GB725642A - Production of anhydrous formaldehyde - Google Patents

Abstract

Anhydrous formaldehyde is obtained continuously by contacting a current of hot trioxane vapour with a solid anhydrous catalyst comprising a non-volatile acidic substance at an elevated temperature. The trioxane vapour is passed over a fixed catalyst bed or alternatively a fluidized bed of catalyst may be used, the catalyst in powder form being maintained in a fluidized form by a sufficiently high velocity of trioxane vapour. The temperature is variable but is below 240 DEG C. to avoid decomposition and side reactions. Suitable catalysts are acidic ion-exchange resins; sulphonated coal or wood; copper pyrophosphate; boric acid; phosphoric acid; potassium hydrogen sulphate or other alkali metal or alkaline earth metal bisulphates; and aluminium sulphate. The catalyst carriers may be granular, or in powder form for fluidized bed operation, and may be inert, e.g. gamma-alumina, porous glass or silicon carbide, or surface-active, e.g. activated silica gel alpha-alumina or activated carbon. The mass velocity of trioxane with respect to catalyst, may vary for example between 0.3 and 2.5 parts by weight per hour. The reaction may be carried out at atmospheric, super atmospheric or sub-atmospheric pressures and the trioxane vapour can be diluted with an inert gas, e.g. methane or carbon dioxide. In examples, (1) trioxane vapour at 210 DEG C. is passed over a catalyst, prepared by mixing 12 parts of 85 per cent phosphoric acid with 90 parts of porous silicon carbide granules, and maintained at a temperature of 210-230 DEG C. An 89 per cent yield of formaldehyde containing no trioxane, carbon monoxide, hydrogen methyl formate or water is obtained; (2) the catalyst is prepared by adsorbing 18 parts of KHSO4 from a saturated solution on 90 parts of activated charcoal granules and drying for 24 hours at 110 DEG C. Trioxane is vaporized and passed over the catalyst at 115-125 DEG C.ALSO:Catalysts used in the production of anhydrous formaldehyde from hot trioxane vapour (see Group IV (b)) are the granular sulphonated copolymers of styrene and divinyl benzene; sulphonated coal or wood; copper pyrophosphate; boric acid; phosphoric acid; potassium hydrogen sulphate or other alkali metal or alkaline earth metal bisulphate; and aluminium sulphate. The catalysts may be supported on carriers which may be granular or powders suitable for fluidized bed operation. The carrier may be inert, e.g., silicon carbide, gamma alumina or porous glass, or may be surface active, e.g., activated silica gel or alpha alumina. In examples the catalyst is prepared by (1) mixing 12 parts of 85 per cent. phosphoric acid with 90 parts of porous silicon carbide granules, (2) adsorbing 18 parts of KHSO4 from a saturated solution on to 90 parts of activated charcoal granules and drying for 24 hours at 110 DEG C.