Quantitative 67Zn, 27Al and 1H MAS NMR spectroscopy for the characterization of Zn species in ZSM-5 catalysts

Abstract

⁶⁷Zn MAS NMR spectroscopy was used to characterize the state of Zn in Zn-modified zeolites ZSM-5. Two ⁶⁷Zn enriched zeolite samples were prepared: by solid-state exchange with metal ⁶⁷Zn (Zn²⁺/ZSM-5 sample) and by ion exchange with zinc formate solution (ZnO/H-ZSM-5 sample), both containing ca. 3.8 wt% Zn. The elemental analysis, TEM, and quantitative BAS and aluminum analyses with ¹H and ²⁷Al MAS NMR have shown that Zn²⁺/ZSM-5 contains zinc in the form of Zn²⁺ cations, while both ZnO species and Zn²⁺ cations are present in ZnO/H-ZSM-5 besides BAS. ⁶⁷Zn MAS NMR has detected the signal of Zn in a tetrahedral environment from ZnO species for both the activated and hydrated ZnO/H-ZSM-5 zeolite. The signal of Zn in an octahedral environment was detected for the hydrated Zn²⁺/ZSM-5 and ZnO/H-ZSM-5 zeolites. This signal may belong to zinc cation [HOZn]⁺ or Zn(OH)₂ species surrounded by water molecules. Quantitative ⁶⁷Zn MAS NMR analysis has shown that only 27 and 38% of zinc loaded in the zeolite is visible for the activated and hydrated ZnO/H-ZSM-5 zeolite, and 24% of Zn is visible for the hydrated Zn²⁺/ZSM-5. Zinc in the form of ZnO species is entirely visible in both the activated and hydrated ZnO/H-ZSM-5 zeolite, while Zn²⁺ cations are not detected at all for the activated sample and only 29% of Zn²⁺ cations is visible for the hydrated zeolite. Detection of only a part of Zn²⁺ cations in the form of [HOZn]⁺ or Zn(OH)₂ species in octahedral environment presumes only partial hydrolysis of the bond of Zn²⁺ cation with framework oxygen and further solvation of the Zn species formed at hydrolysis by the adsorbed water.