Chromium oxidized species

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

C. This is thought to be the contribution of a di-attached chromium oxide species. 

These experiments were repeated with a catalyst made by depositing dibenzenechromium(O) on AIPO4, and similar results were obtained, except that the poisoning was not as severe. Preexposure of the catalyst to CO reduced the activity by about 40%, and again decreased the size of the low-MW peak. A quick preexposure to dry air reduced the activity very little, but it did diminish the GPC peak associated with phosphate, a result that again suggests selective oxidation to a chromium oxide species. 

While chromium oxide species may vary considerably in their local environment because of the heterogeneous nature of an amorphous oxide carrier, Ziegler species are often thought to be more uniform, occup3dng certain places in a microcrystalline environment (56-58). Frequently, donor compounds, such as ethers. 

The use of ordered mesoporous supports has been investigated in parallel as an alternative strategy to obtain the high dispersion of chromium oxidic species for the ODH with CO2. Bi et al. investigated the catalytic behavior for the CO2-ODH of ethane using transition metal-doped M-MCM-41 (M = Ni, Co, Cr) mesoporous materials, prepared by the direct hydrothermal method. Cr-containing catalysts... 

Guesmi H, Tielens F Chromium oxide species supported on silica a representative periodic DFT model J Phys Chem C 116(1) 994-1001, 2012. 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

No other oxide phases below MO2 have been established but a yellow hydroxide , precipitated by alkali from aqueous solutions of chromium(II), spontaneously evolves H2 and forms a chromium(III) species of uncertain composition. The sulfides, selenides and tellurides of this triad are considered on p. 1017. 

Since the publication by the discoverers (3) of chromium oxide catalysts a considerable number of papers devoted to this subject have appeared. Most of them (20-72) deal either with the study of the chromium species on the catalyst surface or with the problem of which of this species is responsible for polymerization. Fewer results have been published on the study of processes determining the polymer molecular weight (78-77) and kinetics of polymerization (78-99). A few papers describe nascent morphology of the polymer formed (100-103). 

So far the problem of active center formation in chromium oxide catalysts amounted mainly to a discussion of the oxidation number of chromium that is necessary for catalytic activity. As an active species chromium ions having practically every possible oxidation number—... 

Kinetic studies of the oxidation of sulphoxides to sulphones by chromium(VI) species have been carried out131-133. The reaction has been found to be first order with respect to the chromium(VI) species and the sulphoxide and second order with respect to acid. At high sulphoxide concentrations the order with respect to sulphoxide is two. The proposed mechanism involves an electron transfer from the sulphoxide to the active chromium(VI) species (HCr03+ in strong acidic media) in the rate-determining step producing a sulphoxide radical cation which further reacts to give the sulphone. 

In the oxidation of 2-propanol no polymer could be detected. However, when benzaldehyde was added, polymerization occurred to a considerable extent. These data suggest that the intermediate chromium(rv) or chromium(V) species formed in the oxidation of alcohol was responsible for the radical products and that the benzaldehyde was involved in the initiation. 

Knowledge of stoichiometry of the induced reaction could help to distinguish whether chromium(V) or chromium(IV) species are involved in the oxidation of benzaldehyde. Thus, the Cr(V) hypothesis predicts that for each molecule of benzaldehyde oxidized two molecules of manganese dioxide should be formed, whereas the Cr(IV) predicts that one molecule of manganese dioxide should be formed for each two molecules of benzaldehyde oxidized. Unfortunately, the attempt to determine the stoichiometry of the induced reaction failed because the oxidized manganese species was not precipitated during the reaction presumably due to formation of acetate complexes in the concentrated acetic acid solution. 

Cerium(III) also proved to be an effective inhibitor of the oxidation of formic acid. As the oxidation of cerium(rri) to cerium(IV) is a 1-equivalent process, the inhibition furnishes additional evidence for the chromium(IV) species as intermediate. 

Luther and Rutter have observed the induced oxidation of iodide during the reactions between chromic acid and vanadium(IV), vanadium(ri[), and vana-dium(II) ions. In all the three systems ci = 2, therefore it is probable that the coupling intermediates are chromium(V) species, these being, especially the two latter systems, too complicated for a detailed kinetic treatment to be given. 

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