STRUCTURE REFINEMENT OF LARGE-PORE ALUMINOPHOSPHATE MOLECULAR-SIEVE, H1, BY RIETVELD METHODS

The structure of Hl, a large-pore molecular sieve, is analyzed using X -ray powder diffraction data collected on two samples from separate pr eparations. Data for one of the samples, Hl-A, prepared using a combin ation of tributylamine and dipentylamine was collected using a synchro tron source, whereas for the other sample prepared using dipropylamine (Hl-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) angstro m for Hl-A and a = 18.9923(4) and c = 8.1206(3) angstrom for Hl-B. Fin al agreement indices were Rp = 0.086 and Rwp = 0.122 for H1-A and Rp = 0.087 and Rwp = 0.112 for Hl-B. The framework structure of Hl in thes e two samples is similar to that of VPI-5 except that the 6-membered r ings in Hl contain disordered phosphate groups. The structure is descr ibed 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 othe r 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 b y four framework oxygens and two water molecules. The large channel is occupied by a network of hydrogen-bonded water molecules. These studi es together with our earlier results on dehydrated samples show that H l has two partially occupied structures, but that their populations va ry from one preparation to the other. The disorder model for Hl clearl y distinguishes it from VPI-5, which does not display this disorder.