coal chemicals[′kōl ‚kem·i·kəlz]
For about 100 years, chemicals obtained as by-products in the primary processing of coal to metallurgical coke have been the main source of aromatic compounds used as intermediates in the synthesis of dyes, drugs, antiseptics, and solvents. Although some aromatic hydrocarbons, such as toluene and xylene, are now obtained largely from petroleum refineries, the main source of others, such as benzene, naphthalene, anthracene, and phenanthrene, is still the by-product coke oven. Heterocyclic nitrogen compounds, such as pyridines and quinolines, are also obtained largely from coal tar. Although much phenol is produced by hydrolysis of monochlorobenzene and by decomposition of cumene hydroperoxide, much of the phenol, cresols, and xylenols are still obtained from coal tar.
Coke oven by-products are gas, light oil, and tar. Coke oven gas is a mixture of methane, carbon monoxide, hydrogen, small amounts of higher hydrocarbons, ammonia, and hydrogen sulfide. Most of the coke oven gas is used as fuel. Although several hundred chemical compounds have been isolated from coal tar, a relatively small number are present in appreciable amounts. These may be grouped as in the table. All the compounds in the table except the monomethylnaphthalenes are of some commercial importance.
|Compound||whole tar, %||Use|
|Carbazole (and other||2.3||Dye intermediates|
|Cresols and xylenols||1.5||Antiseptics, organic syntheses|
|Pyridine, picolines, lutidines,||2.3||Drugs, dyes, antioxidants|
|and other tar bases|
The direct utilization of coal as a source of bulk organic chemicals has been the objective of much research and development. Oxidation of aqueous alkaline slurries of coal with oxygen under pressure yields a mixture of aromatic carboxylic acids. Because of the presence of nitrogen compounds and hydroxy acids, this mixture is difficult to refine. Hydrogenation of coal at elevated temperatures and pressures yields much larger amounts of tar acids and aromatic hydrocarbons of commercial importance than are obtained by carbonization. However, this operation is more costly than other sources of these chemicals. See Destructive distillation