GB782430A - Improvements in the oxidation of organic aldehydes - Google Patents

Abstract

Aldehydes are oxidized by simultaneously and separately introducing an organic aldehyde and a strong alkali into an aqueous suspension of a silver catalyst, the strong alkali being employed in an amount of at least one chemical equivalent for each equivalent of the aldehyde, and passing molecular oxygen through such mixture while maintaining the reaction mixture at a pH of at least 12.5 and at a temperature of at least 20 DEG C., the catalyst being present in a concentration of at least 1 per cent by weight of the reaction mixture. On completion of the oxidation the product acid is liberated from its salt by addition of a strong acid such as sulphuric, hydrochloric, nitric or phosphoric acids. Formic or acetic acid may also be used to liberate weaker or water-insoluble acids. The process may be used for the production of any carboxylic acid such as saturated and unsaturated aliphatic acids, e.g. sorbic, acrylic, methacrylic, crotonic, 2-ethyl-2-butenoic, 2-hexenoic, 2-methyl-2-pentenoic, 2-octenoic, 2 - ethyl - 2 - hexenoic, 2,4,6 - octatrienoic, butyric, hexanoic, 2-ethylbutyric, 2-ethyl-hexanoic, 2-ethyloctanoic and 2-butylhexanoic acids; hydroxy and alkoxy acids, e.g.22-butoxybutyric, 3,5 - diethoxyhexanoic and 2-ethyl-3 hydroxyhexanoic acids; dibasic acids such as glutaric, 3-methylglutaric and 2-ethylglutaric acids; cyclic unsaturated acids, e.g. D 3-tetrahydrobenzoic, D 3-tetrahydro-o-toluic, 2,3-dihydro - 1,4 - pyran - 2 - carboxylic, 2,3-dimethyl - 2,3 - dihydro - 1,4 - pyran - 2 - carboxylic acid and furoic acids; and aromatic acids, e.g. benzoic, salicyclic, cinnamic and p-chlorobenzoic acids. The preferred alkali for use in the process is sodium hydroxide, although lithium, potassium, or barium hydroxides may also be used. The alkali is generally used in an amount of 1-2, preferably 1-1.2 equivalent per equivalent of aldehyde, sufficient being employed to maintain in the reaction mixture a pH of at least 12.5 at all times. The aldehyde and strong alkali are preferably added continuously to a vigorously agitated aqueous suspension of the silver catalyst. The rate of addition of aldehyde should not exceed the rate of which it is oxidized, rates up to 8 pounds per hour per gallon of reaction mixture being suitable. The oxygen may be used in pure form or mixed with other gases as in air, and is preferably used in large excess. The finely divided silver catalyst may be preformed by reduction of silver oxide or a silver salt such as silver nitrate by reaction with an aldehyde and alkali, hydrogen peroxide and alkali, or with aluminium and alkali. The catalyst may also be prepared in situ by adding silver oxide or nitrate to the reaction mixture, the reduction being effected by the aldehyde and alkali. The preferred catalyst concentration is 3 to 10 per cent by weight of the reaction mixture. The reaction is generally effected at 20-100 DEG C., preferably 20-80 DEG C., and pressures from atmospheric to 150 p.p.i.g. are suitable. Pressures in excess of 200 p.p.s.i.g. may also be used. When using air it is necessary to operate at a higher pressure than when using pure oxygen to obtain the same result. The process may be operated batchwise or continuously. In the preferred continuous operation, the product salt solution is continuously withdrawn via an internal filter to eliminate the necessity for recycling the catalyst. When the catalyst becomes inactive it may be reactivated by washing with organic solvents and inorganic solutions. After liberation of the acid from the salt, the acid may be recovered by any appropriate method, such as filtration, solvent extraction, distillation or decantation. Detailed examples are given for the continuous oxidation under varying conditions of hexadienal, acrolein, methacrolein, crotonaldehyde, 2-ethylcrotonaldehyde, 2-hexanal, 2-methyl-2-pentenal, 2-octenal, 2-ethyl-2-hexenal, a mixture of C8 unsaturated aldehydes containing 2,4,6-octatrienal; 4-pentenal, 2-ethylbutyraldehyde, 2-ethylhexaldehyde, 2-ethyloctaldehyde, a mixture of 2-ethyloctaldehyde and 2-butylhexaldehyde; 3-butoxybutyraldehyde, 3,5 - diethoxyhexaldehyde, butyraldol - (2-ethyl - 3 - hydroxyhexaldehyde), glutaraldehyde, 3 - methylglutaraldehyde, 2 - ethyl - 3-methylglutaraldehyde, 2 - ethylglutaraldehyde, D 3 - tetrahydrobenzaldehyde, D 3 - tetrahydro - o - tolualdehyde, acrolein dimer, methacrolein dimer, furfural, benzaldehyde, salicylaldehyde, cinnamaldehyde, and p-chlorobenzaldehyde.