Acrylic or methacrylic acids are prepared from acrolein or methacrolein and/or a compound which gives rise to acrolein or methacrolein under the reaction conditions, by reacting them at elevated temperature in the vapour phase with molecular oxygen in the presence of arsenic or an arsenic oxide, over an oxidation catalyst comprising a molybdate of antimony, cobalt, or tin. Propylene and isobutene give rise to the acrolein and methacrolein respectively, and from 1 to 20% of olefines or aldehydes may be present in the feed and oxygen may be diluted with inert gases e.g. as in air, or with steam. The arsenic in the catalyst may be by admixture, or by coprecipitation followed by thermal decomposition.ALSO:An oxidation catalyst comprising a molybdate of antimony, cobalt or tin, together with arsenic or arsenic oxide, is used for the preparation of acrylic or methacrylic acid by reacting with molecular oxygen at an elevated temperature in the vapour phase, acrolein or methacrolein and/or a compound which gives use to acrolein or methacrolein under the reaction conditions. The catalyst may be prepared by admixture or by co-precipitation from mixed solutions of arsenic salt and metal molybdate, followed by thermal decomposition. In examples; (1) aqueous solutions of cobalt nitrate, ammonium arsenate, and ammonium molybdate were mixed, ammonia added and the precipitate filtered dried, and heated in two stages at 400 DEG C. and at 600 DEG C. with graphite as a lubricant to give a Co: As ratio of 10:1, (2) cobalt malybdate as above was used and arsenious oxide introduced to the inlet gases (3) a tin molybdate catlayst prepared by dissolving tin in nitric acid followed by the addition of molybdenum trioxide, filtration, the addition of graphite on a two stage heating at 750 DEG C. and 1000 DEG C., was used with arsenic pentoxide and (4) antimony pentoxide and molybdenum trioxide, mixed pelleted and heat treated at 500 DEG C., were mechanically mixed with arsenic pentoxide.
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