Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

Abstract

A key challenge for metal-exchanged zeolites is the determination of metal cation speciation and nuclearity under synthesis and reaction conditions. Copper-exchanged zeolites, which are widely used in automotive emissions control and potential catalysts for partial methane oxidation, have in particular evidenced a wide variety of Cu structures that are observed to change with exposure conditions, zeolite composition, and topology. Here, we develop predictive models for Cu cation speciation and nuclearity in CHA, MOR, BEA, AFX, and FER zeolite topologies using interatomic potentials, quantum chemical calculations, and Monte Carlo simulations to interrogate this vast configurational and compositional space. Model predictions are used to rationalize experimentally observed differences between Cu-zeolites in a wide-body of literature, including nuclearity populations, structural variations, and methanol per Cu yields. Our results show that both topological features and commonly observed Al-siting biases in MOR zeolites increase the population of binuclear Cu sites, explaining the small population of mononuclear Cu sites observed in these materials relative to other zeolites such as CHA and BEA. Finally, we used a machine learning classification model to determine the preference to form mononuclear or binuclear Cu sites at different Al configurations in 200 zeolites in the international zeolite database. Model results reveal several zeolite topologies at extreme ends of the mononuclear vs binuclear spectrum, highlighting synthetic options for realization of zeolites with strong Cu nuclearity preferences.

Structure and Reactivity of Binuclear Cu Active Sites in Cu-CHA Zeolites for Stoichiometric Partial Methane Oxidation to Methanol

“…As shown in Figure b, mono­(μ-oxo)­dicopper and mono­(μ-hydroxyl)­dicopper species have similar free energy, which might rationalize why both hydroxyl-bridged , and mono­(μ-oxo)-bridged ,, dicopper sites have been reported in Cu-CHA, suggesting a pool of binuclear Cu structures that are present in some experimental Cu-CHA materials, dependent on synthesis conditions and Cu and Al loading. The sensitivity of site composition to the Al–Al pair site highlights the role of zeolite topology in influencing Cu speciation, helping to rationalize, for instance, the observation that larger average distances between Al pairs in a framework influence speciation between PMO-active and inactive species. ,− …”

“…In contrast to Figure a, these materials are expected to be dominated by binuclear Cu species over a large composition range, including all samples from Table that do not exchange Co 2+ (Samples D–M) and samples that do (but only at higher Cu/Al ratios; Sample A). Wijerathne et al conducted an extensive exploration of the competition between Cu monomers and dimers in various Al pair configurations across different zeolite frameworks. Their estimated phase diagrams resemble those presented in Figure .…”

Copper–oxygen adducts: new trends in characterization and properties towards C–H activation

“…241 Additionally, many techniques and analyses have been applied for this purpose, ranging from rR spectroscopy, isotope labelling experiments and DFT calculations and machine learning. 242,243 The aim of this section is to present the approach dealing with the parameters mentioned above towards a better understanding covering the reusability of the zeolitescontaining copper-heterogenized systems. Cu-O species immobilized in zeolites micropores have been shown to be efficient catalysts for the selective oxidation of methane, among other substrates.…”

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

“…[4] One class of materials that can selectively convert methane to methanol is metal-exchanged zeolites, [5][6][7][8][9][10][11][12] and in particular Cu-exchanged zeolites have attracted attention. After activation at high temperature in an oxidative environment, Cu-oxo [13,14,23,24,[15][16][17][18][19][20][21][22] and Cuhydroxy clusters, [25,26] active in the conversion of methane, are stabilized in these zeolites. Although the catalytic conversion of methane with low conversion [27] rates or low selectivities [28] to methanol has been reported, the reaction is most often pursued in a stepwise process.…”