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Polyhedron 20 (2001) 22612268

Oxovanadium(V)-1-methoxy-2-propanoxide: synthesis andspectroscopic studies a molecular precursor for a

vanadiummagnesium oxide catalyst

L. Albaric, N. Hovnanian *, A. Julbe, G. Volle

Laboratoire des Materiaux et Procedes Membranaires (CNRS UMR 5635), 1919 Route de Mendo, 34293 Montpellier Cedex 5, France

Received 16 October 2000; accepted 8 December 2000

Abstract

A brown liquid oxovanadium(V) alkoxide [VO(OCH(CH3)CH2OCH3)3]n was prepared by heating under reflux ammonium

metavanadate with alcohol and toluene. The exchange processes between monomer and oligomers in pure or diluted medium were

observed by dynamic 51V NMR study. The compound reacted with magnesium 1-methoxy 2-propanoxide to yield in situ a

heterobimetallic alkoxide, which is used in the synthesis of a vanadium magnesium oxide catalyst. Textural and structural

properties of the resulting oxide have been presented. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Oxovanadium(V)alkoxide; 1-Methoxy-2-propanol; 51V NMR; Vanadium magnesium heterometallic alkoxide; Sol gel; Vanadium

magnesium oxide catalyst

www.elsevier.com/locate/poly

1. Introduction

Within the context of research into catalytic vana-

dium magnesium oxide membranes by solgel pro-

cesses, we were interested in oxovanadium alkoxides

since the properties of the alkoxides make them useful

for obtaining high purity ceramic materials of tailored

properties [1].

Oxovanadium alkoxides were first synthesized by the

reaction of alkyl halides with silver vanadate [2]. Other

syntheses from vanadium pentoxide [3], vanadium

oxytrichloride with dry ammonia [4], ethylorthovana-date [5] or ammonium metavanadate [6] have been

studied. Ester-exchange reactions have also been used

[7]. More recently, polyoxoalkoxide clusters of vana-

dium have been synthesized by the hydrothermal reac-

tion of NH4VO3, V2O5 and (Me3NH)Cl with polyols

[8].

We first reported the synthesis and the X-ray struc-

tural characterization of a oxovanadium(IV)alkoxide

derived from the reduction of VOCl3 with sodium

1-methoxy 2-propanoxide [9]. Another reaction using

ammonium metavanadate was performed and this pro-duced a different oxovanadium(V)alkoxide. This paper

presents the synthesis, the characterization and the

solgel studies of this new compound, particularly by

dynamic 51V NMR spectroscopy. The tendency for

alkoxide ligands to bridge allows heterometallic alkox-

ides to be easily obtained by mixing the corresponding

homometallic alkoxides. Nevertheless, they are difficult

to isolate because they can spontaneously dissociate in

solution or under thermal conditions. Here, we report

the reaction of oxovanadium 1-methoxy 2-propanoxide

with magnesium 1-methoxy 2-propanoxide, previouslyreported [10], as well as the textural and structural

properties of the resulting VMg oxide catalyst.

2. Experimental

All manipulations were routinely performed under

argon using Schlenk tubes and vacuum-line techniques

with solvents purified by standard methods. NH4VO3was used as received. 1H and 51V NMR and IR spectra

were run on Bruker AC-250 and IR FT Nicolet ZDX

spectrometers, respectively. l(51V) values are quoted

relative to VOCl3 and the spectra in non-deuterated* Corresponding author: Tel.: +33-46-7613400; fax: +33-46-

7042820.

0277-5387/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 7 7 - 5 3 8 7 ( 0 1 ) 0 0 7 0 4 - 5

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L. Albaric et al. /Polyhedron 20 (2001) 226122682262

solvents (pure or polar and non-polar organic solvents)

were recorded with an external lock (D2O). IR spectra

were obtained as Nujol mulls between KBr plates. Mass

spectra were recorded on a 300 DX JEOL by Electronic

Impact. Analytical data were obtained from the Centre

de Microanalyses du CNRS.

Field emission scanning electron microscopy

(FESEM, Hitachi S-4500) and N2 adsorption desorp-

tion (Micromeritics ASAP2000) were used for the mor-phological and textural characterizations of the oxide

materials.

2.1. Synthesis of

oxo6anadium(V)-1-methoxy-2-propanoxide

NH4VO3 (40 g, 0.34 mol), 1-methoxy-2-propanol

(600 ml, 6.14 mol) and toluene (400 ml, 3.77 mol) were

mixed and heated at reflux for 72 h. The water pro-

duced was removed by continuous azeotropic distilla-

tion with toluene. Excess ammonium metavanadate was

removed by filtration. The filtrate was evaporated todryness in order to eliminate residual solvents. The

yellow viscous liquid corresponding to the alkoxide

compound was then purified by distillation under re-

duced pressure (P=1 mmHg, T=146C). Yield: 60%

based on vanadium. Anal. Found: C, 42.24; H, 7.98; V,

15.30. Calc. for C12H27O7V: C, 43.11; H, 8.08; V,

15.27%. IR (cm1): w(VOC): 1150, 1113, 1079;

w(VO): 988, 976; w(VOV): 872; w(VOR): 705, 671.

SM (m/z, EI), OROCHCH3CH2OCH3: CH2OCH3/

OCH2CH3 (33%), OCH(CH3)CH2 (5.2%), CH(CH3)-

CH2OCH3 (7.2), VO(OCH3) (4%), VO(CH2OCH3)CH2(19.8%), VO(OCH3)(CH2OCH3) (11.5%), VO(OCH2-

CH3)2 (28.6%), VO(OR)(OCH3) (22.4%), VO(OR)-

(OCH2CH3) (65.7%), V(OR)2 (5.2%), VO(OR)2 (100%),

V2O2(OR)2 (0.1%), V2O2(OR)3 (0.7%), V2O2(OR)2-

(OCH2) (0.3%), V2O2(OR)3(OCH2) (0.2%), V2O2(OR)4(0.4%). 1H NMR (CHCl3-d, l ppm): 1.35 d (CH3, 3H);

3.39 s (OCH3, 3H); 3.46 d (CH2, 2H); 5.23 m (CH, 1H).13C NMR (CHCl3-d, l ppm): 20.2 (CH3); 58.8 (OCH3);

77.7 (CH2); 84.2 (CH).51V NMR (pure, ppm): 574.8

(97); 558.7 (3).

2.2. Reaction of [VO(OCH(CH3)CH2OCH3)3]n with[Mg(OCH(CH3)CH2OCH3)3]4

[Mg(OCH(CH3)CH2OCH3)3]4 (26.98 g, 0.133 mol)

(synthesized according to Ref. [10]) was mixed with

[VO(OCH(CH3)CH2OCH3)3]n (6.59 g, 0.02 mol) and

1-methoxy-2-propanol (200 ml). The mixture was

heated under reflux for 24 h. The orange solution

gradually turned green after standing for a few days. It

was evaporated to dryness and yielded a viscous dark-

green oil well adapted to casting. This composition

yielded a vanadium magnesium oxide containing 14

wt% of vanadium (14/VMgO). From a fixed concentra-

tion of vanadium (0.1 mol l1), the quantity of magne-

sium alkoxide can be varied from 0.28 to 6.1 mol l1 to

prepare vanadium magnesium oxide with 2 25 wt% of

vanadium. IR (cm1): w(OH): 3300 3400; l(OH):

1600 1700; w(VOC): 1153, 1095; w(CO): 1257, 1203,

1156; w(Metal O): 591, 531 and 467. SM (m/z, EI),

OROCHCH3CH2OCH3 OCHCH3CH2OCH3 (5%),

Mg(OR)(OCH3) (7.2%), Mg(OR)(OH)2 (26%), VO(OR)

(9%), VO(OR)(CH2OCH3)/Mg(OR)2 (16.5%), VO(OR)2(20%), VO(OR)2(CHCH3CH2OCH3) (4.6%), VOMg-

(OR)3 (5%), VOMg(OR)3(CHCH3CH2OCH3) (2.7%),

V2(OR)5 (2%), V2O2Mg(OR)4(OCH3) (3%), V2O2Mg2-

(OR)5 (3.7%), V2O2Mg2(OR)5(OCH3) (1%), V2OMg2-

(OR)8 (1.2%), V2OMg2(OR)8(CHCH2OCH3) (2%).1H

NMR (CHCl3-d, ppm): 1.01.3 m (CH3); 2.14 s (OH);

2.32 s (OH); 3.1 3.9 m (CH2, OCH3); CH resonance

was not visible. 51V NMR (in parent alcohol, ppm)

around 625 ppm (w1/2=910 Hz).

3. Results and discussion

Two different oxovanadium alkoxides derived from

1-methoxy-2-propanol were obtained depending on the

synthesis and the nature of the vanadium reactant.

VOCl3 reacted with sodium 1-methoxy-2-propanoxide

or 1-methoxy-2-propanol and ammonia gas yielding a

solid vanadium(IV) species {VOCl[OCH(CH3)CH2-

OCH3]}2 (1) [9], whereas ammonium metavanadate

with alcohol and toluene heated under reflux, yields a

liquid oxovanadium(V)alkoxide (2). According to ele-

mental analysis, this yellow viscous compound corre-sponds to the oxovanadium alkoxide [VO(OR)3]n. The

IR spectrum exhibited the characteristic features of

OCH(CH3)CH2OCH3 ligands bound to vanadium and

the absence of w(OH) vibration. The w(VOC) vibra-

tions were located at 1150, 1113 and 1079 cm1, while

strong bands at 988 and 976 cm1 were attributed to

w(VO) vibrations. The w(VOR) vibrations were lo-

cated at 705 and 671 cm1. The band at 872 cm1

could be assigned to w(VOV) vibrations which sug-

gests the presence of associated species at room temper-

ature.

The electronic ionization mass spectrum of com-pound 2 revealed the presence of metallo-organic frag-

ments with two vanadium atoms identified at m/z 312,

343, 401 and 432 as well as an ion with the highest

molecular mass of 490 corresponding to V2O2(OR)4]+;

it suggested that the oxoalkoxide was at least a dimer

with respect to disproportionation reactions in the va-

pour phase. This is in accordance with cryoscopic mea-

surements which suggested that oxovanadium alkoxides

with highly hindered ligands (OR=OtBu, OtAm) were

monomers while those with primary or secondary

groups (OR=OEt, OnPr, OnBu, OiBu) were dimers or

oligomers [11].

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L. Albaric et al. /Polyhedron 20 (2001) 22612268 2263

Table 151V NMR and structural data of VO(OR)3 complexes (Ir: relative intensity)

lV51 (ppm)Composition Remarks and w1/2 (Hz) Solvent. [alkoxide] (M) Ref.

VO(OR)3,

liquidR= iBu 597

liquid599 [21]Methyl-3 butyl

600, 611Dodecyl Ir:1, 8/1; solid CDCl3603Octyl 102101

liquid629i

PrwaxtPentyl 685

monomer;VO(cyclo-C5H9O)3 623

616 dimer CDCl3 [15]

distorted trigonal

bipyramid

w1/2=30VO(OtAm)3 pure683.7 [6]

monomerAm=amyl

tetrahedron

VO(OCH3)3 pentane [13]

octahedron 0.012598.2

0.520547.5

VO(OEt)3 pentane605.4573.0 0.019 [13]

0.770

w1/2=14VO(OiPr)3 CDCl3597

[6]or 538

w1/2=25 pure628.5

641VO(OiPr)3 w1/2=16 C6D6 [22,23]

VO(OnPr)3 548.7 w1/2=50 pure [6]

dimer

VO(OtBu)3 w1/2=12599 C6D6 [22]

w1/2=25 pure672

1H and 13C NMR spectroscopies revealed a single

average environment for the alkoxide groups. Fast ex-

changes between alkoxide groups are possible at room

temperature and prevent the distinction of monomers

and/or oligomers.

3.1. 51V NMR study of 6anadium(V)oxo 1-methoxy

2-propanoxide

The exchange processes between monomers and

oligomers in pure or diluted medium have been studiedby 51V NMR spectroscopy. This sensitive tool gives

valuable information on the characterization of vanadi-

um(V)oxoalkoxides since these compounds are usually

liquids and their molecular structure is not well known

(Table 1). Only a few oxovanadium alkoxides have

been structurally characterized; the vanadium environ-

ment can be described as a distorted octahedron in

[VO(OMe)3] [12] or a trigonal bipyramid in

VO(OCH2CH2Cl)3 [13] and [VO(cyclo-C5H9O)3]2 [14].

In accordance with 51V NMR data, oxovanadium

alkoxides usually form dimers and oligomers in solu-

tion. The presence of associated species depends on

temperature, solvent, concentration and the size of the

R group [11,15]. In other respects, the 51V chemical

shift of VO(OR)3 compounds usually increases with

electronegativity and the steric hindrance of the R

groups. The 51V NMR spectrum of the undiluted com-

pound 2 showed two resonances at 574.8 and

558.7 with an intensity ratio of 97:3. These chemical

shifts were in the range reported for VO(OR)3 (R=nPr,

iPr, tBu, tAm). The high values of the half-widths of the

main peak (146 Hz) and of the secondary peak (383

Hz) can be attributed to an oligomerization of thecompound. Oligomerization leads to an increase in

molecular size and a decrease in local symmetry. Both

effects increase the relaxation rate (via the molecular

correlation time and the field gradient) and hence the

line width. The ability of the alkoxide ligand to act as

a bridging (m2- or m3-) group allows ready oligomeriza-

tion to form condensed units [1]. Generally, dimers or

oligomers are predominant when alkoxides such as

VO(OiPr)3 [16], VO(OnPr)3 [6] and VO(cyclo-C5H9O)3

[14] are pure or in concentrated solutions.

Figs. 1 and 2, respectively, show the variations of the

half-width (w1/2) and the chemical shift (l51) of the

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Fig. 1. Variation of the half-width for the predominant signal present

in the 51V spectrum of the oxovanadium alkoxide undiluted or

diluted in the parent alcohol as a function of temperature.

comparison, the half-width of the alkoxide diluted in

the parent alcohol did not significantly vary with de-

creasing temperature; it can be due to a slower au-

toassociation yielding less condensed species. As a

consequence of monomer/oligomers exchange, the 51V

chemical shift of the diluted and undiluted alkoxide

increased with decreasing temperature. This opposite

trend to the shielding theory has been already shown

for VO(O

i

Pr)3 [16]. The exchange processes betweenoligomers of different condensation degrees were

stopped as soon as the temperature decreased and the

viscosity of the medium increased.

As the dilution of the oxovanadium alkoxide in

parent alcohol increased, the signals shifted slightly

upfield (Fig. 3). The chemical shift difference (Dl51)

between the oxoalkoxide without solvent and the most

diluted oxoalkoxide was less than 1 ppm. In the litera-

ture, Dl51 is high for the oxoalkoxides containing small

R groups (Dl51=99 ppm l mol1 for VO(OCH3)3)

whereas it is low for those with bulky R ligands which

are less able to condense (Dl51=33, 13 and 1 ppm lmol1, respectively, for VO(OCH2CH2CH3)3,

VO(OCHMe2)3 and VO(OCMe3)3) [13]. In our case, the

low value of Dl51 was in favour of a dimer at room

temperature. As the concentration decreased, the main

line split and became thinner, indicating a dissociation

of oligomers into less condensed units (monomers). At

the same time, a very weak peak appeared at 565.6

ppm as the concentration reached 0.34 mol l1 and this

may be attributed to a new solvated species.

The line splitting increased with the polarity of the

solvent (Fig. 4). In n-heptane, a unique and broadresonance at 583 ppm was observed reflecting no

dissociation and/or solvatation. Inversely in CH2Cl2,

every signal split up and a new small signal at 555

ppm appeared. The formation of a new stable solvated

species gave rise to distinct resonance lines. However,

the very low intensity of these new resonances shows

that the solvatation reactions were not predominant.

3.2. EPR study of [VO(OCH(CH3)CH2OCH3)3]2

No EPR signal was observed for the freshly synthe-

sized compound 2, before or after distillation. On thecontrary, an intense and broad signal (centered at

g=1.95) was obtained from a non-distilled and aged

compound.

3.3. Hydrolysis study of [VO(OCHCH3CH2OCH3)3]2

The behavior of the viscous compound 2 towards

hydrolysis was different from that of compound 1 [18].

Generally, the introduction of the functional alcohol in

the coordination sphere increases the coordination

number of the metal; thus it decreases the sensitivity of

the metal to nucleophilic attack and thus to hydrolysis.

Fig. 2. Variation of the chemical shift for the predominant signal

present in the 51V spectrum of the oxovanadium alkoxide undiluted

or diluted in the parent alcohol as a function of temperature.

predominant signal present in the51

V spectrum of thealkoxide without solvent or diluted in the parent alco-

hol as a function of temperature. At room temperature,

the weak signal at 558.7 ppm shifted to 549.5 ppm

at 253 K then disappeared at 213 K as the predominant

signal broadened and became flatter. It was always very

weak irrespective of the concentration, solvent and

temperature and can be assigned to an impurity which

cannot be removed by distillation. The chemical shift

and the half-width of this resonance could be in favour

of a condensed and solvated species according to 51V

NMR studies of compound 2 as a function of thesolvent and the concentration in parent alcohol (see

Section 2.2).

With decreasing temperature, the resonance signals

of dissolved compounds usually broaden due to an

increase in quadrupolar relaxation rates. From 373 to

213 K the half-width increased to a maximum at 233 K

for the undiluted alkoxide.1 At 213 K, the undiluted

oxoalkoxide could be a cluster with a higher symmetry

than that at 233 K because of the thinner half-width. In

1

No results can be obtained for the diluted alkoxide at 60C(213 K) because of the high viscosity of this solution.

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Fig. 3. Variation of the 51V spectrum for oxovanadium alkoxide as a function of the concentration in 1-methoxy 2-propanol.

Fig. 4. 51V spectra of vanadium oxo 1-methoxy 2-propanoxide diluted (1.1 M) in solvents of different polarities at room temperature.

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With compound 1, no gel was formed even with an

excess of water whereas the hydrolysis of compound 2

in the presence of excess water (h= [H2O]/[VO(OR)3]\

10) directly yielded a red colloidal sol. No polymeric

gels (hB10) were obtained; whatever the hydrolysis

conditions, solutions or precipitates formed. This may

account for the very low hydrolysis rate in comparison

with the condensation rate. When non-anhydrous etha-

nol was added to a solution of VO(OCHCH3CH2-OCH3)3 in parent alcohol, an orange sol was obtained

which gelified in a few minutes. The exchange of 1-

methoxy 2-propanoxy ligands by ethoxy groups in-

creased the hydrolysis rate and allowed a polymeric gel

to be obtained. This orange polymeric gel turned green

due to the reduction of V(V) to V(IV) in the presence of

1-methoxy-2-propanol.

The 51V NMR spectrum of compound 2 hydrolyzed

by air for 15 h consists of three lines with the main

signal at 574.5 ppm corresponding to [VO(OR)3]2

and two other signals at 596.6 and 623 ppm whichwere attributed to hydrolyzed VO(OR)3x(OH)x spe-

cies. A similar spectrum has previously been reported

for the partial hydrolysis of VO(OtAm)3 [6]. When the

oxovanadium alkoxide was hydrolyzed with an excess

of water, the spectrum of the resulting red sol was

different; it presented three broad resonances at 422,

511.9 and 532.4 ppm assigned to [Hx

V10O28](6x)

[6,19,20]. These three vanadium environments are dis-

torted octahedra. Thus, with an excess of water,

[VO(OCHCH3CH2OCH3)3]n was entirely hydrolyzed,

contrary to VO(OtAm)3

for which polymeric and deca-

vanadic species were in equilibrium in the red sol.

3.4. Reaction of [VO(OCHCH3CH2OCH3)3]n with

[Mg(OCHCH3CH2OCH3)2]4A magnesium vanadium methoxy-propanoxide was

obtained in situ by mixing under reflux a solution of

[VO(OCHCH3CH2OCH3)3]n and [Mg(OCHCH3CH2-

OCH3)2]4 in parent alcohol with a V/Mg ratio of 1:1.

The reaction medium was evaporated to dryness yield-

ing an orange liquid compound. According to spectro-

scopic characterizations, it was attributed to acondensed and solvated heterometallic alkoxide. Broad

w(OH) and l(OH) bond vibrations were present at

3300 3400 and 1600 1700 cm1, respectively, as well

as characteristic w(VOC) vibrations at 1153, 1095

cm1 w(CO) vibrations at 1257, 1203, 1156 cm1 and

w(Metal O) at 591, 531 and 467 cm1. The 51V NMR

spectrum of this compound consists of a new and very

broad resonance around 625 ppm (w1/2=910 Hz)

indicating the formation of an oligomer. The electronic

ionization mass spectrum showed metallo-organic frag-

ments different from those of the homometallic parent

alkoxides. It shed light on the formation of an het-erometallic alkoxide containing at least two vanadium

and two magnesium atoms (highest molecular mass at

m/z 936) (Fig. 5). The 1H NMR spectrum revealed the

characteristic chemical shifts of the alkoxide groups as

well as the presence of OH groups with peaks at 2.14

and 2.32 ppm. No EPR signal was observed for this

heterometallic alkoxide freshly obtained. Partial disso-

ciation into the homometallic species was observed

when the V Mg alkoxide was hydrolyzed by air as

depicted by the appearance of a resonance peak at

575 ppm in the

51

V NMR spectrum. Stirring for 12 hresulted in a red sol. From sols, V Mg oxides were

Fig. 5. Electronic ionization mass spectrum of vanadiummagnesium methoxy-propanoxide.

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Fig. 6. Comparison of the 14V/VMgO morphology observed by FESEM. (a) powder derived from salts at 600C according to the method

described in Ref. [23] and (b) powder derived from alkoxides at 600C.

prepared with a vanadium content of 2 25 wt% by

varying the V/Mg ratio.

3.5. Characterization of the V Mg oxide catalyst

deri6ed from the synthesized alkoxide precursors

The synthesis of conventional VMgO catalysts for the

oxidative dehydrogenation of propane (ODHP:

C3H8+ (1/2)O2C3H6+H2O) usually involves the

precipitation of Mg(OH)2 and its impregnation with a

solution of ammonium metavanadate [23,24]. It has

recently been demonstrated [24] that a vanadium

content of 14 wt% corresponds to an optimized

composition for this type of catalyst. For this

composition, most of the exposed surface consists of

dispersed vanadia surface units, having intimate

interaction with magnesia crystallites stabilized with a

polar (111) orientation [23].

The vanadium magnesium methoxy-propanoxide

precursors have been synthesised to obtain a

homogeneous medium which is well adapted to casting,

and yields pure V Mg oxide catalysts with a controlled

V/Mg ratio, a high specific surface area and a high

catalytic activity for ODHP (dispersion of active sites).

The sol, derived from the hydolysis by air of the V Mg

heterometallic methoxy-propanoxide, can be easily cast

on a support, dried in vacuum at room temperature and

finally fired in air at 600C yielding the final VMgOcatalyst with a controlled V/Mg ratio.

Apart from the fact that this specific method is well

adapted to catalyst casting on a support (e.g. for

catalytic membrane reactor applications), the derived

VMg oxide powders present original textural and

structural properties compared to conventional V Mg

oxide catalysts derived from precipitated Mg(OH)2. Fig.

6 compares the morphology of the V Mg oxide powder

containing 14 wt.% of vanadium (named 14V/VMgO)

obtained at 600C from the Mg(OH)2 salt precursor [23]

and from the above mentioned original alkoxides. The

14V/VMgO catalyst derived from alkoxide precursors

has a specific surface SBET of 100 m2 g1 with round

shaped particles of about 25 nm whereas that derived

from salt precursors has a specific surface area of about

90 m2 g1 with small grains aggregated in platelets of

about 100 nm in diameter. The alkoxide precursors leadto a high dispersion of amorphous vanadium species on

the surface of MgO particles with (100) orientation,

evidenced by transmission electron microscopy and

electron diffraction [25]. Compared to the salt derived

catalysts, a delayed crystallization of Mg3V2O8 was

evidenced for alkoxide derived materials by X-ray

diffraction studies. This phase generally forms large

crystals which lower the specific surface area of the

catalyst and consequently the propane conversion. The

specific characteristics of the alkoxide derived catalyst

induced attractive catalytic properties for the oxidative

dehydrogenation of propane [26]. Since the synthesis

process was completely different from the salt

precipitation impregnation method, the composition

with 14 wt% of V, which was optimized in the case of

salt-derived catalyst, does not correspond to the

optimum catalyst for the alkoxide method. V Mg oxide

powders with various V/Mg ratio have recently been

investigated in detail and the results help to provide a

better understanding of the nature of the active catalytic

sites for the ODHP reaction. As a result of the

homogeneous distribution of the acidic vanadium

species on the surface of the basic MgO grain with highSBET, a lower quantity of vanadium is needed in the

VMgO catalyst for obtaining an optimum propene

yield.

4. Conclusions

Oxovanadium(V)methoxy-propanoxide has been syn-

thesized and characterized, particularly by 51V NMR

spectroscopy. The pure alkoxide appeared mainly as a

dimer while dilution in parent alcohol led to its dissoci-

ation into a monomer. The monomer/oligomers equi-

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librium shifted towards monomer as temperature in-

creases. The chelating bidendate ligand methoxy-

propanoxide allowed us to obtain soluble homometallic

and heterometallic molecular precursors that reacted

with water in slow hydrolysis reactions. The vanadium

alkoxide reacts with the corresponding magnesium

alkoxide yielding in situ a molecular heterometallic

precursor of vanadium magnesium oxide. The texture

and the structure of the derived oxide powder inducedattractive catalytic properties for the oxidative dehydro-

genation of propane. A V Mg oxide catalyst contain-

ing 14 wt% of vanadium presented round shaped

particles of about 25 nm with a specific surface area of

101 m2 g1; it was composed of crystallized Mg3V2O8with a high dispersion of amorphous vanadium species

on the surface of MgO particles with (100) orientation.

Acknowledgements

Financial support from the Joule programme JOE3-

CT-95 0022 is gratefully acknowledged. Thanks are

also given to J. Perregaard and P.E. Hojlund Nielsen

from Haldor Topsoe for providing the 14V/VMgO

catalyst derived from salts, and to C. Mirodatos for

fruitful discussions.

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