Reactivity of oxygen ions in mixed oxides in dehydrogenation of propane

Abstract

Propane pulse experiments are used as a method to select suitable oxides as oxygen supplier for operation of catalytic dense membrane reactor (CDMR) in oxidative dehydrogenation of propane. Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), La2NiO4+d (LN), La2Ni0.9V0.1O4.15+d (LNV-10) and PrBaCo2O5+d (PBC) powders were used as model catalysts to explore the catalytic properties of surfaces in terms of activity and selectivity. Furthermore, as propane pulses induce variation of the oxidation degree by slowly reducing the powdered oxide, crucial information on the effect of variation of the oxygen content (i.e. the oxidation degree) in the samples on reactivity and selectivity is obtained. PBC and BSCF are not suitable for CDMR because the former showed low selectivity to C3H6 and the latter adsorbs CO2 by forming carbonates. LN and LNV-10 are promising materials for membrane application, provided that they are operated in the optimal window of reduction degree. LNV-10 has shown a constant level of C3H6 production within a broad window of oxidation degree (over-stoichiometric oxygen d < 0.03) at 550 °C, while the performance of LN catalyst deteriorated drastically towards CO, CH4 and coke after just a few pulses. This is because in the case of LN, the activation of lattice oxygen caused the formation of Ni metal, responsible for production of CO and coke, mainly. Contrary, only over-stoichiometric oxygen (d) is activated in LNV-10, which showed promising selectivities to olefins. Additionally, LNV-10 has demonstrated structure stability and reversibility during oxidation-reduction cycles at 550 °C. Testing LN, BSCF and PBC membranes during steady-state experiments revealed that the permeation rate was higher than the reaction rate. Therefore, it was reasonable to assume that intermediate adsorbed oxygen species (O2 -, O2 2-, and O-) were present and possibly responsible for CO2 formation, decreasing the selectivity. In the case of LNV- 10, the presence of intermediate adsorbed oxygen species was excluded but unfortunately, the contribution of the LNV-10 membrane on converting propane was very small although it showed higher yield to olefins at 550 °C.