Xylene or dimethylbenzene is any one of three isomers of dimethylbenzene, or a combination thereof. With the formula (CH3)2C6H4, each of the three compounds has a central benzene ring with two methyl groups attached at substituents. They are all colorless, flammable liquids, some of which are of great industrial value. The mixture is referred to as both xylene and, more precisely, xylenes.
Isomers
Xylene exists in three isomeric forms. The isomers can be distinguished by the designations ortho- (o-), meta- (m-) and para- (p-), which specify to which carbon atoms (of the benzene ring) the two methyl groups are attached. By counting the carbon atoms around the ring starting from one of the ring carbons bonded to a methyl group, and counting towards the second methyl group, the o-isomer has the IUPAC name of 1,2-dimethylbenzene, the m-isomer is 1,3-dimethylbenzene and the p-isomer is 1,4-dimethylbenzene. Of the three isomers, the p-isomer is the most industrially sought after since it can be oxidized to terephthalic acid.
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[8] 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.
| Item | Test Method | Sales Spec. |
| Appearance | Visual | Clear |
| Sp-gr, 15.56 ℃/15.56 ℃ | ASTM D-4052 | 0.86500-0.8750 |
| Color Pt/Co | ASTM D-1209 | Max 20 |
| Acid Wash Color | ASTM D-848 | Max 2 |
| Sulfur Compounds | ASTM D-853 | Free of H2S/So2 |
| Cooper Corrosion | ASTM D-849 | Pass |
| Bromi neIndex,mg/100g | ASTM D-1491 | Max 20 |
| Acidity | ASTM D-847 | No Free Acid |
| Non-Aromatics,VOl% | ASTM D-2360 | Max 2.0 |
| C9+Aromatics,Vol% | ASTM D-2267 | Max 1.0 |
| Range of Distillation, ℃ | ASTM D-850 | 139.3±5.0 |
| Recovery,% | ASTM D-850 | Min 98.0 |
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