Theoretical investigation of Zn-containing species in pores of ZSM-5 zeolites

We report computational studies of various cationic Zn-containing species in pores of ZSM-5 zeolite. The calculations were performed at the gradient-corrected density functional level employing both isolated and embedded quantum clusters for modeling the zeolite framework. We studied zinc species in two models M5 and M7, comprising one and two coupled 5-membered rings with two A1 centers. Our results suggest that ZnOH⁺ species can be transformed into Zn(H₂O)²⁺ complexes when a bridging OH group is available nearby in the zeolite framework. The desorption energy of H ₂O molecules from the later complexes is rather high, 184 (M5) and 130 (M7) kJ/mol. Zn-containing species at the smaller zeolite ring are further stabilized by additional OH^- or H₂O ligands. We found the formation of ZnOZn²⁺ species energetically unfavorable.