Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 31 - 48 (1981)
Update date:2022-08-29
Topics:
Lucas, Jean
Vandervell, Daniel
Waugh, Kenneth C.
The kinetics of the oxidation of benzene to maleic anhydride over a vanadium pentoxide-molybdenum trioxide catalyst have been studied using a combination of transient techniques.Temperature programmed desorption (t.p.d.) has shown (i) the existence of a weakly-bound molecular oxygen species having a desorption activation energy of between 105 and 113 kJ mol-1 and (ii) that the adsorption of benzene is heteroenergetic having three sets of desorption activation energies of between 97.9 and 104.2 kJ mol-1 , 107.9 and 119.2 kJ mol -1 and 130.5 and 133.5 kJ mol-1 (As before, these desorption energies are obtained by solution of the Redhead equation for the peak maximum temperature for an assumed 1E12 s-1 A-factor.) In spite of the high benzene dosages used in these studies (ca. 3E10) Langmuir , 1 Langmuir=1.33E-4 N m-2 s) the total coverages by these sites never exceeded 4E11 cm-2, suggesting that they are an intrinsic defect property of the catalyst.Temperature programmed reaction spectroscopy (t.p.r.s.) has shown that (i) the overall oxidation of benzene to maleic anhydride is rate limited in the desorption of the product, the desorption activation energies of the reactively produced maleic anhydride (in the ranges 143.5-148.1 kJ mol-1 and 166.1-169.9 kJ mol-1) being identical to those obtained for pre-adsorbed maleic anhydride, (ii) the activation energy for the surface oxidation of the adsorbed benzene to adsorbed maleic anhydride is low, having a value of 31.4 kJ mol-1, (iii) the reactively produced maleic anhydride occupies the site upon which its precursor, benzene, was chemisorbed, inhibiting further benzene adsorption on that site and (iv) from the population distribution of the adsorbed maleic anhydride on the energetically heterogeneous surface, it was formed at the benzene adsorption temperature (50 deg C).The immobility of the adsorbed maleic anhydride suggests that the oxygen is transported to the adsorbed benzene, which in combination with the low surface oxidation activation energy appears to preclude the involvement of lattice oxygen in this step, inferring that the oxidant is a mobile chemisorbed species, possibly the weakly bound molecular state seen in the t.p.d. experiments.Gas adsorption chromatography (g.a.c.) shows the heats of adsorption of benzene to be low, 58.6 and 69.5 kJ mol-1, the latter heat being a composite of two heats which cannot be resolved.The maximum number of sites avaible for benzene adsorption is only 2.7E12 cm-2 of which 97.5percent have a heat of adsorption of 58.6 kJ mol-1.Benzene adsorption is therefore weakly activated.
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