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Role of the Hydroxyl−Water Hydrogen-Bond Network in Structural Transitions and Selectivity toward Cesium in Cs0.38(D1.08H0.54)SiTi2O7‚(D0.86H0.14)2O Crystalline Silicotitanate Aaron J. Celestian,*,† John B. Parise,† Ronald I. Smith,‡ Brian H. Toby,§ and Abraham Clearfield⊥ Department of Geosciences, Center for EnVironmental and Molecular Sciences, Stony Brook UniVersity, Stony Brook, New York 11794-2100, ISIS Neutron Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K., AdVanced Photon Source, Argonne National Laboratory, Argonne Illinois 60439-4856, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3133 Received June 22, 2006
The crystal structure of the selective Cs+ ion exchanger D1.6H0.4Ti2SiO7‚D2.66H0.34O1.5, known as crystalline silicotitanate or CST, has been determined in both native (D−CST) and in the Cs+-exchanged forms ((Cs, D)−CST) from angledispersive and time-of-flight neutron diffraction studies. The final fully exchange Cs+ form transformed from D−CST with unit cell parameters a ) 11.0704(3) Å c ) 11.8917(5) Å and space group P42/mbc, to one with a ) 7.8902(1) Å c ) 11.9051(4) Å and space group P42/mcm. Rietveld structure refinements of both D−CST and (Cs, D)− CST suggest the transition, and ultimately the selectivity, is driven by changes in the positions of water molecules, in response to the initial introduction of Cs+. The changes in water position appear to disrupt the D−O−O−D dihedral associated with the CST framework in space group P42/mbc which ultimately leads to the structural transition. The new geometric arrangement of the water−deuteroxyl network in (Cs, D)−CST suggests that Dwater−Ddeuteroxyl repulsion forced by Cs+ exchange drives the structural transformation.
Introduction Crystalline silicotitanates (CST) with the mineral sitinakite topology1,2 along with the H-exchanged (H-CST) and Nbsubstituted forms,2-5 are being tested as possible ion exchangers for the sequestration of 137Cs and 90Sr from radioactive nuclear waste solutions.6-16 The selectivity for Cs+, even in highly alkaline solutions of 1-7 M NaOH and * To whom correspondence should be addressed. E-mail:
[email protected]. † Stony Brook University. ‡ Rutherford Appleton Laboratory. § Argonne National Laboratory. ⊥ Texas A&M University. (1) Menshikov, Y. P.; Sokolova, E. V.; Yegorov-Tismenko, Y. K.; Khomyakov, A. P.; Polezhaeva, L. I. Zap. Vses. Mineral. Obshch. 1992, 121, 94-99. (2) Poojary, D. M.; Cahill, R. A.; Clearfield, A. Chem. Mater. 1994, 6, 2364-2368. (3) Bortun, A. I.; Bortun, L. N.; Khainakov, S. A.; Clearfield, A.; Trobajo, C.; Garcia, J. R. SolVent Extr. Ion. Exc. 1999, 17, 649-675. (4) Clearfield, A.; Tripathi, A.; Nyman, M.; Medvedev, D. Abstr. Pap. Am. Chem. S. 2002, 224, U696-U696. (5) Tripathi, A.; Medvedev, D. G.; Nyman, M.; Clearfield, A. J. Solid State Chem. 2003, 175, 72-83. (6) Clearfield, A.; Sylvester, P.; Bluhm, E.; Bortun, A.; Bortun, L. Abstr. Pap. Am. Chem. S. 1999, 218, U1061-U1061.
10.1021/ic0611387 CCC: $37.00 Published on Web 01/19/2007
© 2007 American Chemical Society
NaNO3,10 and its resistance to crystalline deterioration by both high radiation fields and high pH (>12) solutions, make H-CST a promising candidate for selective removal of Cs+ from waste solutions. The targeted removal of Cs+ from solutions would reduce the overall solution activity and make for safer long-term storage. Ion sequestration processes in crystalline molecular sieves are inherently time and pathway dependent because the (7) Hritzko, B. J.; Walker, D. D.; Wang, N. H. L. AICHE J. 2000, 46, 552-564. (8) Marinin, D. V.; Brown, G. N. Waste Manage. 2000, 20, 545-553. (9) Zheng, Z.; Anthony, R. G.; Miller, J. E. Ind. Eng. Chem. Res. 1997, 36, 2427-2434. (10) Zheng, Z. X.; Gu, D.; Anthony, R. G.; Klavetter, E. Ind. Eng. Chem. Res. 1995, 34, 2142-2147. (11) Anthony, R. G.; Dosch, R. G.; Gu, D.; Philip, C. V. Ind. Eng. Chem. Res. 1994, 33, 2702-2705. (12) McCabe, D. J. Abstr. Pap. Am. Chem. S. 1997, 214, 100-NUCL. (13) Wilmarth, W. R.; Mills, J. T.; Dukes, V. H.; Fondeur, F. F.; Walker, D. D. Abstr. Pap. Am. Chem. S. 2001, 221, U482-U482. (14) Wilmarth, W. R.; Hang, T.; Walker, D. D.; Mills, J. T.; Dukes, V. H.; Fink, S. D. Abstr. Pap. Am. Chem. S. 2000, 219, U761-U761. (15) Sylvester, P.; Clearfield, A. In American Chemical Society Symposium; Ellen, P. G., WHeineman, W. R., Eds.; American Chemical Society: Washington, DC, 2000; Vol. 778, pp 133-145. (16) Clearfield, Solid State Sci. 2001, 3, 103-112.
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Celestian et al. structures of these materials often distort to accommodate the exchanged ions (e.g., gismondine17-21). The irreversibility of Cs+ exchange in CST is a consequence of the site-selective exchange pathway that ultimately results in high affinity for Cs+.22 The mechanisms and dynamics that control ion diffusion processes in zeolitic molecular sieves have only been studied recently with in situ diffraction techniques.22-25 These time-resolved X-ray diffraction (XRD) studies demonstrated that the structural transformation which occur during ion exchange serve to enhance the exchange capacity and/or ion selectivity of the material. Diffraction techniques, either X-ray or neutron, are well suited for studies involving structural and chemical changes because both techniques probe the scattering efficiencies of atoms, which are interpreted as atom type, and their positions within a unit cell. The work presented here includes well-characterized end member structures, as these are an essential prerequisite for successful Rietveld structure refinements of the time-resolved data because the latter necessarily views a more limited region of reciprocal space. Also, the precision of crystallographic information obtained from structural analysis of time-resolved data is further compromised by reduced counting statistics, decreased peak-to-background ratios, multiple phase components, and scattering from ion-exchange media and the in situ cell. Such time-resolved studies elucidate the mechanistic and dynamic nature of cation diffusion processes in zeolites and zeolitic molecular sieves. The CST material is synthesized in the Na form (NaCST, Na2Ti2SiO7‚2H2O),2 and then H+ exchanged (H-CST) in preparation for Cs+ absorption.26 In the general CST form,27 the structure consists of columns of Ti oxide octahedra centered on a 4-fold axis parallel to [001] (Figure 1). The Ti-O octahedra columns are composed of a cluster of Ti4O16 groups collectively generating Ti4O4 cubane-like subunits. These cubane-like groups in the H-CST structure deviate from cube geometry (Figure 2). Columns are connected by silica tetrahedra to form one eight-member ring (8MR) channel along [001] (Figure 1) and three perpendicular 6MR channels along [100], [010], and [110] The 6MR channels have a long axis/short axis (L/S) ratio of 1.25, defined from L ) 04-04 ) 6.02 Å and SdO1-O3 ) 4.87 Å, and have only been observed to be large enough to (17) Nery, J. G.; Mascarenhas, Y. P.; Cheetham, A. K. Micropor. Mesopor. Mater. 2003, 57, 229-248. (18) Bauer, T.; Baur, W. H. Euro. J. Min. 1998, 10, 133-147. (19) Celestian, A. J.; Parise, J. B.; Goodell, C.; Tripathi, A.; Hanson, J. Chem. Mater. 2004, 16, 2244-2254. (20) Celestian, A. J.; Parise, J. B.; Tripathi, A.; Kvick, A.; Vaughan, G. M. B. Acta Crystallogr. C 2003, 59, I74-I76. (21) Tripathi, A.; Parise, J. B.; Kim, S. J.; Lee, Y.; Johnson, G. M.; Uh, Y. S. Chem. Mater. 2000, 12, 3760-3769. (22) Celestian, A. J.; Medvedev, D. G.; Tripathi, A.; Parise, J. B.; Clearfield, A. Nucl. Instrum. Methods B 2005, 238, 61-69. (23) Celestian, A. J. Masters Thesis, Stony Brook University, 2002. (24) Parise, J. B.; Cahill, C.; Chen, J. Abstr. Pap. Am. Chem. S. 1998, 215, U811-U811. (25) Lee, Y.; Reisner, B. A.; Hanson, J. C.; Jones, G. A.; Parise, J. B.; Corbin, D. R.; Toby, B. H.; Freitag, A.; Larese, J. Z. J. Phys. Chem. B 2001, 105, 7188-7199. (26) Bortun, A. I.; Bortun, L. N.; Clearfield, A. SolVent Extr. Ion Exc. 1996, 14, 341-354. (27) Pertierra, P.; Salvado, M. A.; Garcia-Granda, S.; Bortun, A. I.; Clearfield, A. Inorg. Chem. 1999, 38, 2563-2566.
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Figure 1. Crystal structure of D-CST refined from current study. View is down the 8MR. [001] Black box is the unit cell of D-CST, the green dash-dot box is the overlay of the (Cs, D)-CST unit cell. Heavy dashed lines in the left 8MR channel are deuteron bonds from deuteroxyl and water (Ow1), and light dashed lines are deuteron bonds from Ow2 to framework O2-. Block arrows indicate how the Ti-O octahedra columns rotate to obtain the (Cs, D)-CST structure. Water deuterons of site Ow1 point toward the center of the 8MR. Lines B and A are the measured L/S ratio (1.53). Ti (maroon), Si (blue), O (red), D|H (gray).
Figure 2. Cubane-like unit of D-CST. Dihedral angle for D-O-O-D along line P have an angle of 4.089(2)° and along Q are 0.605(1)°. Distorted square planes of Ti-O measured from internal Ti-O-Ti angles of faces perpendicular to [110], [1-10], and [001] are 103.35(1)°, 105.26(1)°, and 106.81(1)°, respectively.
accommodate nothing larger Na+ cations. The diameters of the elliptical 8MR channels are 6.9 Å for the long-axis (L ) O1-O1) and 4.5 Å along the short axis (SdO3-O3) (L/S ) 1.53; Lines A and B, Figure 1). In H-CST, the 8MR channels are solely filled with interstitial water molecules that form a H-bond network anchored at the framework hydroxyls. The charge balancing H+ sites form hydroxides positioned at the edge share union vertex of three Ti octahedra (Figure 2). The vertex sites are the O2- corners of the cubane-like unit. It is these H+ sites are readily exchangeable with larger cations in the 8MR channel. The previous time-resolved XRD investigations of Cs+ exchange into H-CST illustrated that the exchange process is selective by crystallographic site and accompanied by structural transformations.22 There are two crystallographically distinct Cs+ sites in (Cs, H)-CST: one in the center of the 8MR (Cs1) with 8-fold coordination to framework O2-, and one outside of the 8MR (Cs2) with 4-fold
Role of the Hydroxyl-Water Hydrogen-Bond Network
coordination to framework O2- and 2-fold coordination with interstitial water.2 The Cs1 site in the H-CST phase cannot accommodate the large Cs+ ionic radius without structural distortion because of unacceptable short Cs-O bond lengths of less than 3 Å; these short distances result in unexpectedly large bond valence sums, with valences greater than three. In contrast, site Cs2 is able to obtain acceptable bonding geometry but is less energetically favorable for occupancy because of its lower coordination environment. The timeresolved XRD work illustrated that Cs+ first exchanged into site Cs2 with a concomitant change in the crystal structure to form circular (L/S ) 1) 8MR.22 After site Cs2 filled to ∼15% occupancy, site Cs1 became favored for ion exchange due to the structural transformation, which allowed Cs1-O bond distances of ∼3.18 Å. The reverse exchange, H+ into Cs-CST, was also attempted in situ using 5 M HCl, but no Cs+ removal was measured. The structural transformation that favors Cs+ at Cs1 likely explains the difficulty of back exchanged by H+. The exchange mechanisms could not be fully understood previously because the positions and orientations of the H+ sites on the water and hydroxyls could not be resolved by X-ray diffraction techniques. In order to understand the dynamics and mechanics of the Cs+ exchange process in CST and the origin of the structural transformations, the H+ positions of the hydroxyl sites and water orientations of Cs-CST must be determined. Experimental Methods Sample Preparation. The starting material Na-CST (a ) 7.8082(2) Å c ) 11.9735(4) Å, P42/mcm) was first characterized by Poojary et al. (1994)2 and was synthesized according to the procedures of Medvedev et al. (2004).28 Starting gels of molar oxide composition 1.0 TiO2:1.98 SiO2:6.77 Na2O:218 (DI)H2O were prepared by first adding 6.6 mL of TiCl4 to 23.30 mL of H2O in a plastic bottle (A) of 500 mL capacity. To this mixture in bottle (A), 40 mL of 30% H2O2 in H2O was added followed by 150 mL of deionized H2O and 40 mL of 10 M NaOH solution. Then in another 500 mL plastic bottle (B) containing 200 mL of 1 M NaOH, 4.3 g of colloidal silica (Ludox AS-40) was added. The pH of the total solution was adjusted by adding 1 M NaOH solution until a pH of 12.6-12.8 was achieved. The gel was not allowed to age and was immediately treated hydrothermally in 30 and 100 mL Teflon lined stainless steel Parr autoclaves at 210 °C for 10 days in a convection oven. The resulting white powder was filtered using a vacuum flask with 0.45 µm filter paper, rinsed with deionized water, and left to air-dry at room temperature. XRD data were collected on all synthesized samples and showed Na-CST to be the dominant phase. A small amount of impurity phase was present in all preparations. The strongest peak from the impurity phase in the XRD patterns was
