Air as oxidant

Table 1 Selective oxidation of geraniol using air as oxidant. Conditions 0.085mol geraniol in toluene, 5wt.% loading of catalyst (dry weight), 60°C, 3bar air, 600rpm, 6h.

Figure 2 Selective oxidation of 1-octanol and 2-octanol using air as oxidant. Conditions 0.015mol 1-octanol in toluene, reactant metal molar ratio 80 1, 60°C, 600rpm, 3 bar air, 6h.

The hydrogenation of HMF in the presence of metal catalysts (Raney nickel, supported platinum metals, copper chromite) leads to quantitative amounts of 2,5-bis(hydroxymethyl)furan used in the manufacture of polyurethanes, or 2,5-bis(hydroxymethyl)tetrahydrofuran that can be used in the preparation of polyesters [30]. The oxidation of HMF is used to prepare 5-formylfuran-2-carboxylic acid, and furan-2,5-dicarboxylic acid (a potential substitute of terephthalic acid). Oxidation by air on platinum catalysts leads quantitatively to the diacid. [32], The oxidation of HMF to dialdehyde was achieved at 90 °C with air as oxidizing in the presence of V205/Ti02 catalysts with a selectivity up to 95% at 90% conversion [33]. 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

Oxidation of alcohols to carbonyl-compounds or carboxylic acids can be performed under moderate conditions, in aqueous solution and with air as oxidant [1, 2], The selectivity is high, usually above 90 % even at full conversion. The only drawback of the method is the rapid deactivation of Pt- or Pd-based catalysts. An indication of this difficulty is that more than 60 % of the papers, which have been published on alcohol oxidation in the past ten years, describe some sort of catalyst deactivation. 

The following compounds were prepared using air as oxidant ... 

Several important nomadical catalytic oxidations go via organometalhc mechanisms. The commercially useful Wacker process converts ethylene to acetaldehyde with air as oxidant, using Pd(II) and Cu(II) catalysts. The Pd(II) binds to the ethylene to give an organometalhc intermediate, the alkene complex. This complex subsequently uses water as the O source to oxidize the ethylene to acetaldehyde, the Pd being reduced in the process. The resulting Pd(0) is reoxidized to Pd(II) with two equivalents of Cu(n) and the Cu(I) so formed is then reoxidized by air to close the cycle. 

Several plants have been built operating according to the two-stage technology (Figure 4) [48, 49] quite analogously to the acetaldehyde process described above, with air as oxidant and a catalyst cycle. An important by-product in acetone manufacture is propionaldehyde, which is separated by extractive distillation... 

Current densities of more than 1 A cm-2 at 0.5 V have been observed at PEM FC with hydrogen as fuel and air as oxidant. Even... 

Reagents Hydrogen, reformed alcohols, or hydrocarbons are used as fuel and oxygen or air as oxidant. 

For small-scale applications, the cost may be more important than the efficiency of a given plant. In most cases, the partial oxidation reactors give a highly expensive plant layout because of the inherent, high cost of air separation (unless a low-cost source of oxygen is available). The use of air as oxidant makes the final separation very difficult, especially if carbon monoxide is needed either in a pure form or with hydrogen. 

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

An alternative route to produce synthesis gas starting from hydrocarbon feedstock is the partial oxidation reaction (POX) [16]. This reaction utilizes the oxygen in the air as oxidant and results moderately exothermic. The oxygen to carbon ratio is lower than that required by stoichiometric complete combustion. 

Supported Pt and Pd were shown to yield some selectivity in batch conditions using air as oxidant, but the principle products were the unwanted single carbon species such as CO2, HCHO and HCOOH. Under these conditions supported Au catalysts were totally inactive. Using pure oxygen and 3 bar pressure, Au became active, and the formation of Cl by-products was eliminated when NaOH was added. Using 1 wt% Au supported on either graphite or activated carbon, 100% selectivity to glyceric acid was... 

With near ambient pressure air as oxidant, the optimum temperature of operation is about 75°C. Because the air (and fuel) streams must be presaturated to ensure membrane stability, higher temperatures cause excessive dilution of the oxygen in the air stream by water vapor. When oxygen is employed, operational temperatures up to 150°C are possible. 

Analogous condensations, but with a pyrrole aldehyde lead to mesomeric dipyrromethene cations, which play an important part in porphyrin synthesis. Thus, using formyldipyrromethane as the aldehyde and a second mole as the pyrrole component, with air as oxidant, porphine is formed directly, as its magnesium derivative, possibly via a dipyrromethene cationic intermediate. ... 

This version offers the advantage of being able to operate with lower purity ethylene (95 per cent volume) and air as oxidant However, it requires larger capital expenditure. 

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 liquid phase oxidation of alcohols on platinum metal catalysts can be carried out under mild conditions and with air as oxidant. Heptane, ethyl acetate or 2-butanone have been suggested as solvents for water-insoluble alcohols [1]. However, only the aqueous phase oxidation is of practical importance, due to safety reasons [2]. The possibility of applying a "water-detergent" system for water-insoluble substrates will be shown here using the example of the selective oxidation of secondary alcohols to ketones. 

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