Oxidation of p-xylene

Crystallizes in colourless needles m.p. 300° (sublimes). Manufactured by the oxidation of p-xylene and used in the production of Terylene (see also polyesters). U.S. production 1980 2-05 megatonnes. 

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]. 

You will recognize the side chain oxidation of p xylene to terephthahc acid as a reaction type discussed previously (Section 11 13) Examples of other reactions encoun tered earlier that can be applied to the synthesis of carboxylic acids are collected m 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% ... 

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. 

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxides... 

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxide catalysts was studied. While WO3 was active in p-xylene conversion but non-selective for TPAL formation, addition of Sb decreased the activity in p-xylene conversion but increased TPAL selectivity significantly. Structure change was also induced by Sb addition. Evidences from various characterization techniques and theoretical calculation suggest that Sb may exist as various forms, which have different p-xylene adsorption property, reactivity toward p-xylene and TPAL selectivity. Relative population of each species depends on Sb content. 

Selective oxidation of p-xylene was carried out over the temperature range of 450-590°C at an atmospheric pressure using an 8-channel parallel tubular reactor system made in-... 

WO3 was active in selective oxidation of p-xylene conversion but was not selective for... 

In liquid phase aerobic oxidation of p-xylene in acetic acid to terephthalic acid, it is important to eliminate the inherent hazards of this fuel-air mixture. Effects of temperature, pressure and presence of steam on the explosive limits of the mixture have been studied. 

The color of the final product primarily depends on the qualification of the raw materials, TPA, DMT and EG. The content of heavy metals in TPA, residues of catalysts employed during oxidation of p-xylene, and polymer processing affect the final color of the polymer. The tendency of certain catalysts, such as titanium or tin derivatives, to make the polyester yellowish in color is well established. The conversion during esterification is prolonged due to larger TPA particles or their hardness. Color can be influenced by these factors, as well as by chemical impurities in the raw materials, such as water, aldehydes or the quality of insufficiently recovered EG. Similar effects on color can be observed as a result of impurities caused by additives, particularly from less purified Sb2C>3. The quality of the latter can be assessed simply by the color of its solution in EG. 

Figure 15.10. Abridged reaction scheme for catalytic, radical oxidation of p-xylene...

The original route from p-xylene was oxidation in the presence of nitric acid. But the use of nitric acid is always problematical. There are corrosion and potential explosion problems, problems of nitrogen contamination of the product, and problems due to the requirement to run the reactions at high temperatures. Just a lot of problems that all led to the development of the liquid air phase oxidation of p-xylene. Ironically the nitrogen contamination problem was the reason that the intermediate DMT route to polyester was developed, since that was easy to purify by distillation. Subsequently, DMT has secured a firm place in the processing scheme. 

The TPA process. The technology involves the oxidation of p-xylene, as shown already in Figure 18—2. The reaction takes place in the liquid phase in an acetic acid solvent at 400°F and 200 psi, with a cobalt acetate/ manganese acetate catalyst and sodium bromide promoter. Excess air is present to ensure the p-xylene is fully oxidized and to minimize by-products. The reaction time is about one hour. Yields are 90—95% based on the amount of p-xylene that ends up as TPA. Solid TPA has only limited solubility in acetic acid, so happily the TPA crystals drop out of solution as they form. They are continuously removed by filtration of a slipstream from the bottom of the reactor. The crude TPA is purified by aqueous methanol extraction that gives 99 % pure flakes. 

Peracids form as transient species from the oxidation of benzaldehyde during autoxidation. For convenience we have chosen m-chloroperbenzoic acid (MCPBA) as our oxidant since this would be similar to the peracid formed from the very important intermediate 4-carboxybenzaldehyde formed during the oxidation of p-xylene (2). MCPBA would be formed in very low concentrations during oxidation hence we normally study the reaction of MCPBA with an excess of catalyst components i.e. MCPBA < pseudo first order conditions). The sequence of reactions that occurs when MCPBA is reacted with Co(II), Mn(II), and HBr has been previously discussed by Jones (9) in the presence of 5% water in acetic acid. We have repeated much of this work in 10% HjO/HOAc solutions and in general agree with his findings when one accounts for differences in temperatures, concentrations, and water concentrations. 

Terephthalic Acid. The oxidation of p-xylene to terephthalic acid is a more difficult reaction.866 Once one of the methyl groups is oxidized, the second methyl group in p-toluic acid formed is deactivated by the electron-withdrawing effect of the carboxylic group. Bromine-promoted catalytic processes and cooxidation in the presence of suitable additives are usually practiced.970,978,990... 

In the manufacture of terephthalic acid by the oxidation of p-xylene, separation of the xylene from its isomeric mixture is necessary (see Section 2.5.2). An alternative process introduced in Japan uses the oxidation of p-tolualdehyde, which is obtained in good regioselectivity by the HF—BF3 catalyzed carbonylation of toluene without the necessity of separation of the isomers. 

A new reagent, AMiydroxyphthalimide combined with Co(acac)n (n = 2,3), transforms alkylbenzenes to ketones, whereas methylbenzenes give the corresponding carboxylic acids.1121 Phthalimide N-oxyl was found to be the key intermediate. Novel oxoperoxo Mo(VI) complexes mediate the cost-effective and environmentally benign oxidation of methylbenzenes to carboxylic acids.1384 Similar green oxidation of p-xylene to terephthalic acid was reported by using a Ru-substituted heteropolyanion.1385... 

P. de Radzitzky Dr. Kamiya has observed a synergistic effect when manganese is added to the catalytic system Br/Co during the oxidations of p-xylene, cumene, and ethylbenzene, but no effect was observed for Tetralin or dodecane. Although the mechanism advanced by the author has possibly some validity, I think that the explanation is much simpler. 

The heavy metal-catalyzed oxidation of p-xylene to terephthalic acid in stirred autoclaves was greatly accelerated by adding catalytic quantities of nitric acid to the acetic acid solvent (9). Reactions carried out for 2 hours at 200°C. afforded 60-80% yields of terephthalic and p-toluic acids in varying ratios the highest yield of terephthalic acid was 70%. In the absence of either the heavy metal catalyst or nitric acid under otherwise identical conditions p-toluic acid was produced in 20-40% yields, but no terephthalic acid was formed. 

The rate of oxidation of p-xylene with the same concentration of cobalt ion with bromide ion (Figure 6) was six times as fast as without bromide (Figure 7). 

This is one of the most important industrial oxidation processes. Terephthalic acid (TPA) is mostly used for the manufacture of polyester fibers, films and plastics, and its world production capacity reaches 8 Mt/year. Two major processes have been developed. The Amoco-Mid Century process produces terephthalic acid by the one-step oxidation of p-xylene in acetic acid, whereas the Dynamit Nobel process yields dimethyl terephthalate in several steps and in the absence of solvent.83,84,86... 

Since alkylation greatly reduces the ionization potential of benzene derivatives it was hoped that alkylated benzene cations could be prepared by oxidation in sulphuric acid (Bolton and Carrington, 1961b Hulme, unpublished results). However, although oxidation of p-xylene... 

Air and Acetic Acid. The oxidation of p-xylene in acetic acid to form terephthalic acid can result in an explosion.3... 

Peracetic acid can also be formed directly by liquid-phase oxidation at 5 to 50°C with a cobalt salt catalyst. Nitric acid oxidation of acetaldehyde yields glyoxal and the oxidation of p-xylene to terephthalic acid and of ethanol to acetic acid is activated by acetaldehyde. 

Terephthalic acid (boiling point 300°C) and dimethyl terephthalate (melting point 141°C) are derived from p-xylene by oxidation of p-xylene in acetic acid as a solvent in the presence of a variety of catalysts such as cobalt and manganese salts of heavy metal bromides as catalysts at 200°C and 400 psi (Fig. 1). 

The oxidation of p-xylene to terephthalic acid (Table 4, entry 33) would only result in the formation of p-toluic acid. To convert the second group in high yield and selectivity, the oxidation must be influenced in one of three ways ... 

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