GB819188A - Purifying isoprene by distillation - Google Patents

Abstract

Isoprene, such as that which has been obtained by the cracking of petroleum, is purified by adding isopentane to the impure isoprene and submitting the resulting mixture to fractional distillation whereby dimethylacetylene, isopropenylacetylene, dimethyl sulphide and other compounds which act as polymerization-inhibitors are removed as distillate. The purified isoprene can be readily polymerized to produce synthetic rubber. About 4 to 30 per cent of isopentane by volume of impure isoprene should be used in the distillation, depending on the efficiency of the fractionating column employed. Maleic anhydride is also added to the distilland in order to remove cyclopentadiene. The used isopentane, which comes over in the distillate, can be purified by mild catalytic hydrogenation, followed by stripping. Alternatively, the acetylenic compounds can be removed from the isopentane by means of selective solvents or adsorbents. In an example, commercial isoprene containing 2.5 volume per cent of dimethylacetylene and a small proportion of cyclopentadiene and other impurities was mixed with 9 per cent by weight of isopentane and 2 per cent of maleic anhydride; and the mixture was fractionated in a packed column, equivalent to 60 theoretical plates, employing a reflux ratio of 50. Removal of an overhead fraction was continued until substantially complete removal of dimethylacetylene and also of isopentane was accomplished. The purified isoprene was then dissolved in benzene and polymerized by means of tri-isobutyl aluminium and titanium tetrachloride. A yield of 88 per cent of polyisoprene was obtained based on the starting material.