Propane activation on Zn-modified zeolite. The effect of the nature of Zn-species on the mechanism of H/D hydrogen exchange of the alkane with Brønsted acid sites

Zn-modified zeolites exhibit high activity for the aromatization of C₂₊ alkanes. To understand the effect of Zn-species of different nature on propane molecules activation and transformation, the H/D hydrogen exchange of zeolite Brønsted acid sites (BAS) with deuterated propane-d₈ has been investigated. Two samples of the Zn-modified zeolite containing exclusively either isolated Zn²⁺ cations (Zn²⁺/H-BEA) or small (ZnO)_n clusters (ZnO/H-BEA) have been studied. Zn-species of either type work jointly with BAS and may provide the synergy effect for propane C[sbnd]H bond activation resulting in dramatic acceleration of the H/D exchange between propane and BAS. The accelerating effect of isolated Zn²⁺ cations is more pronounced compared to zinc oxide clusters. Moreover, the H/D exchange occurs regioselectively into the methyl groups of propane on Zn²⁺/H-BEA zeolite. For ZnO/H-BEA, the methylene group becomes also involved in the exchange after some induction period. Various mechanisms providing the regioselective H/D exchange on Zn²⁺/H-BEA as well as the involvement of the methylene group after the induction period on ZnO/H-BEA are discussed.