Ordering of aluminium and silicon in synthetic faujasites

Abstract

The distribution of aluminium and silicon atoms on the tetrahedral sites in synthetic faujasites (zeolites X and Y) has been debated for many years. Although the framework structure of the zeolite is known¹, the detailed distribution of Si and Al in the lattice cannot be established by conventional methods, partly because of the very similar scattering powers of silicon and aluminium for X rays and partly because of the difficulties in obtaining crystals of adequate size and perfection. Some inferences have, however, been made on the basis of four indirect approaches: variation of the (cubic) lattice parameter as a function of aluminium content^2,3; electrostatic calculations⁴; change of zeolitic acidity on successive extraction of aluminium from the framework^5–7; and the kinetics of crystallization⁸. All these approaches have assumed the validity of ‘Loewenstein's rule’ which forbids Al atoms from occupying neighbouring tetrahedral sites. High-resolution solid-state ²⁹Si NMR (with magic angle spinning) can determine the detailed Si, Al ordering in zeolites^9–16 . NMR coupled with electron diffraction studies has revealed that Loewenstein's rule, generally treated as axiomatic, is broken in Linde A^10,13,14 and in at least three other zeolites where Si/Al is nominally unity¹⁵. In view of its importance as an archetypal zeolite, particularly commercially, we have re-examined the question of framework ordering in faujasite. We propose new ordering schemes and explain discontinuities in the unit cell constant as a function of composition.