Oxidation of carbonaceous solids

An alternative starting point in chemically reacting fossil fuels is to treat them as if they were graphite. As noted earlier, graphite and larger polynuclear aromatic hydrocarbons are far from inert with respect to electron-transfer reactions, and thus the use of chemistry known to work for graphite may be of possible use in the investigation of coal, petroleum, and their derivatives. In the next two sections, we will discuss aspects of reduction and oxidation of carbonaceous solids and thereby parallel the chapters in this book on the reduction and oxidation of polynuclear aromatic hydrocarbon molecules. 

In general, the intercalation of benzenoid, but nongraphitic, solids with chemical reductants has been studied more than the intercalation of these solids with chemical oxidants. Herold (47) suggested that soft carbons, such as those obtained from petroleum, have acceptor defects, which favor the addition of electrons (reduction) to the benzenoid structure. Hooley (48) discussed the divergences in terms of threshold pressure effects. Nevertheless, the oxidation of carbonaceous solids has been studied, and these studies are discussed in the next section. 

Early approaches to the oxidation of carbonaceous solids were destructive in nature and led to C02 and possibly to organic acids (49-52). More recently, the oxidative intercalation of nongraphitic materials was reported (53-55). 

In several studies, researchers attempted both the reduction and oxidation of carbonaceous solids (48, 54, 62, 63). One group claimed that graphite, oxidatively intercalated by CdCl2, can be later reductively intercalated by sodium (64). In any event, various chemistries will attack both graphite and carbonaceous solids, and various physical techniques can prove this attack. 

The goal of maximum energy generation by oxidation of carbonaceous species often thwarted detailed examination of occasional selective oxidations, such as ethylene oxidation to acetaldehyde on Pd or Au (28, 29, 370) or to ethylene oxide on Ag (330) or methanol and benzyl alcohol oxidation to formates and benzaldehyde, respectively (6-32, 54, 250, 333). Product yields were usually determined at one potential only or even galvanostatically (330), and the combined effects of potential, catalyst, reactant concentration, and cell design or mixing on reaction selectivity are unknown at present. Thus, reaction mechanisms on selective electrocatalysis are not well understood with few exceptions. For instance, ethylene oxidation on solid pal-... 

Similar tests were performed on the La2Cuo.9Pdo.i04+8 catalyst (Fig. 2). The same profiles are obtained as in the case of the LaMno.976Rho.o2403+s catalyst, although the small CO2 peak due to the oxidation of carbonaceous deposits is somewhat bigger than previously (440 ppm CO2 at the maximum). The main difference with the previous catalyst is that the amount of CO2 evolved on introduction of CO on the oxidised solid is much larger (about 400 pmoles), and corresponds in this case to 0.75 to 0.8 mole CO2 per mole catalyst. 

Feedstock Evaluation. A broad spectrum of carbonaceous solids was acquired whose proximate and ultimate analyses are given in Table II. All of these feedstocks as well as oxidized bituminous coal and the coke... 

MacPhee, J.A., Kawashima, H., Yamashita, Y, and Yamada, Y. Solid-state C NMR spectroscopy on coal and coal oxidation. In Botto, R.E., and Sanada, Y. (eds.), Magnetic Resonance of Carbonaceous Solids, Advances in Chemistry Series, Vol. 229, Chapter 17, pp. 323-339. Washington, DC American Chemical Society, 1993. 

Carbon surfaces of various types have been the subject of most studies. However, other types of surfaces, including alumina, fly ash, dust, MgO, V2Os, Fe203, and Mn02, have also been shown to oxidize S02 and/or remove it from the gas phase (Hulett et al., 1972 Judeikis et al., 1978 Liberti et al., 1978 Barbaray et al., 1977, 1978 Halstead et al., 1990). As expected, the rate of removal depends on the nature of the particular surface, the presence of copollutants such as N02, and, as in the case of carbonaceous surfaces, the relative humidity. The increase with increasing water vapor suggests that oxidation of the S02 may occur in a thin film of water on the surface of the solid. 

Reactions of organic compounds over solid catalysts are sometimes accompanied by the formation of heavy by-products which can form a deposit on the surface and lead to catalyst deactivation. For o-xylene oxidation the formation of such compounds has been frequently mentioned [15-17] but no information can be found about their influence on the catalyst deactivation. The present work reports on the formation of carbonaceous deposits over V2O5/T1O2 catalysts used for o-xyletie oxidation. Samples prepared by wet impregnation were used under operating conditions that can lead to the formation of heavy compounds. They were then collected and analysed by FTIR and TFO. The present data help to elucidate the characteristics of such compounds and their influence on the catalytic behaviour. 

In Section 7.2.2 we saw the role that nitric oxide plays in smog formation and the incentive we would have for reducing its concentration in the atmosphere. It is proposed to reduce the concentration of NO in an effluent stream from a plant by passing it through a packed bed of spherical porous carbonaceous solid pellets. A 2% NO-98% air mixture flows at a rate of 1 X 10" mVs (0.001 dm /s) through a 2-in,-ID tube packed with porous solid at a temperature of 1173 K and a pressure of 101.3 kPa. The reaction... 

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