Aromatic hydrocarbons transformation

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

Principal component analysis has been used in combination with spectroscopy in other types of multicomponent analyses. For example, compatible and incompatible blends of polyphenzlene oxides and polystyrene were distinguished using Fourier-transform-infrared spectra (59). Raman spectra of sulfuric acid/water mixtures were used in conjunction with principal component analysis to identify different ions, compositions, and hydrates (60). The identity and number of species present in binary and tertiary mixtures of polycycHc aromatic hydrocarbons were deterrnined using fluorescence spectra (61). 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

In the Figure 23 are summarized the different transformations, discussed before, of the bromoarenes into aromatic hydrocarbons, or more interestingly into arylamines, aryl ethers, phenols or iodoarenes. 

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Herbes SE, LR Schwall (1978) Microbial transformation of polycyclic aromatic hydrocarbons in pristine and petroleum-contaminated sediments. Appl Environ Microbiol 35 306-316. 

Peijnenburg WJGM, MJ t Hart, HA den Hollander, D van de Meent, HH Verboom, NL Wolfe (1992) QSARs for predicting reductive transformation constants of halogenated aromatic hydrocarbons in anoxic sediment systems. Environ Toxicol Chem 11 301-314. 

Richnow HH, A Eschenbach, B Mahro, R Seifert, P Wehrnng, P Albrecht, W Michaelis (1998) The nse of Relabelled polycyclic aromatic hydrocarbons for the analysis of their transformation in soil. Chemosphere 36 2211-2224. 

Ribbons DW, RW Eaton (1982) Chemical transformations of aromatic hydrocarbons that support the growth of microorganisms. In Biodegradation and detoxification of environmental pollutants (Ed AM Chakrabarty), pp. 59-84. CRC Press, Boca Raton. 

Safinowski M, C Griebler, RU Meckenstock (2006) Anaerobic cometabolic transformation of polycyclic and heterocyclic aromatic hydrocarbons evidence from laboratory and field studies. Environ Sci Technol 40 4165-4173. 

Sutherland JB, E Rafii, AA Kahn, CE Cerniglia (1995) Mechanisms of polycyclic aromatic hydrocarbon degradation. In Microbial transformation and degradation of toxic organic chemicals (Eds LY Young and CE Cerniglia), pp. 269-306. Wiley-Liss, New York. 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

M. Bejblova, N. Zilkova, and J. Cejka, Transformations of aromatic hydrocarbons over zeolites, Res. Chem. Interned., 34 (2008) 439 -54. 

Indeed, the extent of disproportionation of NO according to equation (89) clearly depends on the donor strength of the aromatic hydrocarbon.240 For example, hexamethylbenzene which is a strong donor (IP = 7.85 V) promotes the ionization of NO to an extent of 80% whereas the weaker donor durene (IP = 8.05 V) affords less than 25% ion-pair formation. Furthermore, the resulting NO+ cation is a powerful electron acceptor (Erea = 1.48 V versus SCE) in contrast to NO (Ered = 0.25 V versus SCE) and thus readily forms donor/acceptor complexes with a variety of aromatic, olefinic and heteroatom-centered donors. Accordingly, the donor/acceptor complexation and electron-transfer activation are the critical steps in various transformations in Chart 8 as described below. 

Herbes, S.E. (1981) Rates of microbial transformation of polycyclic aromatic hydrocarbons in water and sediments in the vicinity of coal-coking waste water discharge. Appl. Environ. Microbiol. 41, 20-28. 

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