Propylene Transformation on Zn-Modified Zeolite: Is There Any Difference in the Effect of Zn2+ Cations or ZnO Species on the Reaction Occurrence?

The peculiar role of either Zn2+ cations or small ZnO clusters located inside the zeolite pores in propylene transformation to aromatic hydrocarbons on Zn2+ /H-ZSM-5 and ZnO/H-BEA zeolites has been analyzed. C-13 MAS NMR and FTIR spectroscopic investigation has revealed similar properties of different Zn-sites with respect to propylene oligomerization. Both Zn2+ cations and ZnO species are capable to convert propylene to higher C-6-C-9 olefins which represent the precursors of simple benzene-based aromatics. The mechanism of such oligomerization has been inferred to include the following stages: (a) adsorption of propylene on the Zn-sites in the form of a sr-complex; (b) transformation of the sr-complex of propylene to sigma-allylzinc surface species at elevated temperature; (c) formation of C6+ olefins; (d) transformation of oligomeric C6+ olefins to simple aromatic molecules (benzene, toluene, and xylenes). NMR evidences that Bronsted acid sites (BAS) of both zeolite samples are not involved in the oligomerization to a large extent although definitely contribute to oligomers formation. Involvement of BAS in propylene oligomerization on ZnO/H-BEA zeolite is higher because of their larger quantity in the sample. The oligomerization has been monitored to proceed mainly via allylzinc species owing to the olefin insertion into the reactive Zn-C bond. In such a way, selective formation of the C-6-C-9 olefins occurs which accounts for a particular set of aromatic products detected for propylene transformation on Zn-modified zeolites.