High resolution 29Si n.m.r. of dealuminated Y-zeolites. 2. Silicon, aluminium ordering in the tetrahedral zeolite lattice

Abstract

The Si, Al orderings in the tetrahedral framework of dealuminated Y zeolites covering Si/Al ratios in the range 2.5–5.8 have been investigated using relative populations of Si(OSi) 4− n − (OAl) n[=Si( nAl)] building units determined from 29Si n.m.r. spectra. It is concluded that the Si, Al ordering is predominantly determined by the final Si/Al ratio reached after dealumination and is for a given Si/Al ratio independent of the type (thermochemical treatment under deep-bed or steamed shallow-bed conditions, Al extraction by organic acids) and external conditions (temperature, steam pressure) of the dealumination process used. The Si( nAl) populations derived from the n.m.r. spectra of zeolites with increasing Si/Al ratios are compatible with Si, AI ordering schemes generated by combined replacement of Al against Si and mutual exchange of the locations of a pair of adjacent Si and Al atoms. Relations of the proposed Si, Al orderings to the acidic properties of dealuminated Y zeolites are shortly discussed.