Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

Abstract

Copper‐exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time‐resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono‐μ‐oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono‐μ‐oxo species, while the activation energy is two times lower.

Conditions to meet for the [CuOH]+ site to be favorable and reactive toward the conversion of methane to methanol over Cu-MOR zeolite

“…42 Despite this, the E a,app found here, compared to that computed by Heyer et al using a cluster model (19.3 kcal mol −1 ), 42 is closer to the reported experimental value (7.9 ± 2.3 kcal mol −1 ). 23 Comparing the geometries of TS Ho and TS He (Fig. S2 †), we found them similar except that TS He also involves a C⋯Cu interaction in addition to the C⋯H⋯OH interactions.…”

“…In addition, the authors concluded that two kinds of Cu monomers (i.e., Cu 2+ and CuOH + ) coexist in Cu-MOR, but only the latter is CH 4reactive. 23 Whether the Cu 2+ site acts only as a spectator (redox-inert) 38,39 or also plays a primary 40 42 However, in contrast to the findings by Sushkevich et al, 23 this CuOH monomer has been reported to be less reactive than the Cu 2 O dimer. 42 Another example of Cu monomer, i.e., Cu single atom catalyst on different supports, [43][44][45] was also reported to be active for the CH 4 oxidation to CH 3 OH.…”

“…[17][18][19][20] Among them, [Cu 2 O] 2+ has been identified in different zeolite topologies. [21][22][23][24][25] The ZSM-5 zeolite was first reported to host this Cu dimer active site, 26 which is able to activate CH 4 at 175 °C with an activation energy of only 15.7 ± 0.5 kcal mol −1 . The MOR 21,27 and SSZ-13 (ref.…”

“…37 However, Sushkevich et al recently reported experimental evidence for the superior activity of the CuOH site toward CH 4 with activation energy (7.9 kcal mol −1 ) two times lower than that for the Cu dimer site (14.3 kcal mol −1 ). 23 Despite this, the CH 4 reaction at this Cu monomer site is slower than that at the Cu dimer site for unknown reasons. In addition, the authors concluded that two kinds of Cu monomers (i.e., Cu 2+ and CuOH + ) coexist in Cu-MOR, but only the latter is CH 4reactive.…”

“…This study clarifies previous contradictive results on the role, stability, and activity of the CuOH site and provides theoretical support and detailed elucidation for the recent experimental findings. 23,40,42 2 Computational details Spin-polarized calculations were performed using the Kohn-Sham formulation 46,47 as implemented in the Vienna Ab initio Simulation Package (VASP) version 5.4.4. 48,49 The projector augmented wave (PAW) method was employed to describe the interaction between cores and electrons.…”