NMR Characterization and Rietveld Refinement of the Structure of Rehydrated AlPO₄-34

Abstract

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°, β ) 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.

Ionothermal synthesis and characterization of CoAPO-34 molecular sieve

“…Upon calcination, the material should adopt rhombohedral symmetry but, as seen in Fig. 2(a), the pXRD pattern of the calcined material, after exposure to air, shows that the two different hydrated CHA phases, identified by Tuel et al, [19] (both with triclinic symmetry) are present. In addition, the freshly calcined sample is blue but, upon exposure to the air, the material turns blue-green, as previously observed for CoAPO-34.…”

“…[13,14] The fluoride ions present in the material framework are to balance the charge of protonated organic molecule which is located in the CHA cage of the material. [19] Upon calcination ( Fig. 5(b)) the resonance from the octahedral Al disappears and only one tetrahedral resonance is observed (at ca.…”

A Multinuclear NMR Study of Six Forms of AlPO-34: Structure and Motional Broadening

“…[29][30][31][32][33][34] Full details of the syntheses are given in the Supporting Information (S1). The identity of the products was confirmed by comparison of the measured powder X-ray diffraction patterns with those simulated from crystal structures reported in the literature (where available, see Supporting Information (S2)).…”

“…11,17-19, [46][47][48][49] As most of the forms of as-made AlPO-34 considered here have crystal structures refined with disorder of the SDA (emim and cyclam), 29,30,34 water (pyr) 31 or both (pip), 32 DFT calculations were used to help interpret and assign the experimental NMR spectra. One crucial requirement for the prediction of accurate NMR parameters from a given structure is that the structure itself must represent a realistic minimum on the energy landscape.…”

A study of the acidity of SAPO-34 by solid-state NMR spectroscopy

“…6), and more than one tetrahedral 27 Al peaks of trigonal SAPO-34 have not been reported yet. In trigonal AlPO-34, every Al atom is connected to four P atoms through bridged O atoms, and the tetrahedral 27 Al peak is at 33 ppm [32]. In H-SSZ-13 with the same structure as AlPO-34 and SAPO-34, every Al atom is connected to four Si atoms, and the tetrahedral 27 Al peak appears at 56 ppm [33].…”