(314c) Cu-Exchanged SSZ-13 in the Stepwise Conversion of Methane to Methanol

The selective conversion of methane to methanol has remained a major scientific and practical challenge for several decades and has attracted significant attention recently due to the increased availability of natural gas. In this context, Cu exchanged zeolites have attracted significant interest, since they are known to selectively convert methane to methanol in a stepwise process. In this process, the nature of the sites responsible for the conversion of methane to methanol has been an active topic of discussion.

In this contribution, we study the impact of active site structure on the performance of the Cu exchanged zeolite SSZ-13 in the conversion of methane to methanol. In a first step we use phase diagrams obtained from first principles calculations to explore which Cu site structures can be stabilized in Cu-SSZ-13 under any conditions. We then use the information from the phase diagrams to identify activation conditions, where Cu-hydroxy or Cu-oxo dimers are stabilized. Reaction pathways for the different active sites indicate that each Cu-hydroxy dimer will convert one methane molecule to produce one methanol molecule, while each Cu-oxo dimer will convert two methane molecules and produce two methanol molecules. Finally, we experimentally synthesize Cu-exchanged SSZ-13 and choose two different sets of activation conditions to stabilize Cu-oxo dimers in one case and Cu-hydroxy dimers in the other case. In agreement with theoretical predictions, we find that Cu-oxo dimers are able to activate twice as much methane as Cu-hydroxy dimers.

In the future, we expect that the structure-function relationships we developed for methane to methanol conversion will inspire further developments for materials in the direct and selective conversion of methane to methanol.