Inorg. Chem. zyxwvu 1994, 33, zyxwvut 3861-3862 3861 Interaction of Multivalent Cations with Layered Clays. Generation of Lutetium Disilicate upon Hydrothermal Treatment of Lu-Montmorillonite Jose M. Trillo,' Maria D. Alba, Rafael Alvero, Miguel A. Castro, Adela Muiioz-Phez, and Juan Poyato Departamento de QuImica Inorg&nica, Instituto de Ciencia de Materiales, Universidad de Sevilla-Consejo Superior de Investigaciones Cientlficas, zyxwvu P.O. Box 874, Sevilla, Spain Received June 16, 1994 Much interest has been generated in the interaction of multivalent cations, particularly rare earth ions, with clays, in connection with applications such as the preparation of new acid catalysts' and radioactive waste isolation.2 Diverse structural effects which occur upon air-heating of lanthanide-montmorillonite have already been observed by this research group.3 For example, in the case of La(III), extended X-ray absorption fine structurespectroscopy (EXAFS) has shown that an important reaction is the deprotonation of the initially hydrated cations and polyoxocation generati~n.~ This paper describes a basic study on the effect of hydrothermal treatments upon montmorillonite saturated with lutetium. Quan- titative generation of the phase LuzSi207 at temperatures significantly lower than those up to now reported,5 has been observed. This has great significance for the application of diverse clay-based materials, particularly concerning the use of bentonite as a material for nuclear waste repositories and the design of new acid catalysts. Of the different montmorillonjtes employed by us, a sample (MT) from Los Trancos, Almeria, Spain, has been utilized as starting material because of its smaller content and structural distribution of Fe, which facilitates solid-state nuclear magnetic resonance measurements. Ion exchange with either Na(1) or Lu(II1) has provided two modified smectites, Na-MT and Lu- MT, considered as reference and target samples respectively. Among lanthanides, lutetium has been selected because of its high reactivity according to previous assays. Both samples were treated under a water pressure in the range 8.5-10 MPa (approaching those expected in the projected nuclear waste repositories)6 and temperatures from 300 to 500 O C for 24 h. Structural changes occurring in the samples were analyzed studying the long-range order by X-ray powder diffraction (XRD), the chemical environment of the main constituent elements of the lattice by magic-angle spinning nuclear magnetic resonance (MAS-NMR), the microchemical composition by energy- dispersive X-ray (EDX) and the local Lu(II1) environment through EXAFS. From XRD analysis, two different results were observed. Whereas the Na-MT sample was not affected by the hydrothermal treatments even at the higher temperature, the Lu-MT sample was clearly transformed. We have performed our experiments at increasing temperatures and pressures. Under relatively mild conditions, structural modifications affecting the local environ- ment of lanthanide ions had already been observed, but these were not strong enough to be detectable by X-ray diffra~tion.~ At 400 OC, along with the reflections of the montmorillonite, a new set of diffraction peaks appears, compatible with the presence of the crystalline phase Lu2Si207 (Card ASTM No. zyxwvut (1) Figueras, F. Catal. Rev.-Sci. Eng. 1988, 30, 457-499. (2) Beall, G. W.; Ketelle, B. H.; Haire, R. G.; OKelley, G. D. In Radioactive Waste in Geological Storage; Fried, zyxwvutsrq S., Ed.; ACS Symposium Series 100; American Chemical Society: Washington, DC, 1979; pp 201-213. (3) Poyato, J.; Tobias, M. M.; Trillo, J. M. Inorg. Chim. Acta 1987, 140, (4) Mufioz-Pdez, A.; Alba, M. D.; Alvero, R.; Castro, M. A,; Trillo, J. M. (5) Felsche, J. Struct. Bonding 1973, 13, 99-195. (6) Plesko, E. P.; Scheetz, B. E.; White, W. B. Am. Mineral. 1992, 77, 307-308. Jpn. J. Appl. Phys. 1993, 32 (Suppl. 32-2), 779-781. 431437. 0020-166919411333-3861$04.50/0 34-0509). At 500 OC, a more complicated pattern is obtained which can be explained by the coexistence of Si02 and Al+3iOs, together with Lu2SizO7. This result implies the development of a lutetium fixation mechanism based on the formation of Lu2- Si2O7, a crystalline structure described in the literature only at very high temperatures (900-1800 0C).5 With respect to the main constituent elements of the lattice, no variation in the Z7Aland 29Si MAS-NMR spectra is observed for the Na-MT sample with the hydrothermal treatments. In contrast, there are progressive changes in the 27Al and 29Si signals for Lu-MT as reaction temperatures are raised, in agreement with theX-ray diffraction data. While at 400 OC the 29Si spectrum shows a shoulder centered at -89.5 ppm, ascribable7 to a lanthanide disilicate with a SiOSi angle = 180°, both signals undergo substantial modifications at 500 OC. The peak corresponding to tetrahedral aluminum (6 = 67 ppm) has disappeared and several silicon environments with different degrees of condensations from to @, were observed. The reactions that occur under hydrothermal conditions require the aggregation of lutetium cations and must involve a change of the sample microchemical composition. EDX microanalysis has been carried out to check this fact. Hydrothermally treated Na-MT samples exhibit a homogeneous and unchanging com- position, corresponding to that of the untreated one. However, the composition of the Lu-MT sample becomes heterogeneous upon treatment, with aluminum free microvolumes showing Si/ Al/Lu net peak count ratio compatible with formation of the disilicate. The local environment of Lu(II1) ions in the Lu-MT sample after hydrothermal treatment at 400 OC has been studied with EXAFS, searching for the coexistence of isolated interlamellar lutetium cations and other possible ion species not detectable by XRD, together with the crystalline phase Lu2Si207. Lu LIII- edge (E = 9244 eV) spectra of the hydrothermal sample were measured at theSRS (Daresbury Laborat0ryU.K) in transmission mode,g in an "in situ" EXAFS cell.lo The radial distribution function from the k2-weighted uncorrected Fourier transform of the EXAFs spectrum'' shows two well-resolved peaks centered at 1.8 and 3.0 zyxwv 8, approximately. These can be assigned to Lu-0 and Lu-Si + Lu-Lu contributions respectively. Data analysis12 was performed ink and R space using the phase and backscattering amplitude functions obtained from the software provided by (7) Engelhardt, G.; Michel, D. High-Resolution Solid-Stare NMR of Silicates and Zeolites; J. Wiley and Sons: New York, 1987; pp 162- 163. (8) Lippmaa, E.; Magi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. J. Am. Chem. SOC. 1980, 102, 48894893. (9) Measurements were carried out at Station 8.1, using a double crystal monochromator, Si[220], with 30% HHR and ionization chambers as detectors. The ring conditions used were 2 GeV and 250 mA. The monochromator was calibrated using a copper foil (Cu K-edge 8979 eV). (10) Kampers, F. W. H.; Maas, T. M. J.; Van Grandelle, J.; Zimkgreve, P.; Koningsberger, D. C. Rev. Sei. Instrum. 1989, 60, 2635-2643. (1 1) Normalization was done by dividing by the height of the absorption edge, and the background was subtracted using cubic spline routines. Noise level is around 0.003 in the averaged EXAFS spectra. (12) Duivenvoordeu, F. B. M.; Koningsberger, D. C.; Uh, Y. S.; Gates, B. C. J. Am. Chem. SOC. 1986, 108, 6254-6262. 0 1994 American Chemical Society