Methane Activation on Zn²⁺-Exchanged ZSM-5 Zeolites. The Effect of Molecular Oxygen Addition

In relation to the reported methane activation on Zn-modified zeolite ZSM-5 at room temperature to afford the surface methoxy species by Xu et al. (Chem. Sci. 2012, 3, 2932), the activation of methane on Zn2+-exchanged H-ZSM-5 zeolite in the absence and the presence of molecular oxygen has been studied with 13C magic angle spinning (MAS) NMR spectroscopy. It has been established that the methane activation on zinc cationic sites under nonoxidative conditions occurs exclusively by an “alkyl” pathway to form the surface zinc-methyl species. The addition of the molecular oxygen (dioxygen) to methane adsorbed on the Zn2+-exchanged H-ZSM-5 zeolite results in the surface methoxy and other oxygen-containing species, such as formate, acetaldehyde, and acetic acid. The formation of the surface methoxy species occurs by the oxidation with molecular oxygen of zinc-methyl species primarily formed on the zeolite surface. The Zn2+/ZSM-5 zeolite with full substitution of Bronsted acid sites (BAS) by Zn2+ cations offers ...