Patent ID: 8492566

Claim:
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, the catalyst layer comprising a catalyst whose catalytically active material comprises vanadium pentoxide and titanium dioxide, 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 wherein the reactor comprises at least two catalyst layers of different activity arranged in succession in flow direction of the gas stream and the moderator interrupts the catalyst layer further upstream.