Xylene, terephthalic acid

Phthalic acid (1,2-benzene dicarboxylic acid), isophthalic acid (1,3-benzene dicarboxylic acid), and terephthalic acid (1,4-benzene dicarboxylic acid) are made by the selective oxidation of the corresponding xylenes. Terephthalic acid may also be produced from the oxidation of naphthalene and by the hydrolysis of terephthalonitrile. 

Bio-PET and bio-PBT Ethanol, isobutanol Ethylene, MEG, /7-Xylene, terephthalic acid... 

Decarbonylation of aromatic aldehydes proceeds smoothly[71], Terephthalic acid (86), commercially produced by the oxidation of p-.xylene (85), contains p-formylbenzoic acid (87) as an impurity, which is removed as benzoic acid (88) by Pd-catalyzed decarbonylation at a high temperature. The benzoic acid produced by the decarbonylation can be separated from terephthalic acid (86) based on the solubility difference in water[72]. 

Technical-Grade Terephthalic Acid. All technical-grade terephthahc acid is produced by catalytic, hquid-phase air oxidation of xylene. Several processes have been developed, but they all use acetic acid as a solvent and a multivalent heavy metal or metals as catalysts. Cobalt is always used. In the most popular process, cobalt and manganese are the multivalent heavy-metal catalysts and bromine is the renewable source for free radicals (51,52). 

Fig. 2. Terephthalic acid production by catalytic, liquid-phase air oxidation of -xylene.

In many cases simple esters are more easily purified than their intractable parent acids. For example, /7-xylene is difficult to obtain in pure form because of the closeness of its boiling point to the other isomeric xylenes (o-144°C m- 138.8°C p- 138.5°C). It is thus difficult to produce terephthalic acid, which sublimes at 300°C, in a pure form. 

Terephthalic acid is made by air oxidation of /i-xylene in acetic acid with cobalt and manganese salts of metal bromide at 200 °C and 400 tit. 

The carboxylic acid produced in the greatest fflnounts is 1,4-benzenedicarboxylic acid (terephthalic acid). About 5 X 10 Ib/year- is produced in the United States as a starting material for the preparation of polyester fibers. One important process converts p-xylene to terephthalic acid by oxidation with nitric acid ... 

You will recognize the side-chain oxidation of p-xylene to terephthalic acid as a reaction type discussed previously (Section 11.13). Examples of other reactions encountered earlier that can be applied to the synthesis of carboxylic acids are collected in Table 19.4. 

The catalyzed oxidation of p-xylene produces terephthalic acid (TPA). Cobalt acetate promoted with either NaBr or HBr is used as a catalyst in an acetic acid medium. Reaction conditions are approximately 200°C and 15 atmospheres. The yield is about 95% ... 

Another approach is to use an easily oxidized substance such as acetaldehyde or methylethyl ketone, which, under the reaction conditions, forms a hydroperoxide. These will accelerate the oxidation of the second methyl group. The DMT process encompasses four major processing steps oxidation, esterification, distillation, and crystallization. Figure 10-16 shows a typical p-xylene oxidation process to produce terephthalic acid or dimethyl terephthalate. The main use of TPA and DMT is to produce polyesters for synthetic fiber and film. 

A similar oxidation is employed industrially for the preparation of the terephthalic acid used in the production of polyester fibers. Approximately 5 million tons per year of p-xylene are oxidized, using air as the oxidant and Co(lll) salts as catalyst. 

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). 

Consequently, as a result of increasing environmental pressure many chlorine and nitric acid based processes for the manufacture of substituted aromatic acids are currently being replaced by cleaner, catalytic autoxidation processes. Benzoic acid is traditionally manufactured (ref. 14) via cobalt-catalyzed autoxidation of toluene in the absence of solvent (Fig. 2). The selectivity is ca. 90% at 30% toluene conversion. As noted earlier, oxidation of p-xylene under these conditions gives p-toluic acid in high yield. For further oxidation to terephthalic acid the stronger bromide/cobalt/manganese cocktail is needed. 

In the oxidation of p-xylene the first methyl group undergoes rapid autoxidation to afford p-toluic acid (Fig. 8). The second methyl group is, however, deactivated by the electron-withdrawing carboxyl group, and further oxidation of p-toluic to terephthalic acid is much slower, i.e. the relative reactivities of toluene and p-toluic acid are 26 1 (Fig. 8). It is not surprising, therefore, that the autoxidation of p-xylene to terephthalic acid proved to be a difficult proposition. 

Because orr/zo-xylene is more readily isolated and purified (by distillation), it costs less than para-xylene. Like all petrochemicals, prices depend on the price of crude oil but in early 2001, mixed-xylene was about 17 cents/lb while para-xylene was only about 15 cents due to high manufacturing capacity and low demand for use for making terephthalic acid. In the extremely high volumes in which such chemicals are sold, fractions of a penny difference in price can be very important. 

Terephthalic acid (p-TA or TA), a raw material for polyethylene terephthalate (PET) production, is one of the most important chemicals in petrochemical industry. Crude terephthalic acid (CTA), commonly produced by homogeneous liquid phase p-xylene oxidation, contains impurities such as 4-carboxybenzaldehyde (4-CBA, 2000-5000 ppm) and several colored polyaromatics that should be removed to obtain purified terephthalic acid (PTA). PTA is manufactured by hydropurification of CTA over carbon supported palladium catalyst (Pd/C) in current industry [1]. 

An example of this is the commercial process for preparing puru-xylene, the precursor to terephthalic acid, which is polymerised to give polyjethy-lene terephthalate) (PET). In this case, the mixture of xylenes obtained from crude oil is reacted in a zeolite (known as HZSM5). The relative rates of diffusion in and out of the pores are sufficiently different (by a factor of about ten thousand) to allow the extremely efficient and selective conversion of all the isomers to the desired paia isomer, which is the narrowest and can thus move through the structure most rapidly (Figure 4.3). 

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