Terephthalic acid from p-xylene

Liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes,3 "16 including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p-xylene, acetaldehyde and vinyl acetate from ethylene, propylene oxide from propylene, and many others. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. 

Maruzen (1) A process for making terephthalic acid from p-xylene. Similar to the Amoco process but yielding a purer product in one stage. Operated in Japan by Matsuyama Chemical Company. 

Production of acids (i) terephthalic acid from p-xylene, (ii) acetic acid from n-butane,... 

The primary example is the production of terephthalic acid from p-xylene which is used in the production of polyethylene(terephthalate). A recent review describes the oxygenation of at least 251 hydrogens to give 279 products using at least 35 different catalyst combinations. There are a number of reports which include cerium as one of the elements... 

Metal-ion catalysed air oxidations of aromatic compounds are industrially important. Oxidation of a methyl to a carboxylic acid group, such as in the first stage of the manufacture of terephthalic acid from p-xylene, is believed to involve peroxidation of benzylic carbon [formed in eqn (29)] (Andrulis et al., 1966). The reactions leading to benzyl acetates [eqns (29)-(31) for example] must therefore be carried out in the absence of air. Oxidations of alkylaromatics in the presence of oxygen and involving cation radical intermediates have been reported (Onopchenko et al., 1972 Holtz, 1972 Scott and Chester, 1972). 

Terephthalic acid from p-xylene Co(II), Mn(II) salts, bromide ion... 

This method has been licensed world-wide to prepare terephthalic acid from p-xylene. We have used the reaction of m-chloroperbenzoic acid (MCPBA) with mixtures of Co(II) acetate/ Mn(II) acetate/bromide in acetic acid/water solutions to understand the functions of each catalyst component. The sequence of redox reactions that occurs is first the reaction of MCPBA with Co(n) to give Co(III) Co(III) then oxidizes Mn(II) to Mn(III) and finally, Mn(III) oxidizes bromide to bromine. Some of the functions of each component are 1) the cobalt rapidly reacts (very selectively) with the MCPBA (Mn and Br react slowly), 2) Mn lowers the steady state of Co(III) which significantly reduces solvent decomposition and also avoids Co(III) re-arranging into a less reactive form, and 3) bromine reacts rapidly with the methylaromatic compound to generate methylaromatic radicals (Co(III) and Mn(III) react slowly). The dimeric structure of Co(II) in acetic acid is partly responsible for the highly selective nature of the MCPBA oxidation of Co(II). The order of the redox reactions is the opposit to that expected from thermodynamics. 

Whereas benzene and toluene serve as the raw materials for a wide range of products, applications for the three xylene isomers, o-, m- and p-xylene, are basically limited to chemicals arising through oxidation, i.e. phthalic anhydride (PA) from o-xylene, isophthalic acid from m-xylene and terephthalic acid from p-xylene. 

Fig. 22.45. Fiber-grade terephthalic acid from p-xylene and methanol. (Reproduced from Hydrocarbon Processing, p. 171, Nov. 1985 copyright 1985 by Gulf Publishing Co.)...

A variety of transition metal ions accelerate the oxidative degradation of the carbon-chain polymers by catalysing both the formation and the decomposition of hydroperoxides. Typically, cobalt-catalysed oxidation of hydrocarbons is used in the manufacture of terephthalic acid from p-xylene. These prooxidant reactions also accelerate the breakdown of polymer molecules to smaller fragments (see Fig. 12.2) but are effectively inhibited by metal deactivators. All antioxidants have some retarding effect, but the most effective are the peroxide decomposers (PD) that remove hydroperoxides as they are formed by ionic (non-free radical) reactions.Deactivated transition metal ions (e.g. 

The major commercial route to terephthalic acid which is suitable for the direct preparation of poly(ethylene terephthalate) is from p-xylene ... 

Meanwhile attempts to find an air oxidation route directly from p-xylene to terephthalic acid (TA) continued to founder on the relatively high resistance to oxidation of the /Moluic acid which was first formed. This hurdle was overcome by the discovery of bromide-controlled air oxidation in 1955 by the Mid-Century Corporation [42, 43] and ICI, with the same patent application date. The Mid-Century process was bought and developed by Standard Oil of Indiana (Amoco), with some input from ICI. The process adopted used acetic acid as solvent, oxygen as oxidant, a temperature of about 200 °C, and a combination of cobalt, manganese and bromide ions as catalyst. Amoco also incorporated a purification of the TA by recrystallisation, with simultaneous catalytic hydrogenation of impurities, from water at about 250 °C [44], This process allowed development of a route to polyester from purified terephthalic acid (PTA) by direct esterification, which has since become more widely used than the process using DMT. 

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). 

Dimethyl terephthalate is manufactured from terephthalic acid or directly from p-xylene. Esterification of terephthalic acid with methanol occurs with sulfuric acid as the acid catalyst. Direct oxidation of p-xylcnc with methanol present also produced dimethyl terephthalate copper salts and manganese salt are catalysts for this reaction. The dimethyl terephthalate (boiling point 288°C, melting point 141°C) must be carefully purified via a five-column distillation system. 

On the other hand, although o-phthalic acid, or rather its anhydride, had long been produced in enormous amounts for use in the manufacture of alkyd resins, the para derivative was less well known and not available on a large scale. The synthesis is a straightforward one, however, from p-xylene, which is oxidized to terephthalic acid, either by means of nitric acid in the older process or by air (catalyzed) in the newer one. In the early years this compound then was converted to the easily purified dimethyl ester in order to obtain a colorless polymer adequate for the manufacture of commercially acceptable fibers. 

Several other methods were developed for producing the desired dimethyl terephthalate. The Witten (Hercules) process goes from p-xylene to toluic acid by oxidation of one of the methyl groups on the ring, following which the carboxyl group is esterified with... 

Terephthalic acid (TPA), a raw material in the manufacture of polyester fiber, film, and soft drink bottles, is synthesized from p-xylene (PX) in the process shown below. 

The pressure is released on the exit stream, which simultaneously flashes off much of the excess p-xylene and the acetic acid, and cools the residual solution causing terephthalic acid to crystallize out. Residual acetic acid and p-xylene are removed from the crystals by centrifugation. The crude product acid is then slurried in hot water first for washing, and then for hydrogenation to decolorize any residual traces of colored impurities. It is then recrystallized and dried to give fiber-grade material (m.p. >300 °C) in yields of about 90%. 

In 1968 U.S. production of p- and o-xylene was estimated at 1.3 billion and 0.97 billion pounds per year respectively. Production figures for m-xylene have never been published by the Tariff Commission. Its use has, however, remained quite small in relation to p- and o-xylene. It is expected that domestic demand for both p- and o-xylene will continue to increase. Terephthalic acid is the key component required for production of polyester film and fibers and is presently produced only from p-xylene. Phthalic anhydride is produced from both naphthalene and o-xylene. Although o-xylene is not expected to replace naphthalene entirely, its use for phthalic anhydride manufacture is expected to increase. 

The oxidation of alkylaromatic hydrocarbons proceeds particularly easily in the presence of both cobalt and bromide ions (a so-called cobalt-bromide catalysis ). Carboxylic acids are the final products of the reaction. For example, terephthalic acid is selectively formed from p-xylene, the whole process being used in the industrial production of the acid [Ik, 19]. Despite the large number of works on cobalt-bromide catalysis, its mechanism has long remained speculative. 

In addition to the use of p-xylene as a raw material in the production of tere-phthalic acid, processes were also operated in the past to produce terephthalic acid from toluene and phthalic anhydride. 

Resins with long-chain macromolecules obtained by polycondensation have thermoplastic properties. Polyesters ( Terylene ) and polyamides (Nylon) are examples of polycondensations. The synthetic fibre Terylene (known as Dacron in the USA) is a polyester formed by the reaction of ethylene glycol with terephthalic acid the latter is obtained from p-xylene by oxidation ... 

Molecular oxygen oxidizes toluene to benzoic acid and p-xylene to terephthalic acid via p-toluic acid. These reactions are catalyzed by [Co(OAc)3] or [Mn(OAc)3]. The Mn salts, less oxidizing than those of Co , catalyze the oxidation of these alkylaromatics, but not the oxidation of alkanes whose redox potentials are higher than those of alkylaromatics. It is probable that the M ions are regenerated from and the ArCH200" radicals (mechanism below). 

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]. 

The Sandoz company used the dibromoterephthalic acid method. This acid was made from p-xylcnc by brominating it to form 2,5-dibromo-p-xylene and then oxidising this to 2,5-dibromoterephthalic acid. Reaction of one mole of this acid with two moles of an arylamine in the presence of copper(II) acetate gives 2,5-bis(arylamino)terephthalic acid, which can be ring-closed to a linear quinacridone. Unsymmetrical substitution using two different arylamines is possible. 

Although the superior properties of PEN have been known for many years, the unavailability of the naphthalate monomer has delayed the development of commercial markets, until relatively recently (1995) when the Amoco Chemical Company offered high purity naphthalene-2,6-dimethyl dicarboxylate (NDC) in amounts of up to 60 million pounds per year. This diester is produced by a five-step synthetic route, starting from the readily available compounds, o-xylene and 1,4-butadiene [3], Prior to this, the NDC diester was obtained by extraction of 2,6-dimethylnaphthalene (DMN) from petroleum streams, where it was present in relatively low abundance. Oxidation of DMN to crude 2,6-naphthalene dixcarboxylic (NDA) is conducted by a similar process to that used for conversion of p-xylcnc to purified terephthalic acid (TA), crude NDA is esterified with methanol, and is then distilled to yield high purity NDC. Other companies (e.g. the Mitsubishi Gas Chemical Company) followed Amoco s introduction with lesser amounts of NDC. Teijin [4] has manufactured PEN for many years for its own captive uses in films. 

Aromatic polyesters had been successfully synthesized from the reaction of ethylene glycol and various aromatic diacids but commercialization awaited a ready inexpensive source of aromatic diacides. An inexpensive process was discovered for the separation of the various xylene isomers by crystallization. The availability of inexpensive xylene isomers allowed the formation of terephthalic acid through the air oxidation of the p-xylene isomer. DuPont produced polyester fibers from melt spinning in 1953, but it was not until the 1970s that these fibers became commercially available. 

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