The role of oxygen in the oxidative dehydrogenation of propane

Abstract

The oxidative dehydrogenation of propane over Mg-V-O was investigated both under steady-state and transient reaction conditions. Nonlinear regression analysis was used to compare the applicability of various mechanistic models to the steady-state data. The oxygen surface concentration appeared not to be in equilibrium with the gas phase; models in which oxygen reoxidized the catalyst improved the fit of the data. Thus, Mars-Van Krevelen type kinetics were supported in which propane reacts with oxygen on the catalyst surface to produce propene and carbon oxides. Additional steady-state data were also collected over a wider range of oxygen partial pressures at constant propane partial pressure. Large improvements in propene selectivity were observed by operation at steady-state with low oxygen partial pressure. These selectivity improvements could not be attributed to the consecutive reaction mechanism. The increased propene selectivity at lower oxygen partial pressure suggested the reaction mechanism included reaction pathways in which carbon oxides were produced either by separate surface oxygen sites whose concentration were dependent on the gas-phase oxygen partial pressure or by the direct reaction of gas phase oxygen with adsorbed propene.

Transient responses of the reactants and products, during start-up and interruption of the reaction, supported Mars-Van Krevelen type kinetics. Oxygen mass balances confirmed that the catalyst was partially reduced during start-up of the reaction. Also, the catalyst was active in the absence of gas phase oxygen, at least while the catalyst contained sufficient oxygen. During operation of the reaction under transient conditions without gas phase oxygen, the propene selectivity was higher than at steady-state conditions. This was true even at comparable levels of conversion of the propane. Carbon mass balances indicated little to no carbon species accumulated on the catalyst during start-up with gas phase oxygen. However, temperature-programmed oxidation and desorption experiments revealed that significant, strongly bound carbon-containing species must be slowly deposited on the catalyst during steady-state reaction. During nonsteady-state reaction, without gas phase oxygen, weakly bound carbon species were deposited on the catalyst immediately after startup which could be quickly oxidized from the catalyst surface upon subsequent exposure of the catalyst to oxygen.

It was found that by periodic operation of the reaction by alternate feeding of oxygen and propane at a 1:1 cycle split, a higher propene yield could be obtained compared to comparable steady-state conditions at cycle periods from about 50 to 150 s. Propene selectivity for periodic operation was always well above that at steady-state, due to the high selectivity observed without gas phase oxygen. However, it was unclear whether periodic operation was superior to steady-state operation at all conditions. At low propane concentrations, it was found that the propene yield at steady-state conditions could also be improved simply by reducing the partial pressure of oxygen. Periodic operation was also performed without re-oxidation of the catalyst, by alternately feeding propane and pure helium. Under these conditions comparison of the propene selectivity at about the same conversion of propane, at different degrees of catalyst reduction, indicated that the propene yield of the catalyst was proportional to the degree of reduction.

On the whole, the concentration of oxygen both in the gas phase and on the surface of the catalyst is crucial for determining the selectivity of the reaction for producing propene.