ChemInform Abstract: Crystallographic Site Discrimination in Aluminophosphate Molecular Sieves by Solid‐State NMR: Influence of the Field‐Dependent Isotropic Second‐Order Quadrupolar Shift on High‐Resolution 27Al DOR NMR.

Grobet, Piet J.; Samoson, A.; Geerts, H; Martens, Johan A.; Jacobs, Pierre

doi:10.1002/chin.199208011

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“…Using 27 Al double-rotation (DOR) NMR, previous studies have shown that the line at 40 ppm is actually composed of two overlapping resonances from 4-coordinated Al in a 1:1 intensity ratio allowing their differentiation in the MAS spectrum at a lower magnetic field strength . The 2D 27 Al MQ-MAS NMR spectrum of AlPO 4 -54· x H 2 O, exhibits two distinct sites of 4-fold (around 40 ppm) and one site of 6-fold (around −15 ppm) 27 Al.…”

Section: Resultsmentioning

confidence: 99%

Effect of H₂O on the Pressure-Induced Amorphization of AlPO₄-54·xH₂O

et al. 2014

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Microporous AlPO 4 -54•xH 2 O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H 2 O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H 2 O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO 4 -54•xH 2 O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IV Al, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.

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Section: Resultsmentioning

confidence: 99%