Sodium ions in hydrated faujasites as studied by 23Na and 1H NMR

Abstract

Longitudinal ( T 1) and transverse ( T 2) relaxation times and the absolute intensity of 23Na NMR have been studied as a function of temperature, Si/Al ratio and ion exchange. The NMR spectrum of the quadrupole nucleus 23Na of spin I= 3 2 consists of a central line and satellites. Depending on the strength of the quadrupole interaction of the 23Na ions, a whole spectrum, only the central line, or even nothing may be observed experimentally. But the observed 23Na-NMR spectrum can be assigned unambiguously by pulse methods because the NMR signal of a whole spectrum and of a central line depend quite differently on the r.f.-pulse length. By measuring the intensity of the signal as a function of the r.f.-pulse length, it can be shown that a whole spectrum of only a fraction of the Na ions is observed in hydrated NaX and NaY faujasites. In NaY with seven Na ions per cage only four are observed by NMR. It can be concluded from selective ion exchange, and from 1H NMR of water-exchange kinetics between the supercages and the β-cages, that only and all the Na ions in site S2 are detected by NMR. There is extreme motional narrowing above 230 K. The fluctuations of the electrical-field gradient are caused by the diffusion of the sorbed water and not by the diffusion of the Na ions themselves. The quadrupole coupling constant QCC is estimated from T 1. The value of QCC = 3 MHz agrees well with model calculations. In NaX, 2.4 Na per cage are observed. The same value is found in NaY at higher temperatures. No reasonable explanation can be given for this fact.