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

Abstract

High-resolution solid-state *'Si 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 s nthetic faujasites with Si/AI ratio in the range 1.19-2.75. On the basis of previous work which related Si chemical shift to environment, signal intensities corresponding to Si(nA1) 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, A1 ordering beyond the first tetrahedral coordination shell, from the observed ratio of intensities of Si(nA1) units and from arguments based on crystal symmetry and electrostatic energy, a series of Si, A1 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 A1 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, A1 ordering within the framework. Moreover, th,e 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. X9

Electrochemically controlled ion exchange: proton ion exchange with sodium zeolite X and A

“…The broadening of 29 Si lines of less crystalline samples has been previously observed by Lippmaa et al [28]. The Si/ Al ratio for the electrochemical sample is calculated as 1.54 (again the spectral profile agrees with published spectra [26,27] which is equivalent to a 5 % loss in aluminium. The NaA spectra shows a single peak at −90 ppm, which is expected for zeolites with a Si/Al ratio of one (as all silicon atoms have four aluminium neighbours).…”

“…It should be noted that the spectrum was not deconvoluted, and so, the measured ratio is approximate. A comparison of the spectral profile with published experimental and theoretical spectra shows good agreement [26,27]. The corresponding electrochemical sample again shows the five-fingered pattern.…”

Investigation of Al,Si order in K-feldspars using 27Al and 29Si MAS NMR

“…2-step disordering is calculated by setting t2o = t2m = 0 and tlo = tlm for tlo > 0.5, and tlo = tim and t2o = t2m for tlo < 0.5. Chemical shifts for different local environments can be estimated from the empirical observation that substitution of an AI NNN for a Si NNN causes an approximately 4 to 5.5 ppm deshielding at 29Si (Lippmaa et al, 1980;Engelhardt et al, 1981;Klinowski et al, 1982;J.V. Smith et al, 1984;Ramdas and Klinowski 1984).…”

Li+- and H+-Exchanged Low-Silica X Zeolite as Selective Nitrogen Adsorbent for Air Separation

“…19,20 These results indicate that the LSX prepared in this study has high crystallinity and framework Si/Al ratio near 1.0. Figure 3 shows nitrogen and oxygen adsorption isotherms at 20 ºC of fully Li-exchanged and Ca-exchanged LSX.…”