Mechanistic insights into the stepwise oxidation of methane to methanol and formaldehyde over Cu-exchanged SSZ-13

Abstract

The partial oxidation of methane to methanol is investigated over Cu-exchanged SSZ-13 using grid-based projector augmented wave within density functional theory and FDMNES XANES simulations. CH₄ activation at the Cu–O site proceeds, followed by CH₃OH oxidation to HCHO. Projected density of states analysis shows hybridization of the O 2p, Cu 3d, and H 1s orbitals at the transition states, confirming C–H bond activation at the Cu–O moiety. FDMNES simulations reveal a red shift in the Cu K-edge during both steps, indicating Cu reduction and its participation in the redox cycle. These results demonstrate that Cu–O sites are responsible for both methane activation and methanol oxidation with high selectivity toward partial oxidation products.

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1. Introduction

Partial oxidation of methane to methanol is of great interest due to its potential to directly convert abundant natural gas into a valuable liquid fuel and chemical feedstock under mild conditions, offering a more efficient and sustainable alternative to conventional multi-step syngas-based processes.^1–5 Cu-exchanged zeolite is known as a promising catalyst for the selective oxidation of methane to methanol, owing to its isolated Cu sites that can be considered as activating methane at relatively low temperatures with high selectivity toward methanol formation.^6–8 However, the catalytic mechanism of Cu-exchanged zeolites remains unclear. Extensive studies have been conducted to unveil how Cu species are incorporated into the zeolite framework and to identify the nature of the active sites responsible for methane activation.^9–13

In such circumstances, another question also arises, as the partial oxidation of methane over Cu-exchanged zeolites also produces formaldehyde.^13,14 It remains unclear whether formaldehyde is formed as a primary intermediate en route to methanol or as a byproduct through the overoxidation of methanol. The active sites responsible for formaldehyde formation over zeolites are not well understood. Here, SSZ-13 is investigated because Cu-exchanged SSZ-13 has been reported to be an active catalyst for methane oxidation.^15–19 Thus, density functional theory calculations are performed on Cu-exchanged SSZ-13 to identify the reaction mechanisms and identify the active sites involved in the selective formation of methanol and formaldehyde from methane.

2. Methods

Grid-based projector-augmented wave (GPAW) is implemented within density functional theory.²⁰ The exchange correlation of Perdew–Burke–Ernzerhof²¹ along the spin polarization is implemented. Periodic boundary conditions are applied with the special K point of (2 × 2 × 2) with a grid spacing of 0.18 Å.²² A climbing-image nudged elastic band method by considering 6 images between each state is used for searching the transition state.²³ Finite difference method near edge structure (FDMNES) method is used for XANES simulation where the K-edge is used with a radius of 6 Å where the Green’s function formalism is used for the electronic structure calculation, and the energy range is set from −5 eV to +30 eV relative to the Fermi energy.²⁴

3. Results and discussion

Cu embdedded SSZ-13 zeolite is investigated through density functional theory. Atomic model is shown in Fig. 1 where the unit cell parameters in this work are 9.37 Å, 9.46 Å, and 9.44 Å. The atomic model is shown in Fig. 1. Two Si atoms are replaced with Al atoms to create Cu adsorption sites, and subsequently, two Cu atoms are placed at these Al sites. The stepwise conversion of CH₄ to CH₃OH and HCHO proceeds over Cu-embedded SSZ-13. The calculated energy diagram, including transition states and atomic structures, appears in Fig. 2(a) and (b). The first step involves hydrogen dissociation from CH₄, with the H atom binding to a nearby O atom and a transition state energy of 0.44 eV. The dissociated H then relocates to the side of the O atom, while CH₃ adsorbs onto the O atom, forming CH₃OH with a transition state energy of 0.12 eV; thus, CH₃OH is formed. One can see that the CH₃OH adsorption energy is calculated to be −2.26 eV, which is in good agreement with previous work.²⁵ In addition, these results agree with previous reports.²⁶

Fig. 1 Atomic model of the Cu-embedded SSZ-13 structure. (a) Periodic boundary conditions and (b) active site structure from (a).

Fig. 2 Energy diagram with corresponding transition states and atomic structure for CH₄ to CH₃OH (a) and (b), and CH₃OH to HCCO (c) and (d).

Assuming that the produced CH₃OH desorbs from the Cu site, regeneration of an O atom on the same or a neighboring Cu site can provide the active site for further CH₃OH activation to HCHO. Therefore, the CH₃OH to HCHO reaction over the Cu active site is calculated. CH₃OH is placed on the Cu site as shown in Fig. 2(c) and (d). A transition state calculation is performed for H atom dissociation from CH₃OH, where the H atom is adsorbed on the O atom at the top of the Cu site. However, upon forming CH₂OH, the intermediate spontaneously rearranges to CH₂O with the H atom bound to a neighboring O atom during the relaxation process as shown in Fig. 2(c), indicating that CH₂OH is an unstable species. As a result, the calculated transition energy for CH₃OH to HCHO + H + H is 0.23 eV. Thus, the Cu site is considered to be an active site for CH₃OH to HCHO.

Electronic structure of the activation site is investigated via projected density of states (PDOS), as shown in Fig. 3. At TS1 in Fig. 2(a), the p state of the O atom overlaps with the H 1s orbital of CH₄, as well as with the d state of Cu, in an antibonding configuration. This interaction weakens the C–H bond, indicating that the O atom is responsible for activating CH₄, as shown in Fig. 3(b). At TS2 in Fig. 2(a), the s state of the H shifts to lower energy within the bonding region, indicating that the H atom is stabilized at the transition state. Lastly, TS in Fig. 2(c) demonstrates that the s state of H in CH₃OH, the p state of O and the d state of Cu are overlapping at antibonding state. Thus, electronic structures confirm that the Cu–O site is the active site for producing CH₄ and HCHO.

Fig. 3 Projected density of state at (a) CH₄, (b) TS1, (c) CH₃, (d) CH₃, (e) TS2, (f) CH₃OH, (g) CH₃OH, (h) TS, and (i) HCHO.

FDMNES simulations are performed to identify the electronic behavior of Cu during the reaction. The simulated XANES spectra for Cu in the CH₄ to CH₃OH and CH₃OH to HCHO conversion steps are presented in Fig. 4(a) and (b). Upon the formation of CH₃OH, the Cu absorption edge shifts toward lower energy, indicating a partial reduction of Cu during the CH₄ to CH₃OH conversion, as shown in Fig. 4(a). A similar red shift is observed in the CH₃OH to HCHO conversion, suggesting further reduction of the Cu center as shown in Fig. 4(b). This behavior indirectly indicates that Cu actively participates in the redox cycle, serving as the primary site for the activation and transformation of the C–H bond during both reaction steps.

Fig. 4 FDMNES simulation for the Cu atom in (a) CH₄ to CH₃OH and (b) CH₃OH to HCHO.

Adsorption of the produced CH₃OH on Cu is calculated to be strongly exothermic, with an adsorption energy of −2.26 eV, as shown in Fig. 2(a). Such strong binding implies that desorption of CH₃OH from the Cu site must be carefully considered. Therefore, the CH₃OH desorption process is further investigated. Previous study suggests that co-adsorbed H₂O can facilitate the release of CH₃OH from Cu sites.²⁵ To examine this effect, an H₂O molecule is introduced to the CH₃OH adsorbed Cu site as shown in Fig. 5(a). Upon adsorption of H₂O, the CH₃OH adsorption energy becomes significantly weakened to −1.04 eV, while the H₂O adsorption energy is calculated to be −1.01 eV. In addition, the transition state energy barrier for CH₃OH desorption in the presence of H₂O is found to be 0.29 eV. Together, these results indicate that H₂O plays a crucial role in promoting CH₃OH desorption from Cu sites.

Fig. 5 Desorption of CH₃OH upon the adsorption of H₂O. (a) Atomic models and (b) the corresponding energy diagram.

4. Conclusion

Density functional theory and FDMNES simulations are combined to elucidate the mechanism of methane partial oxidation over Cu-exchanged SSZ-13 zeolite. The DFT calculations reveal that CH₄ is activated at the Cu–O site with an energy barrier of 0.44 eV, followed by CH₃OH formation and subsequent oxidation to HCHO with a barrier of 0.23 eV. The calculated energy diagram indicates that the transition states and adsorption energies fall within an energetically accessible range for the reaction. PDOS analysis demonstrates that the overlapping O 2p, Cu 3d, and H 1s orbitals at the transition states facilitate C–H bond cleavage. FDMNES simulations further indicate that the Cu K-edge shifts to lower energy during both reaction steps, evidencing a partial reduction of Cu and confirming its redox participation. Overall, these results establish that the Cu–O site functions as the active center for both CH₄ activation and CH₃OH oxidation, and that the redox dynamics of Cu are central to achieving selective methane oxidation in zeolitic catalysts.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available upon request.

Acknowledgements

This work is funded by the Japan Science and Technology Agency (JST), ERATO grant number (JPMJER1903), JST Mirai Program Grant Number JP-MJMI25G1, and JSPS KAKENHI Grant-in-Aid for Scientific Research (B) Grant Number (JP23H01762) and (24K01241).

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