Copper-exchanged zeolites are active materials for the partial oxidation of methane to methanol (MtM), which is a potential alternative to petroleum-derived building blocks for the petrochemical industry. Many copper-exchanged zeolites of varying structure type are active for this reaction ([1],[2],[3]). During the course of a study to test the performance of copper-exchanged zeolites with 8- and 10-rings, we found that the methanol yield obtained using Cu-omega (MAZ framework type) (4) was significantly higher than that obtained with Cu-MOR, which is the most-often used material for this reaction ([4]). The framework structure of zeolite omega ([5]) is composed of gmelinite (gme) cavities stacked in columns parallel to c. They are connected laterally via ellipsoidal 8-rings to form a hexagonal array of columns that define a round 12-ring channel parallel to c. The 8-ring channels between the gme columns are not unlike those found in mordenite, but they are not accessible from the 12-ring channel. However, they do connect the gme cavities via a convoluted 3-dimensional 8-ring channel system. To investigate the reason for the remarkable catalytic performance of this material, we are using different characterization techniques, such as FTIR, XAS and X-ray diffraction, combined with molecular simulation. In this work, we present the results from a structure analysis using synchrotron powder diffraction data collected in situ during the activation of Cu-omega under oxygen and during the reaction itself. In the activated material, copper species are found in the 6-rings in the gme columns. They are single copper species, as opposed to the dimers generally accepted as the active species in the partial oxidation of methane to methanol ([6]). This material contains a low Cu/Al ratio (0.07) and is not very active in MtM, indicative of low activity of copper ions.
A.B.P. thanks the Energy System Integration (ESI) platform at Paul Scherrer Institute for funding.
The staff at the MS beamline (Swiss Light Source, Switzerland) is acknowledged for their help in data collection.
[1] M.H. Groothaert, P.J. Smeets, P.A. Jacobs, et al., J. Am. Chem. Soc., 2005, 127, 1394-1395.
[2] S. Grundner, M. Markovits, J. Lercher, et al., Nat. Commun., 2015, 6, 7546.
[3] P. Tomkins, S. E. Bozbag, F. Krumeich, M. B. Park, M. Ranocchiari, J. A. van Bokhoven, Angew. Chem. Int. Ed., 2016, 55, 5467-5471.
[4] C. Baerlocher, L. B. McCusker, Database of Zeolite Structures: http://www.iza-structure.org/databases/.
[5] M. B. Park, et al, manuscript in preparation.
[6] E. Galli, Rend. Soc. Ital. Mineral. Petrol., 1975, 31, 599-612.
[7] E.M.C. Alayon, M. Nachtegaal, A. Bodi, M. Ranocchiari, J.A. van Bokhoven, Phys. Chem. Chem. Phys., 2015, 17, 7681-7693.