Abstract
The thermal behaviors of five differently prepared samples (A–E) of the very-large-pore aluminophosphate molecular sieve VPI-5 have been investigated. Thermogravimetric analyses show significant differences in the thermal stabilities of the five samples. Nonetheless, all samples could be completely dehydrated without transformation to AlPO 4-8 if a certain procedure was followed carefully. Although the powder diffraction patterns of all samples in their as-synthesized forms were very similar, one of the patterns of the dehydrated forms was different from the others. This particular sample A d was 3 years old when the experiment was performed, whereas all the others had been freshly prepared. Consequently, structural studies of the dehydrated samples A d and E d were undertaken. Solid state 27Al, 31P and 1H MAS NMR experiments for the two samples yielded almost identical spectra. The 27Al spectra confirmed that all Al were tetrahedral, the 1H spectra showed that no water was present, and the 31P spectra contained seven peaks, indicating that the symmetry was much lower than P6 3 cm (the expected topochemical space group). Rietveld refinement of the structures of the two dehydrated samples resulted in two variants of the VFI topology. Although both showed a clear monoclinic symmetry, the older sample A d required the c-axis (along the 18-ring channel direction) to be doubled. The structure of sample E d did not require this doubling, but two small peaks in the powder pattern remained unindexed. These could be satellite peaks resulting from an incommensurate modulation of the structure along the c-axis.
Published Version
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