Characterization and structural analysis of differently prepared samples of dehydrated VPI-5

The thermal behaviors of five differently prepared samples (A-E) of the ver y-large-pore aluminophosphate molecular sieve VPI-5 have been investigated. Thermogravimetric analyses show significant differences in the thermal sta bilities of the five samples. Nonetheless, all samples could be completely dehydrated without transformation to AlPO4-8 if a certain procedure was fol lowed carefully. Although the powder diffraction patterns of all samples in their as-synthesized forms were very similar, one of the patterns of the d ehydrated forms was different from the others. This particular sample A(d) was 3 years old when the experiment was performed, whereas all the others h ad been freshly prepared. Consequently, structural studies of the dehydrate d samples A(d) and E-d were undertaken. Solid state Al-27, P-31 and H-1 MAS NMR experiments for the two samples yielded almost identical spectra. The Al-27 spectra confirmed that all Al were tetrahedral, the H-1 spectra showe d that no water was present, and the 31P spectra contained seven peaks, ind icating that the symmetry was much lower than P6(3)cm (the expected topoche mical space group). Rietveld refinement of the structures of the two dehydr ated samples resulted in two variants of the VFI topology. Although both sh owed a clear monoclinic symmetry, the older sample A, required the c-axis ( along the 18-ring channel direction) to be doubled. The structure of sample Ed did not require this doubling, but two small peaks in the powder patter n remained unindexed. These could be satellite peaks resulting from an inco mmensurate modulation of the structure along the c-axis. (C) 2000 Elsevier Science B.V. All rights reserved.