How Do Zn²⁺ Ions Promote Methane Activation on Zinc-Modified Zeolites? A Hybrid Quantum-Chemical Study

It was earlier established that Zn²⁺ species loaded in zeolites promoted alkane C-H bond activation by Brønsted acid sites (BAS). It was found that the rate of H/D hydrogen exchange between alkane and zeolite BAS increased dramatically, and the activation barrier decreased for Zn²⁺-modified zeolite compared to H-form zeolite. However, it still remains unclear what the nature of the promoting effect of Zn²⁺ species on C-H bond activation is. In this work, the possible mechanisms of methane hydrogen exchange on the Zn²⁺/H-ZSM-5 zeolite have been analyzed by means of hybrid quantum chemical calculations (PBEh-3c/DLPNO-CCSD(T)). Four models representing various locations of Zn²⁺ ions in H-ZSM-5 differing in Al distribution and local geometry have been investigated. Zn²⁺ sites are found to be rather diverse in their adsorption properties and reactivity. The earlier accepted hypothesis of H/D hydrogen exchange promotion via the formation of a transient molecular complex of methane with Zn²⁺ sites has been disproven by the PBEh-3c/DLPNO-CCSD(T) calculations. Conversely, the two-step mechanism of hydrogen exchange in methane on Zn²⁺/H-ZSM-5 involving zinc-methyl species formation is concluded to be the most probable. Thus, the rationalization of how Zn²⁺ sites affect alkane C-H bond activation by BAS has been provided.