Paraxylene, also written p-xylene or PX, is an aromatic hydrocarbon with the molecular formula C8H10, a molecular weight of 106.17 g/mol and the CAS number 106-42-3. It is one of the three dimethylbenzene isomers, alongside ortho-xylene, CAS 95-47-6, and meta-xylene, CAS 108-38-3. In paraxylene the two methyl groups occupy opposite positions on the benzene ring, the 1,4- or para configuration. The compound is a colourless transparent liquid with a sweet aromatic odour that solidifies slightly above normal room temperature, so storage and transfer lines in cold climates must be heated.
Identity and Physical Properties
| Property | Value |
|---|---|
| CAS number | 106-42-3 |
| Chemical name | 1,4-Dimethylbenzene |
| Molecular formula | C8H10 |
| Molecular weight | 106.17 g/mol |
| Appearance | Colourless transparent liquid |
| Density at 20 °C | 0.861 g/cm3 |
| Melting point | 13.2 °C |
| Boiling point | 138.4 °C |
| Flash point, closed cup | About 27 °C |
| Water solubility | Practically insoluble |
The melting point is the property that shapes most of the logistics around this product: because the material freezes just above 13 °C, piping, pumps and tanks in a cold climate must be traced and insulated, and any transfer that follows a shutdown has to be checked for solidified product before restart.
Raw Materials: Naphtha and Pyrolysis Gasoline
The main raw materials for paraxylene are naphtha, derived from crude oil, and pyrolysis gasoline, a liquid by-product of ethylene production. Neither feedstock is used directly. Both must first be converted into a C8 aromatic stream that contains mixed xylenes, after which the p-xylene fraction is separated and purified.
Naphtha from crude oil: processed in a catalytic reformer to give reformate rich in benzene, toluene and xylenes.
Pyrolysis gasoline: hydrogenated to remove diolefins and other reactive components before its aromatic cut is recovered.
Transalkylation and disproportionation: convert surplus toluene and heavy aromatics into additional xylenes.
In a catalytic reformer the C8 fraction of the reformate contains ethylbenzene and all three xylene isomers, including paraxylene. Pyrolysis gasoline, sometimes called pygas, carries a substantial C6 to C8 aromatic fraction, and once it has been hydrotreated its C8 cut can be fed to the xylene recovery unit. Integrated aromatics complexes deliberately combine both streams so that the separation and isomerisation section receives as much C8 feed as possible.
From Mixed Xylenes to Purified Paraxylene
Because the boiling points of the xylene isomers are extremely close, ordinary distillation cannot separate them efficiently. Industrial practice therefore relies on fractional crystallisation, on selective adsorption over zeolite molecular sieves, or on a combination of adsorption with isomerisation. In the isomerisation unit the less valuable isomers are converted back towards the equilibrium mixture, so every pass increases the total yield of paraxylene. The separated product is finally distilled to reach the purity required for downstream oxidation, typically 99% or higher.
Quality Specification and the PTA-PET Value Chain
More than 95% of global paraxylene is consumed to produce purified terephthalic acid by catalytic oxidation. PTA is then polymerised with monoethylene glycol to make polyethylene terephthalate, the material used for beverage bottles, polyester fibre and packaging film. Because those markets are among the largest polymer markets in the world, paraxylene is the most commercially valuable xylene isomer, and the remainder of demand comes from solvents, herbicides and specialty chemical intermediates.
Commercial paraxylene is specified by purity at 99% or higher, by freezing or melting point, by colour on the platinum-cobalt scale as described in ASTM D1209, and by density. Water content is measured by Karl Fischer titration, while trace toluene and the other xylene isomers are determined by gas chromatography. Buyers should obtain a certificate of analysis and confirm that the batch meets the specification agreed for the downstream oxidation unit, because even small impurity deviations can affect oxidation catalyst performance and product quality.
Handling, Storage and Transport
Paraxylene is a flammable liquid and a volatile organic compound. It must be kept away from ignition sources, transfer equipment should be grounded and bonded to prevent static discharge, and the product should be stored in closed containers in a cool, well-ventilated area. Vapour inhalation can cause dizziness and irritation, so handling areas need adequate ventilation and personnel should wear chemical-resistant gloves and eye protection.
Bulk paraxylene moves in dedicated tank trucks, railcars and ocean tankers, normally fitted with heating systems because the product solidifies near 13 °C, and receiving terminals keep it in heated tanks to preserve pumpability. Product that has been stored for a long period should be re-analysed before use, since moisture pickup and contamination during transfer are the most common reasons for an off-specification batch.
Frequently Asked Questions
Q: What is the main raw material for paraxylene?
The main raw materials are naphtha, converted into aromatic-rich reformate by catalytic reforming, and pyrolysis gasoline from ethylene crackers, both of which supply the C8 aromatic fraction containing mixed xylenes.
Q: Is paraxylene the same as xylene?
No. Xylene is the commercial name for the mixture of three dimethylbenzene isomers, while paraxylene is the single isomer with methyl groups at the 1,4 positions, separated from the mixture by crystallisation or adsorption.
Q: Why is p-xylene more valuable than o-xylene and m-xylene?
Because p-xylene is the feedstock for PTA and PET, which serve the very large polyester fibre and packaging markets, so its demand and value are much higher than those of the other isomers.
Q: Can paraxylene be produced from renewable sources?
Bio-based paraxylene can be made from biomass-derived intermediates such as bio-isobutanol or bio-ethanol through dehydration, aromatisation and alkylation routes, and it is chemically identical to fossil-derived paraxylene.
Q: What purity of paraxylene is required for PTA production?
PTA plants normally require paraxylene with a purity of 99% or higher, because impurities deactivate the oxidation catalyst and reduce product quality.
Q: How is bulk paraxylene transported and stored?
It is shipped in dedicated tank trucks, railcars and ocean tankers with heating systems, because the product solidifies near 13 °C, and it is held in heated tanks at the receiving terminal.