Methane C–H Activation by [Cu₂O]²⁺ and [Cu₃O₃]²⁺ in Copper-Exchanged Zeolites: Computational Analysis of Redox Chemistry and X-ray Absorption Spectroscopy

Abstract

There is an ongoing debate regarding the role of [Cu3O3]2+ in methane-to-methanol conversion by copper-exchanged zeolites. Here, we perform electronic structure analysis and localized orbital bonding analysis to probe the redox chemistry of its Cu and μ-oxo sites. Also, the X-ray absorption near-edge structure, XANES, of methane activation in [Cu3O3]2+ is compared to that of the more ubiquitous [Cu2O]2+. Methane C–H activation is associated with only the Cu2+/Cu+ redox couple in [Cu2O]2+. For [Cu3O3]2+, there is no basis for the Cu3+/Cu2+ couple’s participation at the density functional theory ground state. In [Cu3O3]2+, there are many possible intrazeolite intermediates for methane activation. In the nine possibilities that we examined, methane activation is driven by a combination of the Cu2+/Cu+ and oxyl/O2– redox couples. Based on this, the Cu 1s-edge XANES spectra of [Cu2O]2+ and [Cu3O3]2+ should both have energy signatures of Cu2+ → Cu+ reduction during methane activation. This is indeed what we obtained from the calculated XANES spectra. [Cu2O]2+ and [Cu3O3]2+ intermediates with one Cu+ site are shifted by 0.9–1.7 eV, while those with two Cu+ sites are shifted by 3.0–4.2 eV. These are near a range of 2.5–3.2 eV observed experimentally after contacting methane with activated copper-exchanged zeolites. Thus, activation of methane by [Cu3O3]2+ will lead to formation of Cu+ sites. Importantly, for future quantitative XANES studies, involvement of O– + e– → O2– in [Cu3O3]2+ implies a disconnect between the overall reactivity and the number of electrons used in the Cu2+/Cu+ redox couple.

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

“…Simply, electrons are transferred from methane to copper center(s). We recently used electronic structure analysis of several intrazeolite surface methoxy species to show that methane activation in [Cu 3 O 3 ] 2+ is due to an interesting combination of the Cu 2+ /Cu + and O – /O 2– redox couples . Simply, an electron is transferred from methane to a copper site and another reduces an oxyl.…”

Time-Dependent Long-Range-Corrected Double-Hybrid Density Functionals with Spin-Component and Spin-Opposite Scaling: A Comprehensive Analysis of Singlet–Singlet and Singlet–Triplet Excitation Energies

“…Therein, we showed how the inclusion of a single parameter, which controls the interplay between the long- and short-range regimes, can improve long-range excitations such as Rydberg states and CT excitations without much loss in accuracy for local-valence transitions. We particularly note that both functionals delivered the best TD-DFT results to date for L a and L b transitions in PAHs, and that they have already been successfully used in applications to systems of biological or technological relevance. − …”

Copper–oxygen adducts: new trends in characterization and properties towards C–H activation

“…These pores play a crucial role in MTH, providing not only insights into the distribution of copper within the lattice but also enabling better control over the formation and characterization of various Cu-O species. [262][263][264][265][266][267][268][269][270][271] In general, three of the most active Cu-O species are [Cu2(trans-µ-O2)] 2+ , [Cu2(µ-O)] 2+ and [Cu3(µ-O3)] 2+ , 249 in which the copper atoms are connected to the zeolite scaffold by aluminate units. 272 However, it is noteworthy to mention that not only dicopper and tricopper species are formed within the zeolite lattice but also monomeric species.…”

“…However, depending on the intrinsic kinetics of the Cu site types and the distribution of such types within a given material, this activation temperature has an impact on the overall methanol formation as well as on the selectivity. 250 Additionally, although O 2 as oxidizing gas has been largely used for such catalyst activation, other gases like N 2 O have also been employed showing a higher proportion of [Cu 2 O] 2+ species. 251 species are postulated to be one type of active species under these conditions, their formation ratio can be tuned by applying a self-reduction treatment, usually performed at T > 500 K under He, CO or mixtures of both gases.…”