ORIGINAL PAPER

Textural, structural and catalytic properties of zirconia dopedby heteropolytungstic acid: a comparative study between aerogeland xerogel catalysts

Samir Chakhari • Mohamed Kadri Younes •

Alain Rives • Abdelhamid Ghorbel

Received: 8 September 2013 / Accepted: 28 November 2013 / Published online: 8 January 2014

� Springer Science+Business Media New York 2014

Abstract Zirconia doped by heteropolytungstic acid

HPW have been synthesized by sol–gel method using two

drying techniques of the solvent evacuation. Samples were

analyzed with adsorption–desorption of N2 at 77 K, and the

aerogel catalyst was found to exhibit a higher surface area

and a higher average pore diameter compared to xerogel.

XRD results show that aerogel develops ZrO2 tetragonal

phase, whereas xerogel is amorphous. The thermal analysis

studies show that the aerogel’s thermal stability is better

than the xerogel one. The catalytic behavior of the aerogel

and xerogel toward the nature of the isomerization products

probably depends on the acidity and the presence of car-

bide species. This has been explained by XPS and iso-

propanol dehydration reaction. In fact, the deconvolution

aerogel’s Cls bands reveals the presence of four carbon

species assigned to C–C, C=O, C–O and carbide species.

Keywords Zirconia � Heteropolytungstic acid � Carbide �Aerogel � Xerogel

1 Introduction

Isomerization of n-paraffin is an important reaction that

produces branched alkanes which enhances the hydrocar-

bon octane number and results in cleaner fuels. Several

catalysts were used such as FRIEDEL and CRAFTS cat-

alysts (AlCl3), bifunctional catalysts (Pt/Alumina, Pt/

chlorinated Alumina) and sulfated zirconia (ZrO2–SO42-)

[1–3]. A large number of publications concerning the

decomposition of alcohols on different catalysts [4–9]

have been available in the literature. In our case, Isopro-

panol decomposition was chosen as a probe acidity

reaction.

Zirconia is commonly doped by sulfate ions, in order

to improve its acidity. Currently, other ions are

used similarly such as phosphate and tungstate groups

[10–12]. Among these groups, Heteropolyacids

(H3PW12O40�13H2O) with Keggin type structure are

used because of their strong Brønsted acidity [13],

similar to catalysts of oxidoreduction [14]. The hetero-

polyacids containing tungsten are preferred to those

containing molybdenum because of their stronger acid-

ity and greater stability [15]. Previous works in our

laboratory have shown that sulfated zirconia have high

activity in the n-hexane isomerization thanks to their

strong acidity [16–18]. However, these catalysts

underwent rapid deactivation [19, 20]. In this work, we

have tried to dope zirconia by heteropolytungstic acid

HPW. As a preparation method, we used a sol–gel

method by varying the solvent evacuation mode in order

to study the effect of the drying technique on textural

and structural properties using powder X-ray diffraction

(XRD), Nitrogen physisorption at 77 K, IR, UV–Vis and

XPS Spectroscopy. The activity of the prepared samples

was tested in n-hexane isomerization.

S. Chakhari (&) � M. K. Younes � A. Ghorbel

Laboratoire de Chimie des Materiaux et Catalyse, Departement

de Chimie, Faculte des Sciences de Tunis, Universite Tunis El

Manar, 2092 Tunis, Tunisia

e-mail: samir.chakhari@gmail.com

A. Rives

Unite de Catalyse et de Chimie du Solide, UMR CNRS 8181,

Universite des Sciences et Technologies de Lille, Batiment C3,

59655 Villeneuve d’Ascq, France

123

J Sol-Gel Sci Technol (2014) 69:378–385

DOI 10.1007/s10971-013-3230-3

2 Experimental

2.1 Catalyst preparation

In the first step of preparation, the precursor zirconium

(IV) propoxide (70 % in propanol, Aldrich) was dissolved

in propan-1-ol. Then the heteropolytungstic acid HPW

dissolved in dimethylformamide (DMF, anhydrous,

99.8 %, Aldrich) was added with a ratio of nW/nZr = 0.1.

Finally, the hydrolysis was performed with a dropwise

addition of distilled water using a hydrolysis ratio

h = nH2O/nZr = 3. Drying is the next step of the prep-

aration which consists in removing the solvent (n-propa-

nol) from the wet-gel. Thus, two drying methods were

adopted. The first method is to dry a part of the gel in an

oven under ordinary conditions (P = 1 bar, T = 293 K)

for 24 h. The catalyst obtained under these conditions is

called xerogel. The second method consists in drying the

other part of the gel under the supercritical conditions of

the solvent in an autoclave (P = 51 bar, T = 536.6 K).

The catalyst obtained under these conditions is called

aerogel. Aerogel and xerogel by acronyms are AZrH and

XZrH, respectively.

2.2 Characterization

Adsorption–desorption isotherms of N2 at 77 K were

performed with a Micromeritics ASAP 2020 apparatus

after outgassing the aerogel at 473 K and the xerogel at

393 K for 3 h. The specific surface area and pore size

distribution were determined by BET and BJH methods,

respectively. The errors in the surface area measurements

are 5 %.

XRD patterns were obtained on a Siemens D-5000

powder diffractometer using Ni-filtered Cu Ka radiation

(k = 1.5418 A). The scan rate was 0.02� per second for 2hfrom 5� to 70�.

Infrared spectra were recorded in transmittance mode in

the spectral range 400–4000 cm-1 using a ‘‘Perkin Elmer

Spectrum XT’’ Spectrometer Fourier transform and cou-

pled to a computer. A mass m = 2 mg of the catalyst is

mixed with 200 mg of potassium bromide (KBr), then

crushed and pressed to obtain a pellet. The pellet is then

placed in a sample holder which is introduced in turn into

the spectrometer.

UV–Visible spectra were recorded by a UV–Vis Perkin–

Elmer Lambda 45 instrument coupled with an integrating

sphere type RSA-PE-20 in 200–800 nm with a speed of

960 nm/min and an opening window of 4 nm.

XPS experiments were performed on a Kratos Analyti-

cal AXIS Ultra DLD spectrometer. AlKa = 1486.6 eV

source was used for excitation. The X-ray beam diameter is

around 1 mm. The spectrometer BE scale was initially

calibrated against the Ag 3d5/2 (368.2 eV) level.

Pressure was in the 10-10 Torr range during the experi-

ments. The analyser was operated in constant pass energy

of 40 eV using an analysis area of approximately

700 lm 9 300 lm. Charge compensation was applied to

compensate charging effect occurring during the analysis.

The C1s peak at binding energy (BE) 285.0 eV of adven-

titious carbon was used as internal reference. Peak fitting of

the experimental photo peaks was carried out using Casa

XPS software. Quantification took into account a non-lin-

ear Shirley background subtraction.

TGA/DTA experiments were performed using a Labsys

Evo (Setaram). The thermal treatment was performed

under air flow (30 cm3/min) between ambient temperature

and 1273 K at a heating rate of 10 �C/min.

2.3 Isopropanol dehydration reaction

Mixture of nitrogen and isopropanol (P = 20 torr) was

conducted in a continuous down-flow fixed-bed reactor

over 100 mg catalyst, operating between 423 and 523 K.

The gas products were supplied automatically to ALPHA

MOOS PR2100 gas chromatography equipped with a HP-

innowax column and analyzed by a flame ionization

detector.

2.4 Catalytic reaction

To study the catalytic properties of the materials in

n-hexane isomerization, the catalysts were treated under

the same experimental conditions. A mechanical mixture

of 50 mg of aerogel or xerogel and 50 mg of Pt/Al2O3

(0.35 % wt Pt) was exposed to a stream of helium with a

Fig. 1 Adsorption–desorption isotherm of the aerogel and xerogel

a AZrH; b XZrH

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123

rate of 30 cm3/min at 423 K for 30 min. Once the pre-

processing was performed, n-hexane was fed by a stream of

hydrogen with a partial pressure of 20 torr and a flow rate

of 30 cm3/min. The n-hexane isomerization is studied in a

temperature range from 423 to 493 K.

3 Results and discussion

3.1 Textural properties

N2 isotherms of the aerogel and xerogel are given in Fig. 1

which shows a type IV isotherm. A small hysteresis loop

type H3 is detected [21]. The hysteresis is observed in the

case where the adsorbent forms aggregates. It can be

attributed to capillary condensation taking place in a non-

rigid texture and shows a non defined mesoporosity.

The BET surface areas and average pore diameters of

the aerogel and xerogel are gathered in Table 1. The sur-

face area of the aerogel is higher than that of the xerogel.

This difference between the texture of aero- and xerogel

may be due to a difference in solvent evacuation mode.

Indeed, the simple evaporation in an oven is susceptible to

form a liquid–vapor interface within the gel. This would

generate a surface tension which could act on the pores and

causes their shrinkage. In contrast, when the solvent is

removed under supercritical conditions, the liquid–vapor

interface is avoided. The absence of surface tension allows

the gel to dry without pore shrinkage.

3.2 Structural properties

The XRD patterns of the aerogel and the xerogel are given in

Fig. 2. The aerogel obtained just after solvent evacuation

without further heating is well crystallized and exhibits the

ZrO2 tetragonal phase characterized by the diffraction peaks

at 30�, 50� and 60� with a few percentage of monoclinic

phase characterized by the diffraction peaks at 2h = 24� and

32�. In contrast, the xerogel dried at 393 K is amorphous.

3.3 IR spectroscopy

The four distinct oxygen sites in the Keggin anion are

represented in Fig. 3 and are described as follows:

• 4 Oa belonging to the central tetrahedra PO4;

• 12 Ot terminal oxygens linked to a lone tungsten atom;

Table 1 Textural properties of the aerogel AZrH and xerogel XZrH

catalysts

Catalyst SBET (m2/g) Dp (A)

AZrH 252 138

XZrH 193 61

Dp average pore diameter

Fig. 2 XRD patterns of the aerogel and xerogel catalysts a AZrH;

b XZrH

Fig. 3 Oxygen sites in a Keggin unit

Fig. 4 FTIR spectra of pure HPW (a); ZrO2 aerogel (b); ZrO2

xerogel (c)

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• 12 Ob in W–Ob–W bridges, between two different

W3O13 groups;

• And 12 Oc in a W–Oc–W bridge, in the same W3O13

groups.

The FTIR spectra of the un-doped ZrO2 aerogel and

xerogel are given in Fig. 4b, c. They show a broad band

centered at 3422 cm-1, which can be attributed to the

stretchy vibration of the hydroxyl groups [22]. The band

located at 1636 cm-1 is assigned to the non dissociated

molecules of water d(HOH) [22]. We note also a series of

bands located at 518, 612 and 738 cm-1 assigned to

crystalline zirconia [22] and a series of bands located

at 1543 and 1444 cm-1, related to the vibration of the

connections Zr–O–C [23]. The Keggin anion vibration

bands (Fig. 4a) appearing at 1080 cm-1 characterize the

mas (POa) vibration, those appearing at 982 cm-1 are rela-

ted to the terminal mas(W=Ot) vibration, and those at 891

and 795 cm-1 are assigned to mas(W–Ob–W) and mas(W–

Oc–W), respectively. Furthermore weaker absorptions

appeared at 595 and 525 cm-1 and are assigned to d(O–P–O)

and ms (W–O–W), respectively (Fig. 4).

The FTIR spectra of aerogel AZrH presented in Fig. 5a

shows that the bands related to vibration P=O and W=O

decrease in intensity and shift to 1124 and 950 cm-1

respectively [24]. However the bands of Keggin structure

located at 795, 891 cm-1 disappeared completely. In the

spectra of the xerogel XZrH (Fig. 5b), we note the disap-

pearance of many bands characterizing the Keggin anion

vibration. This may be due to the interaction of the HPW

with the ZrO2 causing a destruction and a loss of symmetry

in the Keggin structure. Moreover, the aerogel obtained by

supercritical drying holds better the different groups

resulting from the decomposition of HPW.

3.4 UV–visible spectroscopy

UV–Visible spectra (Fig. 6) revealing the effect of drying

method on the catalyst’s structure confirmed the existence of

the characteristic band at 210 nm which can be attributed to

the charge transfer from the valence band (O 2p) to the

conduction band (Zr 4d): O2�ð2pÞ ! Zr4þð4dÞ in aerogel’s

quadratic zirconia [25]. However the charge transfer on

absorption spectra of HPW appear in the 200–240 nm

regions, and consist of bands which may be ascribed to

charge transfer from an O2- ion to a W6? ion in the Keggin

unit at W–O and W–O–W bonds, respectively [26–28].

3.5 Thermogravimetry and differential thermal

analysis

The total weight loss between ambient temperature and

1073 K is equal to 10.5 and 18.5 % for aerogel AZrH and

Fig. 5 FTIR spectra of aerogel AZrH (a) and xerogel XZrH

(b) catalysts

Fig. 6 UV-visible spectra of the aerogel and xerogel catalysts

a AZrH; b XZrH

Fig. 7 TGA/DTA curves of aerogel AZrH

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xerogel XZrH catalysts, respectively (Figs. 7a, 8a). This

loss of weight occurs in three distinct areas. The loss of

physisorbed water on the structure is the most important in

the interval of 333–480 K for the two catalysts. The second

zone for weight loss representing 7 % for the aerogel is

between 508 and 823 K, and between 468 and 573 K for

the xerogel with a loss of 9 %. These losses are attributed

to volatilization of alkoxy residual groups of zirconia

propoxide [29–31]. The final weight loss 4.7 % is localized

between 823 and 1073 K for AZrH and 573–1073 K for

XZrH, which is due to the breakdown of tungsten and

phosphorus oxides in catalysts [32]. These results are in

agreement with IR study which shows that aerogel retains

more HPW groups.

The DTA of catalysts studied in the same region from

the ambient temperature to 1273 K are shown in Figs. 7b

and 8b. The corresponding patterns show endothermic peak

at 535 K for aerogel AZrH and 560 K for xerogel XZrH,

which can be attributed to the loss of physically sorbed and

structural water, respectively [24, 33, 34].

Two regions of exothermicity are observed. For aerogel

and xerogel, the broad peaks between 560 and 690 K are

due to the combustion of residual alkoxy groups of zirconia

propoxide. Moreover, for xerogel, a sharp intense peak is

observed at 700 K which is due to the crystallization of the

zirconia.

3.6 XPS spectroscopy

Quantitative XPS results of aerogel and xerogel solids are

presented in Table 2. The Zr3d5/2 photopeak is centered at

a binding energy (BE) of 182.3 eV for AZrH and at a BE of

182.4 eV for XZrH. It can be assigned to Zr(IV) oxidation

state in ZrO2 oxide form [35, 36]. The XPS spectrum of the

aerogel catalyst, when compared to the xerogel one, shows

significant differences in the shape of the W(4f) ? Zr(4p)

region (Figs. 9, 10). The doublet at 37.4 and 35.4 eV

(Table 2) is attributed to W4f5/2 and W4f7/2, respectively,

and is at the same binding energy of tungsten atoms with

?6 formal oxidation number as in WO3 for AZrH and

XZrH [37–41].

Fig. 8 TGA/DTA curves of xerogel XZrH

Table 2 XPS results of aerogel and xerogel catalysts

BE/eV W/Zr

Zr3d5/2 W4f7/2 C1s

AZrH 182.3 35.3 280.3; 285; 286; 288.8 0.04

XZrH 182.4 35.3 285; 288.6 0.08

Fig. 9 XPS spectra of the W4f ? Zr4p energy region of aerogel

AZrH

Fig. 10 XPS spectra of the W4f ? Zr4p energy region of xerogel

XZrH

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The results of XPS aerogel and xerogel catalysts

(Figs. 11, 12) show three main components which can be

distinguished in the C1s peak; a C–C bond with a binding

energy of 285 eV (because of its width and position it is

referred to as a graphite-like peak), a peak at 288.8 eV for

aerogel and 288.6 eV for xerogel corresponding to C=O

and a peak at 286 eV assigned to C–O for aerogel [42–47].

It is interesting to note that a new element in the aerogel

appears in the C1s spectrum at 280.3 eV, which is related

to the presence of the carbide bond [48, 49].

The W/Zr surface ratio of the aerogel and xerogel

obtained by the XPS spectroscopy are summarized in

Table 2. XPS surface analysis shows that there is a

depletion of tungsten for the two solids, but this loss is

greater for the aerogel.

3.7 Isopropanol dehydration reaction

The acid properties of aerogel and xerogel zirconia doped

by heteropolytungstic acid were characterized using iso-

propanol dehydration reaction at 423–523 K. Propylene

and acetone are the main products of dehydration or

dehydrogenation reaction on acid and base sites, respec-

tively. In this reaction, the formation of propylene is cor-

related to the acidic character while the formation of

acetone indicates basic properties. Based on their catalytic

activity in the formation of propylene, these solids can be

classified in order of acidity character:

AZrH [ XZrH. Indeed, the aerogel AZrH is more active

and selective than XZrH xerogel with a conversion of 62 %

against 38 % at 523 K (Fig. 13). The dehydration becomes

predominant for aerogel catalyst with selectivity to pro-

pylene of 98 % against 64 % for the xerogel (Fig. 14).

Fig. 11 XPS spectra of the C1s energy region of aerogel AZrH

Fig. 12 XPS spectra of the C1s energy region of xerogel XZrH

Fig. 13 Propylene selectivity of catalysts a aerogel AZrH and

b xerogel XZrH

Fig. 14 Isopropanol conversion of catalysts a aerogel AZrH and

b xerogel XZrH

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3.8 Catalytic test

The catalytic properties of aerogel and xerogel have been

evaluated in the n-hexane isomerization between 423 and

493 K. All the catalysts were mechanically mixed with Pt/

Al2O3 in a 1/1 (w/w) ratio. The presence of platinum is

necessary to form hydrogen species which cleans the acid

sites and inhibits the formation of coke (responsible for a

rapid deactivation of the catalysts [50]). The corresponding

results show that the evacuation mode of solvent exerts an

impact on the catalytic properties of the solids. Catalysts

with the same HPW content and hydrolysis rate show that

the aerogel develops better catalytic performances at the

stationary state producing the two isomers 2.2-dimethyl-

butane (2.2DMB) and 3-methylpentane (3MP) while the

xerogel produces only 2.2DMB in the same range of

reaction temperature (Table 3). This result was predictable

since the aerogel and the xerogel exhibit different textural

and structural properties. Although the superficial amount

of tungsten is more important in the case of xerogel

(Table 2) it doesn’t exhibit significant activity. It is prob-

able that the tungsten present on the surface of the xerogel

doesn’t favor the presence of active acid sites. The high

activity of aerogel can be explained by the development of

surface area and the presence of carbide species which is

important in catalysis.

4 Conclusion

Aerogel and xerogel zirconia doped HPW samples were

prepared by the sol–gel method with different methods of

solvent evacuation. A comparison of the textural and

structural properties of these solids shows that the solvent

evacuation method affects significantly on the catalytic

properties of the solids in the n-hexane isomerization. The

composition and chemical structures of aerogel and xerogel

were analyzed using XPS measurements. On the basis of

the C1s peak shape analysis and concentrations derived

from the XPS spectra, the presence of carbide at the surface

was found to enhance the catalytic performance. Moreover,

the aerogel exhibits higher activity due to its more devel-

oped surface area and the presence of ZrO2 tetragonal

structure. In fact, the aerogel, obtained by supercritical

drying develops the ZrO2 tetragonal phase just after solvent

evacuation, whereas the xerogel is amorphous. Further-

more the thermal analysis studies showed that thermal

stabilities of the catalysts decrease in the following order

AZrH [ XZrH. This result would indicate that the aerogel

retains better the various groups resulting from HPW

decomposition.

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