Exploring the Impact of Active Site Structure on the Conversion of Methane to Methanol in Cu‐Exchanged Zeolites

Abstract

In the past, Cu‐oxo or ‐hydroxy clusters hosted in zeolites have been suggested to enable the selective conversion of methane to methanol, but the impact of the active site’s stoichiometry and structure on methanol production is still poorly understood. Herein, we apply theoretical modeling in conjunction with experiments to study the impact of these two factors on partial methane oxidation in the Cu‐exchanged zeolite SSZ‐13. Phase diagrams developed from first‐principles suggest that Cu‐hydroxy or Cu‐oxo dimers are stabilized when O2 or N2O are used to activate the catalyst, respectively. We confirm these predictions experimentally and determine that in a stepwise conversion process, Cu‐oxo dimers can convert twice as much methane to methanol compared to Cu‐hydroxyl dimers. Our theoretical models rationalize how Cu‐di‐oxo dimers can convert up to two methane molecules to methanol, while Cu‐di‐hydroxyl dimers can convert only one methane molecule to methanol per catalytic cycle. These findings imply that in Cu clusters, at least one oxo group or two hydroxyl groups are needed to convert one methane molecule to methanol per cycle. This simple structure‐activity relationship allows to intuitively understand the potential of small oxygenated or hydroxylated transition metal clusters to convert methane to methanol.

Spectroscopic Investigation of the Role of Water in Copper Zeolite Methane Oxidation

“…Copper zeolites hold notable potential in this arena; they can oxidize methane at relatively low temperatures and pressures . Two well-defined active sites are competent for this reactivity: a mononuclear [CuOH] + site present in the autoreduced material , and a more reactive binuclear [Cu 2 O] 2+ site that forms upon activation of bicuprous sites in the autoreduced material with O 2 or N 2 O. − There are also proposals for other active sites including a trinuclear copper cluster, hydroxylated copper dimers, − and copper oxo clusters …”