Oxidative dehydroaromatization: III. The mechanism of oxidative dehydrodimerization process of C 3-C 4 olefins over Bi 2O 3-SnO 2 catalyst

Abstract

The mechanism of the oxidative dehydrodimerization from lower olefins was studied from the dependence of the reaction rate on the partial pressure of olefin and from the competitive reaction between two kinds of olefin. The formation rates of dimers such as 1,5-hexadiene from propylene and 2,5-dimethyl-1,5-hexadiene from isobutene, were well expressed by an equation of Langmuir-Hinshelwood type, V = k[ K · P (1 + K · P) ] 2 and those of co-dimers such as 2-methyl-1,5-hexadiene from propylene and isobutene, were written by V = k · K 1K 2P 1P 2 (1 + K 1P 1 + K 2P 2) 2 . Based on these kinetic equations, adsorption constants and rate constants in this reaction were evaluated. From the kinetic equations, the trend of adsorption constants, and the relation between rate constants and dissociation energies of allyl hydrogens, the following reaction steps were proposed: (1) adsorption of olefin, (2) conversion of adsorbed olefins into two π-allyl intermediates coordinated to a single active site, (3) dimerization into hexadiene, and several consecutive steps into aromatics, where step (2) is rate-determining.