Total Oxidation of Aromatic Hydrocarbons

As a general rule, the ease of oxidation of hydrocarbons and derivatives increases in the following order alkanes and isoalkanes aromatics oxygenates soot [20]. Differences also exist within each group. Duprez and coworkers demonstrated that short side chain alkylbenzenes, such as toluene and ethylbenzene, and polymethylbenzenes are more difficult to oxidize than benzene [19]. 

The increase of length of the alkyl group appropriates the oxidation of these hydrocarbons with the oxidation of long-chain alkanes with a better oxidabihty. 

polyalkylbenzenes with hindered heavy alk)d groups are quite easy to oxidize. The behavior of bicyclic or tricyclic hydrocarbons proved to be much more complex. However, partial or complete hydrogenation increases their reactivity and the oxidabihty of bicychc hydrocarbons is in the following order decali-ne tetraline naphthalene. For the heavier aromatics, the reactivity depends on their abiUty to form partial oxidation intermediates or to possess extremely rigid internal C=C bonds. 

the oxidation of imsaturated chlorinated compounds is more difficult than the oxidation of saturated compoimds, which is in direct contrast to the oxidation of unsaturated nonhalogenated hydrocarbons. 

Among mixed oxides, perovskite-type structures received a constant attention since the early 1970s when Voorhoeeve and Tejuca et al. pointed to their potential use as total oxidation catalysts [21,22]. This chapter will discuss the behavior of perovskites in total oxidation of heavy hydrocarbons and related chlorinated compoimds imder thermal and plasma activation conditions [20]. 

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The role of support on the performance of noble metals-based catalysts for the total oxidation of aromatic hydrocarbons is essential [38]. Although with a smaller surface area than the typical supports, perovskites also demonstrate good properties as carriers for noble metals. Thus, perovskites of type LaBOs (B = Co, Mn, Fe, Ni) synthesized using the citrate route were used as support for noble metals in total oxidation of toluene [39]. The performances of these catalysts varied in the order Fe>Mn>Co>Ni, and the superior behavior of iron was attributed to the low temperature of calcination and the high stability of the perovskite lattice irrespective of the nature of the stream it was exposed to. The dispersion of palladium at the different stages of the process remained unchanged. 

The total per-pa s yields of anhydrous oxygenated organic product per gallon of pentane is about 0.151 pounds or about 0.0285 pounds per pound of entering pentane. This should be contrasted with the yields obtained in the case of the oxidation of aromatic hydrocarbons where complete oxidation of entering hydrocarbon to either product or water and carbon dioxide is aimed at. In the absence of data on the amount of destructive oxidation of the pentane no comparison of the heat evolution in the two cases is possible although it is safe to say that in the case of pentane the amount of heat evolved per unit of feed is much less than in the case of the aromatics because of the restricted amount of oxygen present in the former case. 

Mixed oxide systems of well-defined ABO3 structure (perovskites), although not exclusively, are more stable in the presence of such compoimds and appeared as a reasonable alternative. Based on this, the aim of this chapter is to highlight and compare selected examples of perovskites and related oxides in total oxidation of heavy hydrocarbons and aromatics and their halogenated derivatives. The perovskite structure encompasses a wide array of materials with differing physical, chemical, and electronic properties. 

Studies reported to date confirmed the successful replacement of noble metals by perovskites in total oxidation of heavy hydrocarbons and aromatics. Besides activity, these materials allow a good stability, especially in the presence of halogens. Another relevant advantage is the very large composition of these... 

According to the literature, the platinum state does not seem to be a key factor for the catalytic oxidation of chlorobenzene. Certain studies have used platinum in a reduced state and others have used platinum in its oxidised state. However, studies carried out on the oxidation of VOCs showed that reduced Pt (Pt ) deposited on zeoUtes was the most active species for the oxidation of aromatic hydrocarbons and ketones. " In this context, the l.l%PtHFAU(5) catalyst formerly reduced in situ under hydrogen for six hours at 450°C was tested in the oxidation of chlorobenzene at 300°C. The particular effect of this treatment was to slightly increase the total conversion of chlorobenzene but with a much higher number of polychlorinated compounds (from 6.3 to 33.8 ppm) and amount of coke deposited on the catalyst after reaction (from 0.4 to 1.17%) (Table 5.2). 

The yields of secondary organic aerosols from a series of aromatic hydrocarbon-NOx oxidations have been measured by Odum et al. (1997a, 1997b). They showed that the total secondary organic aerosol formed from a mixture of aromatic hydrocarbons can be approximated as the sum of the individual contributions. Based on their experiments, the yield of secondary organic aerosols expressed as the total organic particle mass concentrations formed, AM, (in fxg m 3), divided by the mass concentration of aromatic precursor reacted, A (aromatic), is given by... 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

Literature reports provided arguments demonstrating that the performance of catalysts for the total combustion of alkanes and monoaromatic compounds cannot be directly extrapolated to predict their efficacy for the total oxidation of polycyclic aromatic hydrocarbons [43]. Therefore, the total oxidation of these molecules should consider specific catalytic systems. 

Ntainjua, N.E., Carley, A.F., and Taylor, S.H. (2008) The role of support on the performance of platinum-based catalysts for the total oxidation of polycyclic aromatic hydrocarbons. Catal Today, 137 (2-4), 362-366. 

Ndifor, E.N., Garcia, T., Solsona, B., and Taylor, S.H. (2007) Influence of preparation conditions of nano-crystalline ceria catalysts on the total oxidation of naphthalene, a model polycyclic aromatic hydrocarbon. Appl. Catal B Environ., 76 (3 4), 248 256. 

Catal3Ttic oxidation has been established as one of the most appropriate technologies for VOC abatement. An assessment of the suitability of catalytic oxidation for hydrocarbon control, along with competing processes, is given in Table 3.2. In the literature there are many studies focusing on the catalytic oxidation of VOCs, however, it is beyond the scope of this work to comprehensively review these studies. Rather we will concentrate on the catalytie total oxidation of simple short-chain alkanes and aromatic compounds as illustrative examples of VOC abatement. 

The significance of the total sulfur content of kerosene varies greatly with the type of oil and the use to which it is put. Sulfur content is of great importance when the kerosene to be burned produces sulfur oxides, which are of environmental concern. The color of kerosene is of Htde significance but a product darker than usual may have resulted from contamination or aging in fact, a color darker than specified may be considered by some users as unsatisfactory. Kerosene, because of its use as a burning oil, must be free of aromatic and unsaturated hydrocarbons the desirable constituents of kerosene are saturated hydrocarbons. 

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