Heterometallic [Cu–O–M]2+ active sites for methane C–H activation in zeolites: stability, reactivity, formation mechanism and relationship to other active sites

Abstract

The formation and reactivities of [Cu–O–M]²⁺ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in metal-exchanged zeolites, as well as stabilities of these species towards autoreduction by O₂ elimination are investigated with density functional theory. These species were investigated in zeolite mordenite in search of insights into active site formation mechanisms, the relationship between stability and reactivity as well as discovery of heterometallic species useful for isothermal methane-to-methanol conversion (MMC). Several [Cu–O–M]²⁺ species (M = Ti–Cr and Zr–Mo) are substantially more stable than [Cu₂O]²⁺. Other [Cu–O–M]²⁺ species, (M = Mn–Ni and Ru–Ag) have similar formation energies to [Cu₂O]²⁺, to within ±10 kcal mol⁻¹. Interestingly, only [Cu–O–Ag]²⁺ is more active for methane activation than [Cu₂O]²⁺. [Cu–O–Ag]²⁺ is however more susceptible to O₂ elimination. By considering the formation energies, autoreduction, cost and activity towards the methane C–H bond, we can only conclude that [Cu₂O]²⁺ is best suited for MMC. Formation of [Cu₂O]²⁺ is initiated by proton transfer from aquo ligands to the framework and proceeds mostly via dehydration steps. Its μ-oxo bridge is formed via water-assisted condensation of two hydroxo groups. To evaluate the relationship between [Cu₂O]²⁺ and other active sites, we also examined the formation energies of other species. The formation energies follow the trend: isolated [Cu–OH]⁺ < paired [Cu–OH]⁺ < [Cu₂O]²⁺ < [Cu₃O₃]²⁺. Inclusion of Gibbs free-energy corrections indicates activation temperatures of 257, 307 and 327 and 331 °C for isolated [Cu–OH]⁺, paired [Cu–OH]⁺, [Cu₂O]²⁺ and [Cu₃O₃]²⁺, respectively. The provocative nature of the lower-than-expected activation temperature for isolated [Cu–OH]⁺ species is discussed.