Oxidative Activation of Metal-Exchanged Zeolite Catalysts for Methane Hydroxylation

Abstract

The stepwise approach of methane direct conversion to methanol by Fe- and Cu-exchanged zeolite catalysts involves a pre-activation of the catalysts by an oxidant at high temperature. Since an isothermal process of this approach at temperature below 200 °C is highly desirable, understanding the mechanism of the oxidative activation is important for developing strategies to reduce the activation temperature. In this chapter, I provide an overview of recent theoretical understanding in the mechanisms of the Fe- and Cu-oxo active site formations from N₂O, H₂O₂, and O₂ activations. Density functional theory calculations show that N₂–O, HO–OH, and O=O cleavages on the reduced Fe and Cu centers can actually be achieved with low activation barriers. However, as also suggested by recent experimental works, high temperature is required to form the precursors, i.e. the reduced metal centers. I conclude this overview with forward looking aspects that should be addressed in the future.