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

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