Catalysis by 12-Molybdophosphates

Rocchiccioli-Deltcheff, Claude; Aouissi, Ahmed; Bettahar, M.M.; Launay, S.; Fournier, Michel

doi:10.1006/jcat.1996.0358

Cited by 180 publications

(126 citation statements)

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“…[5][6][7]11 For example, H 3 PMo 12 O 40 clusters supported on SiO 2 (5.75 wt % Mo) gave DMM selectivities as high as 55% at 513 K after the sample was treated at 593 K (Table 2), but CH 3 OH conversion rates were not reported, thus preventing direct comparisons with our results reported here. 6 Mo/MCM-41 (2 mol % Mo) showed a high DMM selectivity (72.6%) and very low CH 3 OH conversion (0.7%) at 543 K, but Mo migration out of MCM channels led to rapid deactivation. 7 The partial conversion of CH 3 OH to DME during DMM synthesis does not present significant hurdles, because pathways are available for the re-formation of CH 3 OH as it is depleted and for DME conversion to HCHO and DMM products on heteropolyacids (Tables 1, Figure 1) and to HCHO on supported MoO x and VO x catalysts.…”

Section: Catalytic Oxidation Of Dimethylmentioning

confidence: 70%

“…11 Mo-containing heteropolyacids with central Si or P atoms catalyze methanol conversion to formaldehyde with minor amounts of DMM side products. 6,11 Here, we report the discovery of very active H 3 -PMo 12 O 40 and H 3+n PMo 12-n V n O 40 structures supported on SiO 2 , which catalyze CH 3 OH reactions at low temperatures with very high selectivities to DMM (80-96%, DME-free basis) and high CH 3 OH conversions (20-40%, DME-free basis). These materials also catalyze DMM synthesis from DME (3CH 3 OCH 3 + O 2 f 2CH 3 OCH 2 OCH 3 + H 2 O) or DME-CH 3 OH reactants.…”

Section: Introductionmentioning

confidence: 90%

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Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H_3+nV_nMo_12-nPO₄₀ Keggin Structures

2003

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The one-step selective synthesis of dimethoxymethane (DMM; CH 3 OCH 2 OCH 3 ) was achieved by oxidation of dimethyl ether (DME) or methanol (CH 3 OH) with O 2 at low temperatures (453-513 K) on unsupported and SiO 2 -supported heteropolyacids with Keggin structures [H 3+n PV n Mo 12-n O 40 (n ) 0-4)]. These materials provide redox and Brönsted acid sites required for bifunctional DMM synthesis pathways. Supported structures at submonolayer coverages (0.1-0.28 Keggin units per nm 2 ) are much more accessible than bulk structures and remove diffusional constraints. Their higher dispersions lead to marked improvements in DMM synthesis rates and selectivities and to lower CO x yields using either CH 3 OH or DME reactants. The presence of H 2 O during DME oxidation increases DMM synthesis rates because of a consequent increase in the rate of DME hydrolysis reactions, which form CH 3 OH molecules required as intermediates in the DMM synthesis reaction sequence. Pure CH 3 OH reactants form DMM at much higher rates than DME reactants. The replacement of some Mo atoms in H 3 PMo 12 O 40 structures with V increases DMM synthesis rates and selectivities while inhibiting the formation of CO x . In fact, CO x was not detected on H 3+n PV n Mo 12-n O 40 (n ) 2, 4; ∼0.1 KU/ nm 2 ) even at high CH 3 OH conversions (∼50%). CH 3 OH converts to DMM via primary CH 3 OH reactions to form formaldehyde (HCHO) and subsequent secondary reactions of HCHO with CH 3 OH in steps requiring both redox and acid sites; CH 3 OH also reacts to form DME on acid sites. These pathways are consistent with the effects of changes in residence time and of the partial removal of acidic OH groups from Keggin structures on reaction selectivities. High CH 3 OH pressures and conversions favor HCHO-CH 3 OH acetalization reactions and DMM synthesis rates and selectivities. Thermal treatments that cause dehydroxylation and loss of Brönsted acid sites without destroying the primary Keggin structures decrease DME formation rates without significant changes in DMM synthesis rates. These findings suggest that acid sites are not involved in the rate-limiting step for DMM synthesis and that much higher DMM selectivities can be achieved by further increases in the ratio of the rates of redox and acid catalysis. This study represents the first report of high DMM selectivity and yields on stable molecular oxide clusters and provides an effective approach to the rational design of oxide materials for the one-step synthesis of dimethoxymethane from either dimethyl ether or methanol.

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Section: Catalytic Oxidation Of Dimethylmentioning

confidence: 70%

Section: Introductionmentioning

confidence: 90%

Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H_3+nV_nMo_12-nPO₄₀ Keggin Structures

2003

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“…IR spectroscopy was chosen as the method of analysis, because unlike calorimetry or TPD, it gives information on the nature of the sites and with the right probes is fairly sensitive to small energetic differences between sites. IR spectroscopy has been applied extensively for the structural characterization of heteropoly compounds at various temperatures and degrees of dehydration [62,63,64,65,66,67,68,69]. This method in addition delivers information on the structure of a sample after each treatment by investigating the lowfrequency range of the IR spectrum.…”

Section: Acid-base Properties Under Relevant Conditionsmentioning

confidence: 99%

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Jentoft

Klokishner

Kröhnert

et al. 2003

Applied Catalysis A: General

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The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-NIR in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides -OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the "active HPA" catalyst. The as-synthesized HPA crystal is thus a pre catalyst and the precursor oxide mixture is the final deactivated state of the catalyst.

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“…Since CO 3 2¹ is typically stable in the interlayer because of its good affinity to the hydroxide layer, decarbonation was carried out following a literature method 15) Figure 1 shows a schematic illustration of the PMoO and HWO polyanions prepared using published methods. 16),17) These polyanions were dissolved in ultrapure water separately, and LDH Cl ¹ was added to these aqueous solutions in a stoichiometric molar ratio. The resulting solutions were then shaken at 40°C and 200 rpm for 4 days.…”

Section: Preparation Of Ldh Hybrid With Polyanion By Ion Exchangementioning

confidence: 99%

Hybridization of Ni–Cr, Cu–Cr, and Zn–Cr layered double hydroxides with polyoxometalates and their catalytic behavior

Mitani

Takei

Yanagida

et al. 2017

J. Ceram. Soc. Japan

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Cr-based layered double hydroxides (LDHs) containing Ni, Zn, or Cu as the divalent cation were prepared with a divalent-totrivalent cation molar ratio of 3. (HWO)) were prepared by ion exchange and hydrothermal processes. The shift of the diffraction line indicating interlayer space in the X-ray diffraction patterns confirmed the intercalation in the LDH. The fraction of PMoO and HWO anions incorporated was 3070 or 6090% of ion exchange capacity, respectively, by charge compensation for Cr 3+ . In the conversion of styrene by epoxidation and oxidation, the hybrid of the LDH composed of Zn and Cr with PMoO showed the maximum catalytic activity, with 70% conversion. For the LDH composed of Ni and Cr, hybridization with a polyanion enhanced the catalytic behavior by a synergistic effect resulting from the basicity of the LDH phase.

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Catalysis by 12-Molybdophosphates

Cited by 180 publications

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Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H_3+nV_nMo_12-nPO₄₀ Keggin Structures

Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H_3+nV_nMo_12-nPO₄₀ Keggin Structures

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Hybridization of Ni–Cr, Cu–Cr, and Zn–Cr layered double hydroxides with polyoxometalates and their catalytic behavior

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Catalysis by 12-Molybdophosphates

Cited by 180 publications

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Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H3+nVnMo12-nPO40 Keggin Structures

Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H3+nVnMo12-nPO40 Keggin Structures

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Hybridization of Ni–Cr, Cu–Cr, and Zn–Cr layered double hydroxides with polyoxometalates and their catalytic behavior

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Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H_3+nV_nMo_12-nPO₄₀ Keggin Structures

Selective One-Step Synthesis of Dimethoxymethane via Methanol or Dimethyl Ether Oxidation on H_3+nV_nMo_12-nPO₄₀ Keggin Structures