CATALYTIC SKELETAL ISOMERIZATION OF LINEAR BUTENES TO ISOBUTENE

The increased demand for isobutene, used for the production of the octane-enhancer methyl tert-butyl ether, has generated tremendous interest in the catalytic conversion of the linear butenes to isobutene. In this review we survey the progress made since the late 1970s in implementing the catalytic skeletal isomerization reaction of these linear alkenes. Halogenated catalysts, especially those based on alumina, and prepared using a variety of compounds of fluorine, chlorine or bromine, have been shown to exhibit both high conversions and selectivities for the reaction, resulting in high yields of isobutene, when water is added to the feed stream. Elution of the halogen from the catalyst leads to the loss of catalytic activity and necessitates the continuous or discontinuous addition of the halogen compound. As a consequence, environmental and other considerations are most likely to weigh against the industrial usage of these catalysts. Another class of catalysts exhibiting high activities and selectivities, again in the presence of water, are the silicated aluminas. No information is, however, available on their long-term stability. Even alumina on its own displays high activity and selectivity, provided water is co-fed with the hydrocarbon stream. More recent results obtained over other types of catalysts such as zeolites and molecular sieves are also presented. Most promising are the results obtained with the zeolite ferrierite which gives high yields of the branched isomer in the absence of any other additive or diluent. The catalyst also appears to be fairly stable showing no decrease in the yield of isobutene after 14 days on-stream. The high yields of isobutene can be ascribed to the small channel diameters which prevent the extensive dimerization or oligomerization of the linear butenes or of the product isobutene. Plans for the first large-scale demonstration plant to produce isobutene from n-butenes using ferrierite as catalyst have already been announced in the United States.