Theoretical nets with 18-ring channels: enumeration, geometrical modeling, and neutron diffraction study of aluminophosphate 54

Abstract

The framework topology of aluminophosphate no. 54 is the same as that of the aluminophosphate-based family of molecular sieves denoted VPI-5. It corresponds to that of theoretical net 81(1), now numbered 520, which has an 18-ring channel. Unit cell parameters for as-synthesized AlPO{sub 4}-54 are a = 19.009 (2) {angstrom}, c = 8.122 (1) {angstrom}, V = 2541.63 {angstrom}{sup 3}. AlPO{sub 4}-54 dehydrated at 275{degree}C has lattice parameters a = 18.549 (1) {angstrom}, c = 8.404 (1) {angstrom}, V = 2504.13 {angstrom}{sup 3}. The crystal structure of dehydrated AlPO{sub 4}-54 was determined from time-of-flight neutron diffraction data. The 13 theoretical nets with crankshaft chains that fit the hexagonal geometry and cell dimensions of AlPO{sub 4}-54 were considered as possible models for the structure. The AlPO{sub 4}-54 framework contains tetrahedrally coordinated Al and P at the vertices of a 3D net obtained by replacing each edge of a (4.4.18){sub 1}(4.6.18){sub 2} 2D net by a crankshaft chain. The average free diameter across the 18-ring channel is 12.5 {angstrom}. (Al,P)-O bond distances from the Rietveld refinement range from 1.44 to 1.70 {angstrom} with a mean value of 1.60 {angstrom}, and (Al,P)-O-(Al,P) angles range from 132 to 180{degree}.