Heterolytic Dissociation of C–H Bond of Methane over Ag+-exchanged Zeolites and Conversion of Methane into Higher Hydrocarbons in the Presence of Ethene or Benzene

Abstract The heterolytic dissociation of CH4 over silver cationic clusters (Agn+) in Ag+-exchanged zeolites leads to the formation of silver hydride (Agn–H) and methyl cations, which then reacts with C2H4 to form C3H6. This process provides methane conversion of 13.2% at 673 K to afford higher hydrocarbons, such as toluene. Under these reaction conditions, H–ZSM-5 only catalyzes ethene conversion to higher hydrocarbons, such as butenes, and no methane conversion occurs. The reaction of CH4 with benzene also proceeds to form toluene and xylenes over Ag–ZSM-5 at 673 K. Zeolites prepared by exchange with other metal cations, including In and Ga, also activate CH4 in the presence of C2H4. Using 13C-labeled methane (13CH4) as a tracer, propene is shown to be a primary product for the ethene reaction based on the observation of a significant proportion of singly 13C-labeled propene (13CC2H6). In–ZSM-5 catalyzes the formation of not only propene, but also benzene and toluene. 13C label atoms are not found in the benzene thus produced, indicating that benzene originates entirely from C2H4. However, the occurrence of singly 13C-labeled toluene (13CC6H8) implies that toluene is formed by the reaction of benzene with 13CH4. The alternative reaction path, involving 13CC6H8 formation by reaction of propene with n-butenes generated by ethene dimerization, can be refuted by confirmation of the toluene origin through direct reaction of 13CH4 with benzene.