCrSG and AlXeCoSG NMR

spectra display a single peak at 2.31 and 2.87 ppm attrib-

uted to octahedral Al species [36–39]. Additional peak at

60 ppm, attributed to tetrahedral Al is observed for AlA-

eCrSG, AlAeCoSG and impregnated catalysts.

Most of peaks are asymmetric due to quadrupolar

interaction. It is known from the literature that paramag-

netic species (like Crn? with n \ 6 [40], or Co3O4 and CoO

[18]) quench 27Al lines in NMR [40–42] due to relaxation

phenomena [43]. The spectra of xerogels show that only

octahedral Al3? of Al2O3 is affected, suggesting that part

of paramagnetic species occupies sites on/in A12O3 in

Table 2 Textural properties of

support and catalysts

a By BET, using the equation:

Dp = 4 * Vp/SBET

b Calculated from the highest

partial-pressure adsorption pointc Most frequent pore size

Samples SBET (m2/g) Dpa (A) Vpb (cm3/g) Pore size distribution (A)

AlAe500 279 116 0.80 35–135 (52)c

AlAeCrI

AlAeCrSG

AlXeCrSG

270

385

423

112

94

87

0.75

0.90

0.90

30–140 (54)

20–160 (47)

20–140 (33)

AlAeCoI 243 122 0.75 30–150 (54)

AlAeCoSG

AlXeCoSG

346

430

112

108

0.96

1.15

20–170 (54)

20–70 (40); 100–450 (265)

Fig. 1 XRD patterns of Cr/Al2O3 and Co/Al2O3 catalysts

J Sol-Gel Sci Technol (2009) 49:170–179 173

123

close vicinity of octahedral 27Al. This, apparently, is not

the case for aerogels.

3.5 DRS

DRS spectra of Cr/Al2O3 catalysts are shown in Fig. 4; a

typical proposed deconvolution in Gaussian bands [44] for

AlAeCrI catalyst is shown in Fig. 5. Spectra consist of

absorption bands in the charge transfer (CT) region

(\500 nm) and monitor no d–d absorptions ([500 nm)

assigned to pseudo-octahedric Cr3? or (Cr2O3).

Such spectrum is typical for metallochromates [44, 45].

The bands are assigned to O2- ? Cr6? CT (see Table 3).

No dichromate band has been detected since dichromate

species occurs on acidic surface such as SiO2 (pHzc = 1–2)

[48]. Only chromate species are stable on Al2O3 surface

(pHzc is a neutral-to basic) [48]. On the other hand, the

appearance of 434–438 nm band may imply that chromium

oxide species are strongly anchored to the surface of alu-

mina [49] and are thermally stabilized since Cr(VI)

compounds are known to decompose to the trivalent state

at a temperature C400 �C [49, 50]. An esterification

reaction between chromate and hydroxyl groups of alumina

is proposed by Weckhuysen et al. [51] (Fig. 6).

DRS spectra of Co/Al2O3 catalysts are shown in Fig. 7

with a proposed deconvolution. Since the UV–visible

spectra were collected under ambient conditions, surface

exposed cations are expected to adsorb gas molecules to

complete their coordination sphere. The spectra are char-

acterized by a triple band at around 627–623 nm (red

region), 580–579 nm (yellow orange region), and 548–

543 nm (green region), which gives rise to the blue col-

oration. This multiple bands are ascribed to the

[4A2(F) ? 4T1(P)] d–d transition of cobalt ions in

Fig. 2 27Al MAS NMR spectra of Cr/Al2O3 catalysts. a AlAeCrSG, b AlXeCrSG, c AlAeCrI

Fig. 3 27Al MAS NMR spectra of Co/Al2O3 catalysts. a AlAeCoSG, b AlXeCoSG, c AlAeCoI

Fig. 4 DRS spectra of Cr/Al2O3 catalysts (solid line) AlAeCrSG, (D)

AlXeCrSG, (h) AlAeCrI

174 J Sol-Gel Sci Technol (2009) 49:170–179

123

tetrahedral coordination as found in the spinel structure of

CoAl2O4 [52–56].

3.6 Raman

The Raman spectra of Cr/Al2O3 catalysts are presented in

Fig. 8. Alumina support does not shows any Raman fea-

tures in the 200–1100 cm-1 region and, therefore, all the

observed Raman bands are assigned to chromium species

vibrations. When we inspect Cr/Al2O3 catalysts spectra,

there is a complete absence of bands at 550 cm-1 (char-

acteristic of crystalline Cr2O3) or at 975 and 550 cm-1

(characteristic of crystalline CrO3). This result suggests

that chromium species are present as a two-dimensional

overlayer up to 5 wt% of metal [57]. The Raman shifts

observed at &875 and 350 cm-1 for AlXeCrSG catalyst

are assigned to the symmetric stretching and bending

modes for an isolated tetrahedral surface Cr(VI) oxide

species, respectively [51, 57–59]. The Raman shifts at 870–

875 and &970–1000 cm-1 for AlAeCrSG and AlAeCrI

catalysts are assigned to the symmetric stretching modes of

O–Cr–O for monomeric Cr(VI), and those of Cr=O bonds

for polymeric Cr(VI) oxide species, respectively [59, 60]. It

seems that ordinary drying leads essentially to the forma-

tion of isolated Cr6? species but mono and polychromates

are present when we use the autoclave method.

It is important to note that in the Raman spectra of

cobalt samples, no bands were detected.

3.7 H2–TPR

TPR profiles of Cr/Al2O3 catalysts were presented in

Fig. 9. On the basis of redox behavior of reference samples

and literature [61], a single peak attributed to the reduction

of Cr6? to Cr3? is observed at around 380 �C.

Quantitative analysis of TPR results is reported in

Table 4. From H2/Cr molar ratio, it is possible to give an

evaluation of Cr6? fraction in the catalysts since a total

reduction of Cr(VI) to Cr(III) necessitates 1.5 mole H2.

For Co/Al2O3 catalysts, TPR experiments up to 1000 �C

are illustrated in Fig. 10. Large reduction peak between

100 and 500 �C is observed for all catalysts. AlXeCoSG

catalyst exhibits in addition, a large reduction in the tem-

perature range of 600–850 �C (Fig. 10b). At relatively high

temperature (above 900 �C), a reduction peak is observed

in the TPR profiles of AlAeCoSG and AlAeCoI catalysts

(Fig. 10a, c). These later is not well resolved in the TPR

profile of AlXeCoSG catalyst.

The H2 consumption in the region of around 100–

500 �C has been attributed to the reduction of cobalt oxide,

as Co3O4 through two steps (Co3? ? Co2? ? Co0), along

with the reduction of Co3? surface ions [53, 62]. The

multiplicity of broad diffuse hydrogen consumption in this

region indicates that there are different populations of

cobalt species reducing at approximately the same tem-

perature. Table 5 illustrates the different peak positions in

the temperature range of 100–500 �C.

As mentioned in literature, reduction peak at T1 is

attributed to the reduction of a small fraction of cobalt

oxide with redox property similar to the Co3O4 bulk oxide

[53, 63, 64]. At T2, the reduction of Co3O4 weakly inter-

acted with alumina support takes place [62]. Furthermore,

Co3O4 crystallites interaction with alumina is responsible

of the apparition of reduction peak at T3 [59, 62]. Because

of the low calcination temperature, solid-state diffusion is

slow and the formation of crystalline Co3AlO6 is unlikely

Fig. 5 Deconvolution of the DRS spectrum of AlAeCrI catalyst

Table 3 DRS band positions and assignment of Cr/Al2O3 catalysts

Bands position (nm) Assignment

AlAeCrSG AlXeCrSG AlAeCrI

205 207 206 Support [45, 46]

257 249 257 (6t2 ? 2e)a [45, 47]

278 277 291 (1t1 ? 7t2)a [45, 47]

367 369 365 (1t1 ? 2e)a [45, 47]

434 434 438 (1t1 ? 2e)b [45, 47]

a Symmetry-allowedb Symmetry-forbidden

Fig. 6 Anchoring reaction of chromate on an alumina support

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(reduced at 600–630 �C [62]). For AlXeCoSG catalyst,

besides Co3? and Co3O4 crystallites, there is evidence of

reduction of Co2? surface ions at 680 �C [62, 65]. The

broad peak situated at 800 �C is attributed, according to

Arnoldy and Moulijn [62], to the reduction of Co2? ions

which are interacting with the support to form CoO–Al2O3

phase. CoO–Al2O3 phase is formed by diffusion of cobalt

ions with the aluminum ions of the support, resulting in

non-stoichiometric spinel structure, which is not cobalt

aluminate, since it is reduced at very high temperatures

[62]. In fact, the peak above 900 �C, shown in the TPR

profiles of AlAeCoSG and AlAeCoI catalysts, has been

ascribed to the reduction of Co2? ions with a high number

of O–Al ligands, like a CoAl2O4 phase [53, 62]. These later

peak is not well resolved in the TPR profile of AlXeCoSG

catalyst.

It can be argued that Co3O4 particles, surface Co3? ions

along with CoAl2O4 spinel are present in the samples dried

in supercritical conditions. For AlXeCoSG catalyst, there is

a pronounced formation of cobalt oxide species together

with CoO–Al2O3 phase as confirmed by H2–TPR results.

3.8 Catalytic activity

Catalytic performances of the different solids in the eth-

ylene ammoxidation as function of reaction temperature

are compiled in Table 6.

A steady state is reached after a transition period of 2 h

characterized by an increase of the catalytic activity

towards acetonitrile. At the same time, the selectivity to

nitrile grows while that to deep oxidation (CO2) decreases

remarkably. Compared to Cr/Al2O3 catalysts, Co/Al2O3

catalysts are more active toward acetonitrile excepting

AlXeCoSG catalyst, which exhibits no catalytic behavior

in the ammoxidation reaction. However, a higher CO2

selectivity was observed for this catalysts.

Fig. 7 Deconvolution of DRS spectra of Co/Al2O3 catalysts. a AlAeCoSG, b AlXeCoSG, c AlAeCoI

Fig. 8 Raman spectra of Cr/Al2O3 catalysts. Inset: Raman spectra of

AlAeCrI catalyst in the 700–1100 cm-1 Raman shift region

Fig. 9 H2–TPR profiles of Cr/Al2O3 catalysts

Table 4 Data derived from H2–TPR profiles of Cr/Al2O3 catalysts

Catalysts H2/Cr (mol/mol) Cr6? (%)

AlAeCrSG 1.79 100

AlXeCrSG 1.58 100

AlAeCrI 1.98 100

176 J Sol-Gel Sci Technol (2009) 49:170–179

123

The active sites in the ethylene ammoxidation on Cr/

Al2O3 are chromate species which are identified by DRS

and H2–TPR techniques. This idea could be confirmed in

this case by the catalytic behavior of CrO3–Al2O3 catalyst.

Raman results confirm that Cr(VI) species are either iso-

lated, monomeric and polymeric depending on the

preparation method.

On the other hand, Co2? in CoAl2O4, which is suspected

to be an active phase, was detected. Ghorbel and co-workers

[27, 66] found that catalytic sites for ethylene ammoxida-

tion are cobalt species present as monomer [Co–OH]? and/

or dimer [Co–O–Co]2? species in exchangeable cationic

sites of ZSM-5 zeolite, isolated cobalt ions and cobalt

phyllosilicate. They also found that agglomerated Co3O4

oxide is inactive in the same reaction.

In our study, the preparation method and the drying

procedure seem to affect the predominance and the

distribution of cobalt species in the catalysts. In fact, during

the preparation of AlAeCoSG sample, an excellent solid

solution is obtained from cobalt acetylacetonate and alu-

minum alkoxide since Al-O–Co binding is formed during

the gelation step. Thus, a strong interaction between cobalt

ions and alumina gel is created at atomic level due to the

good dispersion of Co2? in the medium. Furthermore,

drying in supercritical conditions at &300 �C involves the

migration of Co2? ions into the alumina lattice and leads to

the formation of CoAl2O4 phase as confirmed by the blue

coloration of freshly prepared aerogel (issued from the

autoclave). The calcination treatment performed at 500 �C

seems to stabilise this phase and accomplish the migration

of cobalt ions on the alumina matrix. For AlAeCoI, it is so

hard for cobalt ions to penetrate into the support matrix to

form Al–O–Co binding before thermal treatment. So, after

purging in helium for 12 h at 500 �C, the migration of

cobalt species occurred to form a strong interaction

between alumina and metallic species. Calcination under

oxygen flow can appear to promote the dispersion of Co3O4

which can provide from the eventual decomposition of

residual cobalt precursor.

The above observation and explanation are in agreement

with the good catalytic activity among AlAeCoSG and

AlAeCoI catalysts in which, cobalt aluminate and/or dis-

persed Co3O4 can be probably the active sites. However,

the use of sol–gel method allows the creation of strong

metal-support interaction during the gelation step of Al-

XeCoSG sample but no CoAl2O4 phase is formed before

calcination since drying was performed in ordinary con-

ditions (120 �C). During calcination, cobalt oxide species

was formed at the surface of alumina after decomposition

of cobalt acetylacetonate precursor previously remaining

on the surface of xerogel. CoO–Al2O3 phase can be easily

formed and Co3O4 crystallite can agglomerate on the sur-

face of alumina. H2–TPR result (Fig. 10b) supports this

idea by the existence of reduction peaks at 680 and 800 �C.

Thus, the thermal treatment is primordial for the formation

of CoAl2O4 phase either by diffusion of Co2? into alumina

matrix or by dispersion of Co3O4. To confirm this idea, we

have tried to modify the preparation procedure of

Fig. 10 H2–TPR profiles of Co/Al2O3 catalysts. a AlAeCoSG, b AlXeCoSG, c AlAeCoI

Table 5 Different reduction peak positions in the temperature range

of 100–500 �C for Co/Al2O3 catalysts

Catalysts T1(�C) T2(�C) T3(�C)

AlAeCoSG 140 and 240 350 –

AlXeCoSG 190 295 and 360 480

AlAeCoI 180 350 –

Table 6 Ammoxidation of ethylene over Cr/Al2O3 and Co/Al2O3

catalysts

Catalysts X (%) SAcetonitrile (%) AAcetonitrile

(*104 mol/g/h)

Temp. (�C) 450 475 500 450 475 500 450 475 500

AlAeCrSG

AlXeCrSG

AlAeCrI

3.2

3.1

3.4

4.95

4.6

4.35

8.25

6.8

7.1

68

64

71

77

67

75

83

76

82

0.4

0.4

0.6

0.75

0.65

0.75

1.25

1.05

1.35

AlAeCoSG

AlXeCoSG

AlAeCoI

3.4

–

5.3

6.35

–

7.25

10.75

0.85

10.85

35

–

32

66

–

52

85

48

76

0.25

–

0.3

0.85

–

0.7

1.9

0.1

1.6

CrO3–Al2O3 – – 6.65 – – 43 – – 0.7

Cr2O3–Al2O3 – – 3.65 – – 33 – – 0.3

J Sol-Gel Sci Technol (2009) 49:170–179 177

123

AlXeCoSG catalyst. In this context, the xerogel was treated

in helium atmosphere for 3 h after being dried in an oven at

120 �C. An enhancement in the activity has been observed

(at 500 �C, X = 7.44%, SAcetonitrile = 86%, AAcetonitrile =

1.47 * 10-4 mol/g/h). Nevertheless, there is no activity

with the same catalyst after purging under oxygen stream

during the reaction. This result could be due to the

agglomeration of cobalt oxide on the surface of the catalyst

which favorises the hydrocarbon combustion.

4 Conclusion

Preparation method and operating conditions such as dry-

ing and thermal treatment are important parameters which

affect the nature and concentration of species present at the

surface of the alumina support. Chromium doped alumina

show similar activity in the ammoxidation of ethylene.

After calcination, most of chromium is stabilized as Cr6?

on the alumina surface. Ordinary drying leads essentially to

the formation of isolated Cr6? species but mono and

polychromates are present when we use the autoclave

method. Furthermore, catalytic behavior of cobalt doped

alumina depends on the preparation method and the drying

procedure. Agglomerated Co3O4 and CoO–Al2O3 phase are

present on the alumina upon drying in ordinary conditions.

When drying is performed under supercritical conditions,

the CoAl2O4 phase is easily formed and the catalytic

activity was largely improved. In summary, one could

expect that cobalt containing catalysts are more active in

the ethylene ammoxidation until we choosing the appro-

priate preparation method. Nevertheless, chromium doped

catalysts seems to be more selective.

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