Catalytic properties of defective brannerite-type vanadates: III. Oxidation of o-xylene on Mn 1 − xφxV 2 − 2 xMo 2 xO 6

Abstract

Part I of this series (J. Catal. 81, 298 (1983)) described the catalytic anisotropy of Mn 1 − x φ x V 2 − 2 x Mo 2 x O 6 solid solutions in the oxidation of propylene. In Part II (J. Catal. 81, 311 (1983) a new model of sites active in oxidation reactions has been proposed to explain this phenomenon. This model, designated BSMAS, is based on crystallochemical considerations and on the assumption that the individual catalytic activity of a given surface oxygen atom is proportional to its reciprocal bond-strength sum. In this paper the studies are extended to the oxidation of o-xylene. The conversion and yield of phthalic anhydride, tolualdehyde, phthalide, maleic anhydride, CO, and CO 2 have been determined as a function of temperature, contact time, catalyst composition, and type of preferentially exposed crystallographic plane ((201) or (202̄)). It is shown that the reaction scheme is composed of “one-stage” total combustion and of a one-site “rake-reaction” along which C 8 products are formed. The scheme is common for both planes and it does not alter with catalyst composition, but, as is shown by results from changes of conversion, yields, and selectivities to different products, the rate constant of the elementary steps does depend on the plane exposed and on the composition of the solid. Discussion in terms of the BSMAS model makes it possible to indicate the structure of active sites on which the abovementioned reactions may take place and to explain the plane effect and the dependence of the reaction pattern on catalyst composition.