Chromium oxidation catalysts, alcohol

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

Newer methods of alcohol oxidation (Swem, Dess-Martin) are introduced as environmentally preferable to the older chromium methods, including a description of a general, unifying mechanism of alcohol oxidation to aldehydes and ketones. TEMPO is shown as an oxidation catalyst to enhance hypochlorite oxidation. 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

Secondary alcohols are oxidized by H5IO6 in the presence of various chromium catalysts to ketones [1321-1325], while primary alcohols can be oxidized to aldehydes or to carboxylic acids depending on the catalyst. Primary alcohols in the presence of pyridinium chlorochromate (PCC)[1322] or chromium (III) acetylacetonate, Cr(acac)3 [1321] are oxidized to aldehydes or ketones in excellent yields, while the use of CrOj [1326,1327] or pyridinium fluorochromate [1323] as catalysts results in the oxidation to carboxylic acids. The periodic acid promoted oxidation of primary and secondary alcohols to carbonyl compounds can also be catalyzed by Cu(II) derivatives [1328,1329], by bromide anion [1330] and by TEMPO [1331]. 

The oxidation of alcohols to carbonyl compounds is a fundamental reaction that has synthetic and chemical importance. Using chromium-based catalysts, researchers have developed several catalysts that have impacted alcohol oxidation reactions. Recently, homogeneous catalysts have had problems with catalyst/product separation and suffer from poor catalyst recyclability. Therefore, the quest for a resolution to this problem has led researchers to scaffold salen complexes onto a silica-based material such as MCM-41. Zhou et al. used an ion-exchangeable, layered polysiloxane support to immobili.se their sulfonato-(salen)Cr(m) complex. They reacted benzyl alcohol, cyclo-hexanol and -hexanol with hydrogen peroxide as oxidant in an ionic liquid at 40 °C. Several ionic liquids were investigated [BMImX (BMIm = 1-n-butyl-3-methylimidazolium X =PF6, BF4, NOs")] and compared for each substrate. 

Arene complexes are utilized for the preparation of metallic layers which are deposited on various surfaces (See Table 2.31). They are also used for the preparation of highly pure metals. This is especially true for chromium group compounds. Pyrolytic decomposition of arene complexes found application for deposition of chromium on magnesium oxide in order to prepare the catalyst for dehydration of isopropyl alcohol. 

By passing the alcohol vapour over a copper - chromium oxide catalyst deposit on pumice and heated to 330°, for example ... 

Hydrogenations with coppcr-chromium oxide catalyst are usually carried out in the liquid phase in stainless steel autoclaves at pressures up to 5000-6000 lb. per square inch. A solvent is not usually necessary for hydrogenation of an ester at 250° since the original ester and the alcohol or glycol produced serve as the reaction medium. However, when dealing with small quantities and also at temperatures below 200° a solvent is desirable this may be methyl alcohol, ethyi alcohol, dioxan or methylcyc/ohexane. 

C and 19,600 kPa (2800 psi). The catalyst is a complex aluminum—ca dmium —chromium oxide that has high activity and exceptionally long life. The process is claimed to give a conversion of ester to alcohol of about 99% retaining essentially all of the original double bonds. 

Catalysts used for preparing amines from alcohols iaclude cobalt promoted with tirconium, lanthanum, cerium, or uranium (52) the metals and oxides of nickel, cobalt, and/or copper (53,54,56,60,61) metal oxides of antimony, tin, and manganese on alumina support (55) copper, nickel, and a metal belonging to the platinum group 8—10 (57) copper formate (58) nickel promoted with chromium and/or iron on alumina support (53,59) and cobalt, copper, and either iron, 2iac, or zirconium (62). 

Concentration Effects. The reactivity of ethyl alcohol—water mixtures has been correlated with three distinct alcohol concentration ranges (35,36). For example, the chromium trioxide oxidation of ethyl alcohol (37), the catalytic decomposition of hydrogen peroxide (38), and the sensitivities of coUoidal particles to coagulation (39) are characteristic for ethyl alcohol concentrations of 25—30%, 40—60%, and above 60% alcohol, respectively. The effect of various catalysts also differs for different alcohol concentrations (35). 

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