An 18O 2 Temperature-Programmed Isotope Exchange Study of Transition-Metal-Containing ZSM-5 Zeolites Used for Oxydehydrogenation of Ethane

Abstract

The oxydehydrogenation of ethane was studied over a number of transition-metal containing ZSM-5 catalysts, e.g., Co-H-ZSM-5, Cu-Na-ZSM-5, Nb-ZSM-5, and V-ZSM-5. It has been shown that the introduction of transition metal cations into ZSM-5 zeolite, either as charge balancing cations (Co 2+, Nb 5+) or as lattice cation (V 5+), enhances the activity and selectivity of the catalysts for the oxydehydrogenation of ethane. The 18O 2 temperature-programmed isotope exchange technique was used to probe the likely active sites on the catalysts. It was demonstrated that the introduction of transition metal cations results in the formation of active sites responsible for the formation of single-step double exchange between gas phase oxygen and lattice oxygen. The activity for formation of single-step double exchange 16O 2 is a good indicator for the activity for oxydehydrogenation of ethane to form ethylene. The introduction of transition metal cations (Cu 2+, Co 2+, etc.) into ZSM-5 zeolite lowers the temperature required for isotope exchange between gas phase oxygen and lattice oxygen. The temperature required for isotope exchange depends on the cations introduced. The activity for facilitating single-step double exchange between gas phase 18O 2 and lattice oxygen ( 16O) and the activity for oxydehydrogenation of ethane to ethylene decrease in the same order: Co-H-ZSM-5 > Cu-Na-ZSM-5 > [V]-ZSM-5 > Nb-ZSM-5 > Na-ZSM-5 ∼ H-ZSM-5 ∼ Cu-ZSM-5. It appears that the structural defects created by introducing transition metal cations are responsible for the enhanced activity and selectivity for the oxydehydrogenation of ethane. NH 3-TPD results show that the presence of strong Brønsted acid sites are undesired for oxydehydrogenation reactions. However, the presence of relatively weaker acid sites is not harmful for the reaction.