Patent GB-762738-A (GB)
Title: Catalytic oxidation of aromatic carboxylic acids to phenols
IPC: C07C37/56
Filed: 1954-11-16
Abstract: Phenols are prepared by heating a benzene mono-carboxylic acid having an unsubstituted ortho position or a salt, ester or anhydride thereof while molten or dissolved in an inert solvent, in the presence of oxygen and a copper compound which dissolves in the reaction mixture at least at reaction temperature, to a temperature at which carbon dioxide is evolved.  The benzene mono-carboxylic acid may contain substituent groups such as phenyl, alkyl, alkoxy, nitro and halogen. Inert solvents specified are water, benzene, toluene, xylene and diphenyl.  The following phenols are prepared from the acids mentioned: phenol from benzoic acid, m-cresol from o-toluic acid, o- and p-cresols from m-toluic acid, m-cresol from p-toluic acid, p-nitro-phenol from m-nitrobenzoic acid, m-nitro-phenol from p-nitrobenzoic acid, m-chloro-phenol and phenol from p-chlorobenzoic acid, m-methoxy-phenol and phenol from p-methoxy-benzoic acid, m-phenyl-phenol from p-phenyl-benzoic acid, and 3 : 5-dimethylphenol from 2 : 4-dimethyl-benzoic acid.  Non-phenolic by-products also obtained are benzene from benzoic acid, phthalide from o-toluic acid, toluene from p-toluic acid and anisole from p-methoxy-benzoic acid. The copper compound may be present in a concentration of at least 0.01 mol. of dissolved copper compound per mol. of acid and may be introduced initially as metallic copper, cuprous or cupric oxide, or cupric salicylate.  Other copper compounds mentioned are cupric sulphate, cupric phosphate, cupric benzoate and cupric acetate. Promoters comprising soluble compounds of the metals magnesium, sodium, potassium, cobalt, lithium, barium, or nickel, such as the benzoates of these metals, magnesium chloride or oxide, may also be present and may be added in the form of the metal, metal oxide, hydroxide or salt.  In the continuous process, oxygen in the form of air, oxygen or ozone is passed through the liquid acid containing the dissolved copper compound with or without agitation, steam also being passed through to hydrolyse any ester of the phenol and starting acid which may be formed, while maintaining a temperature of at least 200 DEG  C.  The vapours from the reaction mixture containing phenol, air, steam, unchanged acid and carbon dioxide are separated by known methods, the acid being returned to the reactor and the phenol being recovered.  Accumulation of tar in the reactor is prevented by periodically removing portions of the reaction mixture, separating the tar and returning acid and copper compounds to the reactor.  In modifications of the continuous process, air and steam may be passed alternately through the reaction mixture or air may be passed through the mixture which overflows or is pumped continuously or periodically to a second reactor where steam passes through it.  When inert solvents are used, diphenyl may be used at near atmospheric pressures and benzene or toluene at elevated pressures.  Data is also given showing the effect of replacing the copper compounds used in the invention by compounds of potassium, cadmium, zinc, iron, lead, manganese, tin, aluminium, chromium, silver, cobalt, mercury, indium, vanadium, zirconium, nickel and uranium; only nickel and uranium compounds showed any activity in catalysing the reaction. Further data showing the effect on the process of the invention of variations in water feed rate, temperature, air addition rate, copper ion concentration, type of promoter and reduced pressure are given.  In examples: (1) phenol with a by-product of benzene is obtained by heating benzoic acid, copper benzoate, and magnesium oxide with agitation while air and steam are passed in, the volatile products passing through a distillation column to separate the phenol, tars being removed from the reaction mixture by withdrawing portions and extracting with aqueous methanol, evaporating the extract and returning the acid to the reactor; (2) illustrates use of potassium benzoate as promoter; (3) magnesium benzoate is used as promoter; (4) a mixture of o-, m- and p-cresols is obtained from commercial mixed toluic acids; (5) illustrates the use of a continuous steam supply with an alternated air and nitrogen supply in place of continuous air.