Amoco/MC process

In the late 1950 s two groups - one at ICI (ref. 1) and the other at the Mid-Century Corporation (ref. 2) - independently discovered that p-xylene is oxidized to terephthalic acid in almost quantitative yield when soluble bromides are used together with cobalt and manganese catalysts in acetic acid solvent at temperatures > 130 °C (ref. 3). This discovery formed the basis for what became known as the Mid-Century process and later, when the Mid-Century Corporation was acquired by Amoco, as the Amoco MC process for the commercial production of terephthalic acid. A large part of the ca. 6 million tons of the latter that are manufactured annually, on a worldwide basis, are produced via this method. This makes it the most important catalytic oxidation process (ref. 4). 

In the absence of bromide ion the p-xylene undergoes rapid autoxidation to p-toluic acid but oxidation of the second methyl group is difficult, due to deactivation by the electron-withdrawing carboxyl group, and proceeds only in low yield at elevated temperatures. Although bromide-free processes were subsequently developed (ref. 5) they require the use of much higher amounts of cobalt catalyst and have not achieved the same importance as the Amoco-MC process. Indeed, the... 

In Figure 3 the merits of the two processes for p-xylene oxidation are compared. The main disadvantages of the Eastman Kodak/Toray cooxidation method are the need for a cosubstrate (acetaldehyde of methylethylketone) with concomitant formation of a coproduct (0.21 ton of acetic acid per ton product) and high catalyst concentration. The Amoco MC process, on the other hand, has no coproduct and much lower catalyst concentrations but has the disadvantage that the bromide-containing reaction mixture is highly corrosive, necessitating the use of a titanium-lined reactor. 

Partenheimer has noted (ref. 15) that ca. 270 different substrates have been successfully oxidized using the Co/Mn/Br system. Some examples of aromatic di-and tricarboxylic acids that are commercially produced using the Amoco MC process are shown in Figure 4. 

Hundreds of different carboxylic acids have been produced via this method (1), The industrial process, dubbed the Amoco MC process, produces billions of pounds of terephthalic add, isophthalic add and trimellitic add annually. Industrial processes using just cobalt as the catalyst rather than Co/Mn/Br have also been developed (2-3). The characteristics of the reaction suggest that it is, at least partially, a free radical chain mechanism involving peroxy... 

In the Amoco/MC process terephthalic acid (TPA) is produced by aerobic oxidation of p-xylene. This bulk chemical (>10xl061 a-1) is chiefly used for poly-... 

Mechanism of the Amoco-MC Process (Sheldon and Kochi 1981 Partenheimer 1995) The discussion that follows is based on an exhaustive review by Partenheimer (1995) of metal-Zbromide-promoted auto-oxidation of hydrocarbons. The homolytic oxidation process has three steps commonly encountered in free radical-assisted reactions. Considering toluene as the simplest methylbenzene and I as a radical initiating species, the steps involved have been listed as follows ... 

Reactor Selection for the Amoco-MC Process Majority of liquid-phase oxidations use relatively high pressure (>0.5 MPa). Oxidations are characterized by a substantial heat of reaction that must be removed to prevent side reactions. Therefore, the reactor selected should have good heat transfer characteristics. In the present case of p-xylene oxidation using the Amoco-MC process, the heat of reaction is generally removed by evaporation of the solvent. The solvent vapor is condensed and returned to the reactor. The enthalpy of condensation of the solvent is used to raise high-pressure steam for use elsewhere in the plant The tanperature-pressure condition is auto-related by the refluxing media. 

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