Polyaromatic hydrocarbons oxidation reactions

Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

Although the ECL phenomenon is associated with many compounds, only four major chemical systems have so far been used for analytical purposes [9, 10], i.e., (1) the ECL of polyaromatic hydrocarbons in aqueous and nonaqueous media (2) methods based on the luminol reaction in an alkaline solution where the luminol can be electrochemically produced in the presence of the other ingredients of the CL reaction (3) methods based on the ECL reactions of rutheni-um(II) tra(2,2 -bipyridinc) complex, which is used as an ECL label for other non-ECL compounds such as tertiary amines or for the quantitation of persulfates and oxalate (this is the most interesting type of chemical system of the four) and (4) systems based on analytical properties of cathodic luminescence at an oxide-coated aluminum electrode. 

Much of the study of ECL reactions has centered on two areas electron transfer reactions between certain transition metal complexes, and radical ion-annihilation reactions between polyaromatic hydrocarbons. ECL also encompasses the electrochemical generation of conventional chemiluminescence (CL) reactions, such as the electrochemical oxidation of luminol. Cathodic luminescence from oxide-covered valve metal electrodes is also termed ECL in the literature, and has found applications in analytical chemistry. Hence this type of ECL will also be covered here. 

The marine environment acts as a sink for a large proportion of polyaromatic hydrocarbons (PAH) and these compounds have become a major area of interest in aquatic toxicology. Mixed function oxidases (MFO) are a class of microsomal enzymes involved in oxidative transformation, the primary biochemical process in hydrocarbon detoxification as well as mutagen-carcinogen activation (1,2). The reactions carried out by these enzymes are mediated by multiple forms of cytochrome P-450 which controls the substrate specificity of the system (3). One class of MFO, the aromatic hydrocarbon hydroxylases (AHH), has received considerable attention in relation to their role in hydrocarbon hydroxylation. AHH are found in various species of fish (4) and although limited data is available it appears that these enzymes may be present in a variety of aquatic animals (5,6,7,8). 

Parallel reactions involving selectivity are important in most chemical processes, where they typically control the formation of minor products or pollutants. In combustion, pollutants such as nitrogen oxides, polyaromatic hydrocarbons, and soot are formed by reactions that compete with parallel steps, leading to less harmful products. 

In an attempt to minimize overoxidation we explored the oxidation system m-chloroperbenzoic acid/NaHCC /CE C (34) with three thiophenes and three polyaromatic hydrocarbons. The results are summarized in Table II, where it is seen that the thiophenes are converted to their sulfones after only 30 minutes reaction time and the polyaromatic hydrocarbons are either unaffected by the oxidation or are oxidized much more slowly. The sulfones of the thiophenes listed in Table II are not oxidized further under these conditions. The thiophene content of Syncrude maltene was found to be 6.4% by the present method while the recovery was only 4.2% using the method of Willey et al. (25). Increasing the time of the oxidation reaction in the present procedure from 20 to 60 minutes had only a minor ( 10%) effect on the yield of isolated thiophenes. 

A valuable goal appears to be the anodic conversion of substituted toluenes into the corresponding aldehydes. The reaction can be achieved either in methanol [194] (intermediate formation of a ketal) or in aqueous solution in an indirect manner (presence of Mm11 or/and Cem ions as mediators [195]). The indirect oxidation of polyaromatic hydrocarbons (naphthalene, anthracene) into the corresponding quinones could be achieved in the presence of electrogenerated ceric ions. 

Reactions of curved polyaromatic hydrocarbon hgands like C60 with transition metals is of current interest. Oxidative addition of a strained five-membered ring of a C60-derived molecule to cobalt provides a candidate complex for the inclusion of a metal into the C60 framework [65]. 

We hypothesized that the above treatise of DDL interactions in the presence of an electrical field is a viable model for the explanation of enhanced oxidation-reduction in clay-electrolyte systems. Electrolytic transformations of selected chlorinated hydrocarbons (CHCs) and polyaromatic hydrocarbons (PAHs) have been demonstrated successfully in water and wastewater (Franz, Rucker, and Flora, 2002 Pulgarin et al., 1994). There has been field and laboratory evidence that these transformations can also take place in porous media (Banarjee et aL, 1987 Pamukcu, Weeks, and Wittle, 2004 Alshawabkeh and Sarahney, 2005 Pamucku, Hannum, and Wittle, 2008). As discussed previously, faradic reactions do take place on clay particle surfaces when current pass in the pathways of the DDLs (Grahame, 1951, 1952). Hence, external supply of electrical energy can help drive favorable oxidation-reduction reactions in contaminated clays not only in the bulk fluid but also on clay surfaces, as well as on where most of the contaminants tend to reside because of adsorption or exchange. 

Srzic and coworkers (Kazazic et al., 2005, 2006 Srzic et al., 1997a,b) reported qualitative studies of reactions of U with polyaromatic hydrocarbons (PAHs). The results were complicated by competition between oxidation and adduct formation, but an intriguing observation was the addition of multiple PAH molecules to U, such as in [U(phenanthrene)3] dehydrogenation of PAHs was also observed. Duncan and coworkers also examined the formation of uranium cationic complexes with PAHs using covaporization of uranyl acetate and pyrene (Ayers et al., 2004) or corannulene (Ayers et al., 2005) in a laser plasma source with TOF-MS detection, observing the formation of UO (PAH) species with x=0-2 and y=l, 2. 

Direct evidence for the involvement of solution species in these redox reactions was reported at about this same time by our group. We found that under certain solution conditions, the molecular radical cations (M ) of some divalent metal porphyrins (e.g., Ni octaethylporphyrin (NiOEP), ZnOEP, and VOOEP) formed by this electrochemical process could be observed in positive-ion ES mass spectra.Certain other easy-to-oxidize species like polyaromatic hydrocarbons (PAHs), aromatic amines, and heteroaromatics were also oxidized at the emitter electrode and observed as cationic radicals. Molecular ions formed by loss of an electron had not been observed in ES mass spectra prior to those reports. Our work served to illustrate that analyte species, under the appropriate operational conditions, could be directly involved in the redox reactions in the metal spray capillary and that the products of their reactions could be observed in the gas phase. 

The overall reaction in a partial oxidation reactor is highly exothermic. The desired reactions may be accompanied by thermal cracking of hydrocarbons or oxidative dehydrogenation into nonsaturated compounds including olefins, polyaromatics, and soot. The control of the heat balance and the formation of by-products are important considerations in the design of partial oxidation reactors. 

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