Alternative methods for the synthesis of phenol

The oldest process for phenol production is based on the sulfonation of benzene. This route goes back to Michael Faraday, who recognized as early as 1825 that phenol could be obtained from benzenesulfonic acid by alkali fusion. 

Charles Adolphe Wurtz and August Kekule further improved the process in 1867. The sulfonation route was developed to technical maturity in particular by Bayer and Monsanto. The first stage in the synthesis is the sulfonation of benzene with sulfuric add, generally using a 100% excess of sulfuric acid. In the second stage, the sulfonation product is neutralized with sodium hydroxide or sodium sulfite. The resulting benzenesulfonate, in solid form or as a concentrated aqueous solution, is heated with sodium hydroxide at temperatures of 320 to 340 °C, in cast-iron pans. The alkaline sodium phenolate solution is neutralized ( saturated ) with CO2 or, in the Monsanto process, with SO2. When the water has been distilled off substantially pure phenol with a crystallizing point of 40.5 °C is obtained. 

The disadvantages of the benzenesulfonic acid process are the production of large amounts of sodium sulfite and the highly corrosive reaction conditions of the alkali fusion in addition, the process is only carried out semi-continuously. 

1 and 2 Sulfonation reactors 3 Water separator 4 Benzene vaporizer 5 fte-neufraluation vessel 6 Final neutralization vessel 7 Concentrator 8 Fusion vessel 9 Dissovmg a, 10 and 11 Neutralization vessel (crude phenol precipitation) 12 Crude p eno separa or, 13 Dewatering column 14 Phenol column 15 Residue column 

In the early 1930 s a process was developed by Raschig for the synt esis o p e-nol, the principle of which had already been found in England days of aromatic chemistry, namely, oxychlorination of benzene wit m sequen alkaline hydrolysis. This route had been originally discovered by usart an Ch. Bardy in 1872. 

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