Terephthalic acid

Terephthalic acid

Names
Other names Benzene-1,4-dicarboxylic acid para-Phthalic acid TPA PTA BDC
Identifiers
CAS Number	100-21-0 Y
ChEBI	CHEBI:15702 Y
ChemSpider	7208 Y
Jmol 3D model	Interactive image
RTECS number	WZ0875000
InChI InChI=1S/C8H6O4/c9-7(10)5-1-2-6(4-3-5)8(11)12/h1-4H,(H,9,10)(H,11,12) Y Key: KKEYFWRCBNTPAC-UHFFFAOYSA-N Y InChI=1/C8H6O4/c9-7(10)5-1-2-6(4-3-5)8(11)12/h1-4H,(H,9,10)(H,11,12) Key: KKEYFWRCBNTPAC-UHFFFAOYAF
SMILES c1cc(ccc1C(=O)O)C(=O)O
Properties
Chemical formula	C8H6O4
Molar mass	166.13 g/mol
Appearance	white crystals or powder
Density	1.522 g/cm³
Melting point	300 °C (572 °F; 573 K) in a sealed tube; sublimes at 402 °C (756 °F; 675 K)
Boiling point	Decompose
Solubility in water	0.28 g/100 mL at 20°C
Solubility	polar organic solvents aqueous base
Acidity (pKa)	3.51, 4.82
Structure
Dipole moment	zero
Vapor pressure	{{{value}}}
Related compounds
Related carboxylic acids	Phthalic acid Isophthalic acid Benzoic acid p-Toluic acid
Related compounds	p-Xylene Polyethylene terephthalate Dimethyl terephthalate
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Y verify (what is YN ?)
Infobox references

Terephthalic acid is the organic compound with formula C₆H₄(CO₂H)₂. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced annually.

History

The importance of the terephthalic acid was realized after World War II. The first companies to commercialize the use of the fibers made from polyethylene terephthalate were Imperial Chemical Industries in the UK in 1949 and DuPont in the US in 1953. The fibers were made from dimethyl terephthalate by transesterification with ethylene glycol. Terephthalic acid was produced by oxidation of p-xylene with dilute nitric acid. This type of oxidation involved the use of air in the initial oxidation step to minimize the consumption of nitric acid and difficulty in purification. Growth in the textile industry led to increase in demand of TPA. In the 1960s, all the terephthalic acid produced was converted to its dimethyl ester, since TPA was infusible and difficult to purify. Presently TPA is manufactured by oxidation of p-xylene.

Solubility

Terephthalic acid is poorly soluble in water and alcohols, consequently up until around 1970 terephthalic acid was purified as its dimethyl ester. It sublimes when heated.

Solubility of Terephthalic Acid g/100g Solvent:

Solvent	25 °C	120 °C	160 °C	200 °C	240 °C
Methanol	0.1	-	2.9	15	-
Water	0.0019	0.08	0.38	1.7	9.0
Acetic Acid	0.035	0.3	0.75	1.8	4.5
Formic Acid	0.5	-	-	-	-
Sulphuric Acid	2	-	-	-	-
Dimethyl formamide	6.7	-	-	-	-
Dimethyl sulfoxide	20	-	-	-	-

Vapour Pressure of Terephthalic Acid:

Temp (°C)	Pressure (kPa)
303	1.3
353	13.3
370	26.7
387	53.3
404	101.3

Specifications of Polymer grade Terephthalic acid:

Property	Specification	Test Method
Acid Number	675±2mgKOH/g	Titration
Ash	15 ppm(max)	Pyrolysis
Total Significant metals (Mo,Cr,Ni,Co,Fe,Ti,Mg)	10ppm	Atomic Absorption
4-formyl benzoic acid	25ppm(max)	Polarography
Moisture wt%	0.5(max)	Karl Fischer
5% dimethyl formamide solution color (ALPHA)	10(max)	Colorimetry

Production

Amoco Process

In the Amoco process, terephthalic acid is produced by oxidation of p-xylene by oxygen in air:

The process uses cobalt-manganese-bromide catalyst. The bromide source can be NaBr, HBr or tetrabromoethane where bromine functions as a regenerative source of free radicals. In this process, acetic acid is the solvent and oxygen from compressed air is the oxidant. The combination of bromine and acetic acid is found to be highly corrosive and hence the equipment is lined with titanium. A feed mixture containing p-xylene, acetic acid, the catalyst system, and compressed air is fed to a reactor. The oxidation of methyl group takes place in two steps where p-xylene is converted to p-toluic acid (TA) and the undesirable product 4-carboxyaldehyde along with crude TA. It is further purified to obtain the TA with the removal of 4-carboxyaldehyde using crystallization, centrifugation and filtration. 98% of p-xylene is reacted to give more than 95% mole TPA and the solvent losses are comparatively low and hence Amoco Process is followed by the industries.

This reaction proceeds through a p-toluic acid intermediate which is then oxidized to terephthalic acid. In p-toluic acid, deactivation of the methyl by the electron withdrawing carboxylic acid group makes the methyl one tenth as reactive as xylene itself, making the second oxidation significantly more difficult. The commercial process utilizes acetic acid as solvent and a catalyst composed of cobalt and manganese salts, with a bromide promoter. The yield is nearly quantitative. The most problematic impurity is 4-formylbenzoic acid (commonly known in the field as 4-carboxybenzaldehyde or 4-CBA), which is removed by hydrogenation of a hot aqueous solution. This solution is then cooled in a stepwise manner to crystallize highly pure terephthalic acid.

Despite optimized yields greater than 95% with excellent purity, the synthesis has proven to have shortcomings. Due to high reaction temperature, approximately 5% of the acetic acid solvent is lost by decomposition or ‘burning’. Solvent burning is a significant economic factor in the oxidation process. In addition, product loss by decarboxylation to benzoic acid is common. The high temperature diminishes oxygen solubility in an already oxygen starved system. Pure oxygen cannot be used in the traditional system due to hazards of flammable organic-O₂ mixtures. Atmospheric air can be used in its place, but once reacted needs to be purified of toxins and ozone depleters such as methylbromide before being released. Additionally, the corrosive nature of bromides at high temperatures requires the reaction be run in expensive titanium reactors.

Cooxidation

The cooxidation technique uses an auxiliary substance which is simultaneously oxidized to H₂O₂ which increases the activity of the oxidation catalyst. Auxiliaries include: acetaldehyde (Toray process, Japan), paraldehyde (Eastmann, USA), and methyl ethyl ketone.

Multistage oxidation

This method is developed to reduce concentration of 4-formylbenzoic acid content in final product terephthalic acid to 200-300 ppm without separate purification step. The product is often called Medium-Purity TPA. In this method, heating gives increased TPA solubility. At high temperature, TPA crystals constantly dissolve and reform which increases release of 4-formylbenzoic acid in solution where oxidation can be completed.

Mitsubishi Process

p-Xylene in acetic acid is oxidized as in Amoco oxidation using a cobalt manganese bromine catalyst. A slurry is heated to 235-290 °C and oxidized further in another reactor.

Eastman Chemical

This method includes two stage oxidation processes both at 175-230 °C temperature instead of heating between stages. After multistage oxidation, solid-liquid separation and drying to obtain final product is carried out.

Henkel process (Raecke process)

Alternatively, but not commercially significant, is the so-called "Henkel process" or "Raecke process", named after the company and patent holder, respectively. This process involves the rearrangement of phthalic acid to terephthalic acid via the corresponding potassium salts. Terephthalic acid can be prepared in the laboratory by oxidizing various para-disubstituted derivatives of benzene, including caraway oil or a mixture of cymene and cuminol with chromic acid.

The use of CO₂ overcomes many of the problems with the original industrial process. Because CO₂ is a better flame inhibitor than nitrogen gas, a CO₂ environment allows for the use of pure oxygen directly, instead of air, with reduced flammability hazards. The solubility of molecular oxygen in solution is also enhanced in the CO₂ environment. Because more oxygen is available to the system, supercritical carbon dioxide (Tc = 31 ⁰C) has more complete oxidation with fewer byproducts, lower CO production, less decarboxylation and higher purity than the commercial process.

When reaction run is in supercritical water can be effectively catalyzed by MnBr₂ with pure O₂ in a medium-high temperature. Use of supercritical water instead of acetic acid as a solvent diminishes environmental impact and offers a cost advantage. However, the scope of such reaction systems is limited by the even harsher conditions than the industrial process (T = 300−400 °C, P > 200 bar).

Ketones have been found to act as promoters for formation of the active Co(III) catalyst. In particular, ketones with a-methylene groups oxidize to hydroperoxides that are known to oxidize Co(II). Viable ketones were butanone, triacetylmethane (TAM), 2,3-pentanedione (2,3-PD), and acetylacetone; all of which can stabilize radical formation through resonance.

Reactions run at temperatures as low as 100 ⁰C are possible by using zirconium salts as a cocatalyst in place of bromide and manganese acetate. It is thought that the Zr(IV) acts to oxidize Co(II) to the active Co(III). This alone shortens the induction period, and has been shown to have a synergistic effect with ketones. However, a greater amount of cobalt acetate is required than the common industrial process and is ineffective over 160 ⁰C.3

The addition of a small portion of metalloporphyrin, in particular T(p-Cl)PPMnCl, has a cocatalytic effect with the traditional Co(OAc)₂ catalyst. This requires less acetic acid and does not require bromides. The catalytic effect has been attributed to the ease of peroxide formation over the metalloporphyrin.

The autoxidation of p-xylene is known to proceed through a free radical process. The Mn(III) and Co(III) metals alone are not strong enough oxidizers to start the radical chain reaction, but instead initiate it by forming bromine radicals from the ions in solution. These bromine radicals then decompose hydroperoxides that are ligated to the metals as well as abstract hydrogens from the methyl groups on p-xylene to form free radicals and propagate the reaction. The following are the proposed initiation, propagation and terminations steps for the first of four oxidations involved in the autoxidation:

The radical chain reaction proceeds through a series of intermediates, starting with the oxidation of p-xylene to p-tolualdehyde (TALD), then p-toluic acid (PT), 4-carboxybenzaldehyde (4-CBA), and finally to the terephthalic acid (TA) product.

The kinetics of the oxidation are exceedingly complex, but a general understanding of the mechanism has been established.

Alternative and past technologies

• Oxidation of p-xylene with acetic acid solvent and Cobalt catalyst with acetaldehyde activator at 120-140 C with residence time greater than or equal to 2 hours. Bromine is now being used in place of acetaldehyde. • Mobil Chemicals in US with above processes used Cobalt catalyst with acetic acid solvent and 2-butanone as activator. After reaction, crude TPA was leached by adding pure acetic acid and at high temperature to achieve partial solubility. Final purification was by sublimation and catalytic treatment of the vapor. • Henkel technology (not in practice) : Starting with phthalic anhydride, monopotassium and dipotassium phthalate salts were formed in sequence. Dipotassium salt was isomerized to dipotassiumterephthalate under carbon dioxide at 1000-5000kPa and 350-450 C. This salt was dissolved in water and recycled to the start of process, where TPA crystals formed during production of monopotassium salts. • Process by Lummus (not commercialized) using dinitrile route • Process by Eastman (not commercialized) using the formation of 1,4-diiodobenzene with carbonylation to aromatic acids. • Process by Mitsubishi Gas Chemicals (not commercialized): Here, complex between Toluene and Hydrogen fluoride-Boron trifluoride is formed and carbonylated with carbon monoxide to form p-tolualdehyde complex. After decomposition of complex p-tolualdehyde can be oxidized in water with a manganese-bromine catalyst system to terephthalic acid. • Oxidation using nitric acid but leads to difficulty in purification and NOx emissions.

Applications

• Polyester fibers based on PTA provide easy fabric care, both alone and in blends with natural and other synthetic fibers. Polyester films are used widely in audio and video recording tapes, data storage tapes, photographic films, labels and other sheet material requiring both dimensional stability and toughness.
• PTA derivative, polyethylene terephthalate, (PET) has become the primary container resin for applications such as carbonated beverage bottles, while other polyterephthalates provide dimensional stability, good heat resistance and durability for engineering applications.
• Terephthalic acid is used in paint as a carrier.
• Terephthalic acid is used as a raw material to make terephthalate plasticizers such as dioctyl terephthalate and dibutyl terephthalate.
• It is used in the pharmaceutical industry as a raw material for certain drugs.
• In addition to these end uses, Terephthalic acid based polyesters and polyamides are also used in hot melt adhesives.
• PTA is an important raw material for lower molecular weight saturated polyesters for powder and water-soluble coatings.

Virtually the entire world's supply of terephthalic acid and dimethyl terephthalate are consumed as precursors to polyethylene terephthalate (PET). World production in 1970 was around 1.75 million tonnes. By 2006, global purified terephthalic acid (PTA) demand had exceeded 30 million tonnes.

There is a smaller, but nevertheless significant, demand for terephthalic acid in the production of polybutylene terephthalate and several other engineering polymers.

In the research laboratory, terephthalic acid has been popularized as a component for the synthesis of metal-organic frameworks.

The analgesic drug oxycodone occasionally comes as a terephthalate salt; however, the more usual salt of oxycodone is the hydrochloride. Pharmacologically, one milligram of terephthalas oxycodonae is equivalent to 1.13 mg of hydrochloridum oxycodonae.

Terephthalic acid is used as a filler in some military smoke grenades, most notably the American M83 smoke grenade, producing a thick white smoke when burned.

Toxicity

Terephthalic acid and its dimethyl ester have very low toxicity, with LD50s on the order of grams/kg (oral, mouse).

References

• 1911 Encyclopedia

External links and further reading

• Basic Organic Chemistry: Part 5, Industrial Products, J.M. Tedder, A. Nechvatal, A.H. Tubb (editors), John Wiley & Sons, Chichester, UK (1975).
• International Chemical Safety Card 0330

See also

• Polycyclohexylenedimethylene terephthalate a thermoplastic polyester formed from terephthalic acid