Novel Mechanistic Insights into Methane Activation over Fe and Cu Active Sites in Zeolites: A Comparative DFT Study Using Meta-GGA Functionals

Fe- and Cu-exchanged zeolites are known to oxidize methane directly to methanol at low temperature and have been intensively discussed in recent literature studies including theoretical works based on the density functional theory (DFT). However, there are a number of computational results that are contradictive to each other due to a limitation in accessing accurate methods for realistic models. To address this issue, in this study, we utilize a relatively accurate yet computationally efficient meta-GGA method, including the TPSS, RTPSS, MS0, MS1, MS2, and SCAN functionals, combined with the D2 method of dispersion correction to calculate the homolytic C–H bond cleavage of methane on the periodic structures of [FeO]²⁺-ZSM-5, [Cu₂(μ-O)]²⁺-ZSM-5, and [Cu₃(μ-O)₃]²⁺-MOR zeolites. Besides showing energetics and geometrical comparisons, herein through analysis of projected density of states, we identify [Fe^IVO]²⁺, [Cu^II₂(μ-O)]²⁺, and [Cu^II₂Cu^III(μ-O)₂(μ-O^·)]²⁺ as the preferred electronic structures for the corresponding active species. In addition, we discuss in great detail the fundamental difference in the C–H bond cleavage mechanism for each active species to show the high importance of accurately treating the formed Fe–OH bond on the stability of transition (TS) and radical intermediate (RI) states and to clarify the role of the O atom radical character in preserving the stability of the [Cu₃(μ-O)₃]²⁺ active species when the TS and RI states are formed. We also show the importance of correctly describing (i) weak interactions involved in the methane adsorption state and (ii) Cu–O–Cu bond strengths involved in the TS and RI states for predicting a reasonable reaction energy trend.