The triple helix inside the large-pore aluminophosphate molecular sieve VPI-5

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