Localization of bivalent transition metal ions in high-silica zeolites with the very broad range of Si/Al ratios in the framework probed by low-temperature H2 adsorption

Abstract

H2 adsorption at 77 K with DRIFT control of HH stretching frequency was used as a molecular probe for the study of MOR and ZSM-5 modified with Co2+, Zn2+, or Ni2+ ions. The obtained results indicated localization of these transition metal ions in the zeolites with the very broad range of Si/Al ratios at several different sites. Moderate red shifts of HH stretching frequencies of H2 adsorbed by Zn2+ in the aluminum-rich ZSM-5 are close to those for hydrogen adsorption by ZnY. Therefore it is suggested that similar to HY the moderately perturbing adsorbed hydrogen bivalent cations are localized in the five- or six-membered rings of the pentasil framework with two negatively charged aluminum-occupied oxygen tetrahedra per ring. In contrast, the fraction of the sites of the strongest perturbation of adsorbed hydrogen with the low-frequency shifts of HH stretching frequency over 200 cm−1 is the highest at the highest Si/Al ratios. In this case aluminum ions are distantly separated from each other. Therefore, the bivalent transition metal ions are localized at the singly negatively charged aluminum-centered oxygen tetrahedra. The excessive positive charge of cations at such sites is compensated by the electrostatic interaction with the surrounding negatively charged [AlO2]− tetrahedra with cationic vacancies. In ZSM-5 such acid–base pairs with the distantly placed Zn2+ cations and basic oxygen dissociatively adsorb molecular hydrogen at room temperature or at the moderately elevated temperatures. Formation of the active sites with the excessive positive charge of transition metal ions explains the unusually strong perturbation of adsorbed hydrogen and unusual catalytic properties of the high-silica zeolites modified by transition metal ions.