CATALYTIC SKELETAL ISOMERIZATION OF LINEAR BUTENES TO ISOBUTENE

The increased demand for isobutene, used for the production of the oct ane-enhancer methyl tert-butyl ether, has generated tremendous interes t in the catalytic conversion of the linear butenes to isobutene. In t his review we survey the progress made since the late 1970s in impleme nting the catalytic skeletal isomerization reaction of these linear al kenes. Halogenated catalysts, especially those based on alumina, and p repared using a variety of compounds of fluorine, chlorine or bromine, have been shown to exhibit both high conversions and selectivities fo r the reaction, resulting in high yields of isobutene, when water is a dded to the feed stream. Elution of the halogen from the catalyst lead s to the loss of catalytic activity and necessitates the continuous or discontinuous addition of the halogen compound. As a consequence, env ironmental and other considerations are most likely to weigh against t he industrial usage of these catalysts. Another class of catalysts exh ibiting high activities and selectivities, again in the presence of wa ter, are the silicated aluminas. No information is, however, available on their long-term stability. Even alumina on its own displays high a ctivity and selectivity, provided water is co-fed with the hydrocarbon stream. More recent results obtained over other types of catalysts su ch 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 dec rease in the yield of isobutene after 14 days on-stream. The high yiel ds of isobutene can be ascribed to the small channel diameters which p revent the extensive dimerization or oligomerization of the linear but enes or of the product isobutene. Plans for the first large-scale demo nstration plant to produce isobutene from n-butenes using ferrierite a s catalyst have already been announced in the United States.