Abstract: An improved process for producing maleic anhydride which comprises catalytically oxidizing benzene with a molecular oxygen-containing gas in the vapor phase in a multitubular heat-exchanger-type reactor. The process is characterized in thata catalyst-packed layer of the reactor is divided into a gas inlet side layer and a gas outlet side layer,the activity of the gas inlet side layer is controlled by adding at least one component selected from the group consisting of potassium, cesium, thallium and rubidium as a promoter or by decreasing the amount of phosphorus pentoxide (P.sub.2 O.sub.

Abstract: In a method for the preparation of pyromellitic acid or its anhydride by catalytic vapor phase oxidation of a tetra-C.sub.1 -C.sub.4 -alkylbenzene using a catalyst comprising catalytically active material composed of 1 to 20 parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by weight of TiO.sub.2 and/or SnO.sub.2 and/or ZrO.sub.2, and per one hundred parts by weight of V.sub.2 O.sub.5, TiO.sub.2 and/or SnO.sub.2 and/or ZrO.sub.2 0.02 to 10 parts by weight, calculated as P.sub.2 O.sub.5, of a phosphorus compound, 0.01 to 5 parts by weight, calculated as Nb.sub.2 O.sub.5, of a niobium compound, 0 to 1.2 parts by weight, calculated as oxide, of at least one metal selected from the group consisting of potassium, cesium, rubidium and thallium, and 0 to 10 parts by weight, calculated as Sb.sub.2 O.sub.