Methane Activation on In-Modified ZSM-5: The State of Indium in the Zeolite and Pathways of Methane Transformation to Surface Species

Methane activation pathways as well as methane involvement in the reaction of ethylene aromatization on In-modified H-ZSM-5 zeolite (In/H-ZSM-5) have been studied with solid-state NMR spectroscopy. The state of indium in In/H-ZSM-5 in dependence of the zeolite activation procedure, reductive or oxidative, has been analyzed with X-ray photoelectron spectroscopy (XPS). On the basis of 1H MAS NMR analysis of the evolution of the quantity of Brønsted acid sites (BAS) and XPS analysis of the state of indium in dependence of zeolite activation procedure, it has been inferred that indium exists in the form of either In+ or InO+ isolated cationic species in the zeolite. Methane interaction with different indium cationic species has been analyzed with 13C MAS NMR spectroscopy. In+ species has been concluded to be inactive, whereas InO+ species provides dissociative adsorption of methane to afford primarily the oxyindium–methyl species. The secondary products of oxyindium–methyl species transformation are oxyindium–methoxy, ethane, formate, and acetaldehyde species. Methane can be involved in the reaction of ethylene aromatization on In-modified zeolite H-ZSM-5. This involvement is provided by the reaction of the surface oxyindium–methoxy species with simple aromatic molecules formed from ethylene. Preliminarily, oxyindium–methoxy species are generated by the interaction of oxyindium–methyl species with InO+ cations.