Copper-exchanged zeolites are widely used in automotive emissions control and have been shown as potential catalysts for the methane to methanol reaction. A key challenge for Cu-exchanged zeolites is predicting the Cu cation speciation under reaction and synthesis conditions. Predicting the relative populations of Cu dimers and Cu monomers based on synthesis conditions is essential to estimate the catalytic activity of Cu-exchanged zeolites. The Al distribution in a zeolite is kinetically controlled during synthesis, and Cu speciation is primarily thermodynamically controlled. We used thermodynamics to predict the percentage of Cu dimers formed in two zeolites: chabazite (CHA) and mordenite (MOR). In previous studies, MOR consistently showed higher Cu efficiency than CHA for methane-to-methanol reaction where active sites are proposed to be Cu dimers. We hypothesized that the higher Cu dimer population is responsible for such higher Cu efficiency in Cu-MOR. To verify our hypothesis, we modeled the partition of exchanged Cu into monomers and dimers. First, we evaluated Cu2+ exchange energies at multiple Al configurations using density functional theory (DFT) calculations. We utilized a machine learning interatomic potential and a classical force field to accelerate and minimize the number of DFT calculations required for exchange energy evaluations. We then populated each Al configuration in a repeated supercell (&gt;950 T sites) by iteratively adding Cu2+ according to the Boltzmann probabilities of each species at solid-state ion exchange conditions (100 ppm H2O, 20% O2, and 973 K). For CHA and MOR with random Al distributions, we found that the Cu dimer formation is energetically more favorable in MOR compared to CHA zeolites. In CHA, Cu mainly exchanged as monomers in six-membered rings, and Cu dimers formed only at higher Cu loadings (&gt;1% wt.). In contrast, exchanged Cu2+ in MOR formed a mixture of dimers and monomers even at lower Cu loadings (&lt;0.05 % wt.), resulting in a higher proportion of Cu dimers at a given Cu loading. Higher populations of Cu dimers in MOR materials could explain why MOR is more active for methane to methanol reaction.

Copper cation speciation under solid-state ion-exchange conditions in Chabazite and mordenite zeolites: Computational study

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