SOLID-STATE NMR-STUDIES OF THE ALUMINOPHOSPHATE MOLECULAR-SIEVE ALPO4-18

Solid-state NMR reveals that AlPO4-18 contains three crystallographic sites for both P and Al in a 1:1:1 population ratio. In the as-prepared material two Al sites are 4-coordinated and one is 5-coordinated. The latter becomes 4-coordinated upon calcination. These results are consistent with the structural refinement. There is a reversible structural change from space group C2/c in the calcined form to a lower symmetry space group in the calcined-rehydrated form. As a result of this transition, three P and Al sites in an occupancy 8 in four double six-membered rings in one unit cell give rise to at least 12 crystallographic sites for P and Al.