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

Abstract

The effect of Zn 2+ cations and small clusters of ZnO on isobutene transformation to aromatic hydrocarbons on Zn-modified zeolites (Zn 2+ /H-ZSM-5 and ZnO/H-BEA) has been investigated with 13 C MAS NMR and FTIR spectroscopic techniques. It is inferred that isobutene is mainly stabilized in the form of a π-complex with Zn 2+ cations of the Zn 2+ /H-ZSM-5 zeolite at 296−573 K, whereas the oligomerization of the olefin on Brønsted acid sites (BAS) of ZnO/H-BEA occurs at 296 K. Using isobutene, selectively labeled with the 13 C isotope in either CH 2 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 Zn 2+ /H-ZSM-5 and a complete 13 C-label scrambling in the oligomers formed on ZnO/H-BEA have been established. The detected intermediates include a π-complex of the olefin with Zn species, 2-methyl-σ-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 Zn 2+ /H-ZSM-5 zeolite interact with both BAS and Zn sites at each of the reaction steps of the olefin transformation to aromatic hydrocarbons.

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

“…Further dissociation of isobutene π-complex affords 2-methyl-σ-allylzinc species. [12] The latter species was detected at isobutene transformation on Zn-containing zeolites. [12] The allylzinc species may interact with isobutene yielding a dimeric alkene.…”

“…[12] The latter species was detected at isobutene transformation on Zn-containing zeolites. [12] The allylzinc species may interact with isobutene yielding a dimeric alkene. The latter is further dehydrogenated with the involvement of the Zn sites to give polyenyl species stabilized on the Zn sites in the form of carbanionic species with delocalized negative charge over five carbon atoms.…”

“…The formed propene transforms to the aromatic products by the mechanism [8m] suggested one for isobutene. [12] Ethane, as one of the products of propane hydrogenolysis, may further undergo hydrogenolysis to offer two molecules of methane.…”

“…The signals survive further heating up to 523 K (Figure 1c,h). We have earlier established that such signals belong to the stable π-complex of isobutene with Zn 2 + cations, [12] although the isobutene initially adsorbed on Zn 2 + /H-ZSM-5 exhibits slightly different chemical shifts (208, 90, and 27 ppm) presumably due to various quantities of the formed πcomplex. The observation of the π-complex implies that the dehydrogenation of the alkane occurs [2a,b] at 473 K affording isobutene, which is stabilized on Zn 2 + cations at temperatures up to 523 K.…”

“…At 473 K, isobutane undergoes dehydrogenation, and the signals of the π-complex of isobutene with the Zn sites at 26, 94, and 200 ppm appear in the spectra. [12] At 523 K, the signals from methane (À 8 ppm), ethane (6 ppm), and propane [(17 ppm (À CH 3 ) and 19 ppm (À CH 2 À )] become detectable (Figure S4c,i). Observation of the signal at 17 ppm of propane and the signal of methane, both formed from isobutane-1-13 C labeled in the À CH 3 group, and the signal at 19 ppm for propane formed from isobutane-2-13 C labeled in the > CHÀ group (cf.…”