| 摘要 |
The cracking of 4, 4-dimethylmetadioxane to isoprene is effected in the presence of phosphoric acid on silica of specific surface less than 100 square metres per gram. The catalyst which is preferably quartz in the form of sand or grit may contain 0,3-5% by weight of acid after drying at about 280 DEG C. The catalyst is preferably continuously agitated either by a stirring device or by using a moving or fluidized bed, and may be regenerated by reimpregnation from time to time and, usually at less frequent intervals, heating in air to remove deposits prior to the reimpregnation. Specifications 825,034 and 884,808 are referred to.ALSO:Isoprene and formaldehyde together with small amounts of isobutene are formed from 4,4-dimethylmetadioxane in the vapour phase at elevated temperature in the presence of an inert diluent and phosphoric acid on silica of specific surface less than 100 square metres per gram. The catalyst which is preferably quartz in the form of sand or grit may contain 0.3%-5% by weight of acid after drying at about 280 DEG C. The catalyst is preferably continuously agitated either by a stirring device or by using a moving or fluidised bed. The catalyst is preferably regenerated by reimpregnation from time to time and, usually at less frequent intervals, heated in air to remove deposits prior to the reimpregnation. The two regenerating operations may be performed in the same apparatus. Temperatures of 200 DEG C.-300 DEG C. are used at pressures up to 5 kg./cm.2, preferably atmospheric. Space velocities of 0.2-3 litres/hour/litre of catalyst are used. The diluent may be nitrogen, water vapour or vapours of paraffinic or naphthenic hydrocarbons forming 35%-95% of the total charge. Heat is preferably supplied to the catalyst bed, e.g. by submerged heating coils rather than to the reactants. Unconverted dioxane may be recycled and where the dioxane is obtained from isobutene as in Specifications 825,034 and 884,808 the total effluent from which processes may be passed to the present process, formaldehyde and any isobutene formed may be recycled to the first operation. The effluent is preferably cooled quickly to 80 DEG C.-20 DEG C. by intimate contact, e.g. in a countercurrent scrubber or an ejector, with the aqueous phase of the condensed product after it has been cooled. |
