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 Al and is thermally stable; the other has a deficiency of P and readily converts to AIPO4-8. The two phases are designated VPI-5 and H1, respectively. Both solids yield a P-31 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 the healing of the broken bond and hydration of Al in the 4-ring, which becomes 6-coordinate when sufficient water is present.