Ammoxidation of Substituted Toluenes

Ammoxidation of alkylbenzenes with side-chains of different length leads to reaction at the position a to the benzene ring [38]. Reactivity increases with increasing length of the alkyl group (toluene ethyl benzene isopropyl benzene [58,59]), but in all cases only benzonitrile is formed. 

In addition to toluene higher condensed methyl aromatic compounds and biphenyl derivatives, e. g. 1-methylnaphthaIene (on Cu-Na-mordenite [36]) or p-methylbiphenyl (to / -cyanobiphenyl and terephthalodinitrile [60]) can also be ammoxidized. The ammoxidation route can also be used to insert nitrogen into very highly condensed products, e. g. lignins [61,62], or active carbon [63]. 

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The ammoxidation of substituted toluenes over differently prepared (NH4>2(V0)3(P207)2-and V0(P03)2-phases as well as over (VO)2P207 has been studied by catalytic and in situ-ESR measurements. For effective catalytic performance at least two structural properties were found to be essential i) Closely neighbouring centres must be exposed at the surface which enable the simultaneous adsorption and conversion of the substrate and ii) the catalyst structure must contain building blocks of exchange-coupled ions e. g. in the form of chains or layers which support the electron transport during the redox process. 

The ammoxidation of toluenes substituted with electron-donating groups, for example hydroxy- and alkoxy-substituted toluenes is rather less selective. However, under carefully chosen conditions (choice of the catalyst, feed composition, reaction conditions) adequate yields of nitriles can be achieved. Stabihty of the catalyst performances is typically an issue. 

Toluenes with more than one additional methyl group are also converted to nitriles [70,71], although with higher substitution the number of products resulting from partial ammoxidation increases, e. g. the ammoxidation of mesitylene results in a mixture of l-cyano-3,5-dimethylbenzene, l,3-dicyano-5-methylbenzene and... 

Halogen substituted toluenes are readily converted into nitriles because electron-withdrawing substituents enhance the reactivity of such compounds in the ammoxidation reaction. The fluoro-, chloro-, bromo-, and iodo-substituted toluenes [e. g. 41,74-76] can, therefore, be converted to the corresponding nitriles. Whereas the conversion rate of /7-halotoluenes (over vanadium phosphate catalysts [41,75]) is nearly independent of the nature of the halogen substituent, the selectivity decreases in the sequence p-Cl > /7-Br >> p-l. Ammoxidation of isomeric chloro-toluenes results in different conversion p o > m) and selectivity p > o > m) sequences [41,75,76]. 

Ammoxidation of p-methoxytoluene (protection of the OH group in the p-cresol feed by methylation) over vanadium-titanium oxide catalysts gives p-methoxy-benzonitrile in 65 % yield [81,82]. Because of the greater reactivity of p-methoxy-toluene compared with the m isomer the ammoxidation of m,p-methoxytoluene mixtures results in the formation of only p-methoxybenzonitrile and the m isomer remains mainly unreacted. This presents the possibility of reactive separation of differently substituted toluenes [82]. 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

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