STRUCTURE REFINEMENT OF LARGE-PORE ALUMINOPHOSPHATE MOLECULAR-SIEVE, H1, BY RIETVELD METHODS
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abstract
The structure of HI, a large-pore molecular sieve, is analyzed using X-ray powder diffraction data collected on two samples from separate preparations. Data for one of the samples, H1-A, prepared using a combination of tributylamine and dipentylamine was collected using a synchrotron source, whereas for the other sample prepared using dipropylamine (H1-B), radiation from a rotating anode was used. The space group for these crystals is P63cm with a = 18.9702(3) and c = 8.1163(2) for H1-A and a = 18.9923(4) and c = 8.1206(3) for H1-B. Final agreement indices were Rp = 0.086 and Rwp = 0.122 for Hl-A and Rp = 0.087 and Rp = 0.112 for Hl-B. The framework structure of Hl in these two samples is similar to that of VPI-5 except that the 6-membered rings in HI contain disordered phosphate groups. The structure is described on the basis of two partially occupied models: The first one is a regular VPI-5 framework with an occupancy of about 0.75, and the other is a different orientation of the groups in the 6-membered ring with occupancy of about 0.25. The Al atom of the 4-ring is 6-coordinated by four framework oxygens and two water molecules. The large channel is occupied by a network of hydrogen-bonded water molecules. These studies together with our earlier results on dehydrated samples show that HI has two partially occupied structures, but that their populations vary from one preparation to the other. The disorder model for HI clearly distinguishes it from VPI-5, which does not display this disorder. 1993.
