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 content2,3; electrostatic calculations4; change of zeolitic acidity on successive extraction of aluminium from the framework5–7; 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 zeolites9–16 . NMR coupled with electron diffraction studies has revealed that Loewenstein's rule, generally treated as axiomatic, is broken in Linde A10,13,14 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.