Microporous and Mesoporous Materials 158 (2012) 88–98

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Microporous and Mesoporous Materials

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Infrared investigation on surface properties of alumina obtained using recenttemplating routes

Widad El-Nadjar a,d, Magali Bonne a, Emmanuelle Trela b, Loïc Rouleau b, Andres Mino c, Smain Hocine d,Edmond Payen c, Christine Lancelot c, Carole Lamonier c, Pascal Blanchard c, Xavier Courtois a, Fabien Can a,Daniel Duprez a, Sébastien Royer a,⇑a Université de Poitiers, CNRS IC2MP UMR 7285, 4 Rue Michel Brunet, 86022 Poitiers Cedex, Franceb IFP Energies nouvelles, Direction Catalyse et Séparation, Etablissement de Lyon, Rond-point de l’échangeur de Solaize, BP3, 69360 Solaize, Francec UCCS–UMR CNRS 8181, Université des Sciences et Technologie de Lille, Cité Scientifique, 59655 Villeneuve d’Ascq Cedex, Franced LCAGC, Hasnaoua 1, Université Mouloud Mammeri, Tizi Ouzou 15000, Algeria

a r t i c l e i n f o a b s t r a c t

Article history:Received 2 December 2011Accepted 5 March 2012Available online 13 March 2012

Keywords:MesostructurationAluminaSurface propertiesEISAPrecipitation

1387-1811/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.micromeso.2012.03.006

⇑ Corresponding author. Tel.: +33 (0) 5 49 45 34 79E-mail address: sebastien.royer@univ-poitiers.fr (S

Porous aluminas were prepared according to recent mesostructuration routes, and compared in terms ofstructural, textural and surface properties with classical precipitated and sol–gel alumina supports.Among the different materials, the precipitation in presence of P123 triblock copolymer as pore directingagent does not allow to strongly improve textural properties which remains close to these obtained usingclassical precipitation. In addition, morphology and surface properties remains similar. By sol–gel proce-dure, the P123 leads to completely different pore properties, with extremely large pore size and pore vol-ume, even if the pore directing agent is suggested to act differently than in classical mesostructurationsynthesis. In this case, the emergence of heterogeneous Lewis acidic sites of higher strength is alsoobserved over the sample prepared with P123. Finally, hexagonal structured materials, issued from Evap-oration Induced Self Assembly routes, are the only to present a well-defined pore structure. Unfortu-nately, physical properties remain less interesting than over the sol–gel materials. In addition, aluminaremains amorphous while the materials are presenting the lowest OH density and highest LAS strength.

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1. Introduction

Alumina is a widely used oxide support in heterogeneous catal-ysis. Indeed, alumina found application in many field including assupport of environmental catalysts (oxidation, reduction) [1,2],energy (hydrotreatment, hydrogen production) [3,4], and finechemistry [5,6]. Different routes, generating alumina presentingvarious physical and chemical properties, are well known and aredescribed in Fig. 1 [7,8]. From the Bauxite ore, the purified hydrateis obtained from the Bayer process. Using either electrolytic pro-cess, alkali attack or acid attack, aluminium precursors are ob-tained. Hydrolysis or precipitation of the obtained precursors(including alkoxide, nitrate, sulphate or sodium salts) leads to theproduction of the hydrate of alumina. Therefore, different formsof alumina hydrates can be obtained, depending on the preparationconditions (bayerite, gibbsite, boehmite or amorphous aluminiumtrihydrate). Boehmite is among the most preferred hydrate precur-sors since its thermal treatment at moderate temperature (between400 and 800 �C) leads to the production of the c form of alumina

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; fax: +33 (0) 5 49 45 34 99.. Royer).

having adequate physical and surface properties for the dispersionof an active phase on its surface. Nevertheless, important differ-ences in physical properties are obtained depending on the synthe-sis route used. Indeed, surface areas generally below 250 m2 g�1 areoften reported for alumina synthesized from precipitation routeswhile largely higher surface areas are obtained using controlledhydrolysis of organic precursors (>300 m2 g�1).

With the development of the mesostructuration routes over sil-ica since the beginning of the 1990s using cationic surfactant(CTABr) [9], and the rapid extension of the syntheses using cationicand non-ionic/neutral templates [10,11], numerous attempts rap-idly appeared for the mesostructuration of various oxides, includ-ing zirconia [12–14], titania [15,16] and alumina [17–21]. In thepioneer work of Vaudry et al. [19], synthesis of organized mesopor-ous alumina (OMA) is reported with carboxylic acid as template.Extremely high surface areas (that can reach 700 m2 g�1) are re-ported, but with limited pore sizes (a few nanometers due to thetemplate size used) and wormhole-like porosity. Thereafter, manyattempts to synthesize mesostructured alumina, using non-ionic/neutral or ionic templating agent, were reported. Unfortunately,wormhole-like porosity was obtained in most of the cases[17–21]. It includes the works based on the Yada’s group approach

Tc/ oC

Crystalline Boehmite, γAlOOH

200 400 600 800 1000

Boehmite γ-Al2O3 δ-Al2O3 θ-Al2O3 α -Al2O3

Bauxite

Bayer process Bayer

hydrate

Electrolysis Al Alkoxide

Acid attack

Alkoxide synthesis

Ziegler process

Hydrolysis

Alkali attack Aluminate

Al salt

Precipitation/crystallization

Precipitation/crystallizationH+

OH-

Crystalline Boehmite, γAlOOH

Fig. 1. Main processing routes for alumina synthesis. Adapted from Refs. [7,8].

W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98 89

to generate hexagonal arranged pore structure [22,23], or theprecipitation from aluminium Keggin structures as provided inthe Kolenda’s studies [18,24]. Unfortunately, hexagonal arrange-ment of pore network collapses during templating agent removal[23]. Finally, even if a large number of recent references dealingwith the synthesis of mesostructured alumina is available, theanalysis of these references demonstrated that the gain in proper-ties between conventional alumina and the mesoporous solids issometimes very limited. Only the recent works dealing with thesynthesis of hexagonal organized alumina using Evaporation In-duced Self Assembly (EISA) procedures [24–30] demonstrateddrastically different textural properties than the classicalprecipitated or sol–gel materials.

In a previous work, we demonstrated that the alumina materi-als synthesized using triblock copolymers as template presentedsimilar thermal stability than commercial alumina, with a sinter-ing energy always comprised between 106 and 120 kJ mol�1 (closeto the value obtained for reference alumina, 120 kJ mol�1) [27].This clearly demonstrated that an initial gain in surface area usingmesostructuration routes will lead to higher surface area after sta-bilisation at high temperature compared to classical alumina. Wenow report a systematic comparison of the textural and surfaceproperties of alumina materials synthesized using mesostructur-ation routes and classical precipitation or sol–gel routes, in orderto determine the real potential of these new materials in theperspective of an application as catalyst support.

2. Experimental

2.1. Material synthesis

Due to the possibility to synthesize template-derived aluminausing different synthesis routes, namely precipitation, sol–geland evaporation-induced self-assembly (EISA) routes, materialsobtained from these routes were compared to conventionallyprecipitated and sol–gel samples. The synthesis protocols aredescribed for each material below.

For the chemicals used, Pluronic P123 (M = 5800 g mol�1, EO20-

PO70EO20), Al(NO3)3�9H2O and citric acid (CA) were provided fromSigma–Aldrich. NH4OH was purchased from Prolabo. Butanol andAl(OsecBut) were obtained from Alfa Aesar. HNO3 was obtainedfrom Carlo-Erba, and HCl from Fisher-Scientific.

2.1.1. Route 1, precipitated materialsAluminium nitrate (Al(NO3)3

.9H2O) was used as the aluminiumsource and P123 triblock copolymer was used as the surfactant.According to the procedure proposed by the Pinnavaia group[20], an aqueous solution containing 0.1 mol of aluminium nitratewas mixed with 0.001 mol of surfactant. The resulting mixture wasstirred at 60 �C for 48 h to obtain a transparent mixed solution.

Ammonium hydroxide solution was slowly added under gentlestirring conditions up to a OH�/Al3+ molar ratio of 3.6 (to achievea pH value close to 8). The precipitate was aged in a closed vesselfor 24 h at 100 �C to obtain a surfactant-intercalated boehmitemesophase, denoted MSU-S/B in the original work [20]. TheMSU-S/B was washed with distilled water, and then dried at80 �C during 24 h. Finally, the composite was calcined at 600 �Cfor templating agent removal and crystallization of the c-Al2O3

phase. The final material is denoted P123-P.Similarly, a classical precipitated materials was prepared by

neutralisation of an acidic solution of aluminium nitrate by sodiumaluminate solution at pH = 8.0. After pH stabilization, the precipi-tate was aged for 4 h at ambient temperature, and then hydrother-mally treated at 100 �C for 24 h. After cooling down to roomtemperature, the solid is recovered by filtration and extensivewashing, before being dried at 80 �C during 24 h. Finally, the mate-rial was calcined at 600 �C for crystallization of the c-Al2O3 phase.The final material is denoted P.

2.1.2. Route 2, sol–gel materialsThe procedure is essentially similar to this reported by Zhang

and Pinnavaia [21]. For the synthesis, 0.2 mol of Pluronic P123was dissolved in 12.5 mol of sec-butanol. Then 1 mol of aluminiumsec-butoxide was added. After 1 h of stirring at ambient tempera-ture, a dilute solution of water in sec-butanol (1:2 vol.%) was addeddropwise. The resulting gel was then stirred at 45 �C for a period of40 h. Recovery of the as-synthesized product was achieved by fil-tration and washing with butanol, followed by air drying underambient conditions before drying at 80 �C for 24 h. Finally, thedry powder was calcined at 600 �C before use. The calcined mate-rial is denoted P123-SG.

Reference sol–gel alumina was synthesized using optimizedprotocol from the laboratory, and remained close to the one withtemplating agent. Shortly, aluminium sec-butoxide was dissolvedin butanol, and water (3 mol per mol of aluminium precursor)was added dropwise to the solution under mechanical vigorousstirring. Afterward, the solution was aged at 20 �C for 4 h beforebeing hydrothermally treated at 100 �C for 12 h. The solid wasrecovered by filtration and washing with butanol, and was driedat 80 �C for 12 h. The solid was calcined at 600 �C, and is denotedSG in the text.

2.1.3. Route 3, EISA materialsThe synthesis of two materials issued from the Yuan et al. [25]

study was performed to achieve formation of hexagonal structuredalumina samples. The first material, H1, was prepared using con-centrated nitric acid as acidifying agent. The second material, H2,was prepared using HCl + citric acid (CA) as acidifying agent. Forthe H1 synthesis, 0.014 mol of Pluronic P123 was dissolved in33 mol of ethanol at 20 �C. Then, 2.1 mol of 65 wt.% nitric acid

90 W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98

solution (or 1.7 mmol of HCl with 0.26 ml of CA added for H2), and1 mol of aluminium sec-butoxide, were added into the above solu-tion under vigorous stirring. The mixture was covered with a PEfilm for aging at 20 �C for 5 h. The evaporation-assembly stepwas performed at 60 �C. After 3 days, a light-yellow gel (or whitegel when HCl + CA was used) was obtained. Calcination of the gelwas carried out by slowly increasing temperature from room tem-perature to 600 �C (1 �C min�1) under 100 ml min�1 of oxygen inorder to achieve alumina formation. Compared to sol–gel route,EISA allowed efficient structuration at lower template on aluminaratio, and with low amount of water for hydrolysis.

2.2. Structural and textural properties

N2-sorption isotherms were recorded on a Micromeritics Tristar3000 automated gas adsorption system. All the samples were out-gassed at 250 �C under vacuum prior to N2 adsorption at �196 �C.Surface areas were calculated according to the BET equation, whilepore size distributions were derived from the desorption branchusing the BJH model. Pore volume is evaluated at P/P0 = 0.97.

The small-angle and wide-angle X-ray diffraction (XRD) pat-terns were obtained on a X-ray Diffractometer D5005 BRUKERAXS using a CuKa radiation (k = 0.15418 nm) and equipped witha SolX detector. For small angle analysis, diffractogram was re-corded for 2h between 0.75� and 3�, by step of 0.02� (steptime = 10 s). For wide angle analysis, diffractogram was recordedfor 2h between 10� and 80�, by step of 0.05� (step time = 2 s).

TEM analysis was performed using a JEOL 2100 microscopeoperated at 200 kV with a LaB6 source. Samples were dispersedin ethanol. One drop of the sonicated solution was placed onto acopper grid, and the solvent evaporated before analysis.

2.3. Chemical surface properties

Acidic sites of materials were characterized by carbon monox-ide (CO) adsorption at low temperature, monitored by infraredspectroscopy. Catalysts were pressed into self supported wafers.Materials were heated under vacuum in situ in the IR cell up to450 �C at a rate of 2 �C min�1, and evacuated for 20 h at this tem-perature before analysis. Thereafter, the solid was cooled downto room temperature (RT). At the end of the activation, and beforeprobe molecule adsorption, the residual pressure in the IR cell wasP �1 � 10�4 Pa. IR spectra were recorded on a Nexus Nicolet spec-trometer equipped with DTGS detector (Deuterium TriGlycerideSulfur) and KBr beam splitter. IR spectra were recorded with a res-olution of 4 cm�1 and 128 scans. The spectra presented were nor-malized to a disc of 10 mg cm�2.

CO probe molecule was adsorbed at �173 �C (100 K). CO wasfirst introduced by incremental doses up to 5 Pa, using a calibratedvolume which corresponds to a quantity of about 8 lmol. Then, COwas adsorbed at equilibrium pressures from 5 to 1 kPa.

3. Results

Synthesized materials were prepared according to already pub-lished procedures and textural and structural properties of thestudied materials will be presented succinctly, with only a discus-sion between the properties obtained here with the properties pre-sented in the original articles.

3.1. Physical properties

Large differences in surface areas are reported depending on thesynthesis route (Table 1). Among the different materials, the

aluminas prepared using precipitation route present the lowestsurface areas. The classical precipitated material, P sample in Ta-ble 1, presents a surface of 262 m2 g�1, which is a classical valuefor alumina support obtained by precipitation. The use of templateassisted precipitation process allows slightly increasing the surfacearea, which exceeds 300 m2 g�1 (P123-P sample in Table 1). The ob-tained value, 319 m2 g�1 after calcination at 600 �C, is comparablewith the values reported by Zhang and Pinnavaia [20], always com-prised between 249 and 367 m2 g�1 for alumina prepared usingvarious triblock copolymers and alumina precursors, after stabil-ization at 550 �C. Sol–gel procedure, with or without templatingagent, allows achieving largely higher surface areas, whose reach�400 m2 g�1. Slightly higher surface area is however obtained forthe classical sol–gel sample, compared to the synthesis withP123 (P123-SG). Zhang and Pinnavaia however reported highersurface area over rare earth-doped alumina prepared using similarroute [21]. While we obtained only 388 m2 g�1 for P123-SGalumina, they reported a surface area of 487 m2 g�1 for 1 mol%La–Al2O3 material calcined at 500 �C. This difference can be ex-plained by the incorporation of small amount of rare earth whichis known to stabilize alumina. Finally, the two hexagonal struc-tured materials, prepared according to the EISA route, as proposedby Yuan et al. [25], are presenting surface areas comparable withthose obtained by precipitation.

All N2 adsorption–desorption isotherms exhibits type IV iso-therms with capillary condensation step followed by an adsorptionplateau at high P/P0 (Fig. 2). Nevertheless, differences in hysteresisshape can be observed, depending on the synthesis conditionsused. Indeed, the two precipitated (P123-P, P) and the classicalsol–gel (SG) materials present H2-type hysteresis according tothe IUPAC classification (isotherms obtained for the precipitatedmaterials presented in Fig. 2A). The three other materials are how-ever presenting a hysteresis close to the H1-type (see Fig. 2B,example of the H1 and H2 materials). This means that all the solidsare presenting porosity in the mesopore domain, but that the poremorphology is probably different between the samples. Then, thepresence of H2-type hysteresis suggests the formation of intercon-nected irregular pores having constrictions (‘‘ink-bottle’’ typepores) over samples P, P123-P and SG), while more homogeneouspore morphology should be achieved on P123-SG, H1 and H2materials.

Mean pore diameter in the mesopore range was then alwaysobtained. The two precipitated materials present similar pore size,close to 5 nm, showing a limited effect of the P123 on the proper-ties of the final material obtained using precipitation (Table 1).Contrarily, a large increase in pore size is observed with the useof a templating agent in the case of SG procedure. Then, and as re-ported by Zhang and Pinnavaia [21], a pore size higher than 10 nmis obtained. This result is however difficult to explain, knowing thatthe micelle size of P123 generally gives rise to pore size around 5–8 nm, depending on the temperature of synthesis used. Such valuesof pore size are then logically obtained for the materials preparedby EISA (H1, H2 – pore size at 6.8 nm and 7.5 nm respectively, Ta-ble 1). These values are in agreement with the values obtained forhexagonal-structured alumina materials prepared with P123 asstructuring agent [25–27]. This suggests that P123 effect duringthe synthesis of the P123-SG material plays a different role thanin the case of the synthesis of EISA materials for which cylindricalpores at �6 nm are awaited.

Nevertheless, formation of a large porosity (>10 nm) whilemaintaining a high surface area is extremely beneficial for the dis-persion of an active phase. Such increase in dispersion could leadto increased catalytic activity of the derived material, as previouslyobserved for CoMo-S phase dispersion on mesoporous and com-mercial aluminas [30].

Table 1Properties of the different aluminas.

Sample SBET/m2 g�1 Vp/cc g�1 dBJH/nm Crystal phase d100/nm I2158/I2180a

P 262 0.36 5.1 c – 1.05P123-P 319 0.43 4.8 c 1.22SG 411 0.80 5.5 c – 1.45P123-SG 388 1.50 11.3 c n.d. 1.45H1 310 0.69 6.8 Amorph. 10 1.84H2 282 0.66 7.5 Amorph. 7.5 1.64

SBET, Vp and dBJH are the specific surface area calculated using the B.E.T. model, the pore volume evaluated at P/P0 = 0.98, and the mean pore size evaluated using the B.J.H.model applied on the desorption branch. d100 is the lattice spacing measured at low angle 2h for the mesostructured materials. n.d. not detectable; Amorph., amorphousphase; a data obtained from CO adsorption monitored by infrared spectroscopy (see Section 3.2).

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1

N2

liqui

d vo

lum

e ad

sorb

ed/ c

m3 .g

- 1

P/P0 / -

A

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.2 0.4 0.6 0.8 1

N2

liqui

d vo

lum

e ad

sorb

ed/ c

m3 .g

- 1

P/P0 / -

B

Fig. 2. N2-sorption isotherms recorded for the precipitated samples (A) and theEISA samples (B). j, P123-P; h, P; d, H1 and 4, H2.

20 40 60 80

2θ/°

P123-PP123-P

P

P123-SGP123-SG

SGSGInte

nsity

/a.u

.

Fig. 3. X-ray diffraction patterns recorded on the different aluminas after calcina-tion. Vertical bars: JCPDS reference n�016–0394 for c-Al2O3.

W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98 91

3.2. Structural properties

The synthesized aluminas present different crystallographicfeatures, depending on the synthesis procedure used. Thus, thetwo precipitated materials present reflections characteristic ofthe c-Al2O3 structure. As can be deduced from Fig. 3, the presenceof templating agent has no visible effect on the crystalline charac-

ter of the final precipitated material, with similar pattern for thetwo materials.

As observed in the case of the precipitated aluminas, thecrystalline properties of the materials synthesis using sol–gel routeis not affect by the presence of the templating agent. Nevertheless,the two sol–gel materials present very broad and weak reflectionscharacteristic of the c-Al2O3 phase, showing the poor crystalliza-tion of the c phase. Finally, the two hexagonal materials, H1 andH2 (Fig. 3), remain amorphous even after calcination at 600 �C. Thisresult is however consistent with previous works, reporting anamorphous phase at 400–600 �C [25,29], while crystallinec-Al2O3 phase is detected only after calcination at 700 �C [25].

3.3. Pore morphology and periodicity

Evidence of efficient mesostructure formation was provided bysmall-angle X-ray diffraction. Logically, the materials preparedaccording classical precipitation or sol–gel route do not presentany low angle reflection. The low angle pattern recorded for theP123-P sample display one broad and poorly defined reflectioncentred at 1� (Fig. 4). The presence of only one broad and poorly

0.00 1.00 2.00 3.002θ/°

H1

P123-P

(100)

(110) + (200)

H2

Fig. 4. Small-angle X-ray diffraction of selected alumina samples.

92 W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98

defined reflection, as obtain in the original work from Zhang andPinnavaia [20], suggests the formation of a wormhole-like porestructure, with limited long range ordering. Contrarily to the re-sults obtained by Zhang and Pinnavaia [21], no reflection can bedetected for the P123-SG material, suggesting a low ordering withno periodicity. Finally, analyses of H1 and H2 show the presence ofa first intense peak located around 1�, followed by a broad andweakly intense massif around 1.5–2�. According to the literature[25], the first peak at �1� can be ascribed to the (100) reflection

(A)

(B)

Fig. 5. Representative TEM images obtained for the different alumi

of the hexagonal P6mm pore symmetry, while the massif at higher2h is the sum of the (110) and (200) reflections. The presence ofthe first reflection followed by the two low intensity reflectionssuggests a higher pore organization in these materials than inthe solids obtained by precipitation or sol–gel with P123 addedas structuring agent.

As can be predicted from the low angle X-ray diffraction exper-iments, large differences in morphology can be observed by TEM,depending on the synthesis procedure used. First, the two precip-itated materials present a classical aggregate-like morphology,without any visible organized pore structure (Fig. 5). The SG samplepresents fibrillar morphology. Long fibres are easily detected onthe high magnification image in Fig. 5C. A smooth-like morphologyis obtained for the P123-SG material. However, no organized porestructure can be observed (Fig. 5D). Finally, the two materialsprepared by evaporation-assembly route display clearly observablepore structures (Fig. 5E and F for H1 and H2 respectively). As re-ported by Yuan et al. [25], H1 presents a well defined hexagonalpore structure, which is consistent with the previously presentedexperimental results, i.e. a well defined small-angle X-ray pattern(Fig. 4) and a Type IV isotherm with H1 type hysteresis (Fig. 2B).The morphology of the H2 sample slightly differs from thisobtained for H1. Indeed, a wormhole-like morphology, with easilydetected spherical pores, is observed (Fig. 5F). The deviation fromthe well-organized pore structure for this materials can be explainby the slightly higher water content added during synthesis, sincethe structure is strongly influenced by this parameter [26].

To conclude, only the EISA-derived materials present a well-de-fined pore structure while the syntheses using P123 as directingagent, either by precipitation or sol–gel, lead to the formation ofclassical aggregate-like morphology.

3.4. Surface properties

3.4.1. Free OH group characterizationThe study of infrared spectra in the stretching OH vibration is

now commonly used as usual spectral technique to characterize

(C)

(D)

(E)

(F)

nas. (A) P; (B) P123-P; (C) SG; (D) P123-SG; (E) H1 and (F) H2.

Table 2OH assignment on the surface of aluminas.

m(OH) frequency/cm�1 Type of OH group Structure

3800–3785

(I):

M

H

O

Ib: (AlVI)OH3780–3760 Ia: (AlIV)OH

3745–3740

(II):

H

OM M

IIb: (AlVI)2OH3735–3730 IIa: (AlIVAlVI)OH

3710–3690

(III):

HOH

M M M

III: (AlVI)3OH

3550 3650 3750 3850

A 0.2

3774

37383676

3787

H1H2

SG

P

P123-SG

P123-P

Wave number (cm-1)

Fig. 6. IR spectra in the (OH) region of alumina outgassed at 450 �C.

2120 2160 2200 2240

A 0.02

2198

2195

2120 2160 2200 2240

A 0.01

2200

2196A B

Wave number (cm-1) Wave number (cm-1)

Fig. 7. IR spectra of CO adsorbed at low coverage (increasing doses up to 8 lmol) onaluminas prepared by precipitation route evacuated at 450 �C. (A) P sample; (B)P123-P sample.

W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98 93

the free hydroxyl group of oxides. OH groups of alumina are char-acterized by complex O–H stretching band in the 3900–3500 cm�1

zone. The assignment of these bands has been largely studied inthe literature but is still controversial [31–35]. Among the variousproposed model, the most used remains the one proposed by Tsy-ganenko et al. [33,34] and completed by Knözinger and Ratnasamy

[35]. Five types of OH species are identified on the surface of alu-mina layers, depending firstly on the number of Al3+ bonding tothe OH group and secondly on the coordination number of theAl3+ ion. Assignments are reported in Table 2, and this modelwas used in this work to study the free OH groups over the variousaluminas.

The OH stretching region of the different aluminas activated un-der vacuum at 450 �C is presented in Fig. 6. The IR spectra are con-sistent with those reported in the literature [35], showing thedifferent types of OH groups of alumina that display bands at3787, 3774, 3738 and 3676 cm�1. Assignments are likewise re-ported in Table. However, we observe in Fig. 6 that intensity ofthe various IR bands of OH groups slightly differs from an aluminato another. A common band is nevertheless observed in the over-view of these spectra, at about 3738 cm�1, assigned to the typeIIb ((AlVI)2OH) group of alumina.

Aluminas issued from the precipitation route display sharpbands at 3774 cm�1. The low intensity of the band of the highestwavenumber at 3787 cm�1 is related to the fact that the occupa-tion of octahedral sites by Al3+ is statistically the most accessible.The P123 use during the synthesis leads to a decrease in intensityof this latter band. In parallel, the intensity of the band at3676 cm�1, assigned to triply bridging OH groups ((AlVI)3OH), in-creases when synthesis is performed using structuring agent. Con-cerning samples synthesized by sol–gel process, OH groups are lessdefined than over materials prepared by precipitation. Neverthe-less, the intensity of the band at 3787 cm�1, assigned to the mostbasic hydroxyl species, is enhanced. Finally the overall intensityof m(OH) bands of Al2O3 synthesized by EISA route is significantlylower, due to the intrinsic crystalline properties of the materialsobtained, i.e. the maintaining of an amorphous phase.

The characterization of the free OH groups may be completedby the study of CO adsorbed spectra. In fact, the acidic propertiesof samples can be characterized using carbon monoxide adsorp-tion, which is reported as a powerful probe molecule to evaluatethese surface properties. At low temperature, CO can adsorb on Le-wis acid sites (LAS) or form an H-bond with acidic OH groups. Forhigh coverage, CO in interaction with acidic OH group by hydrogenbond can be observed, which displays wavenumbers usually com-prised between 2180 and 2157 cm�1. In order to discriminate CO ininteraction with Lewis acidic sites from CO in interaction with OHgroup, a study in the m(OH) zone is needed. In fact, CO in interac-tion with acidic OH group leads to the formation of a H-bond lead-ing to a m(OH) band perturbation (Dm(OH/CO)), proportional to theOH group acidity. When CO is adsorbed on LAS at very low cover-age, its vibration frequency can reach 2230 cm�1 and shift to2157 cm�1 for interaction from strong to very weak coordinativelyunsaturated sites (CUS), respectively. Thus, in this work, acidicproperties of aluminas were characterized using both the COadsorption at low and high coverages, to investigate the acidiccharacters of coordinatively unsaturated Lewis sites and BrønstedOH groups respectively.

3.4.2. Acidic properties of the precipitated materials3.4.2.1. Lewis acidity at low CO coverage. IR spectra of CO adsorbedin the m(CO) zone are reported in Fig. 7 for alumina issued fromprecipitation. At low CO coverage, spectra are dominated by a bandcentred at 2200 or 2198 cm�1, respectively for the P and P123-Pcatalysts, and assigned to medium Lewis acidic sites strength(Fig. 7). This band faintly shifts to 2195–2196 cm�1 on increasingthe dose of adsorbed CO up to 8 lmol. The weak shifting of wave-numbers on increasing the introduced CO indicates a relativehomogeneous population of LAS for both samples. We can thenconclude that the presence of templating agent added to the syn-thesis of precipitated materials does not induce any change in Le-wis acidic properties of the final materials.

2080 2120 2160 2200 2240

0.2A

2156

2183

B

2195

2080 2120 2160 2200 2240

0.2A

2158

2183

2196

A

Wave number (cm-1)

Wave number (cm-1)

Fig. 8. IR spectra of CO adsorbed at high coverage (P equilibrium about 1 kPa) onaluminas prepared by precipitation route evacuated at 450 �C. (A) P sample; (B)P123-P sample.

94 W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98

3.4.2.2. Brønsted acidity at high CO coverage. If the study of IR spec-tra of adsorbed CO at very low coverage reveals the strength of Le-wis acid sites, the distribution between acidic type sites (CUS andacidic OH groups) can be evaluated in regards to CO adsorption at

Fig. 9. IR spectra of adsorbed CO at low temperature in the m(OH) ran

equilibrium pressure. For higher CO coverage (Fig. 8), spectra re-ported in the carbonyl stretching region show that CO from thefirst doses essentially coordinates on LAS, as previously observedin Fig. 7.

At the same time IR spectra in the m(OH) stretching zone (notshown) do not present observable hydrogen perturbation for thealumina issued from precipitation procedures. The correspondingcarbonyl wavenumbers are observed at about 2196–2195 cm�1

and gradually shift to 2183 cm�1 with the increase in CO coverage(Fig. 8). This effect is assigned to the consequence of the buildingup of adsorbate–adsorbate interactions. For higher CO coverage, anew band appears at about 2156 and 2158 cm�1, respectively forthe P123-P and P samples. This band is assigned to the H-bondinteraction of CO with surface OH groups. Simultaneously, a broadband with a maximum at 3583 cm�1 grows in the hydroxylstretching region (Fig. 9B). One can note that the whole of OHgroups interact with CO probe, leading to a global perturbed OHband at 3583 cm�1 (Dm(OH/CO) = 145–147 cm�1). At the sametime, in the carbonyl stretching region, the intensity of the bandat 2156–2158 cm�1 gradually increases with the dose of adsorbedCO. Due to the relatively low extent of the Dm(OH/CO) perturba-tion, and to the value of the carbonyl frequency, CO is in interactionwith weak acidic hydroxyl sites.

Finally, the distribution between the amount of Lewis andBrønsted acid sites can be assessed by the ratio of intensities ofbands at about 2158 and 2180 cm�1 (Table 1). It appears that thequantity of LAS on these materials is important in regards to per-turbed OH groups.

3.4.2.3. Main conclusions. The characterization of the precipitatedmaterials allows us to infer that:

(i) The templating agent used during synthesis has no influenceon either Lewis or Brønsted acidic properties;

(ii) The coordinative unsaturated Al3+ sites are of weak strengthbut homogeneous, as determined on the low coverage spec-tra (Fig. 7);

(iii) The amount of LAS is high compared to OH group (Table 1).

3.4.3. Acidic properties of the sol–gel materials3.4.3.1. Lewis acidity at low CO coverage. IR spectra of CO adsorbedat low coverage on alumina prepared by sol–gel route are

ge. (A) P equilibrium about 20 Pa; (B) P equilibrium about 1 kPa.

2120 2160 22002240

A 0.02

2196

2192

2120 2160 2200 2240

A 0.02

2209

2196

A B

Wave number (cm-1) Wave number (cm-1)

Fig. 10. IR spectra of CO adsorbed at low coverage (increasing doses up to 8 lmol)on aluminas prepared by sol–gel route evacuated at 450 �C. (A) SG sample; (B)P123-SG sample.

0.1A

2157

2178

2192

0.2A

2157

2177

2196

2080 2120 2160 2200 2240

A

2080 2120 2160 2200 2240

2177

B

Wave number (cm-1)

Wave number (cm-1)

Fig. 11. IR spectra of CO adsorbed at high coverage (P equilibrium about 1 kPa) onaluminas prepared by sol–gel route evacuated at 450 �C. (A) SG sample; (B) P123-SGsample.

W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98 95

presented in Fig. 10. A first band at 2196 for SG and 2209 cm�1 forP123-SG is observed. The fact that the band of CO coordinated onLAS is observed at various frequencies on both samples indicatesdifferent acidic strength of coordinated unsaturated sites on thesetwo aluminas. Contrary to the materials obtained by precipitationfor which no difference can be evidenced, the presence of a tem-plating agent during the sol–gel synthesis generates LAS fromhigher strength. For increasing CO doses, the band gradually shiftsto lower wavenumbers until 2192 cm�1 for SG sample and to2196 cm�1 for P123-SG sample. Besides, one can note that the shiftof the carbonyl stretching frequency with increasing CO adsorptionis raised more on the P123-SG sample, indicating heterogeneouscoordinatively unsaturated Al3+ sites, compared to over SG samplewhich presents more homogeneous LAS. To conclude, the use ofP123 during the synthesis leads to the emergence of heteroge-neous LAS from higher strength than over sample prepared with-out P123.

3.4.3.2. Brønsted acidity at high CO coverage. As presented for sam-ples prepared by precipitation route, the OH acidic characterizationis obtained by the study of infrared spectra of CO adsorbed at highcoverage for the sol–gel issued materials. IR spectra in the carbonylspectral zone of CO adsorbed at equilibrium pressure for aluminassynthesized by sol–gel route (SG and P123-SG samples) are pre-sented in Fig. 11. Compared to samples prepared by precipitation,OH groups of sol–gel aluminas are perturbed since the first COadsorption doses (Fig. 9). These results suggest that the amount ofLAS is lower on these samples than on precipitated aluminas. Thisinterpretation is supported by the comparison of band intensity as-signed to CO H-bonded to OH groups (m(CO) � 2158–2156 cm�1) tothe one attributed to CO coordinated on LAS (m(CO) � 2183–2177 cm�1), as presented in Table 1. Data reveal that the amountof LAS is higher on aluminas synthesized by precipitation than oversamples prepared by sol–gel, with I2158/I2180 values which varyfrom 1.05–1.22 to 1.45 for precipitated and sol–gel route,respectively.

A broad band observed in the m(OH) zone at 3583 cm�1 revealsthe perturbation of the surface hydroxyl groups of alumina(Fig. 9B). For the first equilibrium pressure, this band is observedat lower frequencies (3561–3568 cm�1, Fig. 9A) that indicateshigher strength hydroxyl perturbation. For the first CO equilibriumpressure, the interaction of CO with Al3+ CUS (m(CO) at 2192 and2196 cm�1) is observed in the carbonyl stretching zone. With theincrease in CO pressure, this band shift to an intense shoulder at

2178–2177 cm�1, depending of the sample. At the same time, theband assigned to the hydroxyl perturbation grows at 2157 cm�1.

3.4.3.3. Main conclusions. To summarize, aluminas prepared by sol–gel route present the following characteristics:

(i) LAS of strength slightly lower than over solids prepared byprecipitation (compare Figs. 8 and 11);

(ii) An acidity of LAS more heterogeneous than on samples syn-thesized by precipitation, as observed at low CO coverage(Figs. 7 and 10);

(iii) An amount of LAS compared to the hydroxyl group H-bonded with CO lower than on samples prepared by precip-itation (Table 1).

3.4.4. Acidic properties of the EISA materials3.4.4.1. Lewis acidity at low CO coverage. Fig. 12 displays the IRspectra of CO adsorbed at 100 K at low coverage on aluminas pre-pared by EISA and evacuated at 450 �C, which are the only materi-als presenting a well-defined pore structure. The m(CO) frequenciesof first adsorbed CO doses are observed at 2211 and 2218 cm�1 forH1 and H2, respectively. These wavenumbers are noticeably higherthan those previously reported for aluminas prepared by precipita-tion or sol–gel route. This indicates LAS of higher strength. Accord-ing to the literature, this band is assigned to CO r-bonded totetrahedral Al3+ defect sites [36]. The formation of such Lewisacidic centres is produced by the reaction of a basic OH group withthe proton of an acidic hydroxyl groups as reported in the follow-ing equation [37]:

Fig. 12. IR spectra of CO adsorbed at low coverage (increasing doses up to 8 lmol)on aluminas prepared by EISA route evacuated at 450 �C. (A) H1 sample; (B) H2sample.

2080 2120 2160 2200 2240

0.1A

2158

2197

2179

2080 2120 2160 2200 2240 Wave number (cm-1)

Wave number (cm-1)

0.2A

2157

2191

2177

A

B

Fig. 13. IR spectra of CO adsorbed at high coverage (P equilibrium about 1 kPa) onaluminas prepared by EISA route evacuated at 450 �C. (A) H1 sample; (B) H2sample.

96 W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98

ðAlIVÞOHþ ðAlIVAlVIÞOH! Al3þIV þ ðAlIVAlVIÞOþH2O ð1Þ

Thus, the presence of CUS Al3+ sites in tetrahedral coordinationshould normally lead to a lower population of basic OH group onthese materials. These results are confirmed by the study of IRspectra in the m(OH) stretching region, especially for the H2 samplewhich obviously presents, after activation, the lower amount of ba-sic OH group, and the LAS of higher strength (Fig. 6).

3.4.4.2. Brønsted acidity at high CO coverage. For increasing CO cov-erage (Fig. 13), the band centred around 2211–2218 cm�1 gradu-ally shifts to lower wavenumbers, up to 2191–2197 cm�1. At thesame time a new band develops at about 2160 and 2157 cm�1

for H1 and H2, respectively. This band can be associated to the per-turbation of OH groups, as reported above for other materials.

Carbon monoxide adsorption at high coverage results in theappearance of a broad band of perturbed OH groups at 3583 cm�1

for H1 and H2 samples (Fig. 9B). Bands of adsorbed CO in the m(CO)region are gradually shifting until about 2177–2179 and 2157–2158 cm�1, for higher coverage. These bands can be assigned respec-tively to coordination on weak Lewis sites and H-bonding (Fig. 13).

3.4.4.3. Main conclusions. Compared to alumina synthesized by pre-cipitation and sol–gel procedure, CO adsorption reveals that:

(i) The synthesis of alumina by EISA route leads to higherstrength of LAS (Fig. 12);

(ii) The amount of LAS sites compared to acidic OH groups is thelowest among all studied materials (Table 1).

4. Discussion on alumina surface properties

The different aluminas were characterized using CO adsorptionat low temperature monitored by infrared spectroscopy. Followingthe increasing amount of CO adsorbed, the Brønsted acid strengthof the free hydroxyl group, as well as the Lewis acid strength of theCUS Al3+ sites, has been characterized.

For high CO coverages, as reported in the Figs. 8, 11 and 13,essentially two bands at about 2180 and 2158 cm�1 are observedon all samples. These bands can be assigned to CO coordinatedon LAS and to CO in interaction with surface OH groups, respec-tively. Due to the slightly higher frequency of the band at2180 cm�1 compared to the free CO molecule (i.e. �2140 cm�1),this band can be assigned to CO bonded to Lewis acid centers ofweak strength (see below).

The spectra evolution in the m(OH) region while increasing theamount of CO adsorbed is presented for the different materials inFig. 9A (20 Pa) and 9B (1 kPa). The broad bands observed in bothfigures at about 3554–3583 cm�1, are assigned to the H-bondinginteraction of CO with the free OH groups of aluminas [38,39]. Thisassignment is supported by the fact that the corresponding car-bonyl band at 2158 cm�1 constantly grows with the broadeningof the m(OH) band. In addition, the interaction of CO by H-bondis proportional to the OH group acidity (Dm(OH/CO)) of the surfaceoxide. At low CO coverage, it appears that the strength of acidic OHgroups fluctuates among the various aluminas characterized in thiswork. However, the shift of 147 cm�1 leading to the broadening ofthe hydroxyl stretching band while increasing amount of CO ad-sorbed (Fig. 9B) is in accordance with literature data, and showssimilar OH strength in all aluminas. Thus, the variation observedat low coverage (Fig. 9A) just allows concluding on the heterogene-ity of the free OH groups depending on the conditions of synthesis.

As shown in Fig. 14, the absorbance of the m(OH) band at3738 cm�1, representing the (AlVI)2OH species, is linearly corre-lated to the intensity of the carbonyl band at 2158 cm�1 (CO ininteraction with isolated acidic hydroxyl groups). The higher theabsorbance of the 3738 cm�1 band is, the higher the carbonyl bandintensity is. This result suggests that, in our applied conditions, CO

Fig. 14. Correlation of the absorbance of the stretching bands of aluminas at3738 cm-1 and the intensity of carbonyl frequency at 2158 cm�1. 4, H1; N, H2; h,P; j, P123-P; s, SG; d, P123-SG.

W. El-Nadjar et al. / Microporous and Mesoporous Materials 158 (2012) 88–98 97

preferentially interacts with octahedral type IIb bridging hydroxylgroups (Table 2). Octahedral aluminium are also involved in theType Ib hydroxyl group ((AlVI)OH), but the absorbance determina-tion of the corresponding at 3787 cm�1 band has a high error mar-gin and cannot be properly integrated to be correlated to thecarbonyl band at 2158 cm�1.

Concerning the characterisation of LAS sites, it appears that COadsorption on the different materials reveals coordinatively unsat-urated sites having different strengths. The band observed at high-er wavenumber for low coverage (i.e. close to 2220 cm�1 on H2prepared by EISA) is assigned to CO r-bonded to AlIV

3+ defect sites[36]. The results then showed that only EISA led to LAS from highstrength.

The attribution of carbonyl band, observed for higher coverage(Figs. 8, 11 and 13) around 2180 cm�1, is still controversial. Someauthors assigned this band to CO ligand coordinated to bulk tetra-hedral Al3+ ions emerging on the surface [36,40], whereas othersassigned it to CO r-bonded to octahedral AlVI

3+ ions [37]. As men-tioned above, at high CO coverage, essentially the intensity be-tween the band at 2158 cm�1 and the one at 2180 cm�1 seems tobe affected by the synthesis route.

5. Conclusions

Numerous routes were recently proposed to prepare mesostruc-tured alumina. Among the different routes, precipitation and sol–gel derived routes are observed to produce materials exhibiting mor-phology close to the morphology classically obtained using conven-tional procedures. Only the EISA (Evaporation Induced SelfAssembly) route is observed to produce organized aluminas, i.e. hav-ing a well defined pore structure. Nevertheless, the textural and mor-phological properties of the final material are strongly dependant onthe synthesis conditions. It was then observed that a synthesis in ni-tric acid media is more efficient to achieve high structuration quality.Unfortunately, a well-organized structure does not result in im-proved physical properties, and sol gel route is probably the mostefficient to maximize surface area. Nevertheless, homogeneous poresize can be achieved only using EISA process.

From the infrared characterization resulting from CO adsorptionat low temperature and at variable coverage over aluminas, itappears that the synthesis route does not alter in depth the acidicproperties of alumina oxide, which maintains conventional proper-ties. At low CO coverage, the adsorption enables us to distinctly

characterize the nature and the strength of LAS. Only materialsprepared by the EISA route seems to present strong LAS comparedto the other materials (precipitated and sol–gel). Properties of iso-lated OH groups were also characterized by CO adsorption. Theacidity of OH groups was almost similar on the whole of the stud-ied samples. However, some heterogeneity on the OH acidity dis-tribution was observed. Finally, the main result presented in thisstudy arises from the distribution between the m(CO) band intensi-ties observed at 2180 and 2158 cm�1 resulting from IR spectra re-corded at high equilibrium CO pressure. It appears that thedistribution between weak LAS and perturbed acidic OH groupswas directly affected by the synthesis route. In fact, whereas theprecipitation route leads to a ratio close to 1 between CO in inter-action with hydroxyl group and CO bonded to Al3+ CUS, EISA syn-thesis route leads to two times more perturbed OH compared tothe available amount of LAS.

Acknowledgements

Widad El-Nadjar acknowledges the Algerian government for the18 months PhD. financial support. Sébastien Royer acknowledgesthe CNRS for the 6 months delegation period. The IFP EnergiesNouvelles group is also acknowledged for the financial support ofthis work through the contract n�782773.

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