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August 25, 2004

Time-Resolved Diffraction Studies of Ion Exchange: K⁺ and Na⁺ Exchange into synthetic aluminogermanate molecular sieve with the gismondine topology

A.J. Celestian¹, J.B. Parise^1,2, C. Goodell¹, A. Tripathi³, and J. Hanson^4
1Department of Geosciences, State University of New York at Stony Brook, NY; ²Department of Chemistry, State University of New York at Stony Brook, NY; ³Department of Chemistry, Texas A&M, College Station, TX; ⁴Department of Chemistry, Brookhaven National Laboratory, Upton, NY

Time-resolved in situ x-ray diffraction was used to determine how ions are exchanged in the potassium (K⁺) and sodium (Na⁺) forms of the aluminogermanate molecular “sieve” (AlGe-GIS) with the topology of the gismondine (GIS) family of zeolite minerals. The AlGe-GIS structure is of particular interest due to its highly flexible framework and its use in industrial processes. This study illustrates the ion site-selective exchange pathways that are created during the substitution process and also determines possible ion exchange mechanisms. Bond valence calculations indicated that the preferred charge-balancing cation in the GIS structure is K⁺, due to its larger framework bonding coordination. Experiments were conducted at beamline X7B.

Zeolites are nanoporous minerals built from a fully corner-shared framework of tetrahedra. In the gismondine (GIS) family of zeolites, the synthetic aluminogermanate (AlGe-GIS) framework is composed of aluminum (Al³⁺) and germanium (Ge⁴⁺) ions, resulting in a net negative (-1) charge for every Al³⁺ ion in the framework. Therefore, extra-framework cations must be incorporated to charge-balance the structure and maintain electro-neutrality (Figures 1 & 2). These extra-framework cations are found in the large channels of the zeolitic framework and can be substituted for other cations. Due to the size, shape, and composition of the zeolitic channels, these materials behave as molecular “sieves” that selectively absorb and desorb cations – a property often used in environmental and industrial applications.

The information obtained from time-resolved in situ x-ray diffraction measurements enables us to directly follow the ion exchange process. The exchange of potassium ions (K⁺) into Na-AlGe-GIS proceeded to 90% (± 1%) completion within the time frame of the experiment (Figures 3 & 4). During the first 10% of K⁺ substitution, K⁺ only entered the [-101] channel (Figures 1 & 4) of the Na-AlGe-GIS structure. After 10%, exchange site-specific substitution could no longer be followed. In the reverse exchange (Na⁺ into K-AlGe-GIS; data not shown), a gradual growth of the Na-AlGe-GIS phase was observed and stopped after approximately 10% substitution (Figure 4) in the time frame of the experiment. Site-specific ion exchange was not observed during the substitution of Na⁺ into K-AlGe-GIS.

The affinity of the GIS framework for the K⁺ ion can be explained through bond valence analysis. In K-AlGe-GIS, K conducts most of its bond valence electrons to the O^2- framework (approximately 51% of its total valence), thus achieving direct framework charge balancing. In Na-AlGe-GIS, Na⁺ conducts most of its bond valence electrons through interstitial water molecules (approximately 71% of its total valence). This effectively reduces the amount of direct charge balancing that Na⁺ can accomplish and results in a less stable bonding configuration relative to the K⁺ bonding environment.

BEAMLINE
X7B

FUNDING
National Science Foundation Division of Materials Research

PUBLICATION
A. J. Celestian, J. B. Parise, C. Goodell, A. Tripathi, J. Hanson, Time resolved diffraction studies of ion exchange: K⁺ and Na⁺ exchange into (Al, Ge) gismondine (GIS) Na₂₄Al₂₄Ge₂₄O₉₆•40H₂O and K₈Al₈Ge₈O₃₂•8H₂O. Chemistry of Materials, 16(11), 2244-2254 (2004).

FOR MORE INFORMATION
Aaron J. Celestian
Center for Environmental and Molecular Sciences
Department of Geosciences
Stony Brook University
Stony Brook, NY
Email: aaron.celestian@stonybrook.edu