Mechanism of H/D Hydrogen Exchange of n-Butane with Bronsted Acid Sites on Zn-Modified Zeolite: The Effect of Different Zn Species (Zn2+ and ZnO) on the Activation of Alkane C-H Bonds

With the aim of clarifying the effect of Zn species of different nature (small clusters of ZnO and Zn2+ cations) on the activation of alkane C-H bonds by Zn-modified zeolite, the mechanism of the H/D hydrogen exchange of n-butane-d(10) with Bronsted acid sites (BASs) of ZnO/H-BEA and Zn2+/H-BEA zeolites has been investigated with H-1 MAS NMR in situ at 413-468 K. It is established that both ZnO and Zn2+ species provide acceleration of the exchange in the methyl groups with a decreased activation energy compared to the exchange in n-butane on pure acid-form zeolite. The effect of Zn2+ cation is more essential. The acceleration has been attributed to the formation of transient complexes of the alkane methyl groups with Zn species preceding the exchange with BAS. For ZnO/H-BEA, the involvement of the methylene groups in the exchange is related to the alkane dehydrogenation to form butene followed by protonation and hydride shift reaction in the formed butyl cation. For Zn2+/H-BEA zeolite, an enhanced rate of the exchange in the methylene groups is due to the contribution of two pathways: direct exchange with the assistance of Zn2+ cations and through the intermediate formation of n-butene. We have concluded that the observed peculiarities of the kinetics of H/D hydrogen exchange of n-butane on Zn2+/H-BEA and ZnO/H-BEA zeolites could be rationalized in terms of a common kinetic scheme realized for two zeolite samples with different kinetic parameters, rate constants, and activation energies for identical reaction steps.