Characterization of H3+xPMo12−xVxO40 heteropolyacids supported on HMS mesoporous molecular sieve and their catalytic performance in propene oxidation

Microporous and Mesoporous Materials 154 (2012) 153–163

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Characterization of H3+xPMo12xVxO40 heteropolyacids supported on HMS mesoporous molecular sieve and their catalytic performance in propene oxidation Siham Benadji a,b,c, Pierre Eloy c, Alexandre Leonard d,e, Bao–Lian Su d, Chérifa Rabia a,⇑, Eric M. Gaigneaux c a Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediene (U.S.T.H.B.), B.P: 32 El-Alia, 16111 Bab-Ezzouar, Alger, Algeria b Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (C.R.A.P.C.), B.P: 248 Alger RP 16004, Algeria c Institute of Condensed Matter and Nanosciences (IMCN) – Division Molecules – Solids and Reactivity (MOST), Croix du Sud 2/17, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium d Laboratoire de Chimie des Matériaux Inorganiques (CMI), I.S.I.S., Facultés Universitaires Notre-Dame de la Paix (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium e Laboratoire de Génie Chimique, B6a, Université de Liège, B-4000 Liège, Belgium

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Article history: Received 26 May 2011 Received in revised form 22 December 2011 Accepted 2 January 2012 Available online 14 January 2012 Keywords: Heteropolyacid (HPA) catalysts Mesoporous materials Propene oxidation Oxidative catalysts Redox properties

a b s t r a c t Materials, consisting of H3+xPMo12xVxO40 heteropolyacids (HPAs) with x = 0–3, supported on a HMS mesoporous pure-silica molecular sieve, have been prepared by means of dry impregnation method and characterized by elemental analysis, X-ray diffraction, transmission and diffuse reflectance (DR) FT-IR, Raman and X-ray photoelectron spectroscopies, nitrogen physisorption and thermal analysis (TG–DTA). The HPA/HMS compositions with HPA loadings from 10 to 50 wt.% display uniformly sized mesopores. HPA retains the Keggin structure on the HMS surface and forms finely dispersed HPA species over the whole range of HPA loadings, crystal phases being absent even at 50 wt.%. Results demonstrate that HPA/HMS exhibit a higher catalytic activity than bulk heteropolyacids in propene oxidation by molecular oxygen. Furthermore, the supported species allow for an enhanced oxidation catalytic activity (formation of acrolein, acetaldehyde and acetic acid) compared to the mother catalysts. The catalytic performance exhibited by H3+xPMo12xVxO40/HMS catalysts was attributed to the fine dispersion of H3+xPMo12xVxO40 species on the HMS mesoporous material via physical adsorption. The HPA/HMS systems, with strong acid sites, high redox power and a regular mesoporous distribution, are promising catalysts for the oxidative reactions. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Polyoxometalates (POMs) are early transition metal–oxygen anion clusters that exhibit a variety in chemical composition and in architecture. Among various POM structural classes, the Keggin-type POMs occupy an important place in the research domain. In the solid state, the POMs are ionic crystals consisting of large polyanions, XM12 On 40 (X: P, Si. . . and M: W, Mo), counter-cations (protons, alkaline, transition metal, ammonium. . .) and crystallization water. The acids corresponding to the POMs, namely heteropolyacids (HPAs), are known to possess a strong Brönsted acidity, stronger than that of many mineral acids or conventional acidic solids and can also display a strong oxidative power. For these reasons, such clusters are widely investigated for being used as catalysts in reactions requiring both acidic and oxidative conditions

⇑ Corresponding author. Tel./fax: +213 21247311. E-mail addresses: [email protected] (S. Benadji), [email protected], crabia@ usthb.dz (C. Rabia). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2012.01.002

and that are performed in homogeneous as well as heterogeneous systems [1–8]. It was shown that POMs, in particular phosphomolybdate compounds, have the ability to activate alkanes at relatively low temperatures and that their efficiency increases when molybdenum atoms are substituted by one, two or three vanadium atoms [9–13]. This is related to the higher oxidative power of vanadium atoms compared to that of molybdenum atoms. However, owing to their small surface area (<10 m2/g), their catalytic performances are often limited in heterogeneous catalysis. For that reason, several authors proposed to disperse them on supports with large surface areas to make the active sites of HPAs more accessible to reactants. Among the carriers, such as carbon [14], silica [15], titania [16] and zirconia [17], the mesoporous silica (MCM-41, SBA-15, HMS. . .) were the subject of several studies [18–24]. These thermally stable materials are made of uniform mesopores and display large specific surface areas (ca. 1000 m2/g) and pore volumes. Among them, Hexagonal Mesoporous Silica (HMS) synthesized from neutral primary alkylamine and neutral inorganic precursors

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at room temperature [25,26] possess thicker framework walls, smaller domain sizes with short channels with consequent larger textural mesoporosity [23,25–28] that allows for a better transport of reactants to active centers and improved diffusion for products to move out. The efficiency of the supported HPAs on mesoporous materials has been demonstrated in many reactions such as the liquid-phase alkylation of 4-t-butylphenol (TBP) with olefins (isobutene and styrene) over PW12/MCM-41 [29], the liquid-phase trans-de-tbutylation of 2,6-di-t-butyl-4-methylphenol over PW12/MCM-41 [30], the methanol conversion and n-butane isomerization over PW12/HMS [31], the esterification of isoamyl alcohol and acetic acid over PMo12 and PW12/SiMCM-41[32], the hydrodesulfurization of dibenzothiophene over PMo12 and PW12 supported on Ti, Zr and Al-HMS materials [33] and the liquid-phase oxidation of anthracene over H3+xPMo12xVxO40 immobilized on MCM-41, MCM-48, and SBA-15 [34]. In our previous work [23], we attempted to immobilize the 11molybdovanadophosphoric, H4PMo11VO40 (HPA) heteropolyacid on Hexagonal Mesoporous Silica (HMS) using three different methods. It was observed that the interaction between the heteropolyacid and the support depends on the impregnation method. In this work, we selected the dry impregnation method to support a series of Keggin-type heteropolyacids, H3+xPMo12xVxO40nH2O where x = 0, 1, 2 or 3 on HMS with HPA loadings of 30 wt.%. The effect of loading (10–50 wt.%) was examined in the case of H5PMo10V2O40 (x = 2). The systems were characterized by nitrogen physisorption, X-ray diffraction (XRD), transmission and diffuse reflectance (DR) FT-IR, Raman and X-ray photoelectron spectroscopies and thermal analysis (TG–DTA) techniques in an attempt to determine the nature of the supported species through the interaction between the heteropolyacid and the support. The second aim is to examine the catalytic properties of these materials that present concomitantly the high acidity and high oxidative power of HPA with the large surface area of mesoporous HMS in the reaction of propene oxidation with molecular oxygen at 350 °C. The catalytic performances of H3+xPMo12xVxO40/HMS systems were compared to that of the H3+xPMo12xVxO40 bulk heteropolyacids.

2. Experimental 2.1. Catalyst preparation The HMS mesoporous material was prepared as previously reported by Tanev et al., using hexadecylamine as template [25,26]. The phosphomolybdic acid sample, H3PMo12O40 was obtained according to the method of Tsigdinos [35] by heating MoO3 with a solution of diluted H3PO4. The synthesis of mono-, di- and trivanadium substituted phosphomolybdic acids, H3+x[PMo12xVxO40] (x = 1–3) was carried out according to the procedure of Tsigdinos and Hallada [36] by acidifying with concentrated H2SO4 an appropriate mixture of Na2HPO4, Na2MoO4 and NaVO3 in appropriate molar ratio. The formed HPAs were extracted with diethyl ether and their compositions were confirmed by XRD, (DR) FT-IR and Raman spectra and elemental analyses. H3[PMo12O40], H4[PMo11VO40], H5[PMo10V2O40] and H6[PMo9V3O40] were denoted V0, V1, V2 and V3, respectively. The 30 wt.% V0, V1 or V3/HMS and 10–50 wt.% V2/HMS samples were prepared by dry impregnation method: a volume of the acid aqueous solution that is ideally equal to the HMS pore volume (pH 0.75), was added dropwise under stirring to the adequate mass of HMS, followed by drying at 50 °C for 20 h under air. The obtained solids were crushed in a mortar. The prepared materials

were thus noted: 30V0HMS, 30V1HMS, 30V3HMS, 10V2HMS, 20V2HMS, 30V2HMS, 40V2HMS and 50V2HMS, respectively.

2.2. Characterization The composition and the weight percentages of P, Mo, V and Si in the samples were measured through inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on an Iris Advantage apparatus from Jarrell Ash Corporation. Following this procedure the metal content could be estimated within an experimental error of ±1.5%. Surface area and porosity measurements were performed by N2 physisorption at liquid nitrogen temperature using a MicromeriticsTristar 3000 equipment after outgassing at 350 °C for HMS or at 130 °C for the other samples under vacuum (5.33–6.67 Pa) for about 15 h. Powder XRD patterns were recorded in the 2h range between 0.5° and 10° using a PanalyticalX’pert Pro, and in the range 10° and 70° on a Siemens D5000 diffractometer, using in both cases Cu Ka radiation (k = 1.5418 Å) at 40 kV and 40 mA. The identification of the phases was achieved by means of the ICDD–JCPDS database. Both transmission and diffuse reflectance (DR) FT-IR spectra of the freshly prepared catalysts were recorded at room temperature with a Equinox 55 (Bruker) spectrometer, in the 4400–370 cm1, with a resolution of 4 cm1, recording 100 scans in the case of analyses made in the transmission mode by using KBr disks (30– 80 wt.% of sample, mixed with dry KBr and pressed as self-supported disks at 4 tons/cm2), and 200 scans in the case of DR. In this latter case, the sample diluted with KBr (1:100) was placed in a Spectratech cell operated in air atmosphere to minimise spurious reduction. Raman spectra were measured under ambient conditions on a Bruker RFS 100/S, in the 1200–200 cm1 range, with a resolution of 4 cm1 and recording 30 scans. The laser used had 785 nm as wavelength and was operated at 200 mW. The destruction of the sample was checked not to occur during the illumination. Thermogravimetry (TG) and differential thermal analysis (DTA) were carried out in a 100 mL min1 air stream with a Mettler Toledo TGA/SDTA 851 apparatus, using 2–50 mg of sample. The samples were heated at a rate of 10 °C min1 from 25 to 500, 600 or 1000 °C for HPA, HPA/HMS and pure HMS, respectively. The surface composition of the sample was determined via ESCA (XPS analyses) on a Kratos Axis Ultra spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromatised aluminum X-ray source (powered at 10 mA and 15 kV). The sample powders were pressed into small stainless steel troughs mounted on a multi specimen holder. The pressure in the analysis chamber was around 106 Pa. The angle between the normal to the sample surface and the lens axis was 0°. The hybrid lens magnification mode was used with the slot aperture resulting in an analyzed area of 700 lm  300 lm. The pass energy was set at 40 eV. In these conditions, the energy resolution gives a full width at half maximum (FWHM) of the Ag 3d5/2 peak of about 1.0 eV. Charge stabilization was achieved by using the Kratos Axis device. The following sequence of spectra was recorded: survey spectrum (with a pass energy of 160 eV), C 1s, O 1s together with Si 2p, P 2p, Mo 3d, V 2p and C 1s again to check the stability of charge compensation as a function of time and the absence of degradation of the sample during the analyses. The binding energies were calculated with respect to the C–(C, H) component of the C 1s peak fixed at 284.8 eV. The spectra were fitted with the CasaXPS program (Casa Software Ltd., UK) with Gaussian/Lorentzian (70/30) product functions and after subtraction of a linear baseline. Molar fractions were

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increasing of the V2 content, indicating a decrease in the total porosity. Table 2 shows BET surface areas (SBET), mean pore diameters (£), pore volumes (Vp) and pore structure parameters (pore wall average thickness WTt and unit cell parameter a0) for the HPAs, the support and the samples loaded with 30 wt.% V0, V1 or V3 and 10–50 wt.% V2. The bulk HPAs have very low BET surface area (3–8 m2/g) with a pore volume of 0.01 cm3/g and a pore diameter that increases from 38.7 to 87.0 Å with the number of vanadium atoms. This increase can be associated to a greater anion–anion repulsion due to the negative charge of Keggin polyanion that passes from 3 to 6. It can be noted that the surface area and the pore volume of the bare HMS support are very high with 1100 m2/g and 1.45 cm3/g, respectively, but with a pore diameter less than that of the HPA (31.0 against 38.7–87.0 Å). Table 2 also shows that the structural properties of HMS are affected by the presence of the heteropolyacid. The surface area and the pore volume decrease by a factor of 2 after impregnation of 30 wt.% HPA (from ca. 1100 to ca. 500–650 m2/g and from 1.45 to ca. 0.6–0.8 cm3/g, respectively). These observations seem to be independent of the composition of the polyanion. Similar results were obtained by supporting PW12 [29], PW12 and their salts [39] H3PW12O40 and H15P5W30O110 [40] on MCM-41 and immobilization of H3+xPMo12xVxO40 on Pd(OAc)2 and HMS systems [41]. The decrease of these parameters (BET surface area and pore volume) could be related either to a blockage of the mesopores of the support by heteropolyanions, to a partial collapse of the support mesopore walls or to the penetration of HPA molecules into the HMS mesopores. On the opposite, the other parameters of supported 30 wt.% HPAs remain practically the same as those of HMS support for all systems (£BJH: 31–32 Å, d100: 45.7–47.3 Å, a0: 52.8– 54.6 Å and WTt: 21.7–23.6 Å). This suggests that the mesoporous structure of the support was preserved in all the synthesized materials. The pore diameters of HPAs, varying between 38.7 and 87.0 Å, are higher than that of the support (£BJH: 31–32 Å) which remains identical even after introduction of the HPA while its pore volume decreases (from 1.45 to ca. 0.6–0.8 cm3/g). These observations suggest that the Keggin units do not penetrate inside the pores but rather stay outside of the surface of the support. Although, in most of investigations, the authors considered that the HPA can enter in the pores of the support [29,39,40], we believe that these conclusions do not take into account the questions of anion–anion repulsion and that of the chemical interaction between the support and the protons of the HPA that could affect the final location of the HPA in or on the support. This hypothesis was already proposed

calculated using peak areas normalized on the basis of acquisition parameters, sensitivity factors provided by the manufacturer and the transmission function. 2.3. Catalytic reaction The propene oxidation was carried out at atmospheric pressure, in a quartz tubular microreactor at 350 °C. The reactor was equipped with a coaxial thermocouple for temperature monitoring. The masses of catalysts used were 210, 90 and 300 mg for HMS, bulk HPA and HPA/HMS, respectively. The reaction temperature was increased with a ramp of 10 °C min1 from the ambient under gas feed consisting of 10% C3H6, 20% O2 and 70% He. The total flow rate was of 30 mL min1. The steady state was reached after 1 h on stream. The catalytic performance was tested for all the catalysts during 5 h. The reactants and reaction products were analyzed on line, by FID gas chromatography for oxygenated compounds, using a Chrompack capillary column CP-Wax 52 CB (50 m, 0.32 mm) and by TCD using an INTERSMAT IGC 12 M equipped with Haysep Q (2 m) and molecular sieve 5A (2 m) columns in series at 80 °C for hydrocarbons, COx and O2. 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. Chemical analysis (ICP) The results of chemical analysis of heteropolyacids (Table 1) were adjusted considering 12, 11, 10 and 9 atoms of molybdenum per Keggin unit according to the nature of HPA, and were found to be in good agreement with the expected ones for phosphorous (0.99–1.02) for all HPAs and vanadium (1.00) for PMo11V. The amount of vanadium was found slightly in excess compared to the desired value for H5PMo10V2 and H6PMo9V3 (2.34 and 3.39, respectively). This excess is attributed to a mixture of vanadomolybdophosphoric acids. Moreover, for the impregnated samples, the Si, P, Mo and V amounts in HPA–HMS samples were close (within 1.5% experimental error) to desired amounts. 3.1.2. Surface and porosity The nitrogen adsorption–desorption isotherms of the HMS and HPAs/HMS materials (Fig. 1) are all of type IV with an H1 hysteresis loop typical of mesoporous solids [25,26,37]. The hysteresis loops for these materials start at P/P0 of about 0.4–0.45, indicating the presence of framework mesoporosity. A second hysteresis is observed at P/P0 > 0.85 and is related to textural inter-particle mesoor macroporosity [38]. Both inflections are attenuated with

Table 1 Chemical composition from ICP of H3+xPMo12xVxO40 (x = 0–3), HMS and HPAs/HMS. Materials

V0 V1 V2 V3 HMS 30V0HMS 30V1HMS 10V2HMS 20V2HMS 30V2HMS 40V2HMS 50V2HMS 30V3HMS a

Content (wt.%) Si

P

Mo

V

– – – – 44.91 30.28 30.27 36.82 32.38 31.04 25.52 21.44 30.61

1.42 1.44 1.58 1.54 – 0.40 0.39 0.15 0.30 0.43 0.61 0.77 0.44

51.90 49.34 48.28 42.14 – 14.83 13.61 4.44 8.80 13.23 18.55 23.31 12.17

– 2.39 6.00 8.43 – – 0.65 0.55 1.07 1.65 2.24 2.83 2.33

Pa

Moa

Va

P/Si

Mo/Si

V/Si

1.02 0.99 1.01 1.02 – 1.00 0.98 1.05 1.05 1.02 1.03 1.02 1.00

12.00 11.00 10.00 09.00 – 12.00 11.00 10.00 10.00 10.00 10.00 10.00 09.00

– 1.00 2.34 3.39 – – 1.00 2.31 2.29 2.35 2.27 2.28 3.24

– – – – – 0.013 0.012 0.004 0.008 0.013 0.022 0.033 0.013

– – – – – 0.157 0.132 0.035 0.080 0.125 0.213 0.318 0.116

– – – – – – 0.012 0.008 0.018 0.029 0.048 0.073 0.040

Atom numbers per Keggin unit [PMo12O40]3, [PMo11VO40]4, [PMo10V2O40]5 and [PMo9V3O40]6.

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1000 HMS

0.07

900

HMS

0.05

3

−1

−1

Pore volume (cm .g .Å )

700

3

−1

Volume adsorbed (cm .g )

800

0.06

600 500

0.04 10V2HMS 0.03

20V2HMS 30V2HMS

0.02

40V2HMS

0.01

10V2HMS

50V2HMS

400

0 20

30

40

50

60

70

80

90

30V2HMS

100

Pore diameter (Å)

300

20V2HMS 40V2HMS

200

50V2HMS

100 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Relative pressure (P/P 0) Fig. 1. Nitrogen adsorption–desorption isotherms and porous distributions determined by BJH of pure HMS and HMS supported by 10–50 wt.% of V2.

Table 2 Physical properties of various samples. Material

V0 V1 V2 V3 HMS 30V0HMS 30V1HMS 10V2HMS 20V2HMS 30V2HMS 40V2HMS 50V2HMS 30V3HMS

H2O contenta (wt.%) (a)

(b)

14.41 13.11 11.31 13.01 1.53 6.25 6.85 5.85 5.45 7.15 7.55 8.25 6.95

1.22 1.62 2.42 2.92 1.04 1.36 3.16 2.56 2.86 3.16 2.56 2.06 3.26

SBET (m2/g)

£BJHd (Å)

Pore volumee (cm3/g)

d100 (Å)

a0f (Å)

WTtg (Å)

8b 6b 4b 3b 1129c 487b 650b 678b 540b 562b 356b 232b 593b

38.7 55.1 63.0 87.0 31.0 31.7 32.0 31.8 32.1 31.9 32.7 34.9 31.8

0.01 0.01 0.01 0.01 1.45 0.64 0.84 0.95 0.77 0.72 0.53 0.34 0.76

– – – – 47.3 45.7 46.5 45.3 47.0 47.3 49.6 51.5 47.1

– – – – 54.6 52.8 53.7 52.3 54.3 54.6 57.3 59.5 54.4

– – – – 23.6 21.1 21.7 20.5 22.2 22.7 24.6 24.6 22.6

a Weight loss from TGA: (a) 1from 25 to 215–240 °C and (b) 2from 215–240 to 500 °C; (a) 3From 25 to 250 °C and (b) 4from 250 to 900 °C; (a) 5From 25 to 150 ± 10 °C and (b) from 150 ± 10 to 600 °C. b BET surface area measured after evacuation at 130 °C. c BET surface area measured after evacuation at 350 °C. d Mean pore diameter determined from BJH desorption dV/dD pore volume. e Single-point adsorption total pore volume at a relative pressure P/P0 = 0.98–0.99. p f Unit cell parameter a0 = 2d100/ 3. g Average thickness of walls WTt = a0  £BJH.

6

by Navez et al. who admitted in the case of the MoO3/SiO2 system, the MoO3 species do not penetrate inside the pores but rather stay outside of the surface of the support [42]. Concerning the V2/HMS series, the amount of impregnated HPA has a more pronounced effect on the structural parameters of HMS support. Surface area and pore volume decrease from 1129 to 678– 232 m2/g and from 1.45 to 0.95–0.34 cm3/g, respectively when the H5PMo10V2 loading increases from 10 to 50 wt.%, while the pore diameter increases gradually from 31.0 to 34.9 Å with HPA content. The unit cell parameter (a0) also becomes more important than that of the bare HMS support when the acid percentage reaches 40 and 50 wt.%, increasing from 54.6 to 57.3–59.5 Å. After introduction of 10–30 wt.% HPA, the pore wall thickness decreased from ca. 24 to 21–23 Å and after impregnation of 40 or 50 wt.% HPA, it

increased to ca. 25 Å. These results show that the structural parameters of the HMS carrier are sensitive to impregnated H5PMo10V2 acid content. Thus, a percentage higher than 30 wt.% acid led to a pronounced change in the morphology of the mesoporous material. Similar results were observed in the case of the introduction of H5PMo10V2 and H3PW12 in SBA-3 support [21]. After introduction of HPA, unlike other parameters, a slight variation of the pore wall thickness is thus observed.

3.1.3. XRD study Fig. 2 depicts the XRD patterns of the HMS support, of V2 and V2/HMS materials with various HPA loadings at low and high angles. The observed XRD patterns of HMS and V2 are typical of the

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Intensity (a.u.)

Intensity (a.u.)

HMS

V2

50V2HMS 40V2HMS 30V2HMS 20V2HMS

10V2HMS 20V2HMS

10V2HMS HMS 10 30V2HMS

15

20

25

30

35

40

45

50

55

60

65

70

2θ (Degree)

50V2HMS 40V2HMS V2 0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

2θ (Degree) Fig. 2. X-ray patterns of V2, HMS and V2/HMS at different V2 loadings at low (2h: 0.5–10°) and high angles (2h: 10–70°).

mesoporous material [26,41,43] and of the Keggin-type heteropolyacid [JCPDS 84-0234], respectively. From low-angle XRD data (2h: 0.5–10°), it was observed that the intensity of the major diffraction peak (2h = 1.9° (1 0 0)) decreased with increasing loadings of acid until its complete disappearance for percentages higher than 30. This result is consistent with that of the textural study that shows the destruction of the structure of the HMS when the acid percentage is high (>30%). The decrease in intensity of the diffraction peaks attributed to the mesostructure was already observed on MCM-41 [39,40], SBA-15 [19] and HMS [31,41] in the presence of heteropolyacids. Moreover, no patterns of the V2 crystal phase are observed even at high HPA loading (40–50 wt.%), indicating that HPA is finely dispersed on the HMS surface, in the form of species with a size of 1 nm, too small to be observed by XRD. Similar results were observed in the cases of supported H3PMo12O40 on MCF [44], in the incorporation of H5PMo10V2 and H3PW12 in SBA-3 [21], PW12 [29], PMo12 [32], H3PW12O40 and H15P5W30O110 [40] on MCM-41 and PW12 [31] and PMo10V2 [41] on HMS. This result can be explained by the fact that with 50 wt.% V2 loading, HPA species occupy a maximum of 1/5 of the total area of the HMS (200 m2 HPA = 0.2 monolayer per 0.5 g of HMS UHPA 12 Å). Therefore, the maximum surface area which may be occupied by HPA is much less than that of the HMS, hence the absence of XRD diffraction lines corresponding to the heteropolyanion. 3.1.4. FT-IR, DRIFT and Raman characterizations FT-IR spectra of bulk V2, pure HMS and V2/HMS (10–50 wt.%) are shown in Fig. 3. The infrared spectrum of bulk H5PMo10V2O40 (Fig. 3b) exhibits typical skeletal vibrations of the Keggin oxoanion in the region 1100–500 cm1 [45,46]. According to literature, the bands at 1061, 961, 864, 779 and 596 cm1 correspond to mas(P–Oa), mas(M@Od), mas(M–Ob–M), mas(M–Oc–M)(M = Mo, V) and d(P–Oa) vibrations, respectively. In a Keggin type unit, Oa refers to the oxygen atom common to PO4 tetrahedron and one trimetallic group Mo3O13; Ob connects two trimetallic groups, Oc binds two octahedral MoO6 inside a trimetallic group and Od is the terminal oxygen

atom. The whole structure has a Td symmetry and corresponds to a isomer. In addition, the vibrational bands attributed to crystallization and constitution water molecules were observed in the ranges 3600–3200 and 1700–1550 cm1. The FT-IR spectrum of HMS (Fig. 3a) exhibits various vibration bands in the 1300–400 cm1 wavenumber region. The vibration band at 1088 cm1and its shoulder at ca. 1160 cm1 (very strong) are assigned to mas(Si–O–Si). The band at 966 cm1 is assigned to the Si–O stretching vibrations of Si–O–R+ groups, (R+ = H+) in the calcined state. The band at 806 cm1 can be assigned to ms(Si–O– Si) in SiO4 groups and the bands at 561 (small) and 465 cm1 (strong) to d(Si–O–Si) bonds [28,42,47–49]. A large and broad band in the hydroxyl region of 3700–2700 cm1 centered at ca. 3430 cm1 is usually attributed to symmetrical and asymmetrical stretching vibrations of water bonded to the external surface. The band at 1630 cm1 is commonly associated to the bending of H–O– H from adsorbed water molecules [49]. It is noted that the C–H bands of surfactant (3120–3050 cm1) were not observed on the spectrum of the calcined HMS sample, showing that all organic products are removed during the thermal treatment of 550 °C. The bands of supported HPA in the 1200–400 cm1 region were partially or fully overlapped with those of the support (Fig. 3c–g). Two strong bands in the IR spectrum of HMS-supported H5PMo10V2O40 appeared at ca. 800 and 962 cm1, coming from the overlapping of the IR absorption bands of silica close to 806 and 966 cm1, and those of the V2 acid at 779 and 961 cm1. A band at about 864 cm1attributed to the mas(Mo–Ob–Mo) vibration is also seen for loadings higher than 10%. All these FT-IR bands are significantly intensified when the HPA loading increases from 10% to 50%. This indicates that the primary Keggin structure is preserved on the mesoporous silica after impregnation. However, for all supported materials, the band assigned to the (P–Oa) asymmetric vibration at 1061 cm1, is strongly masked by the intense band of the support (1160 cm1). Similar observations have been reported in the case of PW12-HMS [31,33,37], PMo12-HMS [33,37] and H5PMo10V2-SBA-3 and H3PW12-SBA-3 [21]. It has been reported that HPA clusters can be decomposed on silica surfaces at very small loadings (below 20 wt.%) due to a

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strong HPA–support interaction. This is generally observed by using the wet impregnation method in which, starting from an aqueous solution of HPA at pH P 2, lacunary and/or unsaturated HPA anions [29,30,50] are formed. In this work, with the dry impregnation method, the pH of aqueous heteropolyacid solutions was lower than 2 even for the very low concentrations of HPAs (<20%), which prevents the decomposition of the Keggin-type species. Fig. 4 shows the DRIFT spectra of HMS, V2 and V2/HMS materials in the 4400–400 cm1 range. The V2 spectrum (Fig. 4a) shows the absorption bands characteristic of the Keggin structure, in

the 1100–700 cm1 region. Absorptions at 787 + 864, 961 and 1061 cm1, are assigned to vibration bands of the mas(M–O–M), mas(M@O) and mas(P–O) bonds (M represents Mo or V), respectively [51]. As shown in Fig. 4a, the stretching vibrations of the silanol groups are clearly identified on pure HMS. The sharp band at 3744 cm1 is ascribed to the isolated terminal silanol groups [37,52]; the bands at 3550 (broad) and 3628 cm1 (shoulder) are assigned to silanol groups inside the HMS channels with and without strong hydrogen-bonding interactions [53]. The strong and broad absorbance band in the 1300–1000 cm1 region is assigned

a

Transmittance (a.u.)

b

c d e f

g

4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800

600

400

−1

Wavenumber (cm ) Fig. 3. FT-IR spectra of (a) HMS, (b) V2, (c) 10V2HMS, (d) 20V2HMS, (e) 30V2HMS, (f) 40V2HMS and (g) 50V2HMS.

Absorbance (a.u.)

Absorbance (a.u.)

a

c d

e

f

g

3780

3770

a

3760

3750

3740

3730

3720

3710

3700

3690

3680

−1

Wavenumber (cm )

c d e f g b 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800

600

Wavenumber (cm−1) Fig. 4. DRIFT spectra of (a) HMS, (b) V2, (c) 10V2HMS, (d) 20V2HMS, (e) 30V2HMS, (f) 40V2HMS and (g) 50V2HMS.

400

159

Intensity (a.u.)

S. Benadji et al. / Microporous and Mesoporous Materials 154 (2012) 153–163

g f

e d c b a

1200

1100

1000

900

800

700

600

500

400

300

200

100

−1

Wavenumber (cm ) Fig. 5. Raman spectra of (a) HMS, (b) 10V2HMS, (c) 20V2HMS, (d) 30V2HMS, (e) 40V2HMS, (f) 50V2HMS and (g) V2.

to the asymmetric stretching mas(Si–O–Si) and that near 800 cm1 to the symmetric stretching mode ms(Si–O–Si). The m(Si–Od) band due to Si–OH and Si–O groups [54] is observed near 950 cm1. The two broad bands between 1800 and 1550 cm1, of medium intensity, are also associated to Si–O lattice vibrations and the one near 1370 cm1 to the H–O–H bending vibrations of physisorbed water [52]. For V2/HMS samples, the band at 3744 cm1of the support is shifted of 2–4 cm1 toward lower wavenumbers. This displacement can be due to a non-homogeneous vibration of the isolated silanol groups induced by the presence of PMo10 V2 O 40 anions. The intensity of this band strongly decreases after impregnation of the HPA on HMS, suggesting a high dispersion of the HPA at the outer surface of the support, in agreement with the XRD and BET results [23,24,55]. In the 1800–400 cm1 region, all vibration bands corresponding to the Keggin structure are fully overlapped by those of the support except those corresponding to metal–oxygen bands (864 and 961 cm1), which are still present at the same positions showing that the Keggin structure is not affected by the silica support. These DRIFT bands are significantly intensified when the HPA loading increases from 10% to 50%. The P–O vibration is masked by the strong adsorption of the silica support between 1000 and 1300 cm1. The Raman spectra of HMS, V2 and V2/HMS materials are shown in Fig. 5. The Raman spectrum of bulk H5PMo10V2O40 (Fig. 5g) is consistent with previously published results [16,45,56]. The main characteristic bands of the Keggin structure are observed in the low wavenumber region (1000–240 cm1) with a shoulders at 1004 (the most intense), 975, 902, 618, and 257 cm1, which are assigned to ms(Mo@Od), mas(Mo@Od), mas(M– Ob–Mo), ms(Mo–Oc–M) and ms(Mo–Oa), respectively (‘‘msMo–Oa’’ with an important bridge stretching character). The Raman spectrum of HMS support does not show any vibration band in this wavenumber region. The Raman spectra, as shown in Fig. 5b–f, provide additional information about the HPA structure of the HPA–HMS samples. In comparison with the standard crystalline HPA (Fig. 5g), no Raman bands attributed to the Keggin polyanion are observed for

HPA–HMS materials. The Raman bands in Fig. 5b–f are very similar to those of the pure HMS. Only the vibration band at 1004 cm1 ms(Mo@Od) is observed. Its intensity increases progressively with loading of HPA. So, as the Keggin unit could be mainly characterized by the stretching mode assigned to metal–oxygen terminal, it can be assumed that the Keggin structure is well preserved on the HMS support, after its impregnation. This observation is in agreement with FT-IR and DRIFT results. 3.1.5. Thermal stability of materials In a previous study, we have observed that the HMS support stabilizes HPA species since the decomposition of H4PMo11VO40 heteropolyacid supported on HMS occurs at 590 °C in comparison to 460 °C for the bulk PMo11V [23]. This stabilization has been attributed to the formation of surface species which are more stable than the free acid form as suggested by Lefebvre [57]. The silanol OH group on the silica surface can react with one proton of þ the H4PMo11VO40 acid to form SiOH2 thus leading to the formation þ of („SiOH2 )(H4 PMo10 V2 O ) surface species. 40 The stability of 10–50 wt.% V2/HMS systems was examined by DTA technique (Fig. 6). The exothermal peak assigned to V2/HMS decomposition is observed at a higher temperature than that of the bulk V2 acid (418 °C), and is visible only for loadings higher than 30 wt.% (ca. 560 °C). At 40 and 50 wt.% loadings, decomposition occurs at ca. 500 °C. These results indicate that the HMS support stabilizes the heteropolyacid even when the amount of acid deposited is high (>30 wt.%) as in the case of V1/HMS [23]. The absence of exothermic peaks for loadings lower than 30 wt.% is probably due to either the very low amount of supported acid or the higher stability of these systems. The stability of V2/HMS can þ be related to the formation of („SiOH2 )(H4 PMo10 V2 O 40 ) surface species, which are more stable than the H5PMo10V2O40 free acid as suggested by DRIFT analysis [23]. This result agrees with that obtained with the H4PMo11VO40 heteropolyacid, supported on HMS. 3.1.6. XPS data XPS results corresponding to the binding energies (BE) of several core levels (O 1s, P 2p, Si 2p, V 2p3/2 and Mo 3d5/2), surface

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4

40V2HMS 30V2HMS

3.75

50V2HMS

3.5

20V2HMS Weight loss (%wt)

3.25 3

10V2HMS

2.75 2.5 2.25 2

V2 1.75 300

325

350

375

400

425

450

475

500

525

550

575

600

Temperature (°C) Fig. 6. DTG curves of bulk V2 and 10–50 wt.% of V2/HMS materials.

Table 3 Binding energies (eV) of core electron levels.

O 1sa O 1sb C 1s C–(C, H) Si 2p P 2p Mo 3d V 2p a b

HMS

V2

10V2HMS

20V2HMS

30V2HMS

40V2HMS

50V2HMS

533.0 – 284.8 103.7 – – –

– 531.2 284.8 – 134.4 233.5 517.8

532.9 530.3 284.8 103.6 133.8 232.6 517.6

533.0 530.6 284.8 103.8 134.1 233.1 517.5

532.8 530.3 284.8 103.9 134.5 232.9 517.6

533.0 530.9 284.8 103.6 133.9 233.3 517.0

533.1 531.0 284.8 103.7 134.2 233.3 517.1

Corresponding to oxygen of support. Corresponding to oxygen of V2.

Table 4 Surface mole fractions (%) from individual spectra.

O 1s C 1s Si 2p P 2p Mo 3d V 2p

HMS

V2

10V2HMS

20V2HMS

30V2HMS

40V2HMS

50V2HMS

62.7 1.7 35.6 – – –

62.3 14.5 – 2.3 17.0 3.9

63.9 4.5 31.0 0.1 0.4 0.1

61.9 4.3 32.6 0.2 0.8 0.2

64.2 2.5 31.7 0.2 1.2 0.2

62.9 4.3 30.5 0.3 1.7 0.3

62.9 7.1 26.7 0.4 2.4 0.5

molar fractions of each element and some atomic ratios obtained on samples V2, HMS and V2/HMS are summarized in Tables 3–5. The BE of Si 2p and O 1s core levels at 103.7 and 533.0 eV, respectively, characteristic of a silicate material [37,38,49], remained unchanged after addition of PMo10V2 (103.6–103.9 eV for Si 2p and 532.8–533.1 eV for O 1s). The well-defined peak of O 1s core level observed in the XPS spectrum (not shown) of bulk heteropolycompound at 531.2 eV (Table 3) can be assigned to Mo–O–Mo bond [16]. A strong O 1s photopeak at 533.0 ± 0.2 eV was observed in the XPS spectra of all supported samples accompanied by a high binding energy asymmetry at 530.3–531.0 eV (Table 3). The first peak is attributed to oxygen of HMS support. The binding energy asymmetry is due to oxygen in polymolybdate structure [16].

The BEs for P 2p core level for all samples are in the 133.8– 134.5 eV region. The values of the position of P 2p photopeak show that phosphorus is in an oxidation state of V [16]. The values of BEs for Mo 3d5/2 core level at 232.9–233.3 eV for bulk and 20–50% supported V2/HMS samples (Table 3) suggest the presence of Mo(VI) in polymolybdates [16,33]. A lower value of BE is observed for the solid 10V2HMS (232.6 eV), suggesting the presence of molybdenum with different oxidation states. The effect is more pronounced after impregnation of 40–50% of V2, with a decrease of BE from 517.8 to 517.0–517.1 eV. The broadening of V 2p3/2 peak suggesting the presence of surface vanadium with lower oxidation states than (V) is due to the photoreduction of the species when the sample is exposed to X-ray source in the chamber of the spectrometer [33].

S. Benadji et al. / Microporous and Mesoporous Materials 154 (2012) 153–163 Table 5 Surface atomic ratios (reported to Si).

O/Si C/Si P/Si Mo/Si V/Si

HMS

10V2HMS

20V2HMS

30V2HMS

40V2HMS

50V2HMS

1.761 0.048 – – –

2.061 0.145 0.003 0.013 0.003

1.899 0.132 0.006 0.024 0.006

2.025 0.079 0.006 0.038 0.006

2.062 0.141 0.010 0.056 0.010

2.356 0.266 0.015 0.090 0.019

Table 6 Propene oxidation by molecular oxygen over HMS, V0, V1, V1, V2, V3 and 30 wt.% H3+xPMo12xVxO40/HMS catalysts (x = 0–3) at 350 °C after 5 h on stream.a Catalyst

b

HMS V0c V1c V2c V3c 30V0HMSd 30V1HMSd 30V2HMSd 30V3HMSd a b c d e

Conversion (%)

2.6 2.7 3.2 4.9 3.5 24.9 22.8 19.4 19.9

Selectivities (%) COxe

Acetaldehyde

Acrolein

Acetic acid

1.4 6.0 5.4 4.1 4.2 42.4 39.4 49.7 39.7

0.0 0.0 0.0 0.0 0.0 5.8 7.2 6.1 4.8

0.0 0.0 0.0 0.0 0.0 2.5 4.0 4.2 3.5

0.0 0.0 0.0 0.0 0.0 2.8 4.3 3.8 2.8

Feed gas: C3H6: 3 mL min1, O2: 6 mL min1, He: 21 mL min1. Catalyst mass; 210 mg. Catalyst mass; 90 mg. Catalyst mass; 300 mg. COx; CO + CO2.

Table 7 Propene oxidation by molecular oxygen over HMS, V2 and H5PMo10V2O40/HMS, with 10, 20, 40 and 50 wt.% V2 at 350 °C after 5 h on stream.a Catalyst

HMSb V2c 10V2HMSd 20V2HMSd 30V2HMSd 40V2HMSd 50V2HMSd a b c d e

Conversion (%)

2.6 4.9 6.8 17.1 19.4 19.0 17.6

Selectivities (%) COxe

Acetaldehyde

Acrolein

Acetic acid

1.4 4.1 11.3 42.7 49.7 46.7 44.1

0.0 0.0 22.3 9.9 6.1 0.0 0.0

0.0 0.0 0.0 4.6 4.2 2.9 0.0

0.0 0.0 0.0 3.2 3.8 3.8 3.7

Feed gas: C3H6: 3 mL min1, O2: 6 mL min1, He: 21 mL min1. Catalyst mass; 210 mg. Catalyst mass; 90 mg. Catalyst mass; 300 mg. COx; CO + CO2.

The atomic ratio Mo/Si estimated from XPS (Tables 4 and 5) for supported samples gradually increases with the amount of HPA. Mo/Si ratios obtained by XPS proved to be lower than those obtained by ICP (Table 1) (0.013 vs. 0.035 for 10V2HMS, 0.024 vs. 0.080 for 20V2HMS, 0.038 vs. 0.125 for 30V2HMS, 0.056 vs. 0.213 for 40V2HMS and 0.090 vs. 0.318 for 50V2HMS). These results suggest an aggregation of Keggin unities on the support surface whatever the amount of impregnated HPA. However, the difference between ICP and XPS results can also be explained by the inclusion of Keggin units in the mesopores of HMS as observed by BET analysis [23]. 3.2. Catalytic performance of H3+xPMo12xVxO40/HMS in the propene oxidation reaction The catalytic results of the propene oxidation by O2 at 350 °C over various catalytic systems are shown in Tables 6 and 7. This

161

temperature was chosen according to the stability of the heteropolyacid to avoid its decomposition to MoO3. It should be emphasized that no product was formed in the reactor in the absence of catalyst and small traces of COx were observed over the bare HMS support. In all samples, carbon is not in balance, which is attributed to the acrylic acid polymerization. Bulk HPAs show a low conversion (<5%) with less than 6% of carbon oxides selectivity. When 30 wt.% of heteropolyacids are supported on HMS, the propene conversion increases strongly from ca. 3–5% to ca. 20–25% (Table 6). The improved conversion can be attributed to the accessibility of the active sites as well as to the fine and homogeneous dispersion of the heteropolyacid on the þ high surface of HMS in the form of („SiOH2 )(H2þx PMo12x Vx O 40 ) surface species as shown by the different characterization techniques. It appears that these surface species seem to be more active than the free acid form (H3+xPMo12xVxO40) since these supported acids favor the formation of oxygenated compounds such as acetaldehyde, acrolein and acetic acid. In the H3+xPMo12xVxO40 (x = 0–3)/HMS series with 30 wt.% loading, the catalytic activity of supported H3PMo12O40 is slightly higher than that of the supported vanadomolybdophosphoric acids, with ca. 25% against ca. 19–23% of propene conversion. However, the formation of reaction products seems to be more sensitive to the presence and number of vanadium atoms in the Keggin unit. H3+xPMo12xVxO40 (x = 1–3)/HMS systems become more selective towards oxidation products when the number of vanadium atoms decreases. Thus, the sum of selectivities to valuable products of oxidation decreases as follows: 30V1HMS > 30V2HMS > 30V3 HMS  30V0HMS. It is noteworthy that the H4PMo11VO40 heteropolyacid seems to be the most interesting in terms of both catalytic activity and valuable oxidation products. The effect of the active phase loading (10–50 wt.%) on the catalytic properties was examined on the V2HMS system (Table 7). The results show that the propene conversion increases from ca. 7% to ca. 19% when the percentage of the active phase increases from 10 to 30 wt.%. Beyond this value (30 wt.%), a slight decrease of the propene conversion from 19.4% to 17.6% is observed. These results indicate that impregnation of 30% of acid is sufficient to achieve the maximum of active sites. Above 30 wt.% loading, there is a þ bad distribution of active species, (SiOH2 )(H3 PMo11 VO 40 ) on the support surface due to the formation of heteropolyanion agglomerates as observed by the structural investigations described above. The lower catalytic activities obtained with percentages of 10% and 20% can be related to the lower number of active sites. The product distribution is also influenced by the loading of HPA on the HMS support. With 10 wt.%, V2/HMS is selective towards acetaldehyde (22.3% of selectivity) and the selectivity of carbon oxides is low (11.3%). Acetaldehyde and acrolein selectivities decrease from 22.3% to 0.0% and from 4.6% to 0.0%, respectively, when increasing the amount of impregnated acid. For loadings of acid between 20 and 50 wt.%, the selectivities towards carbon oxides and acetic acid are similar (43–50% and 3–4%, respectively). The production of selective oxidation compounds was already observed by other authors in propene oxidation over Mx/2H5x[PMo10 V2O40]/HMS (M = Cu2+, Co2+, Ni2+) [58]. The high selectivity observed in the presence of 10V2HMS can be explained by the fact that the mesoporous material leads to a better distribution of active sites and a better stabilization of HPA, preserving its selective catalytic behavior and delaying its degradation to MoO3, known to be less selective. These catalytic results showed that HMS materials impregnated with HPAs appeared to be active catalysts for the propene oxidation with molecular oxygen. The activity of bulk HPA is significantly lower than that of the same amount loaded in HMS demonstrating the importance of using a mesoporous support with a high surface area in improving the catalytic performance of

S. Benadji et al. / Microporous and Mesoporous Materials 154 (2012) 153–163

Absorbance (a.u.)

162

b

a 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800

600

400

600

400

−1

Wavenumber (cm )

Absorbance (a.u.)

Fig. 7. DRIFT spectra of V2 before (a) and after (b) catalytic test for 5 h at 350 °C.

b

a 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 −1

Wavenumber (cm ) Fig. 8. DRIFT spectra of 30V2HMS before (a) and after (b) catalytic test for 5 h at 350 °C.

heteropolyacids. The enhanced stability and the enhanced catalytic activity have been observed by various authors in the use of mesoporous materials such as HMS [31,41], SBA-15 [59] MCM-41 [29,60] and MCF [44]. However no suggestions were made regarding the nature of active sites that may be involved in the alkane oxidation. The originality of our results lies in the fact that we have þ made an assumption on the existence of („SiOH2 )(H2þx PMo12x  Vx O40 ) surface species based on material characterization. On the other hand, the comparison of catalytic results of both bulk and supported HPAs, suggested that the active species in propene oxiþ dation are („SiOH2 )(H2þx PMo12x Vx O 40 ) surface species that are

dispersed homogeneously and whose access is easier for the reactants. 3.3. Characterization of the catalysts after reaction Figs. 7 and 8 show the DRIFT spectra of bulk and supported H5PMo10V2O40 acid before and after 5 h of catalytic test at 350 °C. The DRIFT spectrum of PMo10V2 recovered after propene oxidation is similar to that of HPA before reaction. The characteristic vibration bands of the Keggin structure have the same intensity with a slight displacement of ca. 4 cm1 to higher wavenumbers,

S. Benadji et al. / Microporous and Mesoporous Materials 154 (2012) 153–163

indicating that the Keggin structure remains intact during the catalytic testing. In addition to these bands, a band of low intensity at ca. 670 cm1 can be assigned to reduced V2O5 species [15]. This observation suggests that vanadium migrates from the Keggin anion outside, without destroying its structure. It has been reported in several works that vanadium from the Keggin anion, [PMo11VO40]4, leaves the polyanion structure (anionic position) and moves into a cationic position (counter-cation) during thermal treatment [61–63]. Similarly, the DRIFT spectra of the supported acid, 30V2/HMS, before and after propene oxidation are identical, showing that the Keggin structure was not destroyed during the reaction. Unfortunately, in Raman spectroscopy, the dark color of the samples, reflecting their reduced state after the catalytic test, does not permit their analysis as the incident beam is absorbed by the reduced sample [64,65]. 4. Conclusion In the present work, we have prepared, characterized and investigated the catalytic properties of a series of H3+xPMo12xVxO40 heteropolyacids (HPAs) with x = 0–3, supported on a HMS, mesoporous pure-silica molecular sieve. It appears that physico-chemical characterizations coupled with reactivity study in the propene oxidation in presence of oxygen at 350 °C allowed to point out some important results. BET surface, ICP and XPS results suggested that in addition to the inclusion of Keggin units into the mesopores of HMS, some Keggin units remain outside of the surface of the support. The FT-IR, DRIFT and Raman spectroscopies and XRD analysis showed that Keggin structure of H3+xPMo12xVxO40 heteropolyacids is well preserved on the HMS support, after its þ impregnation and suggested the formation of („SiOH2 )(H2þx  PMo12x Vx O40 ) surface species that are finely dispersed and more stable than the free acid form as suggested by DTA analysis. The catalytic results showed the importance of using a mesoporous support with a high surface area in improving the catalytic performance of heteropolyacid (activity and selectivity to valuable oxygenated compounds, acetaldehyde, acrolein and acetic acid). þ The („SiOH2 )(H2þx PMo12x Vx O 40 ) surface species seem to be the active sites. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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