Methane to Methanol: Structure–Activity Relationships for Cu-CHA

Journal of the American Chemical Society

5 years ago

Methane to Methanol: Structure–Activity Relationships for Cu-CHA

Stian Svelle, Karl Petter Lillerud, Pablo Beato, Bjørnar Arstad, Unni Olsbye, Shewangizaw Teketel, Elisa Borfecchia, Dimitrios K. Pappas, Ilia A. Pankin, Carlo Lamberti, Kirill A. Lomachenko, Silvia Bordiga, Gloria Berlier, Matteo Signorile, Andrea Martini, Michael Dyballa

Cu-exchanged zeolites possess active sites that are able to cleave the C–H bond of methane at temperatures ≤200 °C, enabling its selective partial oxidation to methanol. Herein we explore this process over Cu-SSZ-13 materials. We combine activity tests and X-ray absorption spectroscopy (XAS) to thoroughly investigate the influence of reaction parameters and material elemental composition on the productivity and Cu speciation during the key process steps. We find that the Cu^II moieties responsible for the conversion are formed in the presence of O₂ and that high temperature together with prolonged activation time increases the population of such active sites. We evidence a linear correlation between the reducibility of the materials and their methanol productivity. By optimizing the process conditions and material composition, we are able to reach a methanol productivity as high as 0.2 mol CH₃OH/mol Cu (125 μmol/g), the highest value reported to date for Cu-SSZ-13. Our results clearly demonstrate that high populations of 2Al Z₂Cu^II sites in 6r, favored at low values of both Si:Al and Cu:Al ratios, inhibit the material performance by being inactive for the conversion. Z[Cu^IIOH] complexes, although shown to be inactive, are identified as the precursors to the methane-converting active sites. By critical examination of the reported catalytic and spectroscopic evidence, we propose different possible routes for active-site formation.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b06472

DOI: 10.1021/jacs.7b06472