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Xylene

Xylene

Abbreviation：XYL
CAS NO：1330-20-7
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DETAIL
Xylene is any one of three isomers of dimethylbenzene, or a combination thereof. With the formula (CH3)2C6H4, in each of the three compounds a benzene ring is substituted by two methyl groups. They are all colorless, flammable, slightly greasy liquids. They are of great industrial value. The mixture is referred to as both xylene and, more precisely, xylenes. Mixed xylenes refers to a mixture of the xylenes plus ethylbenzene. The four compounds have identical empirical formulas C8H10. Typically the four compounds are produced together by various catalytic reforming and pyrolysis methods.

Occurrence and production
Xylenes are an important petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel. They also occur in crude oil in concentrations of about 0.5–1%, depending on the source. Small quantities occur in gasoline and aircraft fuels.

Xylenes are produced mainly as part of the BTX aromatics (benzene, toluene, and xylenes) extracted from the product of catalytic reforming known as reformate.

Industrial production
Xylenes are produced by the methylation of toluene and benzene. Commercial or laboratory-grade xylene produced usually contains about 40-65% of m-xylene and up to 20% each of o-xylene, p-xylene and ethylbenzene.The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites.

ZSM-5 is used to facilitate some isomerization reactions leading to mass production of modern plastics.

Applications
Terephthalic acid and related derivatives
p-Xylene is the principal precursor to terephthalic acid and dimethyl terephthalate, both monomers used in the production of polyethylene terephthalate (PET) plastic bottles and polyester clothing. 98% of p-xylene production, and half of all xylenes produced is consumed in this manner. o-Xylene is an important precursor to phthalic anhydride. The demand for isophthalic acid is relatively modest so m-xylene is rarely sought (and hence the utility of its conversion to the o- and p-isomers).

Solvent applications and industrial purposes
Xylenes are used as a solvent in printing, rubber, and leather industries. It is a common component of ink, rubber, and adhesives. In thinning paints and varnishes, it can be substituted for toluene where slower drying is desired, and thus is used by conservators of art objects in solubility testing.[17] Similarly it is a cleaning agent, e.g., for steel, silicon wafers, and integrated circuits. In dentistry, xylene can be used to dissolve gutta percha, a material used for endodontics (root canal treatments). In the petroleum industry, xylene is also a frequent component of paraffin solvents, used when the tubing becomes clogged with paraffin wax.

Precursor to other compounds
In one large scale application, para-xylene is converted to terephthalic acid. The major application of ortho-xylene is as a precursor to phthalate esters, used as plasticizer. Meta-xylene is converted to isophthalic acid derivatives, which are components of alkyd resins.
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