Supported ReO x /Al 2 O 3 catalysts were investigated for propylene metathesis as a function of surface rhenia loading and extensively characterized with in situ UV−vis, Raman, IR, XANES/EXAFS, and isotopic 18 O− 16 O exchange studies. The experimental studies were complemented with DFT calculations using realistic models of the alumina surface. The surface ReO x sites were found to be isolated surface dioxo (O) 2 ReO 2 species, which represent the most stable surface rhenia structures on alumina as shown by DFT. Two distinct surface ReO 4 species, however, were found to be present and only slightly differ in their bridging Re−O−Al bond lengths brought about by anchoring at different sites of the Al 2 O 3 support. The deformed surface ReO 4 −I species preferentially anchor at more basic μ 1 Al IV and μ 1 Al VI sites and are difficult to activate for propylene metathesis. The surface ReO 4 −II species are formed at more acidic μ 2 Al VI and μ 3 Al VI sites and are the catalytic active sites for propylene metathesis. The surface ReO 4 −II sites were readily activated by propylene while the deformed surface ReO 4 −I sites were almost not affected by propylene, with only a few sites being activated. The steady-state propylene metathesis reaction rates are much higher for the surface ReO 4 −II sites than the deformed surface ReO 4 −I sites. The formation of the less reactive deformed surface ReO 4 −I species could be blocked by occupation of the μ 1 Al IV sites with sacrificial surface TaO x species that resulted in catalysts exclusively containing the more active surface ReO 4 −II sites on alumina. This is the f irst study to demonstrate that the surface ReO 4 −II sites are the precursors for the catalytic active sites for propylene metathesis by supported ReO 4 /Al 2 O 3 catalysts and to molecularly design olefin metathesis catalysts that exclusively contain isolated surface ReO 4 −II sites.

International audienceSilica-supported chromium oxide systems are efficient catalysts for many important chemical processes. Despite many years of investigations, the structure of the surface Cr species is not unambiguously determined. In this work, comprehensive DFT investigations of the monomeric Cr(VI) oxide species on silica under dehydrated conditions are performed. A large number of advanced periodic and cluster models of the SiO2 surface, based on the beta-cristobalite structure and different amorphous structures, have been applied. The calculated relative energies of the rnonooxo and dioxo Cr(VI) species depend on their location on the surface and on the structure of the model. It is concluded that the dioxo Cr(VI) species are thermodynamically preferred, but the presence of the monoxo Cr(VI) species, being in minority, cannot be excluded. According to the vibrational frequency analysis, the asymmetric O=Cr=O stretching mode for the dioxo species and the Cr=O stretching mode for the monooxo species can overlap

A periodic density functional theory approach is used to investigate isolated monomeric Mo oxide species on γ-alumina. Eleven potential dioxo and monooxo Mo centers variously located on the (100) and (110) surfaces of γ-alumina are modeled. In these structures, the molybdenum is 2-, 3-, or 4-fold bonded to the surface. Thermodynamic stabilities of the Mo oxide species are compared for a wide range of temperatures, taking into account the hydration/dehydration state of the catalyst. It is predicted that in strict dehydrated conditions, square pyramidal monooxo species are dominant on the most exposed (110) surface of γ-alumina, while tetrahedral dioxo species and five-coordinate dioxo species are most probable on the minority (100) surface. The latter is the potential precursor of the most active sites for alkene metathesis. The presence of 4-fold coordinated monooxo Mo species, especially on the (100) facet, is also possible. At low water exposure, tetrahedral dioxo Mo species are present on both γ-alumina surfaces. It is also predicted that the Mo sites on the (110) γ-alumina are more stable than their analogues located on the (100) facet. A significant increase of the MoO stretching frequency is observed when going from the dioxo species to the monooxo structures on the majority (110) surface of γ-alumina. This agrees with the evolution of the Raman spectra upon calcinations. Such a frequency shift between the dioxo and the monooxo species does not happen on the minority (100) surface.

The structure of monomeric molybdenum oxide species on silica is still a subject under debate. In this work, a large number of advanced silica models are used to study molybdena−silica system with density functional theory. The calculated relative energies of the monooxo and dioxo Mo(VI) species depend on the location of the Mo center on the surface and on the structure of the model. Periodic and cluster calculations employing comparable models of silica give similar results. It is shown that the monooxo Mo(VI) species can be more stable than the dioxo species under dehydrated conditions, provided that the local structure of silica enables preferable 4-fold bonding to the surface. As most locations are favorable for the 2-fold bonded dioxo Mo(VI) species, they should be dominant in the molybdena−silica system, whereas the monooxo Mo(VI) species are predicted to be in minority. The calculated frequencies of the MoO stretching mode for the monooxo Mo(VI) species are generally higher than the frequencies of the symmetric OMoO stretch for the dioxo species, corresponding to the strongest band observed experimentally. The relative energies of the reduced Mo(IV) species on silica are close to the relative energies of the corresponding Mo(VI) precursors.

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Cr/ZSM-5 catalysts with 0.5−2.6 wt % Cr loadings and zeolites with 15−140 Si/Al ratios were characterized with in situ UV−vis, IR, and Raman spectroscopies, including operando Raman measurements under reaction conditions in methane conversion at 773−1123 K with a simultaneous online analysis of reaction products. DFT calculations with cluster and periodic models were performed with DMol 3 , Gaussian 09, and VASP software packages. Isolated Cr(VI) dioxo and Cr(III) mono-oxo structures on framework Al anchoring sites were identified as the dominant species under most conditions. In the absence of gas-phase O 2 (under Ar flow) at 773 K, the Cr(VI) dioxo species on framework Al anchoring sites autoreduce, and the Cr(III) mono-oxo species remain the only observable surface Cr oxide structures. For ZSM-5 zeolites with a relatively low concentration of framework Al atoms (Si/Al ≥ 25), exposure to gas-phase O 2 at 773 K forces surface Cr oxide species to migrate from framework Al anchoring sites to Si sites on the external surface of the zeolite and form dioxo (Si− O−) 2 Cr(O) 2 and mono-oxo (Si−O−) 4 Cr(O) structures. The activity of Cr/ZSM-5 catalysts in methane conversion with the production of benzene and hydrogen as the main products is lower than that of Mo/ZSM-5 catalysts. The rate of benzene formation over Cr/ZSM-5 catalysts, however, is relatively stable with time on stream, in comparison to a rapidly declining rate over Mo/ZSM-5 catalysts. The zeolite-supported Cr species are highly mobile under the reaction conditions and can migrate between zeolite particles.

Ethene metathesis proceeding on Mo-methylidene centers on silica is investigated with density functional theory, applying the cluster approach. Three different locations of the active sites are considered, in which the Mo center replaces a pair of geminal silanols, two silanols from adjacent geminal pairs and two single silanols, respectively. It is shown that metathesis activity of the Mo-methylidene sites strongly depends on their location on silica. Different reactivity of the centers toward alkene is explained by differences in their geometrical and electronic structure parameters. The calculated C-H stretching vibrations of the proposed Mo-methylidene, Mo-ethylidene, and molybdacyclobutane surface complexes are well consistent with the reported IR spectra for the corresponding species generated on real molybdena-silica catalysts. On the basis of the obtained results it is proposed that among the studied cases, the Mo centers replacing two silanols from adjacent geminal pairs of silica surface are the most adequate models of the real active sites.

Periodic DFT calculations have been performed on molybdenum(VI) oxide species supported on the hydroxylated amorphous silica surface. The Mo grafting site has been investigated systematically for the type of silanol (geminate, vicinal, isolated or in a nest) accessible on the surface, as well as its effect on H-bond formation and stabilization, with the Mo-oxide species. Different grafting geometries, combined with different degrees of hydration of the Mo species are investigated using atomistic thermodynamics. The most stable Mo(VI) oxide species resulting from these calculations are confronted with experiment. Finally, calculated vibrational frequencies confirm the experimental evidence of the dominant presence of di grafted di-oxo Mo(VI) species on silica up to 700 K.