P-Xylene
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Product Description
p-Xylene (PX) is a key aromatic hydrocarbon compound and one of the three isomers of xylene. As a core intermediate in the petrochemical industry chain, PX is a major raw material for the production of polyester fibers, plastic bottles, and various chemical products.
Synthesis Methods
p-Xylene is not typically "synthesized" in a laboratory sense but is separated and purified from a mixture of aromatic hydrocarbons produced on an industrial scale.
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Primary Source (Feedstock): It is obtained as a component of "Mixed Xylenes" (a blend of o-, m-, p-xylene and ethylbenzene) produced from two main petroleum refining processes:
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Catalytic Reforming: Reforming of naphtha to produce a high-octane gasoline blendstock (reformate), which is rich in BTX (Benzene, Toluene, Xylenes).
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Steam Cracking: Pyrolysis of naphtha or gas oil to produce olefins (like ethylene and propylene), which yields a liquid byproduct (pyrolysis gasoline) also rich in aromatics including xylenes.
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Separation and Purification: Separating p-xylene from its isomers is challenging due to their similar boiling points (especially m- and p-xylene).
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Crystallization: An older technology that exploits p-xylene's higher freezing point (13.3°C) to crystallize it from the cooled mixture.
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Adsorptive Separation (Modern Dominant Method): Uses specialized zeolite adsorbents (e.g., UOP's Parex process) to selectively adsorb p-xylene from the liquid mixture, followed by desorption with a solvent. This is the most efficient and widely used method.
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Isomerization: To maximize yield, the remaining o- and m-xylene stream is often sent to an isomerization unit where they are catalytically converted back toward an equilibrium mixture, generating more p-xylene for separation.
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Uses and Applications
p-Xylene is a primary petrochemical intermediate. Over 95% of global production is dedicated to a single, massive downstream chain.
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Dominant Use (>95%): Production of Terephthalic Acid Derivatives
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p-Xylene → Purified Terephthalic Acid (PTA) / Dimethyl Terephthalate (DMT): p-Xylene is catalytically oxidized to produce PTA (primary route) or DMT.
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PTA/DMT → Polyethylene Terephthalate (PET): PTA (or DMT) is then polycondensed with ethylene glycol (MEG) to form the polymer PET.
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PET End-Uses:
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Fibers: Polyester fiber for textiles, clothing, carpets, and industrial fabrics.
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Packaging: PET resin for plastic bottles (water, soft drinks), food containers, and thermoforming sheets.
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Films: Biaxially oriented PET (BOPET) film for packaging, electrical insulation, and magnetic tape.
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Minor/Specialty Uses (<5%):
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Solvent: Used as a solvent in coatings, inks, dyes, and adhesives, though this use is limited compared to mixed xylenes.
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Laboratory Reagent: Used as a solvent or standard in analytical chemistry (e.g., HPLC).
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Chemical Intermediate: For the synthesis of other chemicals like di-p-xylene and p-toluic acid.
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Properties and Characteristics
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Chemical & Physical Properties:
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Odor: Sweet, aromatic odor.
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Melting Point: 13.3 °C (55.9 °F)
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Boiling Point: 138.4 °C (281 °F)
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Density: 0.861 g/cm³ at 20°C (less dense than water).
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Solubility: Very slightly soluble in water (~0.2 g/L). Miscible with alcohol, ether, acetone, benzene, and chloroform.
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Vapor Pressure: 8.84 mmHg at 25°C.
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Octanol-Water Partition Coefficient (Log P): ~3.15 (indicates high hydrophobicity).
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Autoignition Temperature: ~528 °C (984 °F).
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Vapor Density: 3.66 (air = 1); vapor is heavier than air and will settle.
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Computed Molecular Descriptors (PubChem):
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Hydrogen Bond Donor/Acceptor Count: 0
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Rotatable Bond Count: 0
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Topological Polar Surface Area: 0 Ų
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Complexity: 48.4
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Safety and Handling
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Hazards Classification (GHS):
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Flammable Liquid (Category 3): Flash Point ~25°C (77°F).
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Skin Irritant (Category 2): Can cause skin irritation and defatting.
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Specific Target Organ Toxicity (Single Exposure) - Narcotic Effects (Category 3): Vapors may cause dizziness, headache, and central nervous system depression.
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Hazardous to Aquatic Environment (Chronic, Category 3).
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Exposure Risks & Health Effects:
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Inhalation: Irritation of nose and throat, headache, dizziness, confusion, nausea, and at high concentrations, respiratory depression.
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Skin Contact: Can cause irritation, redness, and dermatitis due to defatting action.
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Eye Contact: Vapors or liquid can cause severe irritation.
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Ingestion: Can cause gastrointestinal irritation, nausea, vomiting, and central nervous system effects.
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Safe Handling Practices:
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Engineering Controls: Use in well-ventilated areas or with local exhaust ventilation. Use explosion-proof equipment.
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Personal Protective Equipment (PPE): Chemical safety goggles, impervious gloves (e.g., nitrile), and protective clothing. Use respiratory protection if ventilation is inadequate.
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Handling: Avoid contact with skin, eyes, and clothing. Avoid breathing vapor or mist. Ground containers to prevent static discharge. Use non-sparking tools.
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Storage: Store in tightly closed containers in a cool, well-ventilated area away from heat, sparks, open flames, and strong oxidizers (e.g., nitric acid).
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First Aid & Emergency:
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Inhalation: Move to fresh air. Seek medical attention if symptoms persist.
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Skin Contact: Wash thoroughly with soap and water. Remove contaminated clothing.
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Eye Contact: Rinse cautiously with water for several minutes. Remove contact lenses if present. Seek medical attention.
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Fire: Use dry chemical, CO₂, or alcohol-resistant foam. Water spray may be used to cool exposed containers.
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