Oxidative dehydrogenation dimerization of propylene over bismuth oxide: Kinetic and mechanistic studies

Abstract

Classical kinetic experiments together with pulse microreactor studies involving deuterium and carbon-13-labeled isotopic tracers were used to investigate the oxidative dehydrogenation dimerization (OXDD) of propylene to 1, 5-hexadiene and benzene over bismuth oxide between 748 and 898 °K. The kinetic data, which indicated that the OXDD reaction is of variable order with respect to oxygen and propylene concentrations, could be fit to rate equations based on either the Langmuir-Hinshelwood model or the Mars-van Krevelen model, although the former gave more linear Arrhenius plots. A significant kinetic isotope effect ( k H k D = 1.7 at 873 ° K ) shows that the rate-limiting step for the OXDD reaction involves CH cleavage, and there is only a small amount of H/D scrambling among reactant and product molecules. Analysis of liquid products by infrared spectroscopy indicated that both 1, 5-hexadiene and 1, 3-cyclohexadiene are stable reaction intermediates; microreactor results involving unlabeled propylene, 1, 5-hexadiene, 1, 3-cyclohexadiene, and 1, 4-cyclohexadiene as reactants confirmed the infrared findings. Pulse microreactor experiments with 13C-labeled propylene clearly showed that deep oxidation (complete combustion) occurs via a consecutive-parallel network involving the partially oxidized intermediates as well as the starting propylene. Changes in the particle size do not alter the overall activity, although larger particles have lower selectivities for C 6 products than do smaller particles.