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

摘要

The heterolytic dissociation of CH4 over silver cationic clusters (Ag_n~+)in Ag~+-exchanged zeolites leads to the formation of silver hydride (Ag_n-H)and methyl cations,which then reacts with C_2H_4 to form C_3H_6.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 C_2H_4.Using ~(13)C-labeled methane (~(13)CH_4)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 ~(13)C-labeled propene (~(13)CC_2H_6).In-ZSM-5 catalyzes the formation of not only propene,but also benzene and toluene.~(13)C label atoms are not found in the benzene thus produced,indicating that benzene originates entirely from C_2H_4.However,the occurrence of singly ~(13)C-labeled toluene (~(13)CC_6H_8)implies that toluene is formed by the reaction of benzene with ~(13)CH_4.The alternative reaction path,involving ~(13)CC_6H_8 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 ~(13)CH_4 with benzene.