Theoretical studies of the CrOx/sSiO2 catalyst

The Phillips CrOx/SiO2 catalyst is one of the most commonly used system in the industrial production of high-density polyethylene (HDPE). This system has been extensively studied since decades, mainly by means of experimental techniques. However, the progress in determination of the nature of the chromium sites and understanding of surface reactions occurring has been rather not satisfying and many issues are still under debate. Among others, structure of the chromium oxide species on the surface of the reduced catalyst is not well defined. Likewise, the mechanism of the catalyst reduction and active site formation is not well established. Many experimental studies indicate that chromium exists on the surface of amorphous silica in a wide variety of different forms including monomeric, dimeric and polymeric species. Different oxidation states are also possible, as Cr(VI), Cr(V), Cr(IV), Cr(III), Cr(II) were detected. There is a general consensuses that Cr(VI) dominates at the surface after the catalyst preparation, while Cr(II) and Cr(III) are mainly formed after contact with a reducing agent, like ethylene or CO. Nevertheless, the detailed structure of these oxide species, as well as the active sites, is still strongly discussed. As experimental studies do not show a clear picture about the nature of silica-supported chromium catalyst, very helpful can be computational approach which can provide complementary information, not accessible by experimental techniques. In this short review we summarize the recent progress in the field of the Phillips (CrOx/SiO2) catalyst focusing on the most relevant theoretical papers that were published within last couple of years. We also highlight the need of applying advanced models if realistic theoretical description of the CrOx/SiO2 system is to be achieved. Additionally, different computational approaches in modeling of heterogenous catalysts are discussed

Structure of Isolated Molybdenum(VI) and Molybdenum(IV) Oxide Species on Silica: Periodic and Cluster DFT Studies

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

Characterization of tungsten monomeric oxide species supported on hydroxylated silica; a DFT study

International audienceA DFT based characterization of tungsten oxide supported on amorphous hydroxylated silica is presented. The different molecular organizations are investigated on the surface topology and tungsten oxygen coordination. The presence of mono- and di-grafted species is discussed and rationalized, using an atomistic thermodynamic approach. The presence of W[double bond, length as m-dash]O groups are preferred over W–OH groups and the grafting coordination is dominated by the degree of hydration of the silica surface. At room temperature di-oxo digrafted and mono-oxo-tetragrafted species are in competition regulated by the ambient degree of hydration which also affects the silanol density of the silica support. A comparison between tungsten and the other group VI elements confirms a greater chemical difference with Cr than with Mo

Characterization of Molybdenum Monomeric Oxide Species Supported on Hydroxylated Silica: A DFT Study

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Structure of Monomeric Chromium(VI) Oxide Species Supported on Silica: Periodic and Cluster DFT Studies

Silica-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 β-cristobalite structure and different amorphous structures, have been applied. The calculated relative energies of the monooxo 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

Surface ReOx Sites on Al2O3 and Their Molecular Structure Reactivity Relationships for Olefin Metathesis

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