Direct methane utilization through benzene dehydroalkylation catalyzed by Co²⁺ sites in ZSM-5 intersections

Cobalt cations exchanged in ZSM-5 are active for the direct dehydromethylation of benzene with CH₄. While toluene is the primary product, an alternate reaction pathway leads also to the formation of biphenyl and phenyltoluene. This not only diminishes the selectivity but also contributes to the deactivation of the catalyst. Temperature-programmed surface reactions show that benzene adsorbs strongly on the exchanged Co²⁺ cations, essentially blocking the sites for alkylation below 400 °C. At temperatures exceeding 400 °C, the partial desorption of benzene enables its reaction with CH₄, leading to the formation of toluene. The formation of biphenyl is kinetically hindered, and its onset is only observed at temperatures above 450 °C. When CH₄ is allowed to chemisorb on Co²⁺ sites prior to exposure to benzene, the formation of toluene is detected at temperatures below 400 °C. By infrared spectroscopy of benzene adsorption and UV–Vis spectroscopic characterization, we identified the existence of intersections in ZSM-5 where three H⁺ are in proximity. The ion exchange of a zeolite lattice Al³⁺ pair in such positions leads to the formation of a Co²⁺ site near a free Brønsted acid site. We observed a correlation between the concentration of this type of Co²⁺ sites and the catalytic activity of Co-ZSM-5 in benzene alkylation with CH₄. This is further supported by the catalytic tests of Co-ZSM-5 materials with specifically a larger proportion of Co sites in sinusoidal channels, as a result of acidic conditions during ion exchange. Based on this, we propose that toluene formation takes place via heterolytic dissociation of CH₄ on a Co²⁺ site at intersections followed by reaction with benzene strongly interacting with an adjacent BAS.