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J. Phys. Chem. B 2001, 105, 7188-7199
New Insight into Cation Relocations within the Pores of Zeolite Rho: In Situ Synchrotron X-Ray and Neutron Powder Diffraction Studies of Pb- and Cd-Exchanged Rho Yongjae Lee,*,† Barbara A. Reisner,‡,§ Jonathan C. Hanson,| Glover A. Jones,⊥ John B. Parise,†,# David R. Corbin,⊥ Brian H. Toby,‡ Andrea Freitag,| and John Z. Larese| Geosciences and Chemistry Departments, State UniVersity of New York, Stony Brook, New York 11794-2100, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973-5000, and Central Research and DeVelopment, DuPont Company, Experimental Station, P.O. Box 80262, Wilmington Delaware 19880-0262 ReceiVed: January 4, 2001; In Final Form: May 8, 2001
Upon heating, certain cations exchanged into zeolite RHO undergo large shifts in their positions within the pores. In several of these materials, negative thermal expansion is observed in conjunction with these cation relocations. Rather than being the purely temperature-driven effects presumed in previous reports, a combination of in situ time-resolved synchrotron X-ray and neutron powder diffraction studies indicates that the cation relocations and framework distortions observed in Pb- and Cd-exchanged zeolite rho are mediated by the presence of water in specific sites in the pores of RHO. Rietveld refinements using these data reveal that the initial unit cell contraction (50 °C e T e 100 °C) is due to the loss of unbound water in the R-cages of rho. Water molecules in the double eight-ring (D8R) building units persist after this step, bound to the extraframework cations. The framework then contracts as water molecules are gradually removed (200 °C e T e 400 °C). During this period, the extraframework cations migrate from the single eight-ring (S8R) site to the double eight-ring (D8R) sites in a concerted manner with the dehydration at the D8R. Upon complete removal of bound waters (400 °C e T e 500 °C), lead and cadmium cations experience different rearrangements; Cd2+ ions relocate from the D8R and S8R sites to the single six-ring (S6R) site, while all Pb2+ ions migrate from the S8R site to the D8R site. Neither transition is reversible upon cooling to room temperature in vacuo although both are reversible in the presence of water vapor. The role of water in these samples appears to determine the coordination environment of the extraframework cations uniquely since other sorbates, such as Kr, methanol, and CO, do not cause significant changes in either extraframework cation or framework atomic positions.
Introduction Zeolites possess negatively charged crystalline aluminosilicate frameworks with pores and channels of molecular dimensions, within which extraframework cations reside to provide charge balance. Both the shape and size of the pore openings and the position of the extraframework cations impart molecular sieving capabilities and therefore control over the shape and size selectivities for catalytic and separation processes. Recent studies show that some zeolites exhibit framework distortions and changes in cation distributions upon ion exchange,1 sorption,2 or as a function of temperature.3 Due to its exceptional flexibility, zeolite rho is particularly sensitive to these parameters and has been a subject of extensive structural investigations.4 Among them, Sr2+-exchanged zeolite rho (Sr-rho) has been known to show a large negative thermal expansion,5 while cadmium ions in Cd-rho relocate from the eight-ring to the six-ring sites upon heating, opening access to the pores.6 The latter was described as a “trap door” relocation and was used * Corresponding author. † Geosciences Department, State University of New York, Stony Brook. ‡ National Institute of Standards and Technology. § Current address: Chemistry Department, James Madison University, Harrisonburg, VA 22807. | Brookhaven National Laboratory. ⊥ DuPont Company. # Chemistry Department, State University of New York, Stony Brook.
for the entrapment and controlled release of Xe.7 These transformations have been attributed to purely temperature-driven phase transitions. Recent results suggest a role for water and possibly other polar molecules in these phenomena.8 The RHO topology is composed of a body-centered cubic arrangement of truncated cubooctahedra or R-cages linked via double eight-rings of corner-connected tetrahedra (Figure 1). Three sites with unique coordination environments accommodate the extraframework cations: the single eight-ring (S8R), double eight-ring (D8R), and single six-ring (S6R) sites (Figure 1). Extraframework cations preferentially occupy one of these three locations, and water molecules are absorbed both in the R-cages and in the D8R building units. Depending upon the unit cell composition or the temperature,9,10 the framework adopts either a centric (C-form, Im3hm) or an acentric (A-form, I4h3m) structure, and the extraframework cations change their positions (Figure 1). To provide more detailed information on the structural changes observed in zeolite rho and their correlation with the unit cell composition, full structure analyses were undertaken using time-resolved synchrotron X-ray powder diffraction and neutron powder diffraction data collected on Pb- and Cdexchanged rho. The temperature, atmosphere and gas loading were varied in situ, and the structural changes were modeled using these data and the Rietveld techniques.11,12 In this paper,
10.1021/jp0100349 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001
Pb- and Cd-Exchanged Rho
Figure 1. Centric (Im3hm) and acentric (I4h3m) topologies of zeolite RHO. Vertexes represent tetrahedrally coordinated Al or Si. Oxygen atoms are omitted for clarity.
Figure 2. In situ heating cell and imaging plate detection system as used at the X7B beamline of the NSLS. See text for details.
we report the results of this combined in situ synchrotron X-ray and neutron powder diffraction study and suggest new ideas for interpreting structural changes observed in zeolite rho in terms of its interaction with water molecules. Experimental Section Synthesis and Ion Exchange Procedures. Na,Cs-rho, the parent material used for cation exchange, was prepared using a modification of the method described by Robson.13 Batches were prepared by adding 720 mL of colloidal silica (Ludox LS-30) to a mixture containing 200 mL of 4 M Na2AlO2OH, 56 mL of 50% CsOH, and 32 g of NaOH in polytetrafluoroethylene (Teflon) bottles and allowing the solution to stand at room temperature for 6 days. The resulting mixtures were heated at 100 °C for 6 days, then filtered, and thoroughly washed with distilled water. The dried powder was shown to be highly crystalline zeolite rho by X-ray powder diffraction. Chemical analysis gave a unit cell composition of Na7.1Cs3.8Al11.5Si36.5O96‚ wH2O. Pb-rho and Cd-rho were then prepared using standard ion-exchange techniques using aqueous 10% w/w Pb(NO3)2 (10 mL/g) and 10% w/w Cd(NO3)2 (10 mL/g) at 90 °C, respectively. Chemical analysis by inductively coupled plasma and electron probe microanalysis gave unit cell compositions of Pb6.7Cs0.7Al11.7Si36.3O96‚wH2O and Cd5.5Cs0.3Al11.7Si36.3O96‚wH2O for Pb-rho and Cd-rho, respectively. In Situ Synchrotron X-Ray Powder Diffraction. An imaging plate detector (Mar345®, 2300 × 2300 pixels) was coupled to an in situ dehydration cell at the X7B beamline of the National Synchrotron Light Source (NSLS) (Figure 2). A powdered sample (∼0.003 g) was loaded into a 0.5 mm quartz capillary, which was either connected to a vacuum pump (
