GB977496A - Process for preparing acetic acid by catalytic oxidation of propylene - Google Patents

Abstract

Acetic acid is obtained by reacting oxygen or an oxygen-containing gas with propylene in the gaseous phase at elevated temperature in the presence of a catalyst comprising both molybdenum and tin in the form of an oxide or oxides. In addition to acetic acid an appreciable amount of acrolein is obtained as well as small amounts of other products including acetaldehyde, acrylic acid, formic acid and formaldehyde. The catalyst may comprise a mixture of one or more molybdenum oxides with one or more tin oxides or may comprise an oxidic compound of molybdenum and tin, either alone or together with another oxide of one or both of the said metals. The oxidation is preferably carried out at between 250 and 400 DEG C. although higher or lower temperatures e.g. 200 DEG to 250 DEG C. or 400 DEG to 600 DEG C. may be used. It is preferred to operate under normal pressure but higher or lower pressures may be used. The process is preferably effected by leading a mixture of propylene and oxygen-containing gas e.g. air if desired in the presence of an inert gas or vapour, preferably steam, through a fixed bed of the catalysts mass but oxidation may also be effected in a fluidized catalyst bed or in successive fluidized and fixed catalyst beds. When air is used as the oxygen-containing gas good results are obtained by using 0.1 to 2.5 moles of oxygen and from 0.5 to 15 moles of steam per mole of propylene. The catalyst may be prepared by intimately mixing the constituents either alone or together with a carrier material and granulating or tabletting the mixture or by suspending the constituents either alone or together with a carrier material in water, evaporating the suspension with stirring and catalytically activating the resulting mass by drying and heating, e.g., to 400 DEG to 600 DEG C. Alternatively, the catalyst may be obtained by impregnating carrier material with an aqueous solution of a readily decomposable molybdenum salt, e.g. ammonium molybdate, and then adding the other component either as such or in the form of an aqueous solution of a salt of a volatile acid, e.g. stannic chloride, evaporating the suspension and then drying and heating. Suitable carrier materials are silica gel, clay, pumice stone, aluminium phosphate, aluminium oxide, boron phosphate and carborundum. The catalyst used may suitably correspond to the composition Sn-(MoO4)2 or to a mixture of one mole of the latter with one mole of SnO2 or MoO3. The recovery of the oxidation products may be effected by contacting the gas mixture with water at a temperature above the boiling point of acrolein to absorb the acetic acid and then contacting all or part of the gas stream issuing from the acetic acid absorber with water at a temperature below the boiling point of acrolein to absorb the latter. Specification 839,808 is referredALSO:A catalyst used for the oxidation of propylene to form acetic acid (see Division C2) comprises both molybdenum and tin in the form of an oxide or oxides. The catalyst may comprise a mixture of one or more molybdenum oxides with one or more tin oxides or may comprise an oxidic compound of molybdenum and tin either alone or mixed with another oxide of one or both of the said metals. It may be obtained by either (a) intimately mixing the constituents, e.g. MoO3 and SnO2, either alone or with a carrier material and granulating or tableting the mixture, (b) by suspending the constituents, e.g. MoO3 and SnO2, (with or without a carrier) in water, evaporating the suspension with stirring and finally drying and heating the resulting mass, e.g. at 400-600 DEG C., or (c) by impregnating a carrier with an aqueous solution of a readily decomposable molybdenum salt, e.g. ammonium molybdate, adding the other component either as such or in the form of an aqueous solution of a salt of a volatile acid, e.g. stannic chloride, evaporating the suspension and then drying and heating, e.g. at 400-600 DEG C. Specified carriers are silica gel, clay, pumice stone, aluminium phosphate, aluminium oxide, boron phosphate or carborundum. Specification 839,808 is referred to.