A re-examination of Si, Al ordering in zeolites NaX and NaY

Abstract

High-resolution solid-state ²⁹Si nuclear magnetic resonance (n.m.r.) spectra were recorded at 79.80 MHz, with rapid spinning of the sample at the magic angle to the external magnetic field, for a series of phase-pure synthetic faujasites with Si/Al ratio in the range 1.19–2.75. On the basis of previous work which related ²⁹Si chemical shift to environment, signal intensities corresponding to Si(nAl) structural units (n= 0, 1, 2, 3, 4) could be quantitatively determined by accurate deconvolution of the spectrally well-resolved Gaussian peaks. Even though n.m.r. spectra do not by themselves provide direct evidence for Si, Al ordering beyond the first tetrahedral coordination shell, from the observed ratio of intensities of Si(nAl) units and from arguments based on crystal symmetry and electrostatic energy, a series of Si, Al ordering schemes was constructed. These schemes offer greater insight into the structure of the aluminosilicate framework than can be achieved at present by other techniques. Inter alia, they reveal that the Loewenstein rule, which forbids Al atoms to occupy neighbouring framework tetrahedral sites, is strictly obeyed for zeolites of the faujasite structure. Our results and conclusions can also explain observed discontinuites in the plot of the (cubic) unit-cell parameter against aluminium content in terms of Si, Al ordering within the framework. Moreover, the magic-angle-spinning nuclear magnetic resonance (m.a.s.n.m.r.) spectra provide an independent method of determining Si/Al ratios: ratios determined by X-ray fluorescence or analytical electron microscopy agree within a few per cent with those derived from m.a.s.n.m.r., thus lending further credence to the value of solid-state n.m.r. in studies of aluminosilicates. The effect of second-nearest (tetrahedral) neighbours on chemical shifts, as well as the spectral and structural characteristics of faujasites possessing “non-ideal” Si/Al ratios, are also discussed.