Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of the possibility of methane involvement in coaromatization with higher alkanes on this type of zeolites. Herein, two samples of Zn-modified zeolite BEA containing exclusively either small zinc oxide clusters or isolated Zn 2+ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods ( 1 H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn 2+ cations or ZnO small clusters, exists in the void of zeolite pores. The ability of zinc sites of different nature to promote the activation of methane C−H bond with the zeolite Brønsted acid sites (BAS) has been examined in the reactions of methane H/D hydrogen exchange with BAS and the alkylation of benzene with methane. It has been found that both ZnO and Zn 2+ species promote the reaction of H/D exchange of methane with BAS. The rate of H/D exchange is higher by 2 and 3 orders of magnitude for the zeolite loaded with ZnO or Zn 2+ species, respectively, compared to pure acid-form zeolite H-BEA. So, the promoting effect of Zn 2+ cations is more profound than that of ZnO species for H/D exchange reaction. This implies that the synergistic effect of Zn-sites and BAS for C−H bond activation in methane is significantly higher for Zn 2+ cations compared to small ZnO clusters. It has been revealed, however, that only Zn 2+ cations promote the alkylation of benzene with methane, whereas ZnO species do not. The isolated Zn 2+ cations provide the formation of zinc-methyl species, which are further transformed to zinc-methoxy species. The latter is the key intermediate for the performance of the alkylation reaction. Hence, while both zinc oxide clusters and Zn 2+ cationic species are able to provide a synergistic effect for the activation of C−H bonds of methane displayed by the dramatic acceleration of H/D exchange reaction, only the Zn 2+ cationic species perform methane activation toward the alkylation of benzene with methane. This implies that only the Zn 2+ cations in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes.

With regard to a general interest in methane utilization in a rational way the activation and transformation of methane on Ag-modified zeolite ZSM-5 (Ag/H-ZSM-5) have been studied with solid-state NMR. The activation of methane occurs by dissociation of the C–H bond on silver cations via the “carbenium” pathway: methane C–H bond cleavage results in the methoxy groups (O–CH3) and possibly silver-hydride species (Ag–H). The formation of surface methoxy groups on Ag/H-ZSM-5 has been detected experimentally with 13C CP/MAS NMR at 508–623 K for the first time. A comparative analysis of the kinetics of the H/D exchange between methane and acid hydroxyl groups for H-ZSM-5 and Ag/H-ZSM-5 zeolites reveals a significant promoting effect of silver cations on the H/D exchange reaction and therefore on methane activation. This effect has been rationalized in terms of reversible methane dissociation on the surface of Ag/H-ZSM-5 zeolite and further involvement in the exchange of the methoxy groups and the silver-hydride species. Ethane represents the first intermediate product of methoxy group transformation. It is formed by the reaction of a methoxy group with methane. Further, dehydrogenation of ethane offers ethene, producing immediately π-complexes with Ag+ cations, which are stable at temperature as high as 673 K. At 823 K π-complexes decompose and ethene undergoes oligomerization, cyclization, dehydrogenation, and aromatization to give benzene. In the presence of methane, ethene π-complexes decompose and become involved in oligomerization and aromatization reaction at lower temperature, already at 673 K. Methane is also involved in the reaction of coaromatization with ethene. This involvement occurs by the alkylation of aromatics, formed from ethene, with methane. Further demethanation of methylbenzenes in the presence of dihydrogen evolved at the stages of ethene transformation to aromatics produces benzene as the main reaction product.

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 1 H 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 13 C 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.

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