Terephthalic acid methanol esterification

BHET formation is conducted at temperatures of 200 to 250 °C to achieve reasonable reaction rates. The activation energies of the two reactions are of the order of 25 000-30 000 cal/mol [4, 5], The BHET formation is usually conducted under pressure to keep the ethylene glycol in the liquid state. Terephthalic acid is slurried with ethylene glycol for the esterification reaction. Dimethyl terephthalate is dissolved in ethylene glycol and BHET for a liquid-phase transesterification reaction. The synthesis of BHET is driven to this material by the removal of water or methanol. The reactions are reversible at reasonable rates if the concentrations of water or methanol reactants are held high. 

High purity DMT is produced by esterification ofTPA. That is, the terephthalic acid is reacted with methanol to form the ester (actually a di-ester), in Figure 18-4. 

Commodity Phthalate Esters. The family of phthalate esters are by far the most abundantly produced worldwide. Both orthophthalic and terephthalic acid and anhydrides are manufactured. The plasticizer esters are produced from these materials by reaction with an appropriate alcohol (eq. 1) terephthalate esterification for plasticizers is performed more abundantly in the United States. Phthalate esters are manufactured from methanol (C ) up to C17 alcohols, although phthalate use as PVC plasticizers is generally in the range C4 to C13. The lower molecular weight phthalates find use in nitrocellulose the higher phthalates as synthetic lubricants for the automotive industries. 

On the basis of bulk production, polytethylenc lerephthalale) manufacture is the most important ester producing process. This polymer is produced by either the direct esterification of terephthalic acid and ethylene glycol, or hy the transesterilication of dimethyl terephthalutc with ethylene glycol. Dimethyl lerephthalale is produced by the direct esterification of terephthalic acid and methanol. 

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. 

Dimethyl terephthalate (DMT) is produced either by the esterification of terephthalic acid or the esterification of monomethyl terephthalate produced by oxidation of methyl p-toluate. DMT is consumed in the production of polyethylene terephthalate, the polymer used in the manufacture of polyester fibers, films and bottle resins. Terephthalic acid (TPA) is also used in the production of polyethylene terephthalate but does not consume methanol. Since TPA is continuing to increase its share of the market, DMT is expected to exhibit slower growth than the overall market for polyethylene terephthalate. 

C0-p-C6H4-C00CH2CLt20)n-, a synthetic polyester prepared by esterification of terephthalic acid with methanol and subsequent polycondensation of the ester with ethylene glycol. 

Liquid-phase esterification of terephthalic acid with methanol... 

The only practical means for ensuring the desired polymer quality is to use scrupulously pure monomers. Purification of a polymer after it is synthesized would be prohibitively expensive, because these materials are sparingly soluble and are often difficult or impossible to crystallize or free of solvent. The overall least expensive route to good quality polyfethylene terephthalate) was therefore through the dimethyl ester of terephthalic acid as shown in reaction (b). The byproduct methanol was recovered to generate more diester from the acid. In more recent years, methods have been developed to produce the diacid with satisfactory purity, and reaction (a) is now the preferred route to this polymer because the esterification step with methanol can be eliminated. Reactions (d), (e), and others, which the reader may be able to write, will be more expensive in the final analysis for the various reasons mentioned above. 

The discovery of poly(ethylene terephthalate), PET, in the 1940s [1,2] and its commercialization initially by DuPont and by ICI in the 1950s created a large market demand for terephthalic acid and terephthalate esters of polymer purity. Because dimethyl terephthalate, DMT, is readily purified by distillation [3] (and also because the p-xylene oxidation/esteiification intermediate, methyl p-toluate, is more readily kept in solution than is p-toluic acid) the polyester fibers and films industry was initially based on terephthalate ester. With the development of improved oxidation and purification technologies, purified terephthalic acid, TPA, became available in commercial quantities by the mid 1960s. Over 75% of the worldwide PET manufacture (total world PET capacity is over six million tons/year) is currently based on TPA rather than DMT [4]. This preference for TPA results from the less complicated esterification catalysis and the absence of methanol handling when the acid is used directly. 

The first commercial processes for the production of DMT made use of nitric acid oxidation of p-xylene to crude terephthalic acid, followed by esterification with methanol and purification by distillation [3]. Air oxidation of p-xylene displaced the use of nitric acid with the development of the Witten process [5]. In the Witten process, p-xylene is air-oxidized at 140-180 °C and 0.5-2 MPa over a homogeneous cobalt or cobalt/manganese catalyst system to give p-toluic acid, which is then esterified to methyl p-toluate, oxidized again over the cobalt/manganese catalyst, and finally esterified to DMT (see Scheme 1). The four process steps are accomplished in two reactors (see Figure 1). The Witten process uses no solvent. 

Para-xylene may be oxidized to terephthalic acid by means of nitric acid. Liquid-phase oxidation of m- and p-xylene is complicated by the Increased resistance to oxidation of the second methyl group after the first has been oxidized to the carboxyl group. As a consequence of experience with this difficulty, development has been toward oxidation in, two steps, a flrst to the toluic acid stage and a second to the dicarboxylic acid. Esterification of the first carboxyl group results in much easier oxidation of the coond methyl to a carboxyl group. Other p-substituted benzenes such as p-diisopropylbenzene are oxidized by air in the presence of a cobalt catalyst to terephthalic acid. Use is made of this in a recent new approach which permits the use of catalyzed air oxidation of p-xylene and results in formation of dimethyl terephthalate. A four-step process has attained commercial importance air oxidation of p-xylene to toluic acid using oil- Oluble catalysts of cobalt or manganese, esterification with methanol to methyl p-toluate, a second air oxidation to monomethyl terephthalate, and Anally esterification with methanol to dimethyl terephthalate. 

Dimethyl terephthalate is obtained by the esterification of terephthalic acid by methanol. Terephthalic acid is a high-melting (above 3Q0°C), insoluble material and requires special conditions for esterification. Two parts of methyl alcohol, 1 part of terephtiialic add, and 0.01 part of sulfuric acid are placed in a closed, a tated pr sure vessel and heated to 150°C for 2-3 hr. During the last hour, 5-6 parts of additional methyl alcohol is added slowly to the liquid reactants and distOled to remove the water of reaction. By cooling, the dimethyl terephthalate is completely separated from the solution. Yields run high, as 95 per cent. The dimethyl terephthalate can be purified by crystallization from high-boiling solvents, such as xylene, or it may be distilled. 

It was necessary to use the dimethyl ester because of the difficulties in purifying terephthalic acid caused by its poor solubility and high melting temperature. At present, terephthalic acid is being increasingly used in a direct esterification with ethylene glycol. This eliminates the expensive recovery of methanol in the transesterification. 

Because of the difficulty in produdng pure terephthalic add suitable for polymerization, in the early 1950 s processes were developed to produce dimethyl terephthalate. This involves first oxidizing p-xylene to p-methylbenzoic acid, followed by esterification of the first add group with methanol. The subsequent oxidation of the second methyl group can then be carried out more easily, producing terephthalic acid monomethyl ester, which is converted into the diester by further addition of methanol. 

Since the transesterification of dimethyl terephthalate (DMT) requires a methanol recovery plant, and the esterification of terephthalic acid (TPA) with ethylene glycol renders higher yields possible, processes to produce TPA are gaining a greater share of the manufacturing capacity, at the expense of DMT. 

Other commercial manufacturing methods have evolved to a direct esterification of acid and glycol in place of the ester-exchange process. In direct esterification, terephthalic acid and ethylene glycol are reacted rather than esterifying terephthalic acid with methanol to produce the dimethyl terephthalate intermediate. The ester is easier to purify than the acid, which sublimes at 300°C and is insoluble. However, better catalysts and purer... 

Poly(ethylene terephthalate) (abbreviated PET or PETE) is a semi-aromatic thermoplastic polyester obtained by condensation reaction of difunctional reactants and well-known for more than 60 years. PET is commonly produced by esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct or by transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. In order to obtain high molar masses polymers, solid-state polymerization is carried out. PET is one of the most important industrial polymers because of its excellent properties as tensile impact strength, chemical resistance, processability, clarity, thermal stability and others. The main applications of PET are fibers for textiles, films and bottles. Annual world PET production is around 60 millions tons. PET materials were manufactured using extrusion, injection molding and blow molding techniques. 

PBT can generally be produced by reacting 1,4-butanediol with an aromatic diester - dimethyl terephthalate (DMT) or diacid -terephthalic acid (TEA), in the presence of a polyesterification catalyst. Methanol is the major byproduct in the former route, while the water is the major byproduct in the latter route. Although polycondensation to high conversions implicitly requires stoichiometric balance of reacting groups, industrial processes for manufacturing polyesters such as PBT involve initial use of excess BD, which is later removed and recycled in the process. At the time when Pilati published his review [18], most publications described the trans-esterification between BD and DMT. Since that time, the majority of patents have been related to the direct polyesterification of BD by TEA [23-28]. 

The first oxidation step proceeds under conditions that lead only to the reaction of one methyl group and the monoacid is formed. To convert the second methyl group as well, it is necessary to form the monoadd methyl ester in the next step. After the second oxidation step the terephthalic dimethyl ester is formed under the same esterification conditions by reaction with methanol. Terephthahc dimethyl ester is later reacted directly with ethylene glycol to form the PET polymer with liberation of methanol. The total process provides a yield in terephthalic acid dimethyl ester of 85% with respect to p-xylene. 

PolyCethylene terephthalate) is produced by polycondensation of bis(hydroxy-ethyl)terephthalate (BHET) or its oligomers. BHET may be synthesized both by the reaction of dimethyl terephthalte (DMT) and ethylene glycol (EG) and by the direct esterification of EG with terephthalic acid. Although direct esterification has recently gained importance, the DMT method remains the main process for obtaining BHET. This latter process provides the formation of DMT solution in EG, the transesterification of DMT with EG and distillation of methanol with formation of BHET, and finally, BHET polycondensation. The DMT EG molar ratio is 1 2 with an excess of EG (0.2 to 0.5 mol). Preheating of EG to 120 to 160°C and introduction of DMT in the molten state shortens the DMT dissolving time. 

Terephthalic acid (TA) is a starting material for the manufacture of pofyesters, used particularly in fiber manufacture. TA can be produced commercially by hydrofysis of dimethyl terephthalate, the latter compound produced by oxidation of p-)Q lene. Oxidation of / -xylene is achieved in the presence of Co/Mn salt catalysts that yield/7-toluic acid. The oxidation is performed at 140170°C and 0.40.8 MPa pressure. Esterification of the tohxic acid with methanol at 250280°C and 22.5 MPa yields TA. Altemativefy, the oxidation is performed in the presence of a bromine promoter that results in the oxidation of both methyl groups to yield TA [121]. 

Xylenes are used inter alia as solvents, for example in the USA 180,000 t p.a. [13] and in Japan 90,000 t [33], particularly in the paint and printing ink industries (see also [5]). In eastern Europe it is also used for numerous applications in the shoe industry, o-xylene (besides naphthalene) is mainly oxidized to phthalic anhydride from which dyestuffs, phthalodinitrile and - by esterification with alcohols - plasticizers and raw materials for paints and varnishes are obtained [36, 38]. p-xylene is oxidized and processed with methanol to terephthalic acid dimethyl ester, from which polyesters are made [36, 38, 39]. For example, in 1972 2.9 million t polyester fibres were produced worldwide [39] and in 1979 more than 5 million t, which corresponds to 2.8 million t of p-xylene [30 a]. Polystyrene and copolymers (including expanded plastics) in particular are polymerized from styrene, and the world annual production of these products around 1975 was about 5 million t [39, 40],... 

Herm/es/Djnamit JS obe/Process. On a worldwide basis, the Hercules Inc./Dynamit Nobel AG process is the dorninant technology for the production of dimethyl terephthalate the chemistry was patented in the 1950s (67—69). Modifications in commercial practice have occurred over the years, with several variations being practiced commercially (70—72). The reaction to dimethyl terephthalate involves four steps, which alternate between liquid-phase oxidation and liquid-phase esterification. Two reactors are used. Eirst, -xylene is oxidized with air to -toluic acid in the oxidation reactor, and the contents are then sent to the second reactor for esterification with methanol to methyl -toluate. The toluate is isolated by distillation and returned to the first reactor where it is further oxidized to monomethyl terephthalate, which is then esterified in the second reactor to dimethyl terephthalate. 

Esterification ofTerephthalicAcid. Esterification of terephthaUc acid is also used to produce dimethyl terephthalate commercially, although the amount made by this process has declined. Imperial Chemical Industries, Eastman Kodak, Amoco, Toray, Mitsubishi, and Mitsui Petrochemical have all developed processes. Esterification (qv) generally uses a large excess of methanol in a Hquid process at 250—300°C. The reaction proceeds rapidly without a catalyst, but metal catalysts such as zinc, molybdenum, antimony, and tin can be used. Conversion to dimethyl terephthalate is limited by equiHbrium, but yields of 96% have been reported (75,76). 

An alternative route to DMT was introduced in 1953. This was based on air oxidation of y -xylene to /Moluic acid, which was esterified by methanol to form methyl /Moluate, which was oxidised by air to monomethyl terephthalate [40], which in turn was esterified by methanol to make DMT. The two oxidations could be combined so that p-xylene and methyl p-toluate were oxidised in the same vessel, and so could the two esterifications [41], The process was due to Katzschmann of Imhausen, a firm based at Witten and later known as Chemische Werke Witten. This process, known variously by its inventor s name and by various combinations of the names of the companies involved in its development, i.e. Hercules, Imhausen, Witten, and Dynamit Nobel, rapidly replaced the rather unsatisfactory and sometimes hazardous nitric acid oxidation route to DMT. 

Methyl benzoate, [93-58-3], CTfCOOCI Ig, bp, 198—200°C at 101.3 kPa d [ , 1.094 n], 1.5205. Insoluble in water, this is a colorless, transparent liquid solidifying at about 15°C. Methyl benzoate is prepared by the direct esterification of benzoic acid and methanol. It is used in the fragrance industry and in the production of other benzoate esters (via transesterification). A technical-grade methyl benzoate is available as a by-product in the manufacture of dimethyl terephthalate [120-61 -6]. 

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. 

Major end uses for methanol are for the production of formaldehyde, about 30%, which is used for the preparation of phenol-formaldehyde resins. About 20% is used for the production of methyl -butyl ether, which is used as an additive alone, and in blends with methanol as a fuel component. Further uses are for the esterification of terephthalic, and acrylic acids, and for acetic acid preparation, about 10% each. 

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