Investigations of catalytically active surface compounds: XVII. Influence of size and structure of vanadium oxide clusters on selectivity in the oxidation of n-butene

Abstract

n-Butene oxidation on vanadium oxide monolayer catalysts (supported on Aerosil-200) was investigated in the temperature range from 633 to 693 K using two types of active vanadium oxide components differing with respect to the size of the clusters and the coordination of the vanadium ions. Coordination is characterized by uv-vis spectroscopy. The degree of reduction of vanadium in a state close to reaction conditions was determined by titration of suitably pretreated samples. In the conversion range between 7 and 95%, selectivities for butadiene, furan, maleic anhydride, acetaldehyde, CO, and CO 2 were determined, initial selectivities S 0 being estimated by extrapolation to zero conversion. A formal reaction scheme including direct formation of products without desorption and readsorption of preceding intermediates is proposed. From comparison of the values of S 0 and κ (ratio of rate constants for formation and consumption of primary products) characteristic features of the different catalytic behavior of the two active surface phases have been derived. Selectivity is mainly influenced by the amount and the nature of oxygen available in the cluster, by the bond strength of oxygen in the cluster unit, and by the probability of formation of radical oxygen species.