Effect of Copper State in Cu/H-ZSM-5 on Methane Activation by Brønsted Acid Sites, Studied by ¹H MAS NMR In Situ Monitoring the H/D Hydrogen Exchange of the Alkane with Brønsted Acid Sites

Abstract

With regard to possible involvement of zeolite Brønsted acid sites (BAS) in the activation of methane molecules for methane transformation to methanol, the effect of different Cu(II) species loaded in the zeolite on the kinetic parameters of the reaction of H/D hydrogen exchange of the alkane with BAS of Cu-modified ZSM-5 zeolites has been investigated with 1 H MAS NMR in situ at 533−563 K. It is found that the acceleration of the H/D exchange by 1 order of magnitude occurs for zeolite containingsites) compared to pure H-form zeolite. It is thus inferred that both Z 2 Cu(II) and Z 2 [Cu 3 (μ-O) 3 ] sites exhibit the promoting effect of copper on the activation of methane C−H bonds by BAS. Acceleration of the H/D exchange is rationalized by the change of the mechanism of the exchange accepted for the H-form zeolites for the mechanism that involves the formation of a transient molecular complex of methane with Cu(II) species, preceding the H/D exchange reaction. The formation of the complex of methane with both Z 2 Cu(II) and Z 2 [Cu 3 (μ-O) 3 ] sites is confirmed by DRIFTS. BASs with a higher strength than in H-ZSM-5, generated in the zeolite at copper loading, are concluded to not be responsible for the H/D exchange reaction acceleration.

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

“…It would also be reasonable to expect that the protons of methane can be readily exchanged with BASs, in the presence of the copper-oxo species. This is exactly the findings of Gabrienko et al…”

Isobutene Transformation to Aromatics on Zn-Modified Zeolite: Particular Effects of Zn2+ and ZnO Species on the Reaction Occurrence Revealed with Solid-State NMR and FTIR Spectroscopy

“…Monitoring of the transformation of alkanes and alkenes, selectively 13 C-labeled at the specific position of the molecule, with 13 C MAS NMR (MAS – magic angle spinning) allows one to follow the transfer of the 13 C label into the reaction intermediates and products. This results in the identification of the intermediates, establishing their structures, and clarifying the main reaction steps. ,,− Because of high sensitivity and the high speed of data acquisition, FTIR spectroscopy is a valuable tool for studying fast processes such as the initial steps of alkane (alkene) activation on zeolite catalysts. ,,,− The joint use of these spectroscopic methods can help to clarify the roles of BAS and LAS (Lewis acid sites) in the process of isobutene transformation on Zn-containing zeolites and contribute to the determination of the aromatization mechanism. The knowledge obtained about the aromatization mechanism with these spectroscopic methods should promote further investigations in the field of rational catalyst design for the effective utilization of hydrocarbon feedstock to valuable products.…”

Synthesis of Methanesulfonic Acid Directly from Methane: The Cation Mechanism or the Radical Mechanism?

“…With the depleting reserves of fossil fuels, methane has gained significant importance in recent years. − Functionalization of methane for conversion to valuable products − is one of the great contemporary challenges in catalysis. A number of approaches have been explored to activate the strong C–H bond of methane, − among which the direct approaches are of exceptional interest. − For cleaving the extremely stable methane molecule, very aggressive reactants and harsh operating conditions are usually required, leading to sometimes undesired reactions …”