Of methyl aromatics

Selective and Nonselective Pathways in Oxidation and Ammoxidation of Methyl-Aromatic Compounds over Vanadia—Titania... 

The oxidation of / -xylene to terephthalic acid is by far the most important process based on the oxidation of methyl aromatics. However other similar processes are also operated industrially and oxidize toluene to benzoic acid or m-xylene to isophthalic acid. The latter is used as comonomer with terephthalic acid in bottles for carbonated drinks, and for special polyesters, and its production is roughly 2% of that of the terephthalic derivatives. 

The selectivity of a partial-oxidation catalyst can change with slight variations in its composition but is also dependent on the substrate and the reaction conditions. The design of catalysts optimized for a specific reaction should be based on a detailed knowledge and understanding of the reaction mechanism. The state of the art of catalyst development, mechanistic features, kinetics, and reaction technology in the ammoxidation of methyl aromatic compounds was summarized in 1992 by Rizayev et al. [38]. 

A redox mechanism (Mars-van Krevelen) is generally accepted for the ammoxidation reaction of methyl aromatic compounds, thus most catalysts applied contain transition metal oxides (e. g. vanadium, molybdenum) readily enabling changes in valence states. 

The ammoxidation of methyl aromatic and heteroaromatic compounds is a convenient route to many nitriles required for further synthesis of fine chemicals. For example, for the production of amines by hydrogenation or of carboxylic acids and amides by hydrolysis. 

Vapour Phase Oxidation of Methyl Aromatics to the Corresponding Aldehydes... 

Sulfur, and derivatives of sulfur in a low oxidation state, can also be applied for oxidation of methylated aromatic compounds. Thus, under the conditions of a modified Willgerodt reaction, dimethylnaphthalenes can be converted into methylnaphthoic acids (ca. 40%) and naphthalenedicarboxylic acids (ca. 60%) (molar ratios of Hydrocarbon S NH3 H20 = 1 7 5.6 65 ... 

Garrigues, P., Connan, j., Bellocq, j., Parlanti, E. Ewald, M. 1988. Relationship between rank and distribution of methyl aromatic hydrocarbons for condensates of different origins. / Matta-velli, L. Novelli, L. (eds) Advances in Organic Geochemistry 1987. Organic Geochemistrv, 13, 1115-1121. 

In recent years, the liquid phase oxidation of organic substrates using transition metal compounds as catalysts has become a profitable means of obtaining industrially important chemicals. Millions of tons of valuable petrochemicals are produced in this manner annually [1]. Typical examples of such processes are the production of vinyl acetate or acetaldehyde via the Wacker process, equations (1) and (2) the Mid-Century process for the oxidation of methyl aromatics, such as p-xylene to tereph-thalic acid, equation (3) and the production of propylene oxide from propylene using alkyl hydroperoxides, equation (4). 

Busca, G. (1993). Selective and non-selective pathways in the oxidation and ammoxidation of methyl-aromatics over vanadia-titania catalysts FTIR studies, in S. Oyama and J. Hightower (eds.), Catalytic Selective Oxidation, American Chemical Society, Washington, DC,... 

Martin, A., Bentrup, U., and Wolf, G.-U. The effect of alkali metal promotion on vanadium-containing catalysts in the vapour phase oxidation of methyl aromatics to the corresponding aldehydes. Catal. A Gen. 2002, 227, 131-142. 

So, the ethylene production does correlate with coke presence, in particular with aromatics formation as far as the diffusion limitations are not significant. However, it seems that the majority of ethylene is not always formed directly from MeOH [115]. The aromatics and other coke species could be the products of the conversion of primary carbenium ions, which are mobile and could equilibrate each other [28]. This may explain the isotopic distribution in products and retained coke molecules and the coexistence of aromatics and carbenium ions [28], In addition to the coproduction of ethylene with aromatics in olefins interconversion cycle, formation of ethylene via alkylation-dealkylation of methyl aromatics with heavy olefins or with the equivalent carbenium ions like ethyP, propyP, and butyP could be an option. The alkyl aromatics with the side chain length of two carbons or longer are not stable in the pore and dealkylates on the acid sites due to too long residence time and steric hindrances. This may lead to formation of ethylene, other olefins, and alkylaromatics with different structure, namely PMBs [129]. In other words, the ethylene is formed via interaction of the carbenium ions like ethyP, propyP, and butyP formed from MeOH or heavy olefins with aromatics and other coke precursors followed by cracking and in a less extent by a direct alkylation of PMBs with methanol. The speculation is based properly on analysis of the prior arts and is not contradictory with the concept of the aromatic cycle for ethylene formation. 

In general, the ammoxidation reaction of methyl aromatics and/or hetero aro-maties runs via redox mechanism as proposed by Mars and van Krevelen [12]. Most of the catalysts used so far eontain transition metal oxides with easily ehanging valence states (e.g., V, Mo, etc.). Essential steps of the reaction mechanism are (i) chemisorption of the methyl aromatic or hetero aromatic reactant on the catalyst surface followed by H-abstraetion (i.e., C-H bond disassociation) to form a benzylic intermediate, (ii) insertion of nitrogen into a surface bonded partially oxidized intermediate and (iii) desorption of the formed nitrile and iv) reoxidation of the catalyst by gas-phase oxygen. Literature survey [114, 115] revealed that the H-abstraction oeeurs via C-H bond dissociation in three different possible ways, such as (i) heterolytic with the abstraction of hydrogen atom in an anionic form followed by carbocation. 

Figure 28. Oxidative cleavage of methyl aromatics in acetic acid solutions containing sulfuric acid.

---