Activity of Cu–Al–Oxo Extra-Framework Clusters for Selective Methane Oxidation on Cu-Exchanged Zeolites

Abstract

Cu-zeolites are able to directly convert methane to methanol via a three-step process using O 2 as oxidant. Among the different zeolite topologies, Cu-exchanged mordenite (MOR) shows the highest methanol yields, attributed to a preferential formation of active Cu–oxo species in its 8-MR pores. The presence of extra-framework or partially detached Al species entrained in the micropores of MOR leads to the formation of nearly homotopic redox active Cu–Al–oxo nanoclusters with the ability to activate CH 4 . Studies of the activity of these sites together with characterization by 27 Al NMR and IR spectroscopy leads to the conclusion that the active species are located in the 8-MR side pockets of MOR, and it consists of two Cu ions and one Al linked by O. This Cu–Al–oxo cluster shows an activity per Cu in methane oxidation significantly higher than of any previously reported active Cu–oxo species. In order to determine unambiguously the structure of the active Cu–Al–oxo cluster, we combine experimental XANES of Cu K- and L-edges, Cu K-edge HERFD-XANES, and Cu K-edge EXAFS with TDDFT and AIMD-assisted simulations. Our results provide evidence of a [Cu 2 AlO 3 ] 2+ cluster exchanged on MOR Al pairs that is able to oxidize up to two methane molecules per cluster at ambient pressure.

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

“…Further, we can exclude Cu-Al-oxo extra-framework clusters as proposed by Lee et al having an outsize role in explaining our reported material differences as we used both NH 4 and H forms for the ion exchange with copper. 52 It did not lead to distinct activity trends (Table S5). Instead, we postulate that the acid site accessibility degree induced by the Al site distribution influences the reactivity of the subsequently formed Cu-oxo species.…”

“…Quantification of octahedral, extra-framework Al with 27 Al MAS NMR (Figure S8) resulted in largely the same amount for all synthesized MOR. Note that the discrepancy to the 1 H MAS NMR and DRIFTS data can be rationalized by the presence of small extra-framework Al clusters. , Furthermore, it is possible for some more distorted Al to be broadened beyond detection at this field strength. , …”

“…It is clear that the two investigated Cu,H-MOR systems reported here have diverging activities while showing no differences in selectivity. Further, we can exclude Cu-Al-oxo extra-framework clusters as proposed by Lee et al having an outsize role in explaining our reported material differences as we used both NH 4 and H forms for the ion exchange with copper . It did not lead to distinct activity trends (Table S5).…”

“…Note that the discrepancy to the 1 H MAS NMR and DRIFTS data can be rationalized by the presence of small extra-framework Al clusters. 40,52 Furthermore, it is possible for some more distorted Al to be broadened beyond detection at this field strength. 53,54 1 H MAS NMR by itself is not able to resolve spatial differences between the bridging hydroxyl groups (3.8 ppm).…”

In Situ XAFS, XRD, and DFT Characterization of the Sulfur Adsorption Sites on Cu and Ce Exchanged Y Zeolites

“…This may occur in the zeolite samples as the framework structure restricts the location of O atoms around the cation sites. These XANES features have previously been used for analysis of the oxidation states of Cu species in zeolites including faujasite, ZSM-5, mordenite, ,, chabazite, MFI, and others . Additionally, the first derivative XANES data can be used to determine the oxidation state of the copper atoms present.…”