Which Species, Zn2+ Cations or ZnO Clusters, Are More Efficient for Olefin Aromatization? 13C Solid-State NMR Investigation of n-But-1-ene Transformation on Zn-Modified Zeolite

The analysis of n-but-1-ene transformation on Zn-modified zeolite H-BEA, containing zinc exclusively in the form of either Zn2+ cations (Zn2+/H-BEA sample) or small clusters of ZnO (ZnO/H-BEA sample), has been performed with C-13 solid-state nuclear magnetic resonance (NMR) at 296-673 K. The number of intermediates, including pi-complex of n-but-2-ene, methylallylzinc, and delocalized carbanionic species formed by the interaction of oligomeric polyenes with Zn sites, have been identified for both zeolite samples. Methyl-substituted cyclopentenyl cation and cyclohexadienyl cation are additionally identified for the reaction on ZnO/H-BEA. It is inferred that the aromatization of the olefin occurs basically with the involvement of Zn2+ sites on Zn2+/H-BEA. For ZnO/H-BEA, besides aromatization with the assistance of ZnO species, conjunct polymerization process with the involvement of Bronsted acid sites (BAS) contributes notably to the olefin aromatization. The latter process affords also some quantity of C-1-C-4 alkanes. It is concluded that the stronger interaction of the olefin (confirmed by density functional theory (DFT) calculations) and oligomeric polyenes with Zn2+ cations than with ZnO species and different quantities of BAS for two zeolite samples provide peculiar performances of Zn2+/H-BEA and ZnO/H-BEA zeolites for the olefin aromatization. Based on careful analysis of the obtained spectroscopic results, it is suggested that Zn-modified zeolite containing Zn2+ cationic species and some quantity of BAS should exhibit higher efficiency as the catalyst for small olefin and alkane aromatization compared to the zeolite with ZnO species and high concentration of BAS.