Abstract: Catalysts comprising iron and potassium and, if desired, further elements, which catalysts are suitable for dehydrogenating hydrocarbons to give the corresponding olefinically unsaturated hydrocarbons, are prepared by calcining a finely divided dry or aqueous mixture of an iron compound with a potassium compound and, if desired, compounds of further elements in a first step that agglomerates having a diameter of from 5 to 50 .mu.m and formed from smaller individual particles are obtained and, in a second step, preferably after shaping, calcining it at from 300 to 1000.degree. C., with the maximum calcination temperature in the second step preferably being at least 30.degree. below the calcination temperature in the first step. The catalysts thus prepared are useful, in particular, for dehydrogenating ethylbenzene to give styrene.

Abstract: In a process for preparing olefinically unsaturated compounds such as styrene by oxidative dehydrogenation of corresponding hydrocarbons using a previously oxidized oxygen transferer acting as catalyst in the absence of molecular oxygen and reoxidation of the oxygen transferer in at least two reactors, the dehydrogenation and regeneration takes place alternately in time in the two reactors and the reactors are connected to one another in terms of heat via heat exchangers and a common circuit for heat transfer medium.