Molecular insight into the role of zeolite lattice constraints on methane activation over the Cu-O-Cu active site

Understanding the factors that influence the activity of a catalyst toward CH₄ activation is of high importance for tuning the catalyst performance or designing new, better catalysts. Here, we performed a set of density functional theory (DFT) calculations on the H-CH₃ bond cleavage over the Cu-O-Cu active site in the MOR zeolite with various Al-pair arrangements to obtain molecular insight into the structure-activity relation and clarify key parameters that define the Cu-O-Cu reactivity toward CH₄. We found that weakening of the Cu-O-Cu bond during CH₄ activation is crucial for determining the O-H bond strength and thus the Cu-O-Cu reactivity. In this regard, the zeolite lattice constraints are found to play a significant role as, on the one hand, it strengthens the Cu⋯Cu interaction and consequently weakens the Cu-O-Cu bonds and, on the other hand, it forces the Cu-O-Cu bond elongation process to destabilize the active site structure. The non-planar Cu-O-Cu geometry, due to lattice constraints, is also found to make the CH₄ adsorption site, whether positioned closer to the μ-O or the Cu atom, crucial in determining the C-H activation product, i.e., a ˙CH₃ radical or a Cu₂-CH₃^- ligand.