PHYSICAL CHEMISTRY

O. A. GLONTI, Academician of the Academy of Sciences of the Georgian SSR, G. V. TSITSISHVILI, N. A. SHISHAKOV

ON THE LOCATION OF SILVER IONS IN ZEOLITE AgX

Recently, synthetic zeolites of types A and X have been acquiring ever increasing practical importance as highly efficient adsorbents used for the separation of gas mixtures and for the deep drying and purification of gases. They are gradually replacing ordinary desiccants (silica gel, aluminum oxide, etc.) and such purification methods as freezing, distillation, and others.

Only those molecules whose dimensions do not exceed the dimensions of the entrance windows can penetrate into the primary porous structure of zeolites. In this connection, the polarity of the molecules is of great importance. For example, the large and small cavities of zeolite X are accessible to water molecules, which possess this property.

As is known, the degree of ion exchange depends on the nature of the cations. In work \((^1)\) the following exchange series was established: \(\mathrm{Ag}^+ > \mathrm{Pb}^+ > \mathrm{Na}^+ > \mathrm{K}^+ > \mathrm{NH}_4^+ > \mathrm{Li}^+\). It is therefore evident that silver readily displaces sodium. It is also known that sorption properties depend strongly not only on the nature, but also on the location of the cations in the lattice. As M. M. Dubinin \((^2)\) showed, in zeolites of types A and X adsorption changes greatly when \(\mathrm{Na}^+\) ions are replaced by \(\mathrm{Ca}^{2+}\). Therefore, determining the position of cations in the crystal lattice of synthetic zeolites is of great importance for a better understanding of sorption processes.

The aim of the present work was to determine the position of \(\mathrm{Ag}^+\) ions in zeolite of type X.

The synthesis of the silver form of the zeolite was carried out by M. Adolashvili according to the method \((^{3,4})\). This form of zeolite, unlike other forms, is characterized by a number of selective-adsorption properties \((^{4,5})\), by the ability to change color under the influence of moisture \((^{6,7})\), and by other qualities. All of the above indicates that the study of the structure of the silver zeolite is of considerable interest.

The object of our investigation was a synthetic zeolite NaX in which, by ionic exchange, 80% of the \(\mathrm{Na}^+\) ions had been replaced by \(\mathrm{Ag}^+\) ions. The structure of zeolite NaX was determined by Broussard and Shoemaker \((^8)\). The authors assign it to the space group \(Fd3m - O_h^7\) (centrosymmetric case: the origin of coordinates coincides with the center of inversion).

Localized \(\mathrm{Na}^+\) ions are situated at the centers of six-membered prisms (at the origin of coordinates) and in the planes of six-membered windows. They have the following coordinates: \(\mathrm{Na_I}\)—0.00; 0.00; 0.00; \(\mathrm{Na_{II}}\)—0.9911; 0.7589; 0.9911. A discrepancy factor \(R = 10.34\) was obtained. The photographs were taken on an ionization diffractometer. Since, with increasing angle, the distortion of intensities increases, the authors limited themselves to using interplanar spacings down to \(d = 1.262\ \text{Å}\), which corresponds to the sum of squares of the indices \(h^2 + k^2 + l^2 = 387\).

In the present work, the principal method of investigation was the obtaining of X-ray diffraction patterns on photographic film. The method we used is, for a number of reasons, considered of little suitability for determining the structure of a substance; however, if there is a lattice of high symmetry (as in our case), the reliability of indexing the observed reflections is considerably increased, and therefore the structure can sometimes be determined rather accurately.

To increase the reliability of the results, we carried out X-ray photography with two different radiations—iron and copper. The first made it possible to study reflections at comparatively small angles, while the second made it possible to obtain more reflections of higher orders. In this way \(d\) was brought to a value of 0.988 Å, which corresponds to the sum \(h^2 + k^2 + l^2 = 632\). This was important for constructing reliable projections of the electron-density distribution.

For greater accuracy in determining interplanar spacings and reliability in indexing the lines, a standard substance—metallic silicon—was sometimes added to the powder.

For analysis of the intensities of the reflections we used an MF-4 recording microphotometer. From the blackening curves \(S\), the intensities were calculated according to the formula \(I_{\mathrm{e}} = \log(S_0/S)\). On the basis of the line intensities found, the experimental amplitudes \(F_{\mathrm{e}}\) were calculated by the formula

\[ F_{\mathrm{e}} = \sqrt{\frac{I_{\mathrm{e}}}{A\tau^2 L P p}}. \]

The quantity \(B\), entering into the temperature factor \(\tau^2\), was taken equal to 1.4. In the case of overlapping lines, the total intensity was divided in proportion to the quantities \(pF_{\mathrm{T}}^2\).

Fig. 1. Projection of the electron density onto the (001) plane for zeolite AgX

We assume that the ions \(\mathrm{Ag}^{+}\) during ion exchange occupy the positions of \(\mathrm{Na}^{+}\) ions, since in size and polarizability they are close to one another, and therefore, for the calculation of \(F_{\mathrm{T}}\), the coordinates of \(\mathrm{Na}^{+}\) were assigned to the \(\mathrm{Ag}^{+}\) ion.

Table 1

Theoretical and experimental values of the structure amplitudes for AgX

Theoretical amplitudes and the projection of the electron density were calculated on the electronic computer of Moscow State University. The signs \(F_{\mathrm{T}}\) obtained in this way were assigned to the experimental amplitudes, from which the map of the electron-density distribution was calculated and constructed.

onto the (001) plane. Good agreement was obtained between the quantities \(F_t\) and \(F_e\), as is evident from the data in Table 1. From these data the discrepancy factor was found to be \(R = 0.21\).

Figure 1 gives a comparison of the theoretical projection (constructed from the data of work (8)) and the experimental projection. The silver ions are denoted by bold dots, silicon by medium-sized dots, and oxygen by small dots. All these points fit well on the maxima of electron density found by us. The slight displacements of the experimental maxima compared with the theoretical ones are probably caused by the appearance of false maxima and by the superposition of projections of atoms on one another. All this makes it possible to assert that, in ion exchange, the \(\mathrm{Ag}^+\) ions occupy the positions of the \(\mathrm{Na}^+\) ions.

Institute of Physical Chemistry
Academy of Sciences of the USSR

Received
31 III 1965

REFERENCES

1. L. P. Shirinskaya, N. F. Ermolenko, Synthetic Zeolites, Publishing House of the Academy of Sciences of the USSR, 1962, p. 44.
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3. G. V. Tsitsishvili, T. G. Andronikashvili et al., Soobshch. AN GruzSSR, 28, No. 3, 281 (1962).
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8. L. Broussard, D. P. Schoemaker, J. Am. Chem. Soc., 82, 1041 (1960).