Chromium oxidation intermediates

Considering the limiting values of the induction factor it may be postulated that in the case of iodide and bromide the induced oxidation is caused by chromium(V), whereas for induced oxidation of manganese(II) chromium(lV) is the coupling intermediate. Therefore, one has to assume that in the course of reaction between arsenic(ril) and chromium(VI) both chromium(V) and chromium(IV) intermediates are involved. The mechanism below, proposed by Westheimer seems to be in agreement with experiment. 

The mechanism of the induced oxidation of manganese(II) cannot be regarded as sufficiently clear. Thus, the data obtained do not make it possible to decide whether manganese(IV) is formed by interacting with the chromium(V) intermediate... 

It was later shown that aziridine reacts over mixtures of zinc and chromium oxides on alumina at 400°C to give the same products as those obtained from mixtures of NH3 and acetylene [221]. Aziridine, which would form by addition of NH3 to acetylene followed by IH (Scheme 4-8), was thus postulated to be an intermediate in the formation of acetonitrile (by dehydrogenation), monoethylamine (by hydrogenation) and all other heterocyclic bases (by ammonolysis and subsequent reactions) [221]. 

Although providing a satisfactory rationale for experimental facts, an adsorbed cyclo-C4 intermediate still suffers from the problem of high energy, although this should be of lesser importance on chromium oxide catalysts because the reactions are carried out at much higher temperatures than on platinum. 

The products for which the cyclo-C4 isomerization intermediate has been suggested, can also be explained by a sequence of vinyl insertions. Thus, two vinyl insertions would be adequate to explain the formation of m-xylene from 2,3,4-trimethylpentane. Although we have seen in previous sections that extensive reaction sequences are possible on platinum, isomerization by a single vinyl insertion process on chromium oxide is relatively difficult, and the chance of two occurring in sequence would therefore be expected to be very low. In fact, the proportion of m-xylene is comparable to that of o- and p-xylene. 

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]. 

In a more recent study of the dehydrogenation of cyclohexane to benzene over a chromium oxide catalyst at 450°C., Balandin and coworkers (Dl) concluded that benzene was formed by two routes. One of these, the so-called consecutive route, involves cyclohexene as a gas phase intermediate, while the other proceeds by a direct route in which intermediate products are not formed in the gas phase. It was concluded that the latter route played a larger role in the reaction than did the former. These conclusions were derived from experiments on mixtures of cyclohexane and Cl4-labeled cyclohexene, which made it possible to evaluate the individual rates Wi, BY, Wt, and Wz in the reaction scheme... 

In another example of consecutive kinetics, a macrocyclic hydroperoxochromium complex L1(H20)Cr00H2+ undergoes an enzyme-like intramolecular transformation to yield a chromium( V) intermediate that subsequently decays by oxidation of the macrocycle.37 38 In acidic solutions, these reactions are described by Equation 8.67. 

In the presence of chromium oxide catalyst primary aliphatic alcohols of n-carbon atoms are converted to symmetrical ketones of 2n — 1 carbon atoms. The reaction proceeds by an intermediate aldol formation. 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

However, over Ni-kieselguhr in the absence of solvent or in ether and methylcyclo-hexane 32-33% of a diester, ethyl 3-(3 -hydroxybutyryloxy)butyrate (8), was produced along with 68-67% of ethyl 3-hydroxybutyrate and small quantities of dehydroacetic acid, and over copper-chromium oxide 16% of the diester and 7% of dehydroacetic acid were formed in the absence of solvent. It was suggested that the diester is formed through the hydrogenation of the intermediate 9, which results from 2 mol of acetoacetic ester with elimination of 1 mol of ethanol and that the condensation reaction is reversible (Scheme 5.6). Hence, the formation of the diester is depressed in the hydrogenation in ethanol.121 The reaction pathway in Scheme 5.6 has... 

HDA [HydroDeAlkylation] A proprietary dealkylation process for making benzene from toluene, xylenes, pyrolysis naphtha, and other petroleum refinery intermediates. The catalyst, typically chromium oxide or molybdenum oxide, together with hydrogen gas, removes the methyl groups from the aromatic hydrocarbons, converting them to methane. The process also converts cresols to phenol. Developed by Hydrocarbon Research with Atlantic Richfield Corporation and widely licensed worldwide. 

Figure 1. Dehydrogenation of acetoin (to the left, pale dots) and of 2,3-butanediol (to the right) on zinc- chromium oxide catalyst, LHSV=1.6 h" 2, acetoin (as initial material) 3, butadione 1, butanediol (as initial material) 2, acetoin (formed as intermediate from butanediol) 3,butadione.

By a similar mechanism to that proposed for the formation of ethyl ether by dehydration of ethanol, it is possible that the reaction occurs stepwise with the intermediate dehydration of one ethanol molecule to form ethylene which then reacts with another ethanol molecule to form butanol. It is thus possible that higher alcohols may be built up by the reaction of olefins with the lower alcohols. Mixed oxide type of catalysts are used in the process of a nature similar to those which have been found effective in alcohol synthesis from hydrogen and carbon monoxide. It should lie noted here that catalysts which promote the union of carbon atoms must be used, and since potassium oxide promoted catalysts composed of mixtures of zinc, copper, or chromium oxides have been found to be effective in the syuthesis of higher alcohols, such catalysts should be useful in promoting the addition of olefins to alcohols or other oxygenated organic molecules.77... 

This is practically total above 300°C and, even at atmospheric pressure, lowers the residual CO content to less than 20 ppm and to a few ppm under pressure. It takes place in the presence of nickel base catalysts deposited on alumina and doped with chromium oxide. The exothermicity of the reaction (At from 70 to 80°C/per cent CO converted) requires operation with two catalyst beds and intermediate effluent cooling. 

In the following sections it will be shown that several valence states of chromium are present on chromia-alumina catalysts. These are associated with the adsorption of oxygen, and the formation of chemical bonds between the chromium and oxygen ions. Therefore it is of interest to discuss intermediate chromium oxides (49,50) which are analogs of chromates, dichromates, peroxychromates, etc. Hydrated oxides such as CrOOH (51) will not be treated. 

Chromium oxide catalysts, supported on AI2O3 and activated by LiAlH4, have been claimed to cause ROMP of cycloalkenes but the yields are low (Eleuterio 1957). Cr(CO)3(mesitylene) initiates the polymerization of alkynes, possibly via a metallacyclobutene intermediate (Farona 1974 Woon 1974) see Ch. 10. 

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