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 C-13 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 zinc-methyl species from methane at T >= 523 K, whereas Zn2+/H-ZSM-5 with partial substitution (60%) of BAS produces zinc-methyl at room temperature. BAS promotes the formation and decomposition (by the sample evacuation) of zinc-methyl species on Zn2+/H-ZSM-5 at room temperature. Zinc-methyl is readily oxidized by the dioxygen additive to offer methoxy species already at room temperature. Thus, it has been shown that pure methane forms only zinc-methyl species upon its interaction with zinc cationic sites of Zn2+-exchanged H-ZSM-5 zeolite, while the surface methoxide could be formed only by the interaction of zinc-methyl with dioxygen that might be contained in the reactive methane.