On the nature of Pb species in Pb-exchanged zeolite stellerite (STI): A combined experimental and theoretical study.

Structural properties of Pb-exchanged zeolites are of interest because of their applications in environmental remediation and in industrial processes. In this study, we report on a Pb-exchanged stellerite (Pb-STI), with particular focus on the cationic species, which form inside the zeolitic pores as a result of the exchange experiments. The produced zeolite had chemical composition Pb 13.4 (OH) 10 Al 17.4 Si 54.6 O 144 ∙38H 2 O, indicating that Pb was uptaken in form of [Pb(OH) x ](2-x) species. The STI framework maintained the Fmmm space group characteristic of the type material. However, the extraframework occupants, Pb2+, H 2 O and OH−, were characterized by a strong positional-disorder. The latter was resolved and interpreted combining Extended X-ray Absorption Fine Structure (EXAFS) analysis with Molecular Dynamics (MD) simulations. On average, Pb2+ ions are coordinated by 2 OH− and 1H 2 O at distances <2.5 Å, whereas bonds to framework oxygen-atoms were found only at longer distances (>2.8 Å). Pb2+ adopts mainly a sided distorted coordination, indicating a stereochemical activity of the lone pair electrons. The obtained results were compared with those of other mono-cationic forms of STI zeolites. Based on the analysis of the framework distortion experienced after the incorporation of different metal ions, considerations are drawn on the potential effect of Pb2+ on the thermal stability of STI framework type zeolites. [Display omitted] • A STI zeolite was Pb-exchanged in 0.5 M Pb(CH 3 COO) 2 solution at pH = 5-6. • No framework distortion from the Fmmm symmetry was observed. • The EF species (Pb2+, OH−, and H 2 O) were highly disordered in the cavities. • Pb2+ is sided coordinated by OH− and H 2 O. • No significant interactions occur between Pb and framework oxygen. [ABSTRACT FROM AUTHOR]