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

Abstract

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.

Pathways of Methane Transformation over Copper‐Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

“…These observations indicates the direct involvement of copper species to the reaction mechanism . Figure shows 13 C HPDEC NMR spectra of reaction products as observed from 298 to 648 K. At ambient temperature, the only signal was the broad signal at ≈−6 ppm, corresponding to methane . The strong broadening is associated with the presence of paramagnetic Cu II species, which comprise most of the copper fraction in the activated copper‐exchanged mordenite.…”

Progress in Developing a Structure‐Activity Relationship for the Direct Aromatization of Methane

“…It has been proposed for Ag that the cationic species act as a Lewis acid that is able to abstract hydrogen from methane, while methane forms a methoxy‐species with the BAS (Figure ) …”

“…Although most often ethylene, ethane and acetylene are proposed as reaction intermediates, higher intermediates are conceivable as well, especially if ethylene strongly adsorbs on the active metal and has to further react with gas‐phase CH 4 to desorb. This was speculated for Ag . Studying adsorption strength of ethylene at the active site however is difficult, since the structure of the active site formed at reaction conditions is unknown.…”

Catalytic Cracking of Supercritical n-Dodecane over Wall-Coated Nano-Ag/HZSM-5 Zeolites

“…The drop of Brønsted acid site amount is because some Ag + interacts with the oxygen of the bridging hydroxyl groups by replacing the protons (ion exchange effect). One need note that the silver ion-exchanged Brønsted acid sites also have the activity for hydrocarbon activation, 17,18 and it can recover quickly in the presence of hydrogen. 16 Unlike the Brønsted acid sites, both weak and strong Lewis acids increased from 0.0 Ag-NZ to 2.0 Ag-NZ.…”