High-temperature NMR study of zeolite Na-A: detection of a phase transition.
The zeolite Linde 4A was studied by 23Na, 27Al, and 2.Si NMR at ?953 K. 23Na NMR shows that the quadrupole interaction of Na ions sited at 6-rings decreases when the temp. increases. With the aid of 2-dimensional nutation and exchange expts. it can be shown that large-amplitude motions of the Na ions, which in principle could explain a decrease of quadrupole interactions, do not occur. The decrease of the quadrupole interaction can be interpreted in terms of a phase transition. A comparison of the NMR spectra of 23Na and 27Al shows that the zeolite framework undergoes a major structural change upon increasing the temp., before the Na ions are displaced at higher temps. The exchange of Na ions for K ions shifts this transition to higher temps.
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