Terephthalic acid, esterification

Reimschuessel, H. K. and Debona, B. T., Terephthalic acid esterification kinetics 2-(2-methoxyethoxy)ethyl terephthalates, J. Polym. Sci., Polym. Chem. Ed, 17, 3241-3254 (1979). 

Purified terephthalic acid (PTA) is one of the largest-scale chemicals produced worldwide. Main fields of application for PTA include PET chips, PET films, PET fibers, and bottle resins. Two principal processes are used for the purification of raw terephthalic acid esterification distillation and ester cleavage of dialkyl terephthalates, and direct hydrogenation and crystallization [90]. The latter process was discovered by the Standard Oil Co.and later licensed worldwide, becoming the dominant process for PTA. The general process is to hydrogenate... 

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. 

Polyesters are the most important class of synthetic fibers. In general, polyesters are produced by an esterification reaction of a diol and a diacid. Carothers (1930) was the first to try to synthesize a polyester fiber by reacting an aliphatic diacid with a diol. The polymers were not suitable because of their low melting points. However, he was successful in preparing the first synthetic fiber (nylon 66). In 1946, Whinfield and Dickson prepared the first polyester polymer by using terephthalic acid (an aromatic diacid) and ethylene glycol. 

Using excess ethylene glycol is the usual practice because it drives the equilihrium to near completion and terminates the acid end groups. This results in a polymer with terminal -OH. When the free acid is used (esterification), the reaction is self catalyzed. However, an acid catalyst is used to compensate for the decrease in terephthalic acid as the esterification nears completion. In addition to the catalyst and terminator, other additives are used such as color improvers and dulling agents. For example, PET is delustred hy the addition of titanium dioxide. 

Kemkes256 assumes that the overall order relative to the esterification of terephthalic acid by 1,2-ethanediol in oligo(l,2-ethanediyl terephthalate) is two no mechanism has however been suggested. Mares257 considers that during the esterification of terephthalic acid with 1,2-ethanediol, two parallel kinetic paths take place, one corresponding to a reaction catalyzed by non-dissociated add and the other to a non-catalyzed process. In fact, Mares257 is reserved about the existence of protonic catalysis. Some other orders were found for the system terephthalic atid/l,2-ethanediol 0 (overall)318 2 (add) andO (alcohol)203 1 (add) and 1 (alcohol)181 1 (add)194 . These contradictory results could be partly due to the low solubility of terephthalic acid in 1,2-ethanediol. 

In the case of the esterification of terephthalic acid with an excess of 1,2-ethanediol a zero overall order is obtained this is due to the control of the reaction rate by the dissolution of terephthalic add. 

PET is the polyester of terephthalic acid and ethylene glycol. Polyesters are prepared by either direct esterification or transesterification reactions. In the direct esterification process, terephthalic acid is reacted with ethylene glycol to produce PET and water as a by-product. Transesterification involves the reaction of dimethyl terephthalate (DMT) with ethylene glycol in the presence of a catalyst (usually a metal carboxylate) to form bis(hydroxyethyl)terephthalate (BHET) and methyl alcohol as a by-product. In the second step of transesterification, BHET... 

Polymerisation of a diol with a dicarboxylic acid is exemplified by the production of a polyester from ethylene glycol and terephthalic acid either by direct esterification or by a catalysed ester-interchange reaction. The resulting polyester Terylene) is used for the manufacture of fibres and fabrics, and has high tensile strength and resiliency its structure is probably ... 

Esterification of a dicarboxylic acid such as terephthalic acid via the diimidazolides with diols readily leads to the corresponding polyesters.[12]... 

Here the polymer grows by successive esterification with elimination of water and no termination step. Polymers formed by linking monomers with carboxylic acid groups and those that have alcohol groups are known as polyesters. Polymers of this type are widely used for the manufacture of artificial fibers. For example, the esterification of terephthalic acid with ethylene glycol produces polyethylene terephthalate. 

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 is relatively insoluble in EG or the monomer BHET. Experiments on the solubility of TPA in EG or BHET are difficult to evaluate, because at esterification temperatures the dissolution of TPA cannot be performed without its simultaneous reaction. Kang et al. [7] found that TPA is more soluble in EG than in BHET. This is contradicted by the data published by Baranova and Kre-mer, indicating a higher solubility of TPA in BHET [93] which agree with the principle like dissolves like . Data for the solubility of TPA in EG and BHET at different temperatures are summarized in Figure 2.24. 

Yamada, T., Effect of diantimony trioxide on direct esterification between terephthalic acid and ethylene glycol,./. Appl. Polym. Sci., 37, 1821-1835... 

Yamada, T. and Imamura, Y Simulation of continuous direct esterification process between terephthalic acid and ethylene glycol, Polym.-Plast. Technol. Eng., 28, 811-876 (1989). 

Krumpolc, M. and Malek, J., Esterification of benzenecarboxylic acids with ethylene glycol, IV. Kinetics of the initial stage of polyesterification of terephthalic acid with ethylene glycol catalyzed by zinc oxide, Makromol. Chem., 171, 69-81 (1973). 

Prepolymer produced via the terephthalic acid (TPA) monomer route shows an increased reactivity in comparison with that produced by the dimethanol tereph-thalate (DMT) monomer process [49], This behavior is possibly caused by the enhanced CEG content, which is usually higher in products from the TPA process as a result of insufficient conversion of the acid monomer in the esterification reaction (Figure 5.23). The increased reactivity may be caused by an autocatalytic influence of the carboxylic groups which seems to be disturbed by an unbalanced content of OH groups in the case of degradation. 

It has been disclosed [18] that the addition of terephthalic acid after the esterification stage is complete is responsible for a high reaction rate during the SSP process. In this case, the prepolymer has commonly 30-40 meq/kg of CEGs. These observations lead to the conclusion that an increased number of CEGs are... 

Bishydroxyethyl terephthalate (BHET) is the monomer used to make the PET polymer. BHET can be made either by the esterification of terephthalic acid (TPA) with ethylene glycol (EG) ... 

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. 

In the case of the esterification of the diacid, the reaction is self-catalyzed as the terephthalic acid acts as its own acid catalyst. The reverse reaction, the formation of TPA and EG from BHET is catalytic with regard to the usual metal oxides used to make PET, but is enhanced by either the presence of hydroxyl groups or protons. In the case of transesterification of dimethyl terephthalate with ethylene glycol, the reaction is catalytic, with a metal oxide needed to bring the reaction rate to commercial potential. The catalysts used to produce BHET are the same as those needed to depolymerize both the polymer to BHET and BHET to its simpler esters. Typically, titanium, manganese and zinc oxides are used for catalysts. 

The important question of comparative value, mentioned earlier in Section 6, now must be considered. The material output of each process per unit of feed is estimated and multiplied by the market price of the material to arrive at a value of product. The output of the methanolysis processes, DMT and EG, are shown as a methanolysis-type product value in Table 16.5. The stoichiometric ratios are adjusted with a presumed 99 % recovery of DMT and 93 % recovery of EG. The output of the hydrolysis processes, TPA and EG, have the stoichiometric ratios adjusted for a presumed 99 % recovery of TPA and 93 % recovery of EG. The glycolysis processes, including methanolysis/BHET hybrid, are valued at 99 % recovery of terephthalic acid, 95 % recovery of ethylene glycol, and a US 0.022/kg esterification credit for making BHET. The EG recovery is higher for glycolysis-type products because of less loss of useful moieties. The three... 

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. 

Monoesterification of dicarboxylic acids. Chemiabsorption of a dicarboxylic acid on alumina or silica can be used to effect selective esterification of one acid group with diazomethane. The method was demonstrated by conversion of terephthalic acid, C6H4-l,4-(COOH)2) into the monomethyl ester in quantitative yield. 

The largest commercial use of ethylene glycol is its reaction with dicarboxylic acids to form linear polyesters. Polyethylene terephthalate) [25038-59-9] (PET) is produced by esterification of terephthalic acid [100-21 -0] (1) to form bishydroxyethyl terephthalate [959-26-2] (BHET) (2). BHET polymerizes in a transesterification reaction catalyzed by antimony oxide to form PET (3). 

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. 

Polymerization is a two-stage process (Fig. 1) in which the monomer is first prepared either by an ester interchange between dimethyl terephthalate and ethylene glycol, or by direct esterification of terephthalic acid. 

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. 

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