Isobutene Transformation To Aromatics On Zn-Modified Zeolite: Particular Effects Of Zn2+ And Zno Species On The Reaction Occurrence Revealed With Solid-State Nmr And Ftir Spectroscopy

The effect of Zn2+ cations and small clusters of ZnO on isobutene transformation to aromatic hydrocarbons on Zn-modified zeolites (Zn2+/H-ZSM-5 and ZnO/H-BEA) has been investigated with C-13 MAS NMR and FTIR spectroscopic techniques. It is inferred that isobutene is mainly stabilized in the form of a pi-complex with Zn2+ cations of the Zn2+/H-ZSM-5 zeolite at 296-573 K, whereas the oligomerization of the olefin on Bronsted acid sites (BAS) of ZnO/H-BEA occurs at 296 K. Using isobutene, selectively labeled with the C-13 isotope in either =CH2 or =C< groups, for NMR analysis of the evolution of the adsorbed species with temperature, the reaction intermediates have been identified within the temperature range of 296-623 K. At 296 K, an occurrence of the double-bond shift reaction in isobutene on Zn2+/H-ZSM-5 and a complete C-13-label scrambling in the oligomers formed on ZnO/H-BEA have been established. The detected intermediates include a pi-complex of the olefin with Zn species, 2-methyl-sigma-allylzinc species, isobutene oligomers, and delocalized carbanionic species. It is found that similar intermediates are formed in the course of the reaction on both zeolites at T >= 473 K, which implies that similar pathways of isobutene transformation are realized on the zeolites with different Zn species. An analysis of FTIR data reveals that the intermediate species formed on the Zn2+/H-ZSM-5 zeolite interact with both BAS and Zn sites at each of the reaction steps of the olefin transformation to aromatic hydrocarbons.