Methane Activation and Transformation on Ag/H-ZSM-5 Zeolite Studied with Solid-State NMR

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.