Partial Substitution of Copper by Cobalt, Iron, and Nickel in the [Cu3O3]2+ Active Site Motif for Improved Methane to Methanol Conversion in the Zeolite Mordenite

Abstract

We have performed Density Functional Theory (DFT) calculations to examine the effects of partial substitution of copper atoms within the active site motifs involved in methane-to-methanol conversion (MMC) in copper-exchanged zeolites. The model catalyst [Cu3O3(H2O)6]2+ was compared to the [Cu2MO3(H2O)6]2+ species, where M is nickel, iron, or cobalt. The objective was to determine if the replacement of a copper atom with another earth-abundant metal would decrease the reaction barriers associated with methane C-H activation. A reduction in the barrier will lead to increased rates for MMC. We hypothesized that introduction of nickel, iron, or cobalt atoms into the active site motifs will induce novel electronic structure properties that will result in lower C-H activation barriers and increased rates for MMC. We hypothesized that the remaining two copper atoms would allow for high methanol selectivity, as seen in the all-copper system. Our results allowed us to conclude that replacement of a copper center by any of nickel, cobalt or iron results in lower C-H barriers. We also investigated descriptors that can be used to correlate the properties of the catalyst with its C-H transition state barrier. We determined that the hydrogen abstraction energies (HAE) do not correlate with the C-H transition state barriers at the H-O2 position but show correlation for the H-O1 position for the hetero-metallic systems. This suggests that while the correlation between HAE and activation barriers is proven for homo-metallic systems, such relationships only exist for their hetero-metallic analogues when measured in between the hetero-metallic atoms.