Air as oxidizing agent

The metal-catalyzed oxidation of carbohydrates with molecular oxygen is a remarkable example of green chemistry because reactants are obtained from renewable resources, processes are conducted under mild conditions with air as oxidizing agent and water as solvent, and reaction products are environmentally benign because of their biodegradability. In addition oxidized carbohydrate derivatives can often be obtained with high selectivity, and the catalysts are recyclable. These catalytic processes are, therefore, potentially very attractive for the preparation of specialties or intermediates employed in the food, cosmetic, pharmaceutical, and chemical industries. 

Fig. 2 Glucose, xylose, and furfural present in the liquid fraction in wet-exploded wheat straw (5% DM) pretreated with the use of H2O2, O2, or air as oxidizing agents. Results are average of triplicates...

The first step is performed in liquid phase with air as oxidizing agent under pressures of 3.5-5 atm to maintain liquid conditions. With a cobalt naphthenate-catalyst, temperatures in the range of 120-130 C are adequate, whereas without catalyst the temperatures need to reach 145-150 0. An important feature of the process is the relatively low per-pass conversion of about 15 per cent of the cyclohexane charge. Water formed by the oxidation reaction and impurities in the feedstock such as sulfur-containing compounds and other hydrocarbons are removed azeotropically as reaction proceeds. Unless reaction water is removed, the air-oxidation ceases after about 25-30 per cent conversion. Removal of feed impurities and oxidation by-products results in a clean recycle stream. 

Alternatively, a mild and efficient one-pot electrophilic aromatic substitution/ oxidative cyclization without isolation of the intermediate complexes 36 has been achieved using air as oxidizing agent (mode B in Scheme 12). Thus, reaction via mode B leads to tricarbonyl(ri" -4a,9a-dihydrocarbazole)iron complexes 37, which on demetalation with trimethylamine A(-oxide and subsequent catalytic dehydrogenation provide the carbazoles 40. The naturally occurring carbazole... 

The thermogravimetric oxidation tests were performed using air as oxidizing agent. Information trials using oxygen had disclosed very fast oxidation reactions, sometimes so fast that effects are overtaken and concealed which are clearly visible in air.With regard to... 

The reaction is thermodynamically favored by an elevated pressure (Figure 6.3.1), but this effect is rather small as with air as oxidizing agent the decrease in volume is small. 

Iron dissolves in mineral acids. In nonoxidizing acids, such as HCl or H2SO4, and in the absence of air or oxidizing agents, the metal is oxidized to ferrous state (Fe2+) liberating hydrogen ... 

Heumann-Pfleger indigo synthesis. Cycli-zation of phenylglycine to indoxyl followed by oxidation by air or oxidizing agents, such as ferric chloride, to yield indigo. 

Use of air as oxidant instead of stoichiometric quantities of chemical oxidizing agents. 

The reduction of indolones is described together with the reduction of isatogens. The major products of reduction are indoxyls and diindoxyls. In some cases the indoxyls are oxidized by air to benzoxazines (Section III,B). Indolones are comparable with isatogens as oxidizing agents ... 

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