THE TRIPLE HELIX INSIDE THE LARGE-PORE ALUMINOPHOSPHATE MOLECULAR-SIEVE VPI-5

Highly crystalline samples of the pure aluminophosphate form of VPl-5 have been prepared by using diphosphoric acid in the synthesis. The high resolution of the synchrotron powder diffraction data of this material has allowed a detailed refinement of as-synthesized VPl-5 to be performed (a = 18.9752(1) angstrom, c = 8.1044(1) angstrom; R(F) = 0.086, R(wp) = 0.141, R(exp) = 0.108). Initial difficulties encountered in the Rietveld refinement were overcome by reducing the symmetry from the ideal P6(3)cm, which allows for the alternation of Al and P, to P6(3), which has no vertical mirror planes. A series of difference Fourier maps then revealed the presence of seven, fully occupied water positions. Two of these complete an octahedral coordination sphere around the framework Al atom located between the fused 4-rings. Four of the remaining five water positions form a hydrogen-bonded chain between the octahedrally coordinated Al atoms and thus create a triple helix of water molecules following the 6(3) screw axis in the 18-ring channel. The last position, near the center of the channel, links the helices to one another, but is less well defined than the others and is open to interpretation. In contrast to the vast majority of hydrated zeolite structures, this one is not disordered. Both the aluminophosphate framework and the water molecules exhibit the same symmetry. The structure is fully consistent with Al-27 and P-31 MAS n.m.r. results, but requires a reinterpretation of the peak assignment.