What Is Xylene?
Xylene is an organic compound in which two hydrogen atoms of benzene are replaced by methyl groups.
It is also known as xylol, dimethylbenzene, and methyltoluene.
Uses of Xylene
Xylene has three isomers: p-Xylene, o-Xylene, and m-Xylene, which differ in the location where the methyl group is substituted. Mixed xylene, which is a mixture of these isomers before separation, is also used in industry. In addition to the three xylene isomers, mixed xylene contains a large amount of ethylbenzene.
Mixed xylene is used as a raw material for p-xylene, o-xylene, m-xylene, and ethylbenzene by isomer separation, and is also used as a solvent for paints, agricultural chemicals, and pharmaceuticals, as well as a solvent for cleaning fats and oils. Each isomer is also used as a raw material for synthesizing various chemicals, such as the following:
1. P-Xylene
Also known as 1,4-dimethylbenzene, p-Xylene is mainly used as a raw material for terephthalic acid and dimethyl terephthalic acid. Terephthalic acid and dimethyl terephthalate are raw materials for polyethylene terephthalate (PET). It is also used as a raw material for p-toluic acid.
2. O-Xylene
Also known as 1,2-dimethylbenzene, o-Xylene is mainly used as a raw material for phthalic anhydride. Phthalic anhydride is used as a raw material for plasticizers such as dioctyl phthalate and dibutyl phthalate, as well as for diallyl phthalate (DAP) and alkyd resins. It is also used as a raw material for o-phthalodinitrile, xylenol, and xylidine.
3. M-Xylene
Also known as 1,3-dimethylbenzene, m-Xylene is mainly used as a raw material for isophthalic acid. It is used as a raw material for polyester. It is also used as a raw material for meta-xylenediamine and Xylene resin.
Characteristics of Xylene
Like toluene, which is structurally similar to xylene, xylene is a clear, colorless liquid with a distinctive ink-like aroma.
When mass-produced industrially, it is extracted from modified petroleum oil, and has the characteristic of being highly flammable.
Xylene is also highly volatile and inhalation of its gaseous form has been reported to affect the nervous system. Thus, care must be taken when using it.
The three isomers p-Xylene, o-Xylene, and m-Xylene do not differ in appearance, odor, or hazardousness, but they have different physical properties due to differences in molecular structure. In particular, there is a significant difference in melting point, with p-Xylene at 13.3°C, o-Xylene at -25.2°C, and m-Xylene at -47.9°C. The boiling point of o-Xylene is 144.4°C, slightly higher than that of p-Xylene (138.4°C) and m-Xylene (139.1°C), although not as high as the melting point, so they can be separated and purified through distillation.
Other Information on Xylene
How Xylene Is Produced
The following methods are used to separate the three isomers p-Xylene, o-Xylene, and m-Xylene, and ethylbenzene from the industrial mixture of xylenes.
1. O-Xylene
o-Xylene is recovered from mixed xylene by distillation. o-Xylene is present in 20% of the mixed xylene, but can be separated by precision distillation due to the large difference in boiling points between o-Xylene and the other isomers. Ethylbenzene, a mixture of p-Xylene and m-Xylene, and o-Xylene can be separated by precision distillation.
2. P-Xylene
A mixture of p-Xylene and m-Xylene after separation of ethylbenzene and o-Xylene from mixed Xylene is separated by a deep cold separation method. Since the difference in boiling points is within 1°C, separation by distillation is difficult, but the difference in melting points is approximately 60°C. Therefore, separation by deep cooling is possible. Therefore, separation is possible by deep cooling. However, the need to cool to ultra-low temperatures makes it energy inefficient, and p-Xylene has the disadvantage of low yield.
3. M-Xylene
After removing ethylbenzene, o-xylene, and p-xylene from the xylene mixture, m-Xylene is left over. However, the low yield of p-xylene by the deep cold separation method results in low purity. Therefore, an adduct that reacts selectively only with m-Xylene is formed and recovered. The recovered adduct is then decomposed with hydrogen to recover high-purity m-Xylene.