Conditions to meet for the [CuOH]+ site to be favorable and reactive toward the conversion of methane to methanol over Cu-MOR zeolite

Abstract

The nature of the active site in copper-exchanged zeolites catalyzing the direct conversion of CH₄ to CH₃OH has been a long yet important discussion. The mononuclear CuOH active site, in particular, has been a hot discussion, in addition to the mostly reported Cu₂O and Cu₃O₃ active sites. While the binuclear and trinuclear species have been known to be stable and CH₄ reactive, the mononuclear one remains unclear. Herein, we employ density functional theory (DFT) calculations to evaluate the stability and reactivity of CuOH-MOR zeolite toward CH₄ oxidation to CH₃OH. We find that the CuOH site is thermodynamically stable at low oxygen chemical potential and highly reactive with some conditions to meet: (i) the CuOH site must be hosted on specific Al sites and oriented parallel along the 12-membered ring of MOR to stabilize the resultant ˙CH₃ radical and (ii) a Cu²⁺ inactive site must coexist adjacent to the CuOH active site to provide extra electron required to form CH₃OH, although these two Cu monomers are prone to the formation of Cu–OH–Cu dimer when they are too close to each other. Furthermore, we also find that the stability of the (in the homolytic C–H cleavage) or the C–Cu (in the heterolytic C–H cleavage) interaction formed during H–CH₃ activation is mainly responsible for controlling the activation barrier. This paper clarifies previous contradictive results on the role, stability, and activity of the CuOH site and provides theoretical support and detailed elucidation for the recent experimental reports.