STRUCTURE REFINEMENT AND WATER LOCATION IN THE VERY LARGE-PORE MOLECULAR-SIEVE VPI-5 BY X-RAY RIETVELD TECHNIQUES

The structure of the very large pore molecular sieve VPI-5 was solved based on a combination of X-ray power diffraction measurements and modeling techniques. The original structure refinement was hampered by the fact that the water molecules, which represent 25% of the scattering material, could not be located. In the present work, one VPI-5 family member was analyzed using X-ray Rietveld refinement. Rietveld refinement results show that the reported structure is correct. All the water molecules were located and refined in the 18-ring pore. The water molecules form weakly associated layers within the pore, in a manner not unlike that of liquid water. VPI-5, AlPO4·2 H2O, is hexagonal with space group P63cm and cell parameters of a = 18.9777(3), c = 8.1155(1) A ̊; V = 2531.4(1) A ̊3; Dcalc = 1.243 g cm-3. Small errors in the refinement were found to be caused by a second phase of a slightly dehydrated VPI-5. This was the reason for a high final refinement residual, Rwp, of 0.31. © 1990.