STRUCTURE DETERMINATION AND RIETVELD REFINEMENT OF ALUMINOPHOSPHATE MOLECULAR-SIEVE ALPO4-8

The 18-ring pore molecular sieve VPI-5 recrystallizes to the 14-ring sieve AIPO4-8. This recrystallization appears to be reversible under proper conditions, as could be explained on the basis of the structural relationship between the two materials. VPI-5 has an 18-ring channel bounded by six 6-rings and six pairs of adjoining 4-rings. In the conversion to AIPO4-8, two-thirds of the adjoining 4-rings are converted to 6-rings and reconnected to form a smaller, 14-ring channel bounded by eight 6-rings, two pairs of adjoining 4-rings, and two single 4-rings. The space group for AIPO4-8 [Cmc2(1): a = 33.0894(25), b = 14.6832(13), c = 8.3630(6) angstrom at room temperature] is a direct subgroup of that for VPI-5 [P6(3)cm: a = 18.549(l), c = 8.404(1) angstrom for the dehydrated form; orthohexagonal setting: a = 32.128, b = 18.549, c = 8.404 angstrom]. Interconversion of the two structures may be facilitated by extracoordination of framework aluminum atoms with water molecules. The observed small adsorption capacity of AIPO4-8 can be explained by stacking faults, where, within a single channel, different pairs of adjoining 4-rings are left unbroken.