Abstract: The invention relates to a novel structured catalyst bed which optionally comprises at least one part bed comprising at least one catalytically active mixture of oxides of the main group metals and transition metals and additionally comprises at least one catalytically active part bed comprising at least silver, at least one alkali metal and a porous support material and finally and necessarily at least one catalytically active part bed comprising at least one alkali metal phosphate and at least one sheet silicate.

Abstract: A vanadium-, phosphorus- and oxygen-containing catalyst for the preparation of maleic anhydride by heterogeneously catalyzed gas-phase oxidation of a hydrocarbon of at least four carbon atoms has a phosphorus/vanadium ratio of from 0.9 to 1.5, comprises particles having a mean diameter of at least 2 mm and has a composition which, using CuK&agr; radiation (&lgr;=1.54·10−10 m), gives a powder X-ray diffraction pattern which, in the 2&thgr; range from 10° to 70°, has a signal/background ratio of ≦10 for all diffraction lines which are attributable to a vanadium- and phosphorus-containing phase. Said catalyst is prepared and is used for the preparation of maleic anhydride.

Abstract: A catalyst used in a process for preparing maleic anhydride by heterogeneously catalyzed gas-phase oxidation of a hydrocarbon having at least four carbon atoms by means of oxygen-containing gases comprises a catalytically active composition comprising vanadium, phosphorus and oxygen and has a essentially hollow cylindrical structure in which (a) the ratio of the height h to the diameter of the continuous hole d2 is not more than 1.5 and (b) the ratio of the geometric surface area Ageo to the geometric volume Vgeo is at least 2 mm−1.

Abstract: In a process for preparing maleic anhydride by heterogeneously catalyzed gas-phase oxidation of hydrocarbons having at least four carbon atoms using oxygen-containing gases in the presence of a volatile phosphorus compound over a catalyst comprising vanadium, phosphorus and oxygen in a shell-and-tube reactor unit having at least two successive cooled reaction zones, the temperature of the first reaction zone is from 350 to 450° C. and the temperature of the second and further reaction zones is from 350 to 480° C. and the temperature difference between the hottest reaction zone and the coolest reaction zone is at least 2° C.