Partial oxidation of methane over modified Keggin-type polyoxotungstates

Accepted Manuscript Title: Partial oxidation of methane over modified Keggin-type polyoxotungstates Author: S. Mansouri O. Benlounes C. Rabia R. Thouvenot M.M. Bettahar S. Hocine PII: DOI: Reference:

S1381-1169(13)00289-6 http://dx.doi.org/doi:10.1016/j.molcata.2013.08.006 MOLCAA 8827

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

20-3-2013 5-8-2013 6-8-2013

Please cite this article as: S. Mansouri, O. Benlounes, C. Rabia, R. Thouvenot, M.M. Bettahar, S. Hocine, Partial oxidation of methane over modified Keggintype polyoxotungstates, Journal of Molecular Catalysis A: Chemical (2013), http://dx.doi.org/10.1016/j.molcata.2013.08.006 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.

Partial oxidation of methane over modified Keggin-type polyoxotungstates S. Mansouri1, O. Benlounes1, C. Rabia2, R. Thouvenot3, M. M. Bettahar4 and S. Hocine1

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1- Laboratoire de Chimie Appliquée et de Génie Chimique Université Mouloud Mammeri Tizi-Ouzou BP .17 R.P 1500 Tizi-Ouzou Algeria. 2- Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, USTH, B.P 32 El- Alia Bab-Ezzouar 16111 Alger Algeria 3- Institut Parisien de Chimie Moléculaire, UMR 7201, Université Pierre et Marie Curie Equipe Polyoxométallates Case Courier 42, 4 Place Jussieu, 75252 Paris cedex 05, France. 4- UMR CNRS 7565, IJB, Faculté des Sciences, Nancy Université, Boulevard des Aiguillettes BP 70239 54506 Vandoeuvre-lès-Nancy, France

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Abstract

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Partial oxidation of methane by molecular oxygen and nitrous oxide was studied in the presence of catalytic amounts of the Keggin-type heteropolyoxometalates of general formula

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[PW11MO39](7-n)- with M = Co(II), Ni(II), and Fe(III). The catalysts were prepared by refilling the vacant site of the lacunary precursor K7PW11O39 by the metal additives and characterized

M

by 31P NMR, UV-vis and IR spectra, XRD, TGA/DTA and cyclic voltammetry. The oxidation

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reaction was performed at atmospheric pressure at 873 or 923 K. Reaction products observed were methanol, formaldehyde, carbon oxides and water. Most prominent results are the

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following: i) selectivity to oxygenates as high as 48% (conversion 5%) was obtained; ii) cobalt and iron doped polyoxometalates were the most active and selective catalysts; iii) N2O

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was more reactive and selective than O2. The activity rise was correlated with the increase of the oxidant character of the cluster metal. Kinetic study and catalyst behaviour suggested that reaction paths were different for nitrous oxide and molecular oxygen. For N2O, methane would be oxidized by MO2 centers to methoxy species, precursors of both methanol and formaldehyde. For O2, methane activation rather involves hydrogen abstraction by the lattice oxygen on M=O centers to form metal-methyl species, the key-intermediates in the oxidation processes. Key words: methane; methanol; formaldehyde; oxygenates; partial oxidation; polyoxometalates

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1. Introduction Catalytic oxidation of methane for the production of methanol or formaldehyde has received much attention in the recent decades [1-6]. Methanol is a key-intermediate in bulk and fine chemistry whereas methane constitutes one of the most abundant natural carbon

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resources [7-12]. Drawbacks for methane are its undesirable greenhouse gas properties and wastage. Also, in order to solve the twin problems of depletion of fossil fuels and global

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warming, catalytic oxidation of methane has been widely investigated by using homogeneous

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[13-16] and heterogeneous [17-21] catalysts under a variety of conditions. Oxidation of methane with biological enzymes has also been explored [22-25].

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Many heterogeneous catalysts have been investigated for direct selective oxidation of methane to get high selectivity in C1-oxygenates [26-28]. However, the yield of C1-

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oxygenates still remains low, and CO and CO2 are major products. Also there is a great need to develop better catalysts.

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The main types of metal catalysts active for methane oxidation to C1-oxygenates have a degree of oxidation exceeding 3+. They include Pd, Mn, Co, Fe, V, Mo and Ga. Silica-

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supported V2O5 and MoO3 were found to be among the most active and selective catalysts for

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partial oxidation of methane to formaldehyde [29-32] by using either N2O [29,30] or molecular oxygen [31, 32] as oxidants. The former is more active at low temperatures and the latter at high temperatures [33, 34]. Silica-supported Fe2O3 is also claimed as a good catalyst for mild oxidation of methane [32-34, 28]. The mechanism for partial oxidation of methane to methanol or formaldehyde over metal oxide catalysts have been investigated by many researchers [35-37]. However, despite extensive experimental [38, 39] and theoretical [43-48] studies, the chemical processes involved is not entirely understood and still gives rise to controversy. Some studies suggest that methane is directly oxidized to formaldehyde and methanol, and then to CO and CO2 2

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through consecutive reactions [36-37, 46-47]. Others propose the existence of parallelconsecutive scheme [48, 49]. Methane oxidation on PdO is believed to follow a redox mechanism of Mars - van Krevelen type [50, 51]. Transition-metal-substituted molybdophosphoric heteropolyanions (PMo12-xMxO40, M= Co,

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Zr) are reported to be active catalysts in the partial oxidation of methane [52-54]. However, these studies are relatively scarce. Therefore, in this work, CsPWo12-xMxO40 (M= Fe, Co, Ni)

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solids were evaluated as catalysts in the reaction of partial oxidation of methane to methanol

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and formaldehyde, using molecular oxygen and nitrous oxide as oxidants. The catalysts were characterized by X-Ray Diffraction (XRD), 183W Nuclear Magnetic Resonance (NMR), Ultra

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Violet-visible (UV-vis) and Infra-Red (IR) spectroscopy, Thermal Gravimetry Analysis (TGA)/Differential Thermal Analaysis (DTA) and Cyclic Voltammetry. Reaction paths to

M

methanol and formaldehyde formation are suggested based on the polyoxometallates

2. Experimental 2.1. Preparation of catalysts

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reactivity and characteristics.

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The acidic potassium salt of [PW11O39]7- (abbreviated PW11), was prepared according

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to the literature method [54-55]. The lacunary heteropolycompounds modified with transition metal ions [PW11O39M](7-n)-, [M = Ni2+, Co2+, or Fe3+, n = oxidation state of M] (denoted PW11M), were obtained by addition of the metal nitrate according [56-58]. For the modified salts, an aqueous solution of the potassium salt K7PW11.xH2O was treated with an aqueous solution of M(NO3)n under reflux. PW11M samples were precipitated with cesium chloride. They were filtrated, washed with an ethanol/H2O (1/1) mixture and dried at 353K. K7[PW11O39] (PW11). Elemental analysis: calc. (found) for K7[PW11O39] (%): K, 7 (9.13); P, 1 (1.034); W, 68.2 (67.51).

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Cs4[PW11O39Fe] (PW11Fe). Elemental analysis: calc. (found) for Cs4[PW11O39Fe] (%): Cs, 3.90 (4.12); Fe, 1.01 (1.62); P, 0.97 (0.94); W, 57.2 (58.70). Cs5[PW11O39Co] (PW11Co). Elemental analysis: calc. (found) for Cs5[PW11O39Co] (%): Cs, 4.92 (5.22);Co, 1.57 (1.64); P, 0.86 (0.87).

4.95 (6.02);Ni, 1.67 (1.64); P, 0.87 (0.86); W, 53.2 (56.47).

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2.2. Characterization

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Cs5[PW11O39Ni] (PW11Ni). Elemental analysis: calc. (found) for Cs5[PW11O39Ni] (%): Cs,

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UV-vis spectra were recorded at 298K on a UV-vis scanning spectrophotometer (Shimadzu UV-2100PC). A 2.5 mM reaction solution in acetonitrile of each catalyst was

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sampled, and the solution was diluted with 400 ml acetonitrile (0.5 mM). The absorption spectra were recorded with a Bio-rad FTS 60A spectrometer (spectral range = 4000-300 cm-1)

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using the KBr pellet technique. Powder XRD data were obtained with a XR Fischer diffractometer using Cu K radiation in the range 2 = 5°-65°. Thermogravimetric analyses

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were performed with a Setaram LabSys 2000 instrument. The measurements were done in nitrogen using a platinum crucible. The heating rate was 5°C/min and the sample mass was 50

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mg.

2.3. Catalytic reaction

The reaction was conducted in a continuous fixed bed reactor at atmospheric pressure

with a CH4/O2 or CH4/N2O mixture (molar ratio 2.5/1) at a flow rate of 2l/h. The catalyst was pretreated in a flow of nitrogen at the reaction temperatures of 873 and 923 K with a flow rate of 1l/h. After the pretreatment the gaseous reactants entered the reactor via three mass flow controllers. The products were analyzed by a gas chromatograph (Shimadzu 14B) equipped with thermal conductivity (TCD) and flame ionisation (FID) detectors. Carbon monoxide,

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dioxide and water were analyzed on a Porapak Q column and the oxygenate products on Carbosphere column. The experimental data were recorded after 8 h of reaction.

3. Results and discussion

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3.1. Characterization of catalysts The infrared spectra of polyoxometalates (Fig. 1) show strong absorption bands in the

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range 1100-700 cm-1 region, assigned to W=O and W-O-W stretching vibrations, typical of The as(P-O) mode,

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the transition metal modified lacunary heteropolycompounds [59].

observed in the 1110-1040 cm-1 region, is the most informative band in the spectra of the

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tungstophosphates. The vibration of the P-O bonds of the central PO4 tetrahedron gives rise to only one band, at 1080 cm-1, in the spectrum of the parent Keggin anion, [PW12O40]3- [60].

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This band is split into two components (1085-1040 cm-1) in the spectra of the lacunary PW11 and of many PW11M anions, due to the symmetry lowering of the PO4 tetrahedron [61]. For

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the PW11Co anions, the value of the ν PO4 splitting ( = 17cm-1),  is lower than that of [PW11O39]7- (= 45 cm-1) (Table 1). For PW11Fe and PW11Ni filling the hole of the

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octahedral lacuna by the Ni or Fe cation restores the symmetry of the central tetrahedron,

(Oa).

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owing to the interaction between Ni or Fe and the available oxygen of the central PO4 group

Table 1: Characteristic W-O infrared absorption frequencies and decomposition temperature of the metal-modified lacynary heteropolytungstates

Sample

H3PW12O40 K7PW11O39 CsPW11Co CsPW11Ni CsPW11Fe

νasP-Oa (cm-1) 1080 1085 1040 1075 1058 1063 1071

sW=Od asW-Ob-W asW-Oc-W Decomposition  -1 tempertaure(°C) (cm ) (cm-1) (cm-1) (cm-1) 993 895 810 400 45 950 900 808 380 963 886 764 530 17 960 884 813 963 966

886 878

817 798

525 >600

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In the Keggin structure, intense absorption bands at 200 and 260 nm are caused by charge-transfer of the terminal oxygen and bridge-oxygen to metal atoms, respectively (Fig. 2). For PW11 and the metal derivatives, the oxygen to metal charge transfer gives rise to an intense band at 248 (PW11), 266 (PW11Fe) and 252 nm (PW11Ni and PW11Co) respectively.

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(a)

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(b)

(c)

4000

3500

3000

2500

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(d)

2000

1500

1000

500

M

Wavenumber (cm-1)

1,6

252

1,5

252

1,4 1,3

1,0 0,9 0,8

(e)

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266

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1,2 1,1

(a)- PW11 , (b)- PW11Ni, (c)- PW11Co, (d)- PW11Fe

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Figure 1: IR spectra of heteropolytungstates

0,7

(c)

0,6

248

0,5

(b)

0,4 0,3 0,2 0,1

(a)

0,0

200

300

400

500

600

700

800

Figure 2: UV spectra of heteropolytungstates (a)- PW11 , (b)- PW11Ni, (c)- PW11Co, (d)- PW11Fe

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The thermal stabilities of the lacunary PW11 and metal-modified compounds were studied by TG and DTA. The thermal decomposition of the metal-substituted compounds PW11M starts at a higher temperature (773K) in the case of the lacunary PW11 (Fig. 3) (653K). Decomposition of PW11Co and PW11Ni occurs at ca the same temperature (Table 1, Fig. 3)

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(798-803K), whereas PW11Fe does not decompose until 873K (Figure 3d). The DTA diagrams of PW11Ni and PW11Co salts show two unresolved exothermal peaks at 383K and

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another exothermal peak at 803K, assigned to the release of physisorbed water and

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decomposition of the polyoxometalates respectively. However, the PW11Fe sample produced only one exothermic peak, centred at 353K, corresponding to the release of the water

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ed

M

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molecules.

DTA

TGA

Figure 3: Thermal analysis of (a)- KPW11 , (b)- CsPW11Ni, (c)- CsPW11Co, (d)- CsPW11Fe 7

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The redox potentials of Keggin-type polyoxometalates can be varied over a wide range by altering the chemical composition. In fact, Keggin-type anions are able to accept reversibly up to six electrons on the vanadium, tungsten or molybdenum atoms, and thus are potential oxidants. The cyclic voltammograms of the lacunary PW11 and the metal substituted anions,

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PW11M (M= Fe, Co and Ni) show, in the range of potentials studied, two reversible or quasireversible two-electron waves corresponding to reduction of the tungsten atoms (Figure 4,

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Table 2) [62]. Redox processes occurring at the metal M could only be observed for the iron

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and cobalt compounds. The voltammogram of PW11 anion in CH3CN displays three reversible waves with E1/2 = -0.75, -1.36 and -1.54 V/SCE assigned to the successive reduction of

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tungsten centers. Similar results are observed for PW11Ni (-0.95, -1.68 and --1.82 V/SCE). Cyclic voltammetry studies of PW11Fe and PW11Co were particularly informative. The

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number of redox processes identified and the values of the peak potentials are quite similar. It consists of a reversible one-electron system at -0.12 V and -0.7V for the PW11Fe for PW11Co

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respectively, followed by three reversible redox peaks at -0.92, -1.65 and -1.75 V for PW11Fe and at -0.85, -1.67 and -1.8 for PW11Co. They are assigned to the iron (III/II), cobalt (II/I) and

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tungsten (VI/V, V/VI) couples, respectively. Electron-accepting properties of Keggin anion

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are important for oxidative catalysis when electron transfer mechanisms are predominant. In other cases, it is the lability of some oxygen atoms of the clusters or the possibility of being involved in complexation reactions that make them good catalysts for oxidation of organic compounds.

Table 2: Cyclic voltammetry data (in V) for [PW11O39M)](7-n)Epa (V/E.C.S) Epc(V/E.C.S) ½( Epa+Epc) PW11 PW11Ni PW11 PW11Ni PW11 PW11Ni -0.70 -0.8 -0.80 -1.1 -0,75 -0.72 -1.29 -1.65 -1.43 -1.7 -1,36 -1.68 -1.52 -1.8 -1.59 -1.85 -1,54 -1.82 PW11Co PW11Fe PW11Co PW11Fe PW11Co PW11Fe -0.6 0.08 -0.8 -0.17 -0.7 -0.12 -0.8 -0.8 -1.06 -1.04 -0.85 -0.92

Epa-Epc (V/E.C.S) PW11 PW11Ni 0.1 0.38 0,14 0.05 0,07 0.05 PW11Co PW11Fe 0.2 0.25 0.26 0.24

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-1.64 -1.77

-1.6 -1.7

-1.71 -1.84

-1.7 -1.8

-1.67 -1.8

-1.65 -1.75

0.05 0.07

0.1 0.1

(a)

-1,00E+00 E(V)

-5,00E-01

-1,50E+00

0,00E+00

0,00E+00

(d)

-5,00E-01

0,00E+00

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-1,00E+00 E(V)

-2,00E+00

-1,50E+00

M

-1,50E+00

-5,00E-01

E(V)

(c)

-2,00E+00

-1,00E+00

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-1,50E+00

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-2,00E+00

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(b)

-1,00E+00 E(V)

-5,00E-01

0,00E+00

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Figure 4: Cyclic voltammetry of PW11(a) , PW11Ni (b), PW11Co (c) and PW11Fe (d) dmf solution c = 1.3 10-3, supporting electrolyte NBu4PF6 10-1 mol l-1

The data of X-ray powder diffraction are listed in Table 3. In each of the four ranges

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of 2 which are 7-10°, 16-23°, 25-30° and 31-38°, all the compounds show a characteristic peak of HPA anions having the Keggin-type structure. Monocrystals were obtained by

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recrystallization of the synthesised compounds (PW11Ni, PW11Co) powder from acetonitrile solutions at 277K. The structure was solved using Fourier techniques (SHELXTL, V5.10). The crystal structure consists of one Keggin polyoxometallate and five or four cesium ions per asymmetric unit. One metal atom shares the 12 metal atom sites with 11 tungsten atoms and is completely disordered within the Keggin unit. Table 3: Data of X-ray powder diffraction

Compounds Crystal system Space group a/Å b/Å

PW11Ni Tetragonal P4/m 20.99 20.99

PW11Co Tetragonal P4/m 21.06 21.06

PW11Fe Triclinic Plbar 13.74 15.41

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c/ Å /◦ /◦ /◦ V/Å3 Z 31

10.43 90 90 90 4596

10.46 90 90 90 4642

19.35 95.31 93.00 90.79 4090

2

2

2

P NMR spectroscopy is a powerful method to identify and check the purity of P-

containing complexes. An expected single chemical shift is observed in each

31

spectrum for PW11M (M = CoII, NiII or FeIII), as it can be seen in Fig. 5. The

31

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P NMR

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P NMR

spectra of [PW11M] precursors in acetonitrile solution exhibit only one peak according to the

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single phosphorus nucleus and the good degree of purity of the polyoxometalates. The order of line broadening is PW11 (10.35 ppm) < PW11Co (300 ppm) < PW11Ni (408 ppm) < PW11Fe

M

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(528 ppm).

(b)

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(a)

(c)

Figure 5: 31P NMR spectra of PW11 (a) , PW11Ni (b) and PW11Co (c)

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The solid compounds were recovered after the catalytic tests at 873 K and 923 K. In all cases the solids took a deep blue color, indicating their partially reduced state. IR spectra of these used catalysts showed that Keggin units were at least partially preserved, except for PW11Co. In this case, the absence of the splitting of the P-O band could be assigned to the release of Co

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from the substituted polyoxoanions. The relatively low intensity of this P-O band indicates also that the polyoxometalate partly decomposed to corresponding oxides. When the reaction

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was conducted at 923 K and after one day of running time in the reactor in continuous mode,

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the spectrum of PW11Fe shows that the Keggin structure is practically preserved (lines in the region 400-900 cm-1 are still present) (Fig. 6). In contrast, the spectra of the substituted

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ed

M

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lacunary PW11Co and PW11Ni anions show they decompose totally into oxides.

Figure 6: Infrared spectra of CsPW11Fe at 923 K

3.2. Catalytic testing

The results for conversion and selectivity in the reaction between methane and nitrous

oxide at 873 and 923 K and atmospheric pressure over polyoxometalates PW11M (M = Fe, Co or Ni) obtained after 8h on stream are summarises in Table 4. The reaction produced oxygenates, namely methanol and formaldehyde together with COx and H2O. Increase in temperature from 873 to 923 K enhanced conversion of methane (from 2-5% to 8-11%) and lowered the selectivity of methanol (from 11.2-33.6% to 5.0-14.6%). At the same time, 11

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selectivity of COx and formaldehyde increased from 84.7-50.8% to 86.8-61.4% and 4.115.6% to 8.1-24% respectively. The results obtained also indicated that PW11Fe achieved higher methanol (33.6%) or oxygenates (48.2%) selectivities compared to PW11Co (25.7 or 31.8%) and PW11Ni (11.2 or 15.3%) respectively. While for PW11Fe oxygenate selectivity

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increases markedly from 873 to 923 K, this selectivity is not sensitive to the reaction temperature for PW11Co and PW11Ni.

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It is reported that peroxide species (O22-), easily produced from N2O, were more

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reactive for the formation of oxygenates than lattice oxygen species (O2-) [37]. O- species are also believed to favour oxygenate production. In effect, Ostuka and al. [50] claimed that Fe3+

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sites in several oxide catalysts enhanced the formation of O- species, leading to high CH3OH selectivity and yield. Similar conclusions have been made for the FeZSM-5 catalyst [3].

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However, more O- species on the catalysts can induce total combustion of CH4.

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Table 4: Oxidation of methane by N2O catalysed by heteropolyoxotungstates Catalysts T (K) Conv Products Selectivities (%) (%) HCHO CO CO2 CH3OH+ HCHO CH3OH CsPW11Fe 5 33.6 15.6 46.8 4 48.2 873 11 14.6 24 40.4 21 38.6 923 CsPW11Co 3 25.7 15.0 50.9 18.3 31,8 873 10 10.0 20.6 42.9 26.4 30,7 923 CsPW11Ni 2 11.2 4.1 60.7 24 15.3 873 8 5.0 8.1 50.8 36 13.2 923

Table 5 summarized the performance of the polyoxometalates in the catalytic

oxidation of CH4 with molecular oxygen. Deep oxidation of methane is unavoidable since methane combustion requires the use of oxygen as it is apparent from this table. Generally, the conversion of methane and selectivity to oxygenate products were lower than was found for the N2O (Table 4). The order of activity is the reverse (Ni>Co>Fe) than that observed for N2O.

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With M = Ni, conversion of methane is the highest (3-7%) and becomes highly selective to the formation of CO2 (84.2- 94.2%). Activity of PW11Ni was approximately twice that

of PW11Co and PW11Fe at the reaction temperature of 873 K. These

heteropolyoxometalates also gave methanol and formaldehyde but only as minor products,

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CO2 being the major one. The oxygenate products selectivity was quite similar over these catalysts and approximately five times higher than over the CsPW11Ni catalyst.

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Our results show that the product nature of the partial methane oxidation reaction

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depends on the reducibility of the catalyst. On the unreduced catalyst (PW11Ni), CH4 is completely oxidized to CO2. However, on the reduced catalysts as iron and cobalt clusters,

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CH4 can be selectively converted to CH3OH, HCHO and CO with a small amount of CO2. The catalytic results are in good agreement with the electrochemical analysis, which show

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that redox processes occur only on the iron and cobalt compounds. The reactivity is in agreement with the redox potentials for the M(II)/M or M(III)/M reduction pair: Eº Fe(III)/Fe

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= -0.04 V > Eº Ni(II)/Ni = -0.25 V > Co(II)/Co = -0.28 V/ECS. In contrast, it seems that much higher reducibility of the oxides resulted in the combustion of methane and the oxidation of

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methanol.

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Table 5: Oxidation of methane by O2 catalysed by heteropolyoxotungstates Products Selectivities (%) Catalysts T (K) Conv

CsPW11Fe

CsPW11Co CsPW11Ni

873 923 873 923 873 923

(%) 0.8 4 1 6 3 7

CH3OH 17.8 7.3 14.8 6.6 2.2 -

HCHO 16.5 23.1 16.3 22.5 3.8 -

CO 36.8 23.5 46.6 34.9 9.8 5.8

CO2 28.9 46.1 22.3 36.0 84.2 94.2

CH3OH+HCHO 34.3 30.4 31.1 29.1 6.0 -

The dependence of product selectivity on the degree of conversion in initial kinetics was studied at 823K by varying the contact time using PW11Fe as a catalyst and N2O as an oxidant. The results are reported in Fig. 7. It can be seen that the selectivity to methanol and 13

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COx strongly changes with increasing methane conversion from 5 to 20% while the selectivity to formaldehyde only slightly increases from approximately 13% to 23%. The selectivity to methanol falls from approximately 38% to 5% with increasing methane conversion. On the contrary, the selectivity to COx increases with methane conversion from

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48% to 60%. These results indicate that methanol is formed as the primary product in the oxidation of methane and is consecutively converted to formaldehyde. Carbon oxides are as

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well primary products since they are formed at significant amount even at the lowest degree

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of conversion measured. The results also show that methanol and/or formaldehyde are partially converted to these oxides, in additional secondary reactions. Note that production of

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COx may be more complex. Indeed, close inspection of Fig. 8 seems to indicate a change in the slope of the selectivity curves at around 0.6% of conversion, suggesting that carbon oxides

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are rather secondary products stemming from methanol in the early stages of the reaction. Further investigations are needed to confirm these trends and to a better understanding of the

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real mechanism in initial kinetic conditions. 80

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70 50

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Selectivities (%)

60 40 30 20 10

0

0

0,5

1

1,5

Conversion (%)

2

2,5

Figure 7: Selectivity to products as function of methane conversion over PW11Fe at 823K

(■) SHCHO, (▲) SCH3OH, (●) SCOx

3.3. Mechanisms and pathways of methane oxidation

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The present results clearly show that selective oxidation of methane on polyoxometalates is influenced by the nature of the oxidant. Conversion, selectivity, effect of temperature or metal additive are completely different and the reactivity order as well. These facts strongly suggest that the reaction mechanisms are different. This conclusion is

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strengthened by the greater conversions with N2O than with O2, in contradiction with the reaction thermodynamics. This means that exists a lower kinetic barrier for N2O than for O2 in

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catalytic selective oxidation of methane.

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It has been well recognized that the selective oxidation of methane proceeds via redox mechanism, but the pathways and product distribution depend on the nature of catalysts and

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the reaction conditions. Methane activation can occur by a homolytic or heterolytic mechanism [63-71]. The rate-determining step would be the rupture of the C–H bond and the

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formation of CH3 and HO radicals was postulated as the initial step. For CH4 oxidation to methanol and formaldehyde the most strongly supported mechanism consists of consecutive

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conversion scheme: CH4 → CH3OH → CH2O → CO → CO2. 3.3. 1. Mechanism of methane oxidation by N2O

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In situ Raman and IR spectroscopic measurements were performed to analyze methane

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oxidation with N2O over MoO3. ZrO2, Fe0.5Al0.5PO4, Fe-ZSM-5 and Fe ion-exchanged BEA zeolite [63-67]; metal oxygenate species were implicated for the formation of CH3OH. The intermediates identified were methoxy (CH3O*), formaldehyde (H2CO), dioxymethylene (H2COO), formate (HCOO), hydrogenocarbonate (OC(OH)O) and carbonate (CO3) species. Based on our experimental results with polyoxometalates as catalysts, we propose reaction paths involving similar oxygenate species (Fig. 7). CH4 would be oxidized by peroxo species MO2 (a) formed by the reaction of N2O with a M=O center (M= W, Fe, Co and Ni). M=O centers may be the active sites for methanol formation. They have been evidenced in polyoxometalates. In the first step of the oxidation, CH4 adds across the peroxide to form an 15

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M(OH)(OCH3) specie (b). Methanol is then formed via the transfer of a hydrogen atom from the hydroxyl group to the methoxide (step3). Surface methoxy groups may also decompose to adsorbed HCHO which desorbs and reacts with water to lead to the final products and a reduced catalyst surface (step 4). Reoxidation of the catalyst by N2O restores the lattice

ip t

oxygen atom of the M metal. This sequence is represented by: M + N2O  M=O + N2. CO and CO2 were possibly produced through several oxygenate intermediates:

cr

bidentate formate (H2COO) (d) formed via dehydratation of M(OH)(OCH3) species (b) (steps

us

5 and 6), formate (e) (step 8), monodentate hydroxymethylene species (f) (step 9), bicarbonate (g) (step 11) and finally carbonates (i). Note that there is a second possible

an

pathway for the formaldehyde formation through formate evolution (step 7). The kinetic results obtained on the oxidation of CH4 by N2O (Fig. 7) showed that

M

carbon oxides are probably primary products and may also stem from methanol or formaldehyde as secondary products. The direct pathway to carbon oxides could be ascribed

ed

to the existence of surface sites where the intermediate oxygenates expected are strongly attached and not allowed to desorb in the gas phase. In contrast, formation of methanol or

pt

formaldehyde would rather correspond to oxygenate precursors more easily desorbed because

Ac ce

less retained at the catalyst surface.

M O

N N O

N2O

O

M

O C

CO2

O

O M (i)

CH3OH

H 1

O

O M (a)

N2

CH4 2

12 N2O H2O N2

CH3 O

O

C O H O M (g)

O M (f)

9 N2O

CO + H2O

N2

N2O 6

O

O

N2

H

10

CO2 + H2

(c)

OH

O

11

M

H2O

C

H

H2 C

5

M (b) H

HO

HCHO + H2

4

3

O

C O H 7 M (e)

C

H

O M (d)

HCHO

8

CO + H2

Figure 8: Reaction pathways for the partial oxidation of methane by N2O over M=O centers.

16

Page 16 of 23

3.3. 2. Mechanism of methane oxidation with molecular oxygen

When employing O2, MO2 peroxide species are not generated on the polyoxometalates surface, also CH3O species cannot be directly produced by CH4 adsorption on the catalyst

ip t

surface. CH4 activation rather involves hydrogen abstraction by the lattice oxygen (M=O) [68-71]. The role of lattice oxygen was confirmed by isotopic labelling experiments working

cr

with Pd18O/ZrO2 [71]. It was found that at 573K, at least 20% of the CO2 produced from

us

pulses of CH4/16O2 contained oxygen from the lattice. At higher temperatures, the exchange of oxygen from CO2 and the lattice is so rapid that it was not possible to determine the fraction

an

of CO2 produced by the redox mechanism. It follows that a metal-methyl compound is most probably formed, where the methyl is negatively charged. Therefore, oxidative

M

dehydrogenation (ODH) of methane on polyoxometalates would occur via parallel and

Fig. 9.

ed

sequential reactions, with formaldehyde as the most abundant primary product as shown in

In the proposed scheme, Step 1 involves the activation of the C-H bond in methane

pt

using lattice oxygen to form an adsorbed methyl species and an OH group (a). Step 2 involves

Ac ce

the recombination of an OH group and CH3 to form methanol, leaving behind an oxygen vacancy. The irreversible dissociative chemisorption of O2 then reoxidizes the reduced M centers to form the M=O oxide domains required for a new catalytic cycle. This Mars-van Krevelen mechanism resembles those shown to account for ethane and propane ODH on VOx and MoOx catalysts. In step 3, the methyl group of (a) reacts with the vicinal OH group to form a metal carbene intermediate (b), via the elimination of water. Further oxidation would lead to dioxomethylene (d) (step 4) then formate (e) (step 5), two major reaction intermediates for methane partial oxidation over polyoxometalates catalysts. HCHO and CO would be produced mainly via decomposition of formate (e) as shows steps 6 and 7. On the other hand, 17

Page 17 of 23

surface formate can be further oxidized to surface vicinal hydroperoxide and formate (f), which decomposes either to CO and H2O via dehydration or to surface carbonate (g) (step 10) after dehydrogenation. Obviously, selectivity to formaldehyde depends on the relative rates of both

ip t

decomposition and oxidation of surface formate, which vary with reaction conditions, temperature, and nature of the catalyst and the ratio of CH4/O2. The O2- superoxide species is

cr

known to be very reactive and capable of promoting deep oxidation of the hydrocarbons to

us

COx. The difference in the selectivity of oxygenates with PW11Fe content is attributed to the changes in the structure of iron. It was suggested that the tetrahedral surface Fe species is an

[72].

HCHO + M=O

CH3OH + M

CH4 1

M (a)

CH3

CH2 3

M (b)

O2

4

H2O

10

M (g)

H

6

C O O O O H 5 M M (e) (d) O H

O

C

H

2

O C

O

H

9 H2O

O

7 8

C O O-OH CO + H2O + M M (f)

Ac ce

pt

CO2 + M=O

O

M O (c)

ed

M O

HO

H H C

M

2

an

active site for the selective oxidation of methane to methanol on the iron phosphate catalysts

Figure 9: Reaction pathways for the partial oxidation of methane by O 2 over M=O centers.

4. Conclusion

The transition-metal modified heteropolytungstates [PW11M] (M=Co, Ni, Fe) were synthesized and characterized by different physicochemical methods.

31

P NMR, UV-visible,

IR spectroscopy and XRD analyses are in agreement with the Keggin structure. The thermal

18

Page 18 of 23

decomposition of the cesium salts of PW11Ni and PW11Co occurs at 803K whereas that of [PW11Fe] is stable up to 873K. We carried out partial oxidation of methane with N2O and O2 over the polyoxometalates at 873 and 923 K. Selectivity to oxygenates as high as 48% (conversion

ip t

5%) was obtained, cobalt and iron doped polyoxometalates being the most selective catalysts and N2O the most active and selective oxidant. The order of activity was: PW11Fe > PW11Co

cr

> PW11Ni for N2O and the reverse for O2. The increase of activity was correlated with the

us

increase of the oxidant character of the substituting metal. Kinetic study and catalyst behaviour suggested that reaction paths are different for nitrous oxide and molecular oxygen.

an

The key steps in selective oxidation of methane would be the reaction of CH4 with MO2 centers for N2O (producing methoxy active species) and with M=O centers for O2 (producing

M

methyl active species). Moreover, cyclic voltammetry analysis showed that the oxygenate products were most probably formed via a redox process. Finally, study of used catalysts

Ac ce

REFERENCES

pt

polyoxometallate stability.

ed

indicated that formation of water during the reaction had a positive effect on the

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HPA CH4

HCHO

ip t

CH3OH

Ac ce

pt

ed

M

an

us

cr

Graphical Abstract

22

Page 22 of 23

Highlights ► The PW11MO39 (M = Co+2, Ni+2 and Fe+3) compounds were prepared by refilling the vacant site of the lacunary precursor K7PW11O39. ► Application in methane partial oxidation. ► High

selectivity

to C1 oxygenates. ► Reaction pathway

for methane

Ac ce

pt

ed

M

an

us

cr

ip t

oxidation was proposed.

23

Page 23 of 23