NMR Characterization and Rietveld Refinement of the Structure of Rehydrated AlPO 4 -34 Alain Tuel,* ,² Stefano Caldarelli, ² Anton Meden, ‡ Lynne B. McCusker, § Christian Baerlocher, § Alenka Ristic, | Nevenka Rajic, | Gregor Mali, | and Venceslav Kaucic | Institut de Recherches sur la Catalyse, CNRS, Villeurbanne France, UniVersity of Ljubljana, Faculty of Chemistry and Chemical Technology, Ljubljana, SloVenia, Laboratory of Crystallography, ETH, Zurich, Switzerland, and National Institute of Chemistry, Ljubljana, SloVenia ReceiVed: February 4, 2000 The triclinic form of AlPO 4 -34, a microporous aluminophosphate with the chabazite (CHA) topology, adopts a rhombohedral symmetry upon calcination. The framework structure of this phase remains intact under ambient conditions, but it distorts dramatically, though reversibly, in the presence of water. Following these structural changes in situ by X-ray diffraction revealed that there are actually two stable rehydrated phases, which differ from each other by one water molecule in the channel. Both of these phases have triclinic unit cells that are closely related to that of the calcined rhombohedral phase. The structure of the low-temperature (10 °C), fully rehydrated phase (phase B) was elucidated by combining high-resolution synchrotron powder diffraction with solid-state NMR techniques. Coordination of three of the six Al atoms to water molecules causes the deformation of the framework and the reduction of the symmetry. Rietveld refinement of the structure of phase B in the triclinic space group P1(a ) 9.026, b ) 9.338, c ) 9.508 Å, R) 95.1°,  ) 104.1°, and γ ) 96.6°) converged with R F ) 0.079 and R WP ) 0.176 (R exp ) 0.087). Framework connectivities derived from the structure were used to assign 31 P NMR lines as well as part of the 27 Al NMR signal. Introduction The synthesis of novel crystalline aluminophosphate molec- ular sieves (AlPO 4 -n) 1 has been a matter of increasing interest over the two past decades. In contrast to aluminosilicate zeolites, the aluminum species in AlPO 4 -n materials can be four, five, or six-coordinate, 2-4 and this explains why such a large number of structures have been reported. Interest in the field has also been stimulated by the possibility of incorporating tetravalent or divalent cations into the frameworks of these materials and thereby forming potentially active catalysts (MeAPO’s) for acidic or redox reactions. 5-8 Originally, microporous aluminophosphates were prepared in basic media with organic molecules, such as primary amines, as structure-directing agents. The introduction of fluoride ions in the preparation of aluminophosphates allowed novel struc- tures, which could not generally be obtained using the standard route, to be synthesized. 9,10 This strategy was used to make the triclinic form of AlPO 4 -34, an aluminophosphate with the chabazite (CHA) topology. 9-12 Indeed, attempts to synthesize pure AlPO 4 -34 in the absence of fluoride ions, using conditions that led to the crystallization of MeAPO-34 or SAPO-34, were not successful. However, a synthesis without fluoride using nonconventional phosphorus sources such as Al(H 2 PO 4 ) 3 or H 10 P 8 O 25 13,14 and tetraethylammonium hydroxide as the tem- plating species has been reported. Fluoride ions not only decrease the nucleation rate of aluminophosphates by complex- ing aluminum but also can play a role as costructuring agents or even be covalently bonded to the framework. In triclinic AlPO 4 -34, two fluoride ions bridge between two Al atoms in 4-rings connecting double-6-rings of the structure. These two F - ions neutralize the protonated organic molecule located in the chabazite cage. 11,12 Upon calcination, both the organic and fluoride species are expelled from the pores, and the template-free material adopts the rhombohedral symmetry of the CHA topology. However, the diffraction pattern of this material changes rapidly in the presence of air. 9 Structure modifications of calcined alumino- phosphate molecular sieves in the presence of water molecules have been widely reported in the literature. 15-20 They result from the ability of framework aluminum species to modify their coordination in the presence of water. The structure changes are generally reversible, and the original material can be restored by evacuation at relatively low temperature. Structural studies of rehydrated AlPO 4 -n materials are scarce. One of the reasons is that the peaks in the X-ray diffraction patterns of such rehydrated compounds tend to be broad, so structure analysis using Rietveld refinement techniques is difficult. 16,17 Neverthe- less, Mentzen et al. 21 were able to refine the structure of rehydrated AlPO 4 -11 to determine the position of extraframe- work water molecules inside the channels. In the present paper, two complementary techniques, X-ray powder profile refinement and solid-state NMR, were applied to elucidate the structure of one of the rehydrated forms of AlPO 4 -34. Experimental Section Synthesis. The reactants used were piperidine (99%, Aldrich), 85% phosphoric acid (Fluka), 40% hydrofluoric acid (Fluka), and aluminum isopropoxide (Aldrich). The relative molar composition of the reaction mixture used in the preparation of AlPO 4 -34 was Al 2 O 3 :P 2 O 5 : HF: 2 piperidine: 100 H 2 O. The mixture was prepared by successive addition of phosphoric acid, * Corresponding author. E-mail: tuel@catalyse.univ-lyon1.fr. Fax: (+33) 4 72 44 53 99. ² Institute de Recherches sur la Catalyse. ‡ University of Ljubljana. § Laboratory of Crystallography. | National Institute of Chemistry. 5697 J. Phys. Chem. B 2000, 104, 5697-5705 10.1021/jp000455a CCC: $19.00 © 2000 American Chemical Society Published on Web 05/26/2000