Isobutane Transformation to Aromatics on Zn-Modified Zeolites: Intermediates and the Effect of Zn2+ and ZnO Species on the Reaction Occurrence Revealed by 13 C MAS NMR

Abstract

To clarify the effects of different Zn species, zeolite topology and acidity (quantity of Brønsted acid sites, BAS) on alkane aromatization, isobutane transformation on Zn²⁺ /H-ZSM-5, Zn²⁺ /H-BEA, and ZnO/H-BEA zeolites has been monitored with ¹³ C MAS NMR. The alkane transformation has been established to occur by aromatization and hydrogenolysis pathways. Zn²⁺ species is more efficient for the aromatization reaction because aromatic products are formed at lower temperatures on Zn²⁺ /H-BEA and Zn²⁺ /H-ZSM-5 than on ZnO/H-BEA. The larger quantity of BAS in ZnO/H-BEA seems to provide a higher degree of the hydrogenolysis pathway on this catalyst. The mechanism of the alkane aromatization is similar for the zeolites of different topology and containing different Zn species, with the main reaction steps being the following: (i) isobutane dehydrogenation to isobutene via isobutylzinc; (ii) isobutene stabilization as a π-complex on Zn sites; (iii) isobutene oligomerization via the alkene insertion into Zn-C bond of methyl-σ-allylzinc formed from the π-complex; (iv) oligomer dehydrogenation with intermediate formation of polyene carbanionic structures; (v) aromatics formation via further polyene dehydrogenation, protonation, cyclization, deprotonation steps with BAS involvement.

Keywords: 13C MAS NMR; Zn-modified zeolite; aromatization and hydrogenolysis; isobutane; mechanism.