Mechanistic Insight into the Propane Oxidation Dehydrogenation by N2O over Cu-BEA Zeolite with Diverse Active Site Structures

Abstract

The present work theoretically investigated propane oxidation dehydrogenation by utilizing N2O as an oxidant (N2O-ODHP) over Cu-BEA with three different types of active site, including monomeric Cu ([Cu]+), dimeric Cu ([Cu−Cu]2+), and distant monomeric Cu sites ([Cu]+—[Cu]+). Energetically, we calculated that the monomeric [Cu]+ is favorable for the αH dehydrogenation step (∆E = 0.05 eV), which, however, suffers from high barriers of N2O dissociation and βH dehydrogenation steps of 1.40 and 1.94 eV, respectively. Although the dimeric [Cu−Cu]2+ site with a Cu—Cu distance of 4.91 Å is much more favorable for N2O dissociation (0.95 eV), it still needs to overcome an extremely high barrier (∆E = 2.15 eV) for βH dehydrogenation. Interestingly, the distant [Cu]+—[Cu]+ site with the Cu—Cu distance of 5.82 Å exhibits low energy barriers for N2O dissociation (0.89 eV) and ODHP steps (0.01 and 0.33 eV) due to the synergistic effect of distant [Cu]+. The microkinetic analyses quantitatively verified the superior activity of the distant [Cu]+—[Cu]+ site with a reaction rate being eight to nine orders of magnitude higher than those of the monomeric and the dimeric Cu sites, and this is related to its ready charge-transfer ability, as shown by the partial Density of State (PDOS) analysis and the static charge differential density analysis in this study. Generally, the present work proposes that the distance between the [Cu]+ sites plays a significant and important role in N2O-ODHP over the Cu-based zeolite catalyst and modulates Cu—Cu distance, and this constitutes a promising strategy for highly-efficient Cu-zeolite catalyst design for N2O-ODHP.