Terephthalic acid Amoco process

Several large-scale industrial processes use homogeneous catalysts [e.g., hydrofor-mylation, hydrocyanation (DuPont), ethene-oligomerization (SHOP), acetic add (Eastman Kodak), acetic acid anhydride (Tennessee-Eastman), acetaldehyde (Wacker) and terephthalic acid (Amoco)] as well as smaller scale applications [e.g., metolachlor (Novartis), citronellal (Takasago), indenoxide (Merck) and glycidol (ARCO, SIPSY)]. 

Other processes for manufacturing dimethyl tercphthalate pass through the intermediate of fixe terephthalic acid Amoco, Du Pont, Eastman Kodak, ICl (Imperial Chemical Industries). Mitsubishi, Mitsui, Petrochemicals. Toray, etc) which is esterified around 250 to 300°C in the presence of catalysts based on antimony, tin, molybdenum, zroc, etc, with molar yields of 96 per cent and complete conversion, provided that water is removed as soon as it is formed. 

Purified terephthahc acid became commercially available from Amoco Chemical Co. in 1965, by which time a considerable polyester industry based on dimethyl terephthalate had already developed. The Amoco process involves purification of cmde terephthahc acid by a separate step to attain the high product purity required for polyester manufacture. The Amoco technology is the most-used worldwide, but other processes have been developed and are operating commercially. 

Fig. 4. The Amoco purification process for polymer-grade terephthalic acid.

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). 

Amoco Amoco Chemicals Company, a subsidiary of Amoco Corporation, formerly Standard Oil Company (IN), is best known in the chemicals industry for its modification of the Mid-Century process for making pure terephthalic acid. /7-Xylene in acetic acid solution is oxidized with air at high temperature and pressure. Small amounts of manganese, cobalt, and bromide are used as catalysts. The modification allows the use of terephthalic acid, rather than dimethyl terephthalate, for making fiber. The process can also be used for oxidizing other methylbenzenes and methylnaphthalenes to aromatic carboxylic acids. See also Maruzen. 

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

Mid-Century Also called M-C. A process for oxidizing p-xylene to terephthalic acid, using oxygen in acetic acid and catalyzed by a mixture of cobalt and manganese bromides. Developed in the 1950s by Halcon International and commercialized by Standard Oil Company (Indiana). The first plant was built at Jolet, IA, in 1938. The Amoco and Maruzen processes are improved versions. 

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. 

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. 

As outlined above a purity of only 99.5% is not sufficient for a polymer feedstock and the mono-acid intermediate has to be removed either by oxidation under more forcing conditions (Mitsubishi) or reduction (Amoco). In the Amoco process (not shown) the crude di-acid is dissolved in water (275 °C) (15 % weight) and hydrogenated over a palladium on carbon catalyst. The intermediate 4-formylbenzoic acid is hydrogenated to 4-toluic acid which has a much higher solubility and does not cocrystallise with terephthalic acid. The solution is carefully cooled while the product crystallises and the by-product remains in the water. The final content of 4-formylbenzoic acid is as low as 15... 

Prior to polymerization, p-xylene is first oxidized to terephthalic acid (TA) or dimethyl terephtalate (DMT). These diacid or dimethyl ester monomers are then polymerized via a condensation reaction with ethylene glycol to form the polyester. Prior to the development of a method to purify TA to make purified terephtahc acid (PTA, >99% pure) by the Mid-Century Corporation in the 1950s [10], DMT was the primary way to obtain the purified dicarboxylate. The Amoco Oil Company, now part of BP International, made several improvements to the PTA process since its inception [11]. Since the advent of the availability of PTA, it has become the monomer of choice over DMT. PTA avoids the complications of including methanol to enable purification and handling the methanol evolved during the polymerization to polyester. 

Terephthalic acid is commonly abbreviated TA or TPA. The abbreviation PTA (P = pure) is reserved for the product of 99% purity for polyester manufacture. For many years polyesters had to be made from dimethyl terephthalate (DMT) because the acid could not be made pure enough economically. Now either can be used. TA is made by air oxidation of /7-xylene in acetic acid as a solvent in the presence of cobalt, manganese, and bromide ions as catalysts at 200°C and 400 psi. TA of 99.6% purity is formed in 90% yield. This is called the Amoco process. 

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... 

The Dynamit Nobel process produces dimethyl terephthalate (DMT) by a complicated series of oxidation and esterification stages (equation 241).83,84,86 In the oxidation section, p-xylene is oxidized at 150°C and 6 atm without solvent and in the presence of cobalt octoate to TPA and p-toluic acid. These oxidation products are sent to another reactor for esterification by methanol at 250 °C and 30 atm. Fiber grade DMT is purified by several recrystallizations, and monoesters are recycled to the oxidation reactor. The overall yield in DMT is about 80%, which is lower than in the Amoco process. However, this process is competitive because it is not corrosive and requires lower investments. It provides high-quality fiber-grade dimethyl terephthalate. 

Commercially two main processes, that of Mid-century/Amoco and Dynamit Nobel/Hercules, are operated. In the former acetic acid is used as a solvent. Mixtures of cobalt and manganese bromide and acetate salts are used to catalyze the initiation step. The reaction conditions, a temperature of about 220°C and a pressure of 15 atm, are relatively severe. Under these conditions bromine and CH2C02H radicals are formed. These radicals can effect new initiation steps. In the overall process, though toluic acid is an intermediate, it is never isolated. The final isolated product is terephthalic acid (see reaction 8.10). 

The advantage of the Amoco process is that high-purity terephthalic acid is produced in one step. The solubility of terephthalic acid in acetic acid is low its separation with high purity by crystallization is therefore relatively easy. However, the corrosive nature of the acids and the relatively drastic conditions make it necessary to use special material of construction for the reactors. 

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-... 

The syntheses of terephthalic acid and DMT are basically similar but, during the early years of the polyester industry, in the 1950s and 1960s, DMT could be made in a purer form than the acid and was the preferred raw material. In the 1960s however with the development of the Amoco Mid-Century process, terephthalic acid gained acceptance and, since then, it has become the preferred feedstock (based on cost and performance) so that today nearly all of the new PET capacity is based on it. Thus we consider only the synthesis of terephthalic acid in this section. 

The AMOCO Chemical Corp. developed from the late 1950s until the middle of the 1960s, a liquid-phase oxidation process [3f, 4], performed at 190-205 °C and 1.5-3 MPa, to obtain in one step fiber-grade terephthalic acid (99.99 %) reaching... 

The dominant current commercial process for the production of isophthalic acid (lA) and terephthalic acid (TA) is the catalytic oxidation of m-xylene and p-xylene (PX). Xylenes are oxidized to lA or TA, respectively, in the so called Amoco process at 110-205 °C and 15-30 bar and in the presence of 95% acetic acid [128]. Furthermore, Co and Mn need to be added as catalysts and NIttBr and tetrabromoethane as co-catalysts. It is important to realize that some routes towards bio-based lA and TA may involve the production of the xylenes as an intermediate, whereas others might not. Furthermore, production routes may rely on technologies generating a slate of products comprising mostly fuel components next to chemicals like PX, whereas other technologies rely on the dedicated production of PX. 

We will begin with the carbonylation of Mel which in situ is generated from MeOH for acetic acid production because of its industrial importance. Acetic acid is an important chemical commodity with a wide range of appUcations in organic chemistry. In organic synthesis, acetic acid is mainly used as a raw material for vinyl acetate monomers and acetic anhydride synthesis, as well as a solvent for producing terephthalic acid from xylene via the oxidation process. In 1998 the world s capacity of acetic acid production was approximately 7.8 milUon tons, of which more than 50 % were produced by BP-Amoco and Celanese. 

An emphasis throughout this Chapter (and indeed in much of the literature on homogeneous catal)dic oxidations) is on selectivity in product formation using 0-atom transfer mechanisms, and indeed this is a worthy aim. However, it should be noted that the major industrial homogeneous catalytic process (from an economic standpoint) currently in operation is the O -oxidation of p-xylene to terephthalic acid (the Mid-Century Amoco process), which is catalyzed by an inexpensive mix of Co and Mn acetates and bromides in acetic acid, the system operates at 225°C under 15 atm Oj. This is a radical-initiated process that has been in operation for over 30 years, and the details of the mechanistic pathways continue to be studied . In essence, the system has been fine-tuned from an empirical point of view, and is a classic case of the practical industrial approach versus the basic academic approach a mechanistically messy process but one that is commercially extremely important... 

Figure 12.157 Production processes of terephthalic acid, including Amoco Process...

---