(528a) Selective Oxidation of Methane to Methanol in Zeolites: A Window of Opportunity

Despite decades of active research, selective oxidation of methane to methanol remains an unsolved ‘grand challenge’ of heterogeneous catalysts.¹ Although, previous experimental and computational^2-4 works have studied the mechanism in specific cation-exchanged zeolites; a universal, molecular level understanding of the methane activation and methanol extraction process is not yet available.

In this work, we use periodic DFT calculations to develop a unified framework that describes methane activation reaction and subsequently, enables us to identify strategies for improving the performance of cation-exchanged zeolites. Specifically, we explore a range of active site motifs, transition metal cations, zeolite topologies as well as binding sites for this reaction. Using the activation energy and formation energy of the active site as metrics, we obtain a ‘window of opportunity’ where we predict active site motifs to be both stable and sufficiently reactive to selectively oxidize methane. Encouragingly, a number of the known active sites (i.e. [Cu-OH]⁺, [Cu-O-Cu]²⁺, [Cu₃O₃]²⁺, copper-oxide clusters and [Fe-O]⁺²) lie within this region. More importantly, we also identify [M-O]⁺ (M = Ni, Co, Fe), [M-OH]⁺² (M = Ni, Co, Fe), and [bis(-O)Ni]²⁺as alternative target motifs for future catalyst design and synthesis. Finally, the origin of the high methanol selectivity can be explained by the (a) strong binding energy of methanol and (b) formation of surface methoxy species.

The atomistic and mechanistic insight presented in this work will guide future experimental syntheses, and will lead to the design and discovery of highly active cation-exchanged zeolite catalysts for the direct partial oxidation of methane to methanol.

References

1. Olivos-Suarez, A.I.; Szécsényi, À.; Hensen, E.J.M.; Ruiz-Martinez, J.; Pidko, E.A.; Gascon, J., ACS Catal. 2016, 6 (5)

2. Zhao, Z.-J.; Kulkarni, A. R.; Vilella, L.; Nørskov, J. K.; Studt, F., ACS Catal., 2016, 6 (6)

3. Kulkarni, A.R.; Zhao, Z.-J.; Siahrostami, S.; Norskov, J.K.; Studt, F., ACS Catal. 2016, 6 (10)

4. Latimer, A.A.*; Kulkarni, A.R.*; Yoo, J.S.; Aljama, H.; Tsai, C.; Abild-Pedersen, F.; Studt, F.; Nørskov, J.K., Nat. Mater. 2016 (DOI: 10.1038/nmat4760)