Journal of Saudi Chemical Society (2012) 16, 445–449

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Effect of calcination temperature on the structure

of vanadium phosphorus oxide materials and their

catalytic activity in the decomposition of 2-propanol

M. Sadiqa,*, M. Bensitel

a,*, K. Nohaira, J. Leglise

b, C. Lamonier

c

a Laboratoire de Catalyse et Corrosion des Materiaux, BP 20, El Jadida 24000, Moroccob DRRT Basse-normandie, CITIS, Avenue de Tsukuba, 14209 Herouville Saint-Clair, Francec Unite de Catalyse et Chimie du Solide, UMR CNRS 8181, 59650 Villeneuve d’Ascq, France

Received 22 November 2010; accepted 18 February 2011Available online 24 February 2011

*

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KEYWORDS

Vanadyl phosphate;

Vanadyl pyrophosphate;

XRD;

DTA–TGA;

IR;

UV–Visible;

Isopropanol conversion

Corresponding author. T

3342187.

-mail addresses: sadiqmham

el@hotmail.com (M. Bensite

19-6103 ª 2011 King Saud

sevier B.V.

er review under responsibilit

i:10.1016/j.jscs.2011.02.014

Production and h

Open access under C

el.: +2

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osting by E

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Abstract The synthesis of vanadyl phosphate by reaction of an alcoholic solution of V2O5 and

o-H3PO4 has been studied. The solids obtained were investigated by various physico-chemical

techniques as in situ X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential

thermal analysis (DTA) under inert atmosphere. The compounds have been further characterized

by infrared and UV–Visible spectroscopies. The isopropanol conversion is carried out to evaluate

the catalytic activity of the samples. The isopropanol conversion increases with the reaction temper-

ature from 433 to 503 K for vanadyl phosphorus oxides calcined at a different temperature. How-

ever, the conversion attains 100% at 503 K, and the solid calcined at 973 K exhibits high selectivity

towards propene (100% at 503 K).ª 2011 King Saud University. Production and hosting by Elsevier B.V.

12 523343003; fax: +212

ail.com (M. Sadiq), medben-

y. Production and hosting by

Saud University.

lsevier

Open access under CC BY-NC-ND license.

ND license.

1. Introduction

Vanadium phosphates (VPO) have many important applica-

tions. They have attracted much attention among catalysisresearchers and electrochemists. The most active phase ofVPO catalysts for synthesis of maleic anhydride (MA) is made

up of a well-crystallized (VO)2P2O7 which is considered to pos-sess unique structural and surface features to allow the activa-tion of alkane (Centi, 1993; Ballarini et al., 2006). Vanadiumphosphorus oxide catalysts have also been widely investigated

for the partial oxidation and oxidative dehydrogenation ofother lower hydrocarbons (Centi, 1993; Pillai and Sahle-Demessie, 2003; Pillai and Sahle-Demessie, 2002; Kandi

et al., 2003; Savary et al., 1996; Michalakos et al., 1993). Cat-alysts based on vanadyl phosphate (VOPO4Æ2H2O) were used

446 M. Sadiq et al.

in some reactions involving Bronsted and Lewis acid sites, such

as dehydration (Carlini et al., 2004) and isomerization (Beneset al., 1997). Taufiq-Yap and Saw (2008) have studied theeffect of different calcination environments on the vanadiumphosphate catalysts for selective oxidation of propane and n-

butane. In lithium battery research, d- and especially e-VOPO4,the most recently discovered polymorph (Lim et al., 1996)show promising electrochemical properties (Kerr et al., 2000;

Azmi et al., 2002; Azmi et al., 2003; Song et al., 2005). In bothfields of research, knowledge of the crystal structure of therespective phases is desirable, since it is a prerequisite to

uncover structure-(re)activity relationships.Decomposition of 2-propanol is frequently used as a test

reaction to determine acid–base properties of different cata-

lysts by many groups (Aramendıa et al., 1996; Chen et al.,2001; Gervassini et al., 1997; Ortiz-Islasa et al., 2005; El-Mollaet al., 2004; El-Shobaky et al., 2006; Oukerroum et al., 2001).It has been reported that the decomposition of 2-propanol oc-

curs by two parallel reactions: (i) the dehydration carried outin acid sites giving the propene and the isopropyl ether (DIPE)(Ortiz-Islasa et al., 2005; Satoh et al., 2000) and (ii) the dehy-

drogenation to acetone occurring on basic sites or both typesof site (acid and basic) via a concerted mechanism (Lopezet al., 2000).

The aim of this paper is to study one of the methods ofpreparation leading to (VO)2P2O7 solid, and to examine thetreatment temperature effect on the transformation of precur-sor to active phase in the inert atmosphere and catalytic dehy-

dration of isopropyl alcohol at temperatures within 433–503 K.

2. Experimental

2.1. Solid synthesis

An organic protocol of the VPO precursor preparation con-

sists of refluxing V2O5 (6 g) in a 100 ml of tert-butanol for6 h at a reflux temperature of 373 K under agitation. An85% ortho-phosphoric acid solution was then added to the

resulting vanadium suspension and was refluxed for a further16 h at 393 K. The reactant P:V mole ratio was 1:1. The result-ing solid was recovered by filtration, washed by ethanol anddried at 383 K.

2.2. Characterization techniques

Powder X-ray diffraction (XRD) measurements were recordedon a Siemens D5000 diffractometer using CuKa radiation. Thepatterns were recorded with a step of 0.015� using a counting

time of 0.2 s per step over the 2h range from 10� to 80�, underan N2 flow (2L/h).

TGA–DTA analysis has been recorded with a TA Instru-

ments balance, model SDT 2966 (TGA–DTA). It allows to ob-tain simultaneous DTA and TGA diagrams under similarexperimental conditions. Experiments were performed underan N2 flow, from 298 K to 1073 K using a heating rate of

5 K min�1.Diffuse reflectance infrared spectra for the synthesized sol-

ids were recorded from 4000 to 400 cm�1 on a Nicolet 460

spectrophotometer. Samples were prepared by mixing 1 mgof powdered solid with 150 mg of KBr.

UV–Visible spectra in diffuse reflectance mode were re-

corded by a Varian Cary 100 Scan spectrometer, from 200 to800 nm.

2.3. Catalytic test

Decomposition of isopropanol was carried out in a fixed-bedtype reactor with a continuous flow system at atmospheric

pressure. The catalyst (100 mg, 200–500 lm particle size) waspacked in a glass reactor (Pyrex tube, 10 mm I.D) placed ina furnace. Before carrying out such catalytic activity measure-

ments, each catalyst sample was activated by heating in astream of nitrogen for 2 h at 673 K then cooled to the catalyticreaction temperature. The reactant, 2-propanol (3.7 kPa), was

diluted in nitrogen by bubbling the gas through the liquid reac-tant in saturator maintained at 291 K. The isopropanoldecomposition was performed at a temperature between 433and 503 K. The products were analysed by on line gas chroma-

tography (Varian-3700 GC with a flame ionization detectorFID) using a 10% 0 V–101 on Chromosorb-WHP (80/100mesh) packed column (4 m) maintained at 353 K. A blank test

showed that insignificant thermal reaction developed in the ab-sence of the catalyst.

3. Results and discussion

3.1. XRD Results

Fig. 1 shows the evolution of the XRD patterns of the dried

precipitated VPO solid calcined at different temperatures.The most intense diffraction lines at 14� present in the diffrac-tion pattern of the dried VPO solid is attributed to the crystal-line phase VOPO4ÆH2O (JCPDS 47-0949) which disappeared

completely at 473 K. The XRD pattern of the sample calcinedat 473 K shows the formation of two anhydrous VOPO4 allo-tropic phases characterised by low intense diffraction lines at

2h = 21.6� for d-VOPO4 and at 28.6� for aI-VOPO4 phase.The crystallinity of these phases increases with the temperatureup to 773 K. These results are in agreement with those re-

ported by Guliants et al. (1996) and Cavani et al. (2010).The diffraction lines at 2h = 23� and 30�, observed at 673 K,have been assigned to (VO)2(P2O7) (JCPDS 83-2388) whichprogressively crystallised with increasing of the temperature

of calcination. Beyond 1073 K, a crystalline material couldbe clearly identified as vanadium phosphate, VPO4 (JCPDS76-2023) which remained stable after return to an ambient

temperature.

3.2. DTA and TG analyses

Fig. 2 shows the thermogravimetric and differential thermalanalysis diagrams of samples VPO. Below 773 K, the TG curve

for VPO exhibits two successive mass losses. The first one oc-curs between the ambient temperature and 364 K. It corre-sponds to the loss of water potentially adsorbed on itssurface. The second loss of mass takes place between 364 K

and 720 K. It is due to the crystallization water from the VO-PO4ÆH2O phase. In this temperature range (364–720 K), theexperimental mass loss amounts to 9.5% of the initial vana-

dium phosphate mass, which is very close to the theoreticalmass loss (10%) calculated from the content of water in the

10 20 30 40 50 60 70 80

Inte

nsity

(a.u

)

2theta (degree)

373 K

473 K

573 K

673 K

773 K

873 K

973 K

1073 K

1173 K

ret.300 K

(VO)2(P

2O

7)

Figure 1 Evolution of the VPO XRD patterns with the

temperature treatment under N2 flow.

1000 2000 3000 4000 Wavenumber (cm-1)

Abso

rban

ce

973K

dried

723K

680

9921084

12431150

1063 941

16173565

3366

1000 2000 3000 4000

973K

dried

723K

680

9921084

12431150

1063 941

16173565

3366

Figure 3 FT-IR spectra of vanadium–phosphorus oxide : dried,

Effect of calcination temperature on the structure of vanadium phosphorus oxide materials 447

VOPO4ÆH2O structure. The DTA curve for this solid presentstwo endothermic peaks at 333 K corresponding to the loss ofphysisorbed water, and at 412 K corresponding to the conver-

sion of VOPO4ÆH2O into VOPO4 structure. An endothermicpeak observed around 840 K and without loss mass can beattributed to transition phase from d-VOPO4 to aI-VOPO4

phase. This result is in agreement with XRD analysis (seeFig. 1). The endothermic peak at about 1005 K is attributedto the conversion of VOPO4 phase into crystalline (VO)2(P2O7)previously deduced from the XRD patterns. It gives place in

TGA to a loss of weight of 2.8% corresponding to thereaction:

2VOPO4 ! ðVOÞ2ðP2O7Þ þ 1=2O2

80

85

90

95

100

-10

-8

-6

-4

-2

0

300 400 500 600 700 800 900 1000 1100

perc

enta

ge o

f mas

s (%

)

Heat Flux (W

/g)

temperature (K)

333k412k

1005k

840K

Figure 2 TGA-DTA curves for VPO solid.

3.3. IR spectroscopy

Infrared spectra of vanadium–phosphorus oxide VPO calcinedat different temperatures are given in Fig. 3. Table 1 summa-

rizes the normal modes of different phases and their attribu-tion according to Imai et al. (2007), Kamiya et al. (2003)and Sadiq (2007). From the absorption bands, we observe that

the structure of VPO evolves during the heat treatment whichconfirms the results XRD and TGA–DTA.

3.4. UV–Vis diffusive reflectance spectroscopic

The UV–Vis spectra of the VPO samples dried and calcinedare compared in Fig. 4. The shape of the bands observed indi-

cates the presence of different vanadium species on the surfaceof vanadium–phosphate solids. All spectra of VPO samples ex-hibit one major CTLM (charge transfer ligand–metal) band

centred at about 286 nm broadened by two shoulders at 242and 310 nm, that is, normally attributed to vanadium V5+ in

calcined at 723 and 973 K.

Table 1 FT-IR absorption bands (cm�1) for normal modes of

VPO and their attribution.

Samples m (cm�1) Normal modes

VOPO4ÆH2O and VOPO4 3565, 3366 m(O–H), H2O

1617 d(H2O)

1171(sh), 1084 mas(P–O)

992(sh) m(V‚O)

948 ms(P–O)

680 d(V–OH)

(VO)2(P2O7) 1243, 1217, 1186, 1150 mas(PO3)

1063 ms(PO3)

986 m(V‚O)

941, 927 mas(P–O–P)

798 m(V–O‚V)

745 ms(P–O–P)

634, 563, 514, 430 d(PO3)

sh: shoulder.

0

5

10

15

20

200 300 400 500 600 700 800wavelength (nm)

F(R

)

a

b

c

260286

310

420500 700

650

Figure 4 UV–Vis DRS spectra of dried (a) and calcined at 723 K

(b) and 973 K (c) VPO samples.

0

20

40

60

80

100

433 K 473 K 503 KTemperature (K)

Con

vers

ion

(%)

VPO calcined at 723 K VPO calcined at 973 K

Figure 5 Isopropanol decomposition as function of the reaction

temperature on VPO catalyst.

448 M. Sadiq et al.

tetrahedral coordination (VO4) (Eon et al.,1994; Busca et al.,2000; Lindbald et al., 1994; Concepcion et al., 1995); in addi-

tion, the VPO samples dried and calcined at 723 K exhibit abroader band with a maximum at about 420 nm that has beenattributed to V5+ species in square-pyramidal coordination(Busca et al., 2000; Sadiq et al., 2007). But then, after calcina-

tions at 973 K, the presence of absorption band at 650 nm cor-responding to d–d transitions of vanadyl VO2+ ions in the(VO)2(P2O7) phase indicates the reduction of V5+ species to

VO2+ (Cavani et al., 2010).

3.5. Catalytic activity

Table 2 shows the activity and selectivity in the isopropanoldecomposition in the gas phase using the studied solids(VPO calcined at 723 and 973 K). The main reactions pro-

moted by these catalysts are dehydration of the 2-propanolto the corresponding alkenes (propene) and dehydrogenationto the carbonyl derivative (acetone).

As it can be seen from Table 2 and Fig. 5 the activity of sol-ids increases with the increasing reaction temperature in all in-stances. The calcined solid at 723 K (VOPO4) exhibits catalytic

activity higher than that calcined at 973 K ((VO)2P2O7) at areaction temperature of 433 K, and similar results obtainedat a reaction temperature P 473 K. The latter catalyst

((VO)2P2O7) exhibits high selectivity to propene in all reactiontemperatures. This higher selectivity is attributed to the in-crease of active site numbers on the surface of catalysts. Itcan be also correlated to the acidity of the catalyst (Zhu

et al., 2006; Elassal et al., 2010). This selectivity attains100% at 503 K (Table 2). In other words, the solid contains

Table 2 Isopropanol conversion (%) and selectivity towards propen

over VPO catalyst calcined at 725 and 973 K.

Temperature (K) Catalysts

VPO calcined at 723 K

Conversion (%) Selectivity (%)

Propene Acetone

433 30 49 49

473 97 99 1

503 100 99.5 0.5

(VO)2P2O7 phase favored dehydration over dehydrogenation,so the solid can be assumed to be essentially acid. All values

shown were obtained at t = 175 min, beyond which conver-sion remained constant.

4. Conclusion

The results obtained in this work show that the synthesis of

vanadium phosphate depends on the procedure of preparation.The thermal treatment at high temperature under inert atmo-sphere leads to vanadyl pyrophosphate (VO)2(P2O7) whereas

the treatment below 973 K leads to VOPO4, as shown byXRD analysis and confirmed by DTA–TGA results. The re-sults obtained from FT-IR and UV–Vis spectroscopy are in

agreement with the previous data, showing that this techniqueis efficient for the analysis of vanadium–phosphate materials.The synthetic catalysts showed high activity and selectivity to-wards propene for the isopropanol decomposition at low reac-

tion temperature in comparison with the others catalysts. Itwas found that the VOPO4 samples exhibited high catalyticactivity in the 2-propanol decomposition than (VO)2P2O7 sam-

ples. The 2-propanol dehydration to propene is the predomi-nant reaction and proceeds via an elimination E1-likemechanism catalysed by acid sites.

Acknowledgement

The authors are grateful to the Comite Mixte InteruniversitaireFranco-Marocain (CMIFM) for financial support throughgrants MA/02/36 and MA/06/145.

e, acetone and diisopropyl ether (DIPE) at various temperatures

VPO calcined at 973 K

Conversion (%) Selectivity (%)

DIPE Propene Acetone DIPE

2 18 69 31 0

0 97 99.5 0.5 0

0 100 100 0 0

Effect of calcination temperature on the structure of vanadium phosphorus oxide materials 449

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