Solid-state NMR study of 18-ring large pore aluminophosphate molecular sieves
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A magic angle spinning solid-state NMR study of 18-ring aluminum phosphates has been carried out as a function of the hydration level of the solid phases. Two samples of aluminum phosphate were considered. One has a one to one ratio of P to Al and is thermally stable; the other has a deficiency of P and readily converts to AlPO4-8. The two phases are designated VPI-5 and H1, respectively. Both solids yield a 31P spectrum, when completely dry, that consists of three resonances. These spectra cannot arise from the 18-ring VPI-5 framework of alternating 4-coordinate Al and P. It is suggested that in the dry state the P-O-Al bond in every other 4-ring is broken to produce a P=O site and 3-coordinate aluminum. At low water contents six resonances are observed, and these eventually transform to the familiar three-peak 1:1:1 pattern for the fully hydrated samples. The changes observed are described on the basis of a healing of the broken bond and hydration of Al in the 4-ring, which becomes 6-coordinate when sufficient water is present. © 1991 American Chemical Society.
author list (cited authors)
Perez, J. O., Chu, P. J., & Clearfield, A.