Abstract: A catalyst for gas phase oxidations comprises an inert support and a catalytically active material which comprises vanadium oxide and titanium dioxide and has been applied thereto. The titanium dioxide has a content of sulfur compounds, calculated as S, of less than 1000 ppm and a content of calcium compounds, calculated as Ca, of less than 150 ppm. The catalyst has a relatively high activity and/or selectivity and thus enables relatively high yields of the desired target product, for example phthalic anhydride. Also described is a process for preparing phthalic anhydride, wherein a gas stream which comprises molecular oxygen and o-xylene, naphthalene or mixtures thereof is contacted with the catalyst.

Abstract: The present invention relates to a catalyst system for preparing carboxylic acids and/or carboxylic anhydrides, which system comprises a plurality of superposed catalyst layers arranged in a reaction tube, where vanadium antimonate is introduced into the active material in at least one of the catalyst layers. The present invention further relates to a process for gas-phase oxidation, in which a gaseous stream comprising at least one hydrocarbon and molecular oxygen is passed through a plurality of catalyst layers and the maximum hot spot temperature is below 425° C.

Abstract: What is described is a process for starting up a gas phase oxidation reactor for oxidation of o-xylene to phthalic anhydride, said reactor comprising at least one catalyst layer and being temperature-controllable by means of a heat carrier medium, wherein a) the catalyst layer is interrupted by a moderator layer which is less catalytically active than the catalyst layer or is catalytically inactive, b) a gas stream is passed through the reactor with an initial loading of o-xylene and at an initial temperature of the heat transfer medium, c) the loading of the gas stream is increased to a target loading and, in parallel, the temperature of the heat transfer medium is lowered to an operating temperature. The introduction of the moderator layer allows the loading to be increased more rapidly and the startup time to be shortened.