Vapor-phase oxidation of aromatic hydrocarbons

The commercial production of maleic acid by oxidation of benzene has, in a large measure, been due to the early work of Weiss and Downs on this 

The stability of benzene and the fact that nine atoms of oxygen is required for the oxidation of a molecule of benzene to maleic anhydride necessitate the use of high air hydrocarbon ratios in the oxidation. Thus, the theoretical requirement would be about 106 cu ft of dry air at room temperature per pound of benzene oxidized to maleic anhydride. In practice, higher ratios than this are used, and yields of 60-75 lb of maleic acid per 100 lb of benzene reacted are obtained. This represents an efficiency of conversion of benzene to maleic acid of about 40-50 per cent. 

The heat theoretically liberated in the oxidation of benzene to maleic acid is about 10,500 Btu per lb,of benzene reacting, and the heat liberated in the complete combustion of benzene is approximately 18,000 Btu per lb. In practice, where 40 per eent or more of the benzene may undergo complete combustion during reaction, the heat liberated would be 13,500 Btu or more per pound of benzene reacted. It is imperative that this reaction heat be removed from the catalyst zone and that the catalyst tempterature be maintained at the proper operating level. Special means have been provided for this in commercial opteration. 

A variety of catalysts have been pmtented for this oxidation, and the oxides of metals of the fifth and sixth groupis of the pieriodic system have been particularly stressed. Of these, vanadium p entoxide has been shown to be one of the best when used as a single-eomponent catalyst supported on an inert carrier such as aluminum turnings or diatomaceous earth. Vanadium pentoxide becomes active in promoting the oxidation at a temperature of about 300°C and reaches its maximum usefulness in the rangp of 400-500°C. 

Toluene. The oxidation of hydrocarbons having an aromatic nucleus and one or more side chains may be effected in the side chain without marked rupture of the ring itself, since each component behaves more or less as it would if it alone constituted the major part of the molecule. Thus, the ring component exhibits the characteristic stability of the aromatic compounds, and the aliphatic substituent shows the relatively greater ease of oxidation of the aliphatic hydrocarbons. Under specific conditions, oxidation of such substituted aromatic hydrocarbons may be controlled to give satisfactory yields of the side-chain products viz., toluene may be oxidized to benzaldehyde or benzoic acid o-xylene, to phthalic anhydride ethylbenzene, to benzoic acid etc. 

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