Solid state NMR study of 18-ring large pore aluminophosphate molecular sieves

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 AI and is thermally stable while the other has a deficiency of P and readily converts to AIPO4-8. The two phases are designated as VPI-5 and H1, respectively. Both solids yield a 31P spectrum, when completely dry, which consists of three resonances. These spectra cannot arise from the 18-ring VPI-5 framework of alternating 4-coordinate AI and P. It is suggested that in the dry state the P-O-AI 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. The changes observed are described on the basis of a healing of the broken bond and hydration of AI in the 4-ring which transforms to 6-coordination or hydration of the aluminum atoms in alternate 6-membered rings at high levels of water content.

Solid state NMR study of 18-ring large pore aluminophosphate molecular sieves Conference Paper