The Alkyl and Carbenium Pathways of Methane Activation on Ga-Modified Zeolite BEA: 13C Solid-State NMR and GC-MS Study of Methane Aromatization in the Presence of Higher Alkane

By using C-13 solid-state NMR spectroscopy and GC-MS analysis, the activation of methane and coaromatization of methane and propane have been monitored on gallium-modified zeolite BEA at 573-823 K. A noticeable degree involvement of the C-13-label from methane-DC into the aromatic reaction products (benzene, toluene) has been demonstrated. The major intermediate of the methane activation represents gallium-methyl species, which are formed by methane dissociative adsorption on Ga2O3 species of the zeolite. The minor species of methane activation, Ga-methoxy groups, provide the involvement of methane into aromatics by the methylation of aromatic molecules, which are generated exclusively from propane, by the mechanism of electrophilic substitution. Ga-methyl species can serve as methylating nucleophilic agent for the reaction of nucleophilic substitution with participation of aromatic molecules, which contain the electron-withdrawing substitutes.