Analysis of the factors affecting selectivity in the partial oxidation of benzene to maleic anhydride. Part 1.—Detailed kinetics of maleic anhydride adsorption

Abstract

The adsorption of maleic anhydride on a vanadium pentoxide–molybdenum trioxide catalyst has been studied using the transient techniques of temperature programmed desorption and gas adsorption chromatography. The desorption spectra obtained over a range of adsorption temperatures show two peaks having maxima at between 220 and 250 °C and 340 and 350 °C. Solution of the Redhead equation for an assumed value of 10¹³ s^–1 for the desorption A-factor at the peak maxima gave desorption energies of between 138 and 148 kJ mol^–1 and 171 and 181 kJ mol^–1. In spite of the high dosages used (ca. 10⁷ Langmuirs), the surface coverages never exceeded 6 × 10¹² sites (molecules) cm^–2 with the number of high energy sites remaining constant at 3 × 10¹² sites cm^–2 over the large range of adsorption temperatures; it is inferred that these sites are a defect property of the mixed oxide. The gas adsorption chromatographic peak shapes are also analysed in terms of two adsorption bonds from which the heats of adsorption of 79.9 and 104.2 kJ mol^–1 are derived. The adsorption of maleic anhydride on the mixed oxide catalyst is therefore seen to be activated, having adsorption activation energies of 63 and 71 kJ mol^–1. These not inconsiderable heats of adsorption and desorption activation energies imply the possibility of product inhibition in the selective oxidation of benzene to maleic anhydride.