Unusual localization of zinc cations in MFI zeolites modified by different ways of preparation

Comparison of DRIFT spectra of OH groups in MFI with framework Si/Al ratios equal to 25 or 40 with the spectra of the same zeolites after reaction with zinc vapour at 773 K indicated quantitative substitution of acidic hydroxyls by Zn2+ ions. Complete substitution of protons indicates that at high Si/Al ratios some of the Zn2+ cations are localized at the isolated aluminium occupied oxygen tetrahedra with single negative charges. This results in the formation of sites with only partially compensated excessive positive charges of bivalent cations. In parallel, the same amount of negatively charged aluminium occupied oxygen tetrahedra remains without compensating cations or protons. Their negative charges are neutralized indirectly by the electrostatic interaction with the surrounding positively charged cationic sites. Zinc or other bivalent ions with only partially compensated positive charges exhibit an unusually high chemical activity. This explains the specific catalytic and adsorption properties of high-silica zeolites modified with the bivalent transition metal cations. In the present study, this was demonstrated by very large red shifts of H–H stretching vibrations of H2 adsorbed by zinc cations at 77 K and by heterolytic dissociative adsorption of molecular hydrogen at moderately elevated temperatures. Application of low temperature H2 adsorption as a molecular probe indicated that similar sites of unusual localization of bivalent zinc cations also exist in zeolites modified by ion exchange or incipient wetness impregnation, but the amount of such sites is less than for the samples modified by reaction with zinc vapour.