Oxidations Using Catalytic Chromium Compounds

A great effort is dedicated to the development of methodologies for the oxidation of alcohols, involving catalytic quantities of chromium compounds, which are re-oxidized with other oxidants present in excess.406 Using chromium compounds in catalytic amounts is environmentally sound, and often facilitates the work-ups. 

Chromium compounds used in catalytic amounts for the oxidation of alcohols to aldehydes and ketones include  

As oxidants (used in excess), the following reagents were tried t-butyl hydroperoxide407,408,410,414,409,418 cumyl hydroperoxide, 414b hydrogen 

Catalytic chromium compound Oxidant used in excess Molar ratio chromium compound/oxidant in excess/alcohol Observations References 

Although the oxidations using catalytic chromium compounds are industrially attractive, none of them has found a widespread use in organic synthesis, because its versatility and efficiency in complex substrates have not been demonstrated. 

---

Table 1.4. Lists the combinations of catalytic chromium compounds and oxidants (used in excess) employed in the oxidation of alcohols to...

The Econ-Abator system is a fluidized-bed catalytic oxidation system. Catalytic fluidized beds allow for destruction of volatile organic compounds (VOCs) at lower temperatures than conventional oxidation systems (typically 500 to 750°F). The technology uses a proprietary catalyst consisting of an aluminum oxide sphere impregnated with chromium oxide. 

The keto group of a keto ester may be preferentially reduced by catalytic hydrogenation. Excellent yields of hydroxy esters are obtained. Copper-chromium oxide catalyst has been employed in the preparation of methyl p-(a-hydroxyethyl)-benzoate and several aliphatic -hydroxy esters. The last compounds have also been made by hydrogenation over nickel catalysts.Substituted mandelic esters are prepared by catalytic reduction of aromatic a-keto esters over a palladium catalyst. Similarly, platinum oxide and copper-chromium oxide have been used in the aliphatic series for the preparation of the a-hydroxy diester, diethyl... 

The catalytic oxidation of alcohols selectively to carbonyl compounds is one of the more important transformations in the synthetic organic chemistry. A large number of oxidants have been reported in the literature and most of them are based on transition metal oxides such as chromium and manganese [1-3]. A serious drawback to these reagents is the need to use them in large amounts, very... 

Another method of abating tail gas from ammonia-oxidation processes that normally do not exceed 2000 ppm nitrogen oxides is selective catalytic reduction (SCR). The term selective is used because the ammonia fuel only reacts with the nitrogen oxides and not with the oxygen first as in other abatement systems. Catalyst materials include vanadia/ titania and iron-chromium compounds. 

Although all of the above elements catalyze hydrogenation, only platinum, palladium, rhodium, ruthenium and nickel are currently used. In addition some other elements and compounds were found useful for catalytic hydrogenation copper (to a very limited extent), oxides of copper and zinc combined with chromium oxide, rhenium heptoxide, heptasulfide and heptaselen-ide, and sulfides of cobalt, molybdenum and tungsten. 

The most important (and also the most expensive) grapefruit aroma compound is the bicyclic terpene nootkatone. It is manufactured by oxidation of valencene, which is extracted from Valencia oranges. Figure 1.23 shows two routes for this oxidation, a stoichiometric reaction using chromium trioxide, and a catalytic alternative using sodium hypochlorite (bleach) in the presence of 1 mol% osmium tetraoxide catalyst. 

The oxidation of primary and secondary alcohols into the corresponding carbonyl compounds plays a central role in organic synthesis [1, 139, 140]. Traditional methods for performing such transformations generally involve the use of stoichiometric quantities of inorganic oxidants, notably chromium(VI) reagents [141]. However, from both an economic and environmental viewpoint, atom efficient, catalytic methods that employ clean oxidants such as 02 and H202 are more desirable. 

Catalytic reductions have been carried out under an extremely wide range of reaction conditions. Temperatures of 20 C to over 300 C have been described. Pressures from atmospheric to several thousand pounds have been used. Catal3rsts have included nickel, copper, cobalt, chromium, iron, tin, silver, platinum, palladium, rhodium, molybdenum, tungsten, titanium and many others. They have been used as free metals, in finely divided form for enhanced activity, or as compounds (such as oxides or sulfides). Catalysts have been used singly and in combination, also on carriers, such as alumina, magnesia, carbon, silica, pumice, clays, earths, barium sulfate, etc., or in unsupported form. Reactions have been carried out with organic solvents, without solvents, and in water dispersion. Finally, various additives, such as sodium acetate, sodium hydroxide, sulfuric acid, ammonia, carbon monoxide, and others, have been used for special purposes. It is obvious that conditions must be varied from case to case to obtain optimum economics, yield, and quality. 

Mixed vanadium-chromium oxide compounds present a wide range of interesting properties for instance, they have excellent catalytic properties, and recently they were shown to be potential candidates for anodes in lithium-ion batteries. DTA, TG and powder XRD analyses were used [101] to monitor the dehydration/crystallization and phase transitions upon heat treatment of the hydrated vanadates obtained through the reaction of peroxo-polyacids of vanadium and chromium, and to determine the ranges of coexistence of the phases in equilibrium. 

---