Reactions with Oxygen or Air

Oxidation is a complex process that consists of parallel bnt competing/interacting reaction processes. Although at least four such processes are believed to exist, the exact number, nature, and kinetics of such processes are not very clearly nnderstood. Althongh aerial oxidation of coal is essentially a chemical reaction process, it is inflnenced by, apart from its original chemical composition, other factors like temperature, moisture, catalytic effects of water, and components in the mineral matter. Furthermore, the effects of the physical and snrface properties play a role that is not properly understood. 

A different consequence of coal oxidation is the development of spontaneous combustion (Chapters 9 and 14), when the heat generated by in situ oxidation canses the coal to smolder and nltimately bnrn without any external heat source. The liability to oxidation is mainly determined by the coal s rank, in conjunction perhaps with the maceral content (Chapter 4) and the content of mineral matter (Chapter 7). Low-rank coal is particularly prone to spontaneous combustion and factors, snch as access of air to coal stockpiles, may need to be controlled to reduce the ever-present risk of spontaneous ignition and combustion. 

The oxidation of coal is of considerable interest as a means of modifying the physical and chemical properties of the coal. In particular, the caking of the coal on carbonization may be reduced or prevented by oxidation treatment. The degree of oxidation is important, about 1% of oxygen being adsorbed on, or combined with, the coal. It may be estimated by chemical analysis or trial carbonization, but for the control of oxidation plant, a more rapid indication is required of any departure from the correct operating conditions. 

The reaction of coal with aerial oxygen has been the subject of many investigations (Van Krevelen and Schuyer, 1957 Dryden, 1963). Weathering studies, the incorporation of functional groups, and the mechanisms of spontaneous ignition of coal are only three of the many reasons of interest (Wender et al., 1981). Furthermore, a standard test method is available for determining the relative degree of oxidation in bituminous coal by alkali extraction (ASTM, 2011). 

The oxidation reaction is, in fact, a general occurrence over a wide range of conditions (Given, 1984) but, more specifically, increases readily with temperature and decreases with particle size. This latter phenomenon is an indication of the function of surface area as one of the reaction parameters. If the coal contains a substantial proportion of indigenous moisture, the oxidation reaction rate will be relatively high. 

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The mechanism of the dihydroxylation reaction with oxygen or air is presumed to be similar to the catalytic cycle presented by Sharpless et al. for the osmium-cata-... 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

Nakajima and coworkers [30] observed that, in the presence of CuCl(OH).-TMEDA with oxygen or air as the oxidant, 2-naphthol 103a is transformed into l,l -bi-2,2 -naphthol 104a. A wide variety of substrates undergo oxidative coupUng in excellent yields (Scheme 28). It is worth noting that the reaction requires as little as 1 mol % of the catalyst. 

Kiyama, R., J. Osugi and S. Kusuhara Studies on explosive reactions of tetra-fluoroethylene and acetylene with oxygen or air. Rev. Phys. Chem. Jap. 27, 22-41 (1957). 

The most dramatic results obtained so far with gold catalysts have been with the liquid phase processes. They are conducted with oxygen or air, often using water as solvent, and are therefore felt to be environmentally benign. Particular success has been obtained with reducing sugars (Section 8.3.2) and other aldehydes (Section 8.3.3), and with alcohols and other hydroxy-compounds (Sections 8.3.4-8.3.7). Reactions that use soluble gold complexes to catalyse selective oxidation are reported in Chapter 12. 

Although Fig. 6.10 is presented in a general form, very many specific examples are published in the literature because, often, it is important to characterize the conditions in which different reaction modes are observed before any other detailed investigations are made [125-127]. The reactant pressures and vessel temperatures at which the different features occur are dependent not only on the particular fuel and its proportions with oxygen or air but also on the size and shape of the vessel [121]. Surface reactions may play some part also, especially in the initiation processes, so the quantitative details are usually specific to the particular system employed... 

Indeed, Fujiwara and coworkers [4b, 20] discovered that when copper(II) acetate or silver(I) acetate is employed together with oxygen (or air), the palladium-acetate-assisted alkene arylation reaction proceeds catalytically with respect to both palladium and copper (or silver). For example, styrene (4a) reacted with benzene (2a) in the presence of 10 mol% Pd(OAc)2, 10mol% Cu(OAc)2 and 50atm oxygen, producing tra -stilbene (3a) in 45% yield (Equation (9.7)) [20]. 

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