Transformation of AlPO-53 to JDF-2: Reversible Dehydration of a Templated Aluminophosphate Studied by MAS NMR and Diffraction

Abstract

We describe a detailed study of the aluminum phosphate AlPO-53 in both its as-made and calcined forms. In its as-made state, AlPO-53(A), the material is templated by methylammonium cations and contains occluded water molecules and also hydroxide ions that bridge pairs of aluminum atoms, increasing their coordination number to 5. Solid-state NMR experiments confirm the local environment of the aluminum and phosphorus atoms proposed in a previous structural model from powder X-ray diffraction. 31P NMR shows the presence of four distinct resonances with an intensity ratio of 1:1:2:2, consistent with the expected six crystallographic P sites. 27Al triple-quantum MAS NMR resolves five aluminum peaks, two with NMR parameters characteristic of four-coordinate Al and three of five-coordinate Al. One of these latter signals has a greater intensity than that of the others, consistent with the presence of two overlapping signals from two distinct crystallographic Al sites. First-principles calculations of NMR parameters provide a complete spectral assignment and confirm our interpretation of unresolved spectra. AlPO-53(A) is found to convert easily into a second crystalline phase on moderate heating (upon spinning in the NMR rotor for an extended period, for example), and variable-temperature powder X-ray experiments, together with TGA, suggest that this is a dehydration process yielding a second aluminophosphate, JDF-2. This is confirmed using both 31P and 27Al NMR, with the spectral assignment of JDF-2 supported by first-principles calculations. Calcination of AlPO-53(A) or of the dehydrated material, JDF-2, at 300 °C yields the microporous open-framework material AlPO-53(B), a tetrahedral network with three Al and three P sites, as confirmed by NMR and first-principles calculations. In addition to demonstrating the power of the combined use of NMR, first-principles calculations, and diffraction for detailed structural investigations, we show that the possibility of a reversible dehydration in as-made AlPO-53 and similar systems is an important consideration in structural studies and provides evidence that the published structural model for AlPO-53(A) may be incomplete.

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

“…We have previously observed such behaviour for the as-made AlPO, JDF-2. 17 However, as motion was detected in all six materials, including the anhydrous AlPO-34(dmim), AlPO-34(emim) and AlPO-34(cyclam), it is clear that dynamics of the SDAs also occurs on the microsecond timescale (as previously observed for AlPO-14 22 ) and further work, for example including molecular dynamics simulations and experimental 2 H NMR, will be required in order to characterise these motions more fully.…”

“…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. 17,47,49,50 In order to ensure this is the case, the atomic coordinates and, often, unit cell parameters can (or, indeed, must) be optimised to an energy minimum. Sneddon et al 50 recently investigated the computational parameters most suited to the optimisation of AlPOs and showed that the optimisation of all atomic coordinates and unit cell parameters with the inclusion of semi-empirical dispersion correction schemes yielded results in good agreement with both crystallographic and spectroscopic measurements, without applying any a priori constraints to the structural model.…”

Recent developments in solid-state NMR spectroscopy of crystalline microporous materials

“…67 Similarly, the 10 inequivalent oxygen sites in the 17 O spectrum of the SiO 2 zeolite ferrierite were able to be assigned using GIPAW calculations. 68 DFT calculations were also crucial in understanding the reversible phase transition that was observed in situ during NMR experiments on AlPO-53(A).…”

“…This latter point is particularly important for as-made AlPOs and MOFs, where organic molecules are present in the structure. For example, for AlPO-14, 67,69 poor agreement between experiment and calculation was obtained, unless the structural models were "optimised" (typically achieved by reducing the forces upon the atoms) prior to the calculation of the NMR parameters. Although the structural changes resulting from this optimisation were small, this did produce significant changes in the NMR parameters, reflecting the sensitivity of NMR as a probe of local structure.…”

Studying brain cytoarchitecture with MRI—Present, future and promises of high field

“…23 Na has spin of 3/2, and thus it is possible to generate double and triple quantum coherences (DQC, TQC) by means of the appropriate pulse sequences (Jaccard et al, 1986;Pekar and Leigh, 1986). Triple quantum filtered (TQF) 23 Na-NMR has been shown to be particularly attractive, since in the case of slow molecular motion, the TQF generates a visible signal, whereas in fast motion the two triple-quantum transverse relaxation components cancel each other and the TQF signal is then nulled (Navon, 1993;Ashbrook et al, 2009). Most interestingly, it has been shown that the TQF 23 Na signal in living tissue has an almost exclusively intracellular contribution, stemming probably from the higher macromolecular content in the intracellular space (Navon, 1993).…”