Abstract
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, Pb 2+ , 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, Pb 2+ 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). Pb 2+ 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 Pb 2+ on the thermal stability of STI framework type zeolites. • 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 (Pb 2+ , OH − , and H 2 O) were highly disordered in the cavities. • Pb 2+ is sided coordinated by OH − and H 2 O. • No significant interactions occur between Pb and framework oxygen.
Highlights
Cation exchange is undoubtedly one of the most exploited properties of natural and synthetic zeolites
A high Pb content was measured at all analytical points (Table 2), resulting in a surplus of Pb species theoretically needed to balance the net negative charge of the aluminosilicate framework (13.4 instead of 8 Pb per for mula unit) if all extraframework cations were represented by Pb2+ ions
The surplus of extra framework (EF) cationic charges [16] was reported for other Pb-zeolites in literature [17,18,19,20,21] and it implies the presence of OH− groups inside in the zeolitic channels, likely associated with the EF Pb, in order to reach charge neutrality
Summary
Cation exchange is undoubtedly one of the most exploited properties of natural and synthetic zeolites. The ability to exchange the extra framework (EF) cation content with different cationic species has been widely applied for the removal of contaminated substances from water and soils [1,2] and to produce zeolite structures with tailored features [3,4,5]. The modification of the EF cation-content strongly affects the microporous properties, such as sorption, catalytic and dehydration behaviour that can significantly differ from those of the original untreated zeolite. Zeolites exchanged with Zn, Cu, Ag, Fe, Cd, were found to be highly efficient for catalytic [6,7,8], environmental-remediation [9,10] and selective-gas-sorption processes [3,4,11 and references ]. Baekelant et al [15] demonstrated that Pb clusters with high lumines cence properties can be confined in Linde Type A (LTA) zeolite
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