Influence of pore size of SBA-15 on activity and selectivity of H3[PMo12O40] supported on tailored SBA-15

Accepted Manuscript Influence of pore size of SBA-15 on activity and selectivity of H3[PMo12O40] supported on tailored SBA-15 R. Zubrzycki, T. Ressler PII:

S1387-1811(15)00234-6

DOI:

10.1016/j.micromeso.2015.04.022

Reference:

MICMAT 7090

To appear in:

Microporous and Mesoporous Materials

Received Date: 10 February 2015 Revised Date:

7 April 2015

Accepted Date: 15 April 2015

Please cite this article as: R. Zubrzycki, T. Ressler, Influence of pore size of SBA-15 on activity and selectivity of H3[PMo12O40] supported on tailored SBA-15, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.04.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT 1

Influence of pore size of SBA-15 on activity and selectivity of

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H3[PMo12O40] supported on tailored SBA-15

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Technische Universität Berlin, Institut für Chemie, Sekr. C2

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Strasse des 17. Juni 135, D-10623 Berlin, Germany

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[*] E-mail:

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Phone: (+49) 30 314 79736, Fax: (+49) 30 314 21106

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[email protected]

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Keywords: Structure-activity relationships; EXAFS spectroscopy; PMo12-SBA-15;

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Selective Propene Oxidation; Tailored SBA-15

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ACCEPTED MANUSCRIPT 1 2 3

Abstract Large pore SBA-15 was successfully synthesized and used as support material

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for molybdenum based oxidation catalysts. H3[PMo12O40] was supported on SBA-15

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with modified pore radii (10, 14, 19 nm). All samples were prepared with a similar

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surface coverage of 1 Keggin ion per 13 nm2 independent of the pore radii. Structural

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evolution and catalytic activity of H3[PMo12O40] supported on SBA-15 (PMo12-SBA-

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15) with different pore radii (10, 14, 19 nm) were investigated under selective propene

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oxidation conditions by in situ X-ray absorption spectroscopy investigations. PMo12-

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SBA-15 (10, 14, 19 nm) formed a mixture of mostly tetrahedral [MoO4] and octahedral

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[MoO6] units during thermal treatment in propene oxidation conditions. A higher

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concentration of octahedral [MoO6] units and higher oligomerized [MoxOy] units were

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detected for act. PMo12-SBA-15 (10 nm) compared to act. PMo12-SBA-15 (14, 19 nm).

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The higher concentration of [MoxOy] units present in act. PMo12-SBA-15 (10 nm)

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resulted in an increased catalytic activity compared to activated PMo12-SBA-15 (14, 19

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nm) with a lower concentration of [MoxOy] units. Selectivities towards oxidation

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products during propene oxidation were comparable and largely independent of the pore

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radii of act. PMo12-SBA-15 (10, 14, 19 nm). Apparently, tailoring the pore radius of

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silica SBA-15 permitted to prepare Mo oxide model systems to investigate correlation

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between activity and structure of characteristic oxide species at similar surface

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coverage.

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ACCEPTED MANUSCRIPT 1. Introduction

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Molybdenum oxides constitute active heterogeneous catalysts for selective oxidation of

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alkenes and alkanes with gas phase oxygen [1]. However, structure-activity

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relationships are often difficult to obtain for the various mixed oxides with different

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chemical compositions and corresponding crystal structures. Molybdenum based

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heteropolyoxomolybdates (HPOM) with Keggin structure exhibit a broad compositional

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range while maintaining their characteristic structural motifs [2–4]. Therefore,

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substituting Mo atoms with addenda atoms (i.e. V, W, Nb) makes Keggin type HPOM

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suitable model systems to study structure-activity relationships. Thus, HPOM have been

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frequently studied as active catalysts for selective oxidation reactions [5].

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In addition to using bulk model catalysts for determining structure-activity

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relationships, supported metal oxide catalysts have received increasing attention [6,7].

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Supported catalytic species posses high dispersions and an improved surface to bulk

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ratio. Hence, differentiating between bulk and surface structures is no longer necessary.

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Therefore, structure-activity relationships can be readily deduced from the characteristic

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oxide species observed on the support material under catalytic reaction conditions.

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Suitable support materials for catalyst model systems should possess a large surface

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area and a homogeneous internal pore structure with sufficiently large pores.

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Furthermore, the support should interact with the precursor to stabilize particular

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structural motifs without participating in the catalytic reaction. Nanostructured SiO2

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materials such as SBA-15 [8,9] represent suitable support systems for oxide catalysts

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[10,11]. For investigating structure-activity relationships, model systems are required

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that combine compositional invariance with structural variety [12,13]. Supported

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HPOM can be used to vary the chemical compositions while maintaining good

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accessibility of the supported molybdenum based catalysts. Studies on H4[PVMo11O40]

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supported on SBA-15 revealed a certain structure directing behavior of the silica

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ACCEPTED MANUSCRIPT support [14]. H4[PVMo11O40] supported on SBA-15 formed a mixture of tetrahedrally

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and octahedrally coordinated and connected [MoO4] and [MoO6] units under catalytic

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conditions [14].

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Previous studies have shown that catalytic activity and selectivity scales with both the

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concentration and the degree of oligomerization of tetrahedral [MoO4] and octahedral

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[MoO6] units at the surface [15]. Isolated [MoO4] units supported on MgO for instance

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were nearly inactive for propene oxidation. The catalytic activity and selectivity towards

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oxygenates increased with an increasing amount of [MoxOy] species. Previously, the

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degree of oligomerization was adjusted by either varying the metal loading or altering

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the surface acidity of the support material [11,16–18]. In addition, only few other

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characteristics of supported model systems are conceivable to alter the connectivity of

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supported MoOx species. Hence, varying the pore radii of the support material may be a

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complimentary approach. This could lead to modified structure directing effects on

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supported HPOM at constant metal oxide surface coverage and identical surface acidity.

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Subsequently, the resulting [MoxOy] structures on tailored SBA-15 can be used to

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further elucidated structure-activity relationships.

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Here, H3[PMo12O40] was supported on SBA-15 with modified pore radii (10, 14,

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19 nm). The samples were prepared with a surface coverage of 1 Keggin ion per 13 nm2

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independent of the pore radii. Correlations between structure of the resulting [MoxOy]

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units and their catalytic activity during propene oxidation are presented.

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ACCEPTED MANUSCRIPT 2. Experimental

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2.1 Physisorption measurements

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Nitrogen physisorption isotherms were measured at 77 K on a BEL Mini II volumetric

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sorption analyzer (BEL Japan, Inc.). Silica SBA-15 samples were treated under vacuum

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at 368 K for about 20 min and at 448 K for about 17 h before starting the measurement.

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Data processing was performed using the BELMaster V.5.2.3.0 software package. The

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specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method

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in the relative pressure range of 0.03–0.24 assuming an area of 0.162 nm2 per N2

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molecule[19]. The adsorption branch of the isotherm was used to calculate pore size

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distribution and cumulative pore area according to the method of Barrett, Joyner, and

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Halenda (BJH) [20].

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2.2 Powder X-ray diffraction (XRD)

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XRD measurements were conducted on an X’Pert PRO MPD diffractometer

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(Panalytical, θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel

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PIXcel detector. Wide-angle scans (5–90° 2θ, variable slits) were collected in reflection

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mode using a silicon sample holder. Small-angle scans (0.4–6.0° 2θ, fixed slits) were

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collected in transmission mode with the sample spread between two layers of Kapton

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foil.

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2.3 Thermal analysis

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Thermogravimetric (TG) measurements were conducted using a Seiko SSC5200 thermo

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balance. The gas flow through the sample compartment was adjusted to 100 ml/min

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(20% O2 / 80% He). Samples were measured at a rate of 2 K/min in the range from 298

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K to 823 K.

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ACCEPTED MANUSCRIPT 2.4 X-ray absorption spectroscopy (XAS)

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Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at

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beamline X at the Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a

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Si(311) double crystal monochromator. In situ experiments were conducted in a flow

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reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min,

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temperature range from 303 to 723 K, heating rate 4 K/min). The gas phase composition

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at the cell outlet was continuously monitored using a non-calibrated mass spectrometer

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in a multiple ion detection mode (Omnistar from Pfeiffer).

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X-ray absorption fine structure (XAFS) analysis was performed using the

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software package WinXAS v3.2. [21] Background subtraction and normalization were

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carried out by fitting linear polynomials and 3rd degree polynomials to the pre-edge and

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post-edge region of an absorption spectrum, respectively. The extended X-ray

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absorption fine structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a

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smooth atomic background µ0(k) The FT(χ(k)·k3), often referred to as pseudo radial

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distribution function, was calculated by Fourier transforming the k3-weighted

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experimental χ(k) function, multiplied by a Bessel window, into the R space. EXAFS

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data analysis was performed using theoretical backscattering phases and amplitudes

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calculated with the ab-initio multiple-scattering code FEFF7 [22]. Structural data

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employed in the analyses were taken from the Inorganic Crystal Structure Database

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(ICSD).

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Single scattering and multiple scattering paths in the H3[PMo12O40] (ICSD 209

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[4,23]) and hexagonal MoO3 (ICSD 75417 [24]) model structure were calculated up to

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6.0 Å with a lower limit of 4.0% in amplitude with respect to the strongest

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backscattering path. EXAFS refinements were performed in R space simultaneously to

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magnitude and imaginary part of a Fourier transformed k3-weighted and k1-weighted

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experimental χ(k) using the standard EXAFS formula [25]. This procedure reduces the 6

ACCEPTED MANUSCRIPT correlation between the various XAFS fitting parameters. Structural parameters allowed

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to vary in the refinement were (i) disorder parameter σ2 of selected single-scattering

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paths assuming a symmetrical pair-distribution function and (ii) distances of selected

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single-scattering paths. The statistical significance of the fitting procedure employed

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was carefully evaluated in three steps as outlined in [26].

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2.5 Catalytic testing - selective propene oxidation

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Quantitative catalysis measurements were performed using a fixed bed laboratory

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reactor connected to an online gas chromatography system (Varian CP-3800) and a non-

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calibrated mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a

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SiO2 tube (30 cm length, 9 mm inner diameter) placed vertically in a tube furnance. In

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order to achieve a constant volume and to exclude thermal effects, catalysts samples (~

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38-76 mg) were diluted with boron nitride (Alfa Aesar, 99.5%) to result in an overall

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sample mass of 375 mg. For catalytic testing in selective propene oxidation a mixture of

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5% propene (Linde Gas, 10% propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas,

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20% O2 (5.0) in He (5.0)) in helium (Air Liquide, 6.0) was used in a temperature range

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of 293-723 K Reactant gas flow rates of oxygen, propene, and helium were adjusted

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with separate mass flow controllers (Bronhorst) to a total flow of 40 ml/min. All gas

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lines and valves were preheated to 473 K. Hydrocarbons and oxygenated reaction

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products were analyzed using a Carbowax capillary column connected to an

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AL2O3/MAPD column or a fused silica restriction (25 m·0.32 mm each) connected to a

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flame ionization detector. O2, CO, and CO2 were separated using a Hayesep Q (2 m x

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1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as precolumns combined with a

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back flush. For separation, a Hayesep Q packed column (0.5 m x 1/8``) was connected

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via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector (TCD).

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ACCEPTED MANUSCRIPT 1

2.6 Sample preparation Silica SBA-15 samples with a pore diameter of ~10 nm were prepared according

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to Ref. [8]. 16.2 g of triblock copolymer (Aldrich, P123) were dissolved in 294 g water

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and 8.8 g hydrochloric acid at 308 K and stirred for 24 h. After addition of 32 g

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tetraethyl orthosilicate, the reaction mixture was stirred for 24 h at 373 K. The resulting

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gel was transferred to a glass bottle and the closed bottle was heated to 388 K for 24 h.

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Subsequently, the suspension was filtered by vacuum filtration and washed with a

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mixture of H2O/EtOH (100:5).

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Silica SBA-15 samples with large pores were prepared according to Refs.

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[27],[28]. Silica SBA-15 with a pore diameter of ~14 nm (or ~19 nm) was prepared as

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follows. 9.6 g of triblock copolymer (Aldrich, P123) were dissolved in 336 ml

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hydrochloric acid (1.3 M) at 288 K (290 K). After addition of 0.108 g ammonium

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fluoride the solution was stirred for 16 h (or 24 h). 20.7 g tetraethyl orthosilicate and

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8.08 g 1,3,5-triisopropylbenzene were added to the solution. The resulting gel was

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transferred to a glass bottle and the closed bottle was heated to 393 K (or 373 K) for

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29 h (or 48 h). Subsequently, the suspension was filtered by vacuum filtration and

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washed with a mixture of EtOH/HCl/H2O (100:10:100). The resulting white powders

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were dried at 378 K for 3 h and calcined at 453 K for 3 h and at 823 K for 5 h.

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H3[PMo12O40] were prepared as follows [5]. 19.72 g MoO3 (Sigma Aldrich)

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were dissolved in 650 ml water and heated under reflux. 95 ml of 0.12 M phosphoric

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acid were added dropwise to the reaction mixture. The resulting suspension was heated

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for 3 h and left for 24 h at 298 K until a clear yellow solution was obtained. The

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remainder was filtered of and the volume of the resulting yellow solution was reduced

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to ~30 ml using an evaporator. H3[PMo12O40] (PMo12) crystallized during storage at

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277 K for several days. H3[PMo12O40] was supported on SBA-15 through incipient

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wetness impregnation. PMo12-SBA-15 (10 nm) were prepared as follows. 0.218 mg 8

ACCEPTED MANUSCRIPT PMo12 was dissolved in water (2 ml) and deposited on 1 g sillica SBA-15 (10 nm).

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PMo12-SBA-15 (14 nm) were prepared as follows. 0.136 mg PMo12 was dissolved in

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water (1.5 ml) and deposited on 1 g sillica SBA-15 (14 nm). PMo12-SBA-15 (19 nm)

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were prepared as follows. 0.102 mg PMo12 was dissolved in water (1 ml) and deposited

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on 1 g sillica SBA-15 (19 nm). The amount of molybdenum was adjusted to 10 wt.%,

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6.7 wt.%, and 5.2 wt.% on SBA-15 (10, 14, 19 nm).

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3 Results and discussion

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3.1 Structure of support material

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Pore size distributions and specific surface areas of the synthesized support

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materials were calculated from N2 adsorption/desorption isotherms. SBA-15 (10 nm),

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SBA-15 (14 nm), and SBA-15 (19 nm) showed typical type IV isotherms indicative of

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mesoporous materials. Adsorption and desorption branches in the hysteresis range were

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nearly parallel for SBA-15 (10 nm) and SBA-15 (14 nm) indicating regular shaped

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pores (Figure 1). N2 isotherm for SBA-15 (19 nm) showed a slight broadening of the

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hysteresis loop. BET surface areas were calculated from adsorption isotherms. The

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tailored SBA-15 samples exhibited areas between 400 and 850 m2/g. Figure 1 shows the

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pore size distribution derived from BJH analysis resulting in three different pore

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diameters of ~10, ~14, and ~19 nm. Small-angle X-ray diffraction patterns of the

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tailored SBA-15 are presented in Figure 2. The second derivates of small-angle X-ray

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diffraction patterns are shown for clarity in Figure 3. SBA-15 (10 nm) and SBA-15

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(14 nm) exhibited the typical patterns with low-angle 10l, 11l, and 20l peaks

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corresponding to the two-dimensional hexagonal symmetry. Small-angle X-ray

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diffraction pattern of SBA-15 (19 nm) showed one peak at low values of 2Ɵ.

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ACCEPTED MANUSCRIPT 1

3.2 Structure of supported HPOM Figure 4 shows the Mo K edge FT(χ(k)·k3) of PMo12-SBA-15 (10, 14, 19 nm).

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The shapes of the FT(χ(k)·k3) resembled that of bulk PMo12 indicating a similar local

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structure around the Mo centers in supported and unsupported HPOM Keggin ions. For

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a more detailed structural analysis the H3[PMo12O40] Keggin structure (ICSD 209

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[23,4]) was chosen as model structure. Theoretical and experimental Mo K edge

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FT(χ(k)·k3) of PMo12-SBA-15 (14 nm) are shown in Figure 5. Comparing the distances

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R and disorder parameters σ2 of PMo12-SBA-15 (10, 14, 19 nm) (Table 2) exhibited no

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significant differences between the initial Keggin ion structure and Keggin ions

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supported on SBA-15 with different pore radii. The good agreement between theory and

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experiment for PMo12-SBA-15 (10, 14, 19 nm) confirmed the maintained Keggin ion

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structure upon supporting PMo12 on SBA-15 [14].

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3.3 Thermal stability of PMo12 supported on SBA-15 with different pore radii

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Figure 6 depicts the measured thermogravimetric data of PMo12-SBA-15 (10,

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14, 19 nm) in 20% O2 in He. The mass loss between 303 K and 373 K (range I) was

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ascribed to desorption of physically adsorbed water on the surface of the materials.

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Relative mass loss increased for samples with large pore diameter. Afterwards a nearly

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constant mass in range II (373-448K) was detected. Range III (448-523 K) showed a

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mass loss of ~1% (PMo12-SBA-15 (10nm)), ~0.7% (PMo12-SBA-15 (14nm)), and

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~0.5% (PMo12-SBA-15 (19nm)). Sample mass in range IV (523-823 K) decreased

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slightly for all samples. Comparable behaviour was shown for pure silica samples in

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vacuum [29]. Silica dehydrated between room temperature and 453 K followed by

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dehydroxylation of silanol groups between 453 and 673 K. This resulted in the

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formation of siloxane groups and a decrease of silanol density from 4.6 OH/nm2 (473

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K) to 2.3 OH/nm2 (673 K) [29,30].

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ACCEPTED MANUSCRIPT Comparing dehydration and dehydroxylation processes of supported HPOM and

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bulk HPOM revealed a correlation between dehydration and thermal stability of the

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Keggin ion. Bulk PMo12 loses 1.5 molecules of constitutional water between 673-713 K

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under dry air conditions. This decomposition is accompanied by the formation of MoO3

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[31]. Structures resulting for supported HPOM after thermal treatment under oxidizing

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conditions were comparable to stabilized two dimensional hexagonal MoO3 on SBA-15

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[14,32]. It has been shown that structural characteristics of supported model systems

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like MoOx-SBA-15 and VOx-SBA-15 depended mainly on their hydration states and

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previous calcination processes [26,33]. A comparable effect may be responsible for the

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structural evolution of HPOM supported on SBA-15. Adsorbed water and silanol

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groups from the support material may possess a structure stabilizing effect on the

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Keggin ion. This effect would be comparable to that of water of crystallization and

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constitutional water in bulk HPOM under ambient conditions [31]. Therefore,

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dehydroxylation of SBA-15 may be the driving force for the structural decomposition of

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the Keggin ion resulting in the formation of Mo oxide species on SBA-15.

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3.4 Structural evolution of PMo12- SBA-15 (10, 14, 19 nm) under catalytic

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conditions

PMo12-SBA-15 (10, 14, 19 nm) samples were investigated by in situ XAS under

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catalytic conditions. Figure 7 shows the K edge FT(χ(k)·k3) of PMo12-SBA-15 (10 nm)

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after thermal treatment in 5% propene and 5% oxygen in helium. The FT(χ(k)·k3) of

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PMo12-SBA-15 (10, 14, 19 nm) exhibited features similar to that of previously reported

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activated MoOx-SBA-15 or PVMo11-SBA-15 under catalytic conditions [32,14]. The

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resulting structures exhibited an increased concentration of tetrahedral [MoO4] units.

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The pre-edge peak features in the Mo K edge XANES spectra (Figure 8) can be

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employed to elucidate the local structure around the Mo center. Using the pre-edge peak 11

ACCEPTED MANUSCRIPT height sufficed to quantify the contribution of tetrahedral [MoO4] and distorted [MoO6]

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units present under catalytic conditions. Figure 8 shows the Mo K edge XANES spectra

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of PMo12-SBA-15 (14 nm) and a spectrum calculated from a linear combination of

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XANES spectra of bulk MoO3 and bulk Na2MoO4. The linear combination represented

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the amount of distorted [MoO6] and tetrahedral [MoO4] units. Quantitative evolution of

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the structural units was used to visualize changes in the structure of PMo12-SBA-15 (10,

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14, 19 nm) during temperature programmed treatment in 5% propene and 5% oxygen.

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Figure 9 depicts the calculated concentration of tetrahedral [MoO4] units during thermal

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treatment under catalytic conditions. No significant structural changes of PMo12-SBA-

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15 (10, 14, 19 nm) were detected in the temperature range between 303 K and 448 K.

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Apparently, the Keggin structure was stable on silica SBA-15 in the temperature range I

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(303-448 K). Subsequently, the concentration of tetrahedral [MoO4] units considerably

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increased in the temperature range between 448 K and 598 K (range II). The onset of

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structural rearrangement was identical for all PMo12-SBA-15 (10, 14, 19 nm) samples

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and independent of the pore radii of the support materials. The stability of the Keggin

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ion seemed to depend only on the nature of the support material. The structural

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evolution in the temperature range II (448- 598 K) correlated with dehydration and

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dehydroxylation of SiO2 under oxidizing conditions (20% O2 in He). Apparently, the

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dehydroxylation process was also the driving force for the structural rearrangement of

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the PMo12-SBA-15 (10, 14, 19 nm) samples under catalytic conditions. At a temperature

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of about 598 K the concentration of tetrahedral [MoO4] units for PMo12-SBA-15 (14, 19

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nm) amounted to ~70% compared to ~65% for PMo12.SBA-15 (10 nm). Hence, the

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concentration of tetrahedral [MoO4] units increased with the larger pore radii of the

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support material. Quantification of tetrahedral [MoO4] and octahedral [MoO6] units in

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the temperature range between 598 and 723 K confirmed this assumption. The [MoO4]

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concentration of PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm) were comparable

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ACCEPTED MANUSCRIPT 1

and reached the highest concentration with 80% [MoO4] units at 723 K. PMo12-SBA-15

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(10 nm) reach a maximum of tetrahedral [MoO4] units (~71%) at 657 K. Subsequently,

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a decreasing concentration of tetrahedral [MoO4] units to 64% at 723 K was determined.

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3.5 Influence of the pore radii on the resulting structure of [MoxOy] species

Figure 10 shows the Mo K edge FT(χ(k)·k3) of activated PMo12-SBA-15 (10, 19

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nm) after thermal treatment under propene oxidation conditions. The FT(χ(k)·k3) of act.

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PV2Mo10-SBA-15 exhibited features similar to that of previously reported dehydrated

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molybdenum oxides and HPOM supported on SBA-15 [14,32]. For a more detailed

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structural analysis hexagonal MoO3 was chosen as model structure. Theoretical XAFS

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phases and amplitudes were calculated for Mo-O and Mo-Mo distances and used for

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EXAFS refinement. The results of the refinement are given in Table 3. The first peak in

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the Mo K edge FT(χ(k)·k3) of act. PMo12-SBA-15 (10 nm) exhibited differences

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compared to act. PMo12-SBA-15 (14, 19 nm). The first peak in the FT(χ(k)·k3)

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originated mainly from the tetrahedral species on the SBA-15 support and could be

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sufficiently simulated using four Mo-O distances. These four distances sufficiently

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accounted for the minor amount of octahedral [MoO6] species. The 1st and 2nd disorder

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parameters (1st-σ2, 2nd-σ2) were higher for act. PMo12-SBA-15 (10 nm) and indicated a

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lower amount of tetrahedral units. Additionally, the 4th disorder parameter (4th-σ2) was

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smaller than that of act. PMo12-SBA-15 (14, 19 nm) with larger pores. This disorder

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parameter mainly represented the fraction of octahedral [MoO6] species, and

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corresponded to an increasing amount of octahedral structural motifs in act. PMo12-

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SBA-15 (10 nm) compared to act. PMo12-SBA-15 (14, 19 nm).

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A distinct peak at ~3 Å in the FT(χ(k)·k3) indicated a significant amount of dimeric or

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oligomeric [MoxOy] units on SBA-15 (10, 14, 19 nm) independent of the pore radii [15].

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ACCEPTED MANUSCRIPT Hence, isolated tetrahedral [MoO4] units can be excluded as major molybdenum oxide

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species. The obtained Mo-Mo distances were identical for act. PMo12-SBA-15 (10, 14,

3

19 nm) samples and, thus, were independent of the pore radii. The disorder parameters

4

σ2 of the Mo-Mo distances for act. PMo12-SBA-15 (10 nm) were slightly increased

5

compared to act. PMo12-SBA-15 (14, 19 nm). This indicated a decreased

6

oligomerization degree of Mo species on silica SBA-15 with larger pore diameters.

7

Apparently, concentration of [MoxOy] species reached a minimum on the large pore

8

samples PMo12-SBA-15 (14, 19 nm). These large pore SBA-15 (dMeso= 14- 19 nm)

9

materials possessed a larger angle of inclination because of the decreased curvature of

10

the pores in these samples. This may reduce the contact area between the decomposing

11

Keggin ions eventually resulting in a higher concentration of dispersed and isolated

12

oxide species. Conversely, thermal decomposition of Keggin ions on small pore support

13

materials was prone to result in connected [MoxOy] species. The Mo-O, Mo-Mo

14

distances and disorder parameters for act. PMo12-SBA-15 (14 nm) and act. PMo12-SBA-

15

15 (19 nm) were nearly identical and different from those of PMo12-SBA-15 (10 nm).

16

This confirmed the results of the quantification of tetrahedral [MoO4] (~ 70%) and

17

distorted [MoO6] (~30%) units, present on large pore materials under catalytic

18

conditions. Apparently, the formation of oligomeric [MoxOy] units mostly consisting of

19

tetrahedral [MoO4] units depended on the pore radius of the silica SBA-15. Hence, act.

20

PMo12-SBA-15 (10 nm) with smaller pores favored the formation of more extended

21

structures on the support material.

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22 23

3.6 Influence of the resulting structures on catalytic activity

24

PMo12-SBA-15 (10, 14, 19 nm) samples were tested under catalytic conditions

25

for selective propene oxidation. Table 4 shows a comparison of the selectivities towards

26

oxidation products and the reaction rates. The oxidation product distributions were 14

ACCEPTED MANUSCRIPT comparable for all three PMo12-SBA-15 (10, 14, 19 nm) samples. The reaction rates for

2

PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm) were also nearly identical. In

3

contrast to the samples with larger pores, PMo12-SBA-15 (10 nm) showed a ~14%

4

higher reaction rate during propene oxidation. The theoretical Mo coverage of 0.9

5

Mo/nm2 was similar for all PMo12-SBA-15 (10, 14, 19 nm) samples. However, a

6

significant difference between all SBA-15 (10, 14, 19 nm) materials was the curvature

7

of the surface in the pores. The pore structure of mesoporous SBA-15 corresponds to

8

that of hollow cylinders. Thus, the curvature of the walls of these cylinders decreases

9

with higher pore radius. Therefore, arrangement of spherical Keggin ions on an area

10

along the inner surface of pores with different pore radii leads to a decreasing distance

11

between the spheres at smaller pore radius. Hence, assuming a volume of 1 nm3 per

12

Keggin ion and considerating the various curvatures for SBA-15 (10, 14, 19 nm) lead to

13

an increased effective coverage at smaller pore radius. Therefore, the increased effective

14

distance of the Keggin ion on PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm)

15

resulted in a lower concentration of [MoxOy] units compared to PMo12-SBA-15 (10

16

nm). Hence, the catalytic activity in propene oxidation increased with higher

17

concentration of [MoxOy] units under catalytic conditions. [MoxOy] units orginating

18

from PMo12-SBA-15 (10 nm). This resulted in an enhanced catalytic activity without

19

significant influence on the product distribution. Therefore, a higher concentration of

20

[MoxOy] units at similar surface coverage improved the catalytic activity towards

21

propene oxidation. Comparable results have been shown for PVMo11 supported on SiO2

22

(Aerosil 300: 295 m2g-1, Nippon Aerosil Co., Ltd.) with different loadings during

23

oxidation of methacrolein [18]. Selective propene oxidation requires the transfer of

24

more than two electrons [34]. Therefore, [MoxOy] sites are necessary to selectively

25

oxidize the propene molecule [35],[36,37]. Catalytic activity of supported vanadium

26

oxide based catalysts depended also on the concentration of [VxOy] units comparable to

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ACCEPTED MANUSCRIPT 1

[MoxOy] [16,38]. Therefore, [MoxOy] units seemed to be necessary for catalytic activity

2

while their concentration increased with the effective coverage.

3 4

6

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5 4 Summary

Structural evolution of H3[PMo12O40] supported on SBA-15 (PMo12-SBA-15)

8

with different pore radii (10, 14, 19 nm) was examined by in situ X-ray absorption

9

spectroscopy investigations at the Mo K edge during propene oxidation conditions.

10

Large pore SBA-15 was successfully used as support material for molybdenum based

11

oxidation catalysts. Supporting heteropolyoxo molybdates on large pore SBA-15

12

resulted in regular Keggin ions on the support material. During thermal treatment in

13

propene oxidation conditions PMo12-SBA-15 (10, 14, 19 nm) formed a mixture of

14

mostly tetrahedral [MoO4] and octahedral [MoO6] units. The onset temperature of

15

structural changes of PMo12-SBA-15 (10, 14, 19 nm) during thermal treatment in

16

propene oxidation conditions was largely independent of the pore size of SBA-15. The

17

stability of the Keggin ions depended mostly on the nature of the support. Apparently,

18

dehydroxylation of silanol groups of the support material was the driving force for the

19

structural instability of the Keggin ion. The resulting [MoxOy] structures present under

20

catalysis conditions depended on the pore size of the support material. A higher

21

concentration of octahedral [MoO6] units and higher oligomerized [MoxOy] units was

22

detected for act. PMo12-SBA-15 (10 nm) compared to act. PMo12-SBA-15 (14, 19 nm).

23

The higher concentration of [MoxOy] units present in act. PMo12-SBA-15 (10 nm)

24

resulted in an increased catalytic activity compared to to activated PMo12-SBA-15 (14,

25

19 nm) with a lower concetration of [MoxOy] units. Selectivities towards oxidation

26

products during propene oxidation were comparable and largely independent of the pore

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16

ACCEPTED MANUSCRIPT 1

radii of act. PMo12-SBA-15 (10, 14, 19 nm). Apparently, tailoring the pore radius of

2

silica SBA-15 permitted to prepare Mo oxide model systems to investigate correlations

3

between activity and structure of characteristic oxide species at similar surface

4

coverage.

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5 5 Acknowledgments

7

The Synchrotron Radiation Laboratory HASYLAB, Hamburg, is acknowledged for

8

providing beamtime for this work. We are grateful to J. Scholz, A. Müller, S. Kühn, and

9

G. Koch for contributing to the characterization of the materials. The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft, DFG.

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ACCEPTED MANUSCRIPT 1

6 References

2

[1] B. Grzybowska-Świerkosz, Top. Catal. (Topics in Catalysis) 11/12 (2000) 23–42.

3

[2] T. Ressler, O. Timpe, F. Girgsdies, J. Wienold, T. Neisius, J. Catal. (Journal of

4

Catalysis) 231 (2005) 279–291. [3] T. Ressler, O. Timpe, F. Girgsdies, Z. Kristallogr. 220 (2005) 295–305.

6

[4] T. Ressler, O. Timpe, J. Catal. (Journal of Catalysis) 247 (2007) 231–237.

7

[5] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199–218.

8

[6] C. Hess, J. Catal. (Journal of Catalysis) 248 (2007) 120–123.

9

[7] I.E. Wachs, Catal. Today 100 (2005) 79–94.

14 15 16 17 18 19

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13

[9] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024–6036.

[10] G.M. Dhar, G.M. Kumaran, M. Kumar, K.S. Rawat, L.D. Sharma, B.D. Raju, K.R.

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12

Stucky, Science 279 (1998) 548–552.

Rao, Catal. Today 99 (2005) 309–314.

[11] D.E. Keller, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 110 (2006) 14313–14325.

EP

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[8] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Friedrickson, B.F. Chmelka, G.D.

[12] M. Bettahar, G. Costentin, L. Savary, J. Lavalley, Appl. Catal. A (Applied Catalysis A: General) 145 (1996) 1–48.

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10

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5

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[13] R. Schlögl, Top. Catal. (Topics in Catalysis) 54 (2011) 627–638.

21

[14] T. Ressler, U. Dorn, A. Walter, S. Schwarz, A. Hahn, J. Catal. (Journal of

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Catalysis) 275 (2010) 1–10. [15] J. Scholz, A. Walter, A. Hahn, T. Ressler, Microporous Mesoporous Mater. 180 (2013) 130–140. [16] J. Scholz, A. Walter, T. Ressler, J. Catal. (Journal of Catalysis) 309 (2014) 105– 114. 18

ACCEPTED MANUSCRIPT 1

[17] G. Deo, I.E. Wachs, J. Phys. Chem. 95 (1991) 5889–5895.

2

[18] M. Kanno, T. Yasukawa, W. Ninomiya, K. Ooyachi, Y. Kamiya, Journal of

3

Catalysis 273 (2010) 1–8. [19] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309–319.

5

[20] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373–380.

6

[21] T. Ressler, J. Synch. Rad. (Journal of Synchrotron Radiation) 5 (1998) 118–122.

7

[22] J. J. Rehr, C. H. Booth, F. Bridges, and S. I. Zabinsky, Phys. Rev. B 49 (1994)

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12347–12350.

[23] J. Boeyens, G. McDougal, J. Van R. Smit, J. Solid State Chem. 18 (1976) 191–199. [24] J.-D. Guo, P.Yu. Zavalij, M.S. Whittingham, Eur. J. Solid State Inorg. Chem. 31

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(1994) 833–842.

[25] T. Ressler, S.L. Brock, J. Wong, S.L. Suib, J. Phys. Chem. B 103 (1999) 6407– 6420.

[26] A. Walter, R. Herbert, C. Hess, T. Ressler, Chem. Central J. 4 (2010) 3–23.

15

[27] M. Kruk, L. Cao, Langmuir 23 (2007) 7247–7254.

16

[28] L. Cao, T. Man, M. Kruk, Chem. Mater. 21 (2009) 1144–1153.

17

[29] L.T. Zhuravlev, The surface chemistry of amorphous silica. Zhuravlev model,

18

2000, http://www.sciencedirect.com/science/article/pii/S0927775700005562,

19

accessed 13 January 2014.

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[30] E.F. Vansant, Voort, P. van der, K.C. Vrancken, Characterization and chemical

21

modification of the silica surface, Elsevier, Amsterdam, New York, 1995.

22 23 24 25

[31] L. Marosi, E. Escalona Platero, J. Cifre, C. Otero Areán, J. Mater. Chem. 10 (2000) 1949–1955. [32] J.P. Thielemann, T. Ressler, A. Walter, G. Tzolova-Müller, C. Hess, Appl. Catal. A: Gen. 399 (2011) 28–34.

19

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[33] T. Ressler, A. Walter, Z. Huang, W. Bensch, J. Catal. (Journal of Catalysis) 254 (2008) 170–179. [34] I.E. Wachs, Applied Catalysis A: General 391 (2011) 36–42.

4

[35] R.K. Grasselli, Topics in Catalysis 15 (2001) 93–101.

5

[36] G. Jander, A. Winkel, Z. anorg. allg. Chem. 200 (1931) 257–278.

6

[37] K.F. Jahr, J. Fuchs, Angew. Chem. 78 (1966) 725–735.

7

[38] B. Solsona, A. Dejoz, M. Vázquez, F. Márquez, J. López Nieto, Applied Catalysis A: General 208 (2001) 99–110.

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ACCEPTED MANUSCRIPT 7 Tables

2

Table 1

3

Specific surface area aBET (calculated by BET method), external surface area aEXT

4

(calculated as the difference between aBET and aMeso), area corresponding to the

5

mesopores aMeso, pore diameter dpore (calculated by BJH method), mesopore volume

6

VMeso, d10l-values (derived from low-angle XRD), unit cell constants a0 (corresponding

7

to the hexagonal pore arrangement) of SBA-15 (10 nm), SBA-15 (14 nm), and SBA-15

8

(19 nm).

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1

aBET (m2/g) aExt (m2/g) aMeso (m2/g) dpore (nm) VMeso (cm3/g) d10l (nm) a0 (nm) 843

145

698

10.3

1.233

SBA-15 (14 nm)

525

83

442

13.8

1.344

SBA-15 (19 nm)

395

50

345

18.5

0.957

9

10.52

12.14

12.52

14.46

14.77

17.05

M AN U

SBA-15 (10 nm)

Table 2

11

Type and number (N), and XAFS disorder paramters (σ2) of atoms at distance R from

12

the Mo atoms in as prepared PMo12-SBA-15 (10, 14, 19 nm). Experimental parameters

13

were obtained from a refinement of H3[PMo12O40] model structure to the experimental

14

Mo K edge XAFS χ(k) of PMo12-SBA-15 (10, 14, 19 nm) (k range from 3.0-13.7.0 Å-1,

15

R range from 0.9 to 4.0 Å, E0= ~ 2.3, residuals ~11.3-12.5 Nind= 22, Nfree=9) Subscript c

16

indicates parameters that were correlated in the refinement.

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Keggin model

act. PMo12-SBA-15

act. PMo12-SBA-15

act. PMo12-SBA-15

(10nm)

(14nm)

(19nm)

2

2

2

2

N

R(Å)

R(Å)

σ (Å )

R(Å)

σ (Å )

R(Å)

σ2(Å2)

Mo-O

1

1.68

1.65

0.0019

1.65

0.0029

1.65

0.0021

Mo-O

2

1.91

1.79c

0.0038c

1.78c

0.0040c

1.79c

0.0032c

Mo-O

2

1.92

1.96c

0.0038c

1.95c

0.0040c

1.95c

0.0032c

Mo-O

1

2.43

2.40

0.0014

2.39

0.0010

2.40

0.0007

Mo-Mo

2

3.42

3.43

0.0058c

3.41

0.0056c

3.42

0.0054c

Mo-Mo

2

3.71

3.74

0.0058c

3.74

0.0056c

3.74

0.0054c

17 18 21

ACCEPTED MANUSCRIPT Table 3

2

Type and number (N), and XAFS disorder paramters (σ2) of atoms at distance R from

3

the Mo atoms in act. PMo12-SBA-15 (10, 14, 19 nm). Experimental parameters were

4

obtained from a refinement of a hexagonal MoO3 model structure to the experimental

5

Mo K edge XAFS χ(k) of act. PMo12-SBA-15 (10, 14, 19 nm) (k range from 3.6-16.0 Å-

6

1

7

indicates parameters that were correlated in the refinement.

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1

, R range from 0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.5 Nind= 26, Nfree=12) Subscript c

act. PMo12-SBA-15

act. PMo12-SBA-15

SC

act. PMo12-SBA-15

hex-MoO3 model

(10nm)

(14nm)

2

2

R(Å)

R(Å)

σ (Å )

R(Å)

σ (Å )

R(Å)

σ2(Å2)

Mo-O

2

1.67

1.67

0.0015

1.67

0.0009

1.67

0.0009

Mo-O

2

1.96

1.89

0.0038

1.88

0.0024

1.88

0.0024

Mo-O

1

2.20

2.19

0.0038c

2.17

0.0024c

2.17

0.0024c

Mo-O

1

2.38

2.35

0.0011

2.34

0.0030

2.34

0.0029

Mo-Mo

2

3.31

3.49

0.0068

3.49

0.0054

3.49

0.0058

Mo-Mo

2

3.73

3.63

0.0068c

3.62

0.0054c

3.62

0.0058

Mo-Mo

2

4.03

3.73

0.0100

3.73

0.0089

3.72

0.0096

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8

2

(19nm)

N

M AN U

2

Table 4: Reaction rate and product distribution over PMo12-SBA-15 (10, 14, 19 nm) at

10

445°C during propene oxidation under isoconversional conditions (25-30% propene

11

conversion).

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9

AC C

rate

(µmol/g[Mo])

Selectivity (%)* CO2

CO

Aa

Pa

Ac

Ar

AcA

PMo12-SBA (10nm)

90.4

14

22

28

5

3

19

9

PMo12-SBA (14nm)

77.7

12

20

31

9

3

18

7

PMo12-SBA (19nm)

78.1

13

20

31

8

3

18

7

12

* Aa: acetic aldehyde; Pa: propionic aldehyde; Ac: acetone; Ar: acrolein, AcA: acetic

13

acid.

14 15

22

ACCEPTED MANUSCRIPT 1

8 Figure Captions

2

Figure 1

3

SBA-15 (14 nm) (circle), and SBA-15 (19 nm) (triangle) and pore distributions of of

4

silica SBA-15 (10 nm) (square), SBA-15 (14 nm) (circle), and SBA-15 (19 nm)

5

(triangle)(inset).

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Nitrogen physisorption isotherms of silica SBA-15 (10 nm) (square),

6 Figure 2

Low-angle X-ray diffraction patterns of SBA-15 (10 nm), SBA-15

8

(14 nm), and SBA-15 (19 nm).

SC

7

9 Figure 3

2nd derivates of low-angle X-ray diffraction patterns SBA-15 (10 nm)

11

(square), SBA-15 (14 nm) (circle), and SBA-15 (19 nm).

M AN U

10

12

Mo K edge FT(χ(k)·k3) of [PMo12O40]3- supported on SBA-15 with

Figure 4

14

different pore radii.

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13

15

Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of

Figure 5

17

[PMo12O40]3- supported on SBA-15 (14 nm).

EP

16

18 Figure 6

20

PMo12-SBA-15 (19 nm) at 20% O2 in He (298- 823 K; 2 K/min).

21

Thermograms of PMo12-SBA-15 (10 nm), PMo12-SBA-15 (14 nm), and

AC C

19

Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of

22

Figure 7

23

activated PMo12-SBA-15 (10 nm) after thermal treatment in propene oxidation

24

conditions at 723 K.

25

23

ACCEPTED MANUSCRIPT 1

Figure 8

Refinement of sum (dotted) of XANES spectra of references MoO3 and

2

Na2MoO4 (dashed) to Mo K edge XANES spectrum of activated PMo12-SBA-15

3

(14 nm) after thermal treatment in propene oxidation conditions at 723 K.

4 Figure 9

Evolution of MoO4/MoO6 ratio of PMo12-SBA-15 (10 nm) (square),

6

PMo12-SBA-15 (14 nm) (circle), and PMo12-SBA-15 (19 nm) (triangle) during thermal

7

treatment in propene oxidation conditions.

9

Figure 10

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5

Mo K edge FT(χ(k)·k3) of activated PMo12-SBA-15 (10 nm) and

activated PMo12-SBA-15 (19 nm) after thermal treatment in propene oxidation

11

conditions at 723 K.

M AN U

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12 13

17 18 19 20 21

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ACCEPTED MANUSCRIPT 1

9 Figures 2

Figure 1 3 4

6

600

10 15 20 25 dp [nm]

5

400

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-1

V [ml g ]

5

dVp/d

800

7 200 0 0.0

9

11 Figure 2

12

13 norm. intensity

14

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15 16 17 18

23 24 25 26

AC C

Figure 3

27

-1.5

SBA-15 (10nm) SBA-15 (14nm) SBA-15 (19nm) -1.0 -0.5 0.5 1.0

1.5

2.0

21

2nd dev. SBA-15 (10 nm) 2nd dev. SBA-15 (14nm) 2nd dev. SBA-15 (19nm)

28 -1.0

29

1.0

norm. intensity

20

EP

-2.0

19

22

0.2 0.4 0.6 0.8 Relative Pressure p/p0

M AN U

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-0.5 0.5 2Θ [°]

1.0

25

ACCEPTED MANUSCRIPT 1 2 Figure 4 3 4

0.25

PMo12-SBA-15 (19 nm)

5 6

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0.20

7

PMo12-SBA-15 (14 nm)

0.15 3

FT(χ(k)·k )

8

10 11

0.10

SC

9

PMo12-SBA-15 (10 nm)

0.05

13

0.00

14 -0.05

15

0

16 17

19 Figure 5 20

1

2

3 4 R [Å]

5

6

5

6

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18

M AN U

12

PMo12-SBA (14 nm)

22 23 24 25 26

AC C

3

FT(χ(k)·k )

21

EP

0.04 0.02 0.00

-0.02 Experiment Theory

-0.04 0

1

2

3 R [Å]

4

27 28 29 30 31 32 26

ACCEPTED MANUSCRIPT 1

Figure 6

Normalized Mass [%]

100

PMo12-SBA-15 (10 nm) PMo12-SBA-15 (14 nm) PMo12-SBA-15 (19 nm)

98 96

92 90

I

II 373

III 473

IV 573

673

2

4

Figure 7

M AN U

5 0.08

act. PMo12-SBA-15 (10 nm)

6 0.04 3

FT(χ(k)·k )

7 8 9

0.00

TE D

-0.04

10

0

11

AC C

13

19 20

Normalized absorption

18

3 R [Å]

4

5

6

1.00 0.75

Na2MoO4

0.50 0.25 MoO3 0.00

21

2

14

Figure 8

17

1

Experiment Theory

EP

12

16

SC

3

15

773

T [K]

RI PT

94

20.0

20.1 Photon energy [keV]

20.2

22 23 27

ACCEPTED MANUSCRIPT 1 Figure 9

2

100

4 5 6

80 60 40 20

7

0

8

I

II

III

373

473 573 T [K]

673

9

SC

10 11

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12 Figure 10 13

0.08

PMo12-SBA-15 (10 nm) PMo12-SBA-15 (19 nm)

14

19 20 21 22 23 24 25

TE D

18

-0.04

-0.08 0

1

2

3 R [Å]

4

5

6

EP

17

0.00

AC C

16

FT(χ(k)·k3)

0.04

15

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MoO4/MoO6 ratio

3

PMo12-SBA-15 (10nm) PMo12-SBA-15 (14nm) PMo12-SBA-15 (19nm)

26 27 28 29

28

ACCEPTED MANUSCRIPT - Supporting heteropolyoxo molybdates on SBA-15 resulted in regular Keggin ions. - PMo12-SBA-15 (10, 14, 19 nm) formed a mixture of mostly [MoO4] and [MoO6] units. - The resulting [MoxOy] structures depended on the pore size of the SBA-15. - The stability of the Keggin ions depended mostly on the nature of the support.

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- Tailoring the pore radius of SBA-15 permitted to prepare Mo oxide model systems.