Chemistry

A. A. Slinkin, E. A. Fedorovskaya

EPR SPECTRA OF PRODUCTS OF THE HIGH-TEMPERATURE INTERACTION OF CrO₃ WITH K₂Cr₂O₇, K₂CrO₄, K₂CO₃, KCl, AND KOH

(Presented by Academician A. A. Balandin, January 29, 1963)

Topochemical reactions occurring in the preparation of multicomponent catalysts ultimately determine their chemical composition and catalytic properties. Therefore, the study of chemical reactions in the solid state is of considerable theoretical and practical interest. We have studied the EPR spectra of the products of the interaction of CrO₃ with K₂Cr₂O₇ at high temperature, since both components are used jointly to obtain alumopotassium chromia catalysts.

Fig. 1. EPR spectra of the mixture CrO₃ + K₂Cr₂O₇ calcined at different temperatures:
a—450° \((\chi = 17.5 \cdot 10^{-6})\); b—600° \((\chi = 24.4 \cdot 10^{-6})\); c—700° \((\chi = 39.4 \cdot 10^{-6})\); d—850° \((\chi = 45.0 \cdot 10^{-6})\)

The reaction of CrO₃ with K₂CrO₄ at 208–220° was investigated by V. I. Spitsyn and co-workers (¹), who showed that it begins in the melt and proceeds with the formation of K₂Cr₂O₇. Zukhov et al. (²) found that treatment of K₂Cr₂O₇ and K₂CrO₄ with hydrogen at 300–700° forms a black product of composition KCr₃O₈, in which the authors assumed the presence of Cr⁵⁺ ions. However, a magnetochemical study carried out by Klemm (³) showed that in this compound only Cr³⁺ and Cr⁶⁺ ions are present. Rohde, Kazanskii, and Pecherskaya (⁴) investigated the EPR spectra of the decomposition products of CrO₃ (chromium bichromate, chromium dichromate, chromium monochromate) and the EPR spectrum of CrO₂. For all compounds, one line with \(\Delta H = 130\text{–}150\) Oe and \(g \simeq 1.97\) was found. Thus, in all these compounds only Cr³⁺ and Cr⁶⁺ ions are present.

The reaction of CrO₃ with K₂Cr₂O₇, K₂CrO₄, K₂CO₃, KCl, and KOH was carried out by us in the solid phase by heating in air at a rate of ~7 deg/min and holding at constant temperature for 6 hr. The products of the interaction of CrO₃ with K₂Cr₂O₇ obtained at 450, 600, 700, and 850° were examined, and with the remaining salts at 600°. The ratio of components in the initial mixture corresponded to a content of ~10% K₂O in the final mixture.

The EPR spectra were recorded on an RE-1301 spectrometer at a frequency of 9326 MHz at room temperature and at temperatures of 43, 54, and 80°, produced by blowing hot air through the resonator. Magnetic susceptibility was measured by the Faraday method in the field-strength interval \(H = 1300\text{–}4500\) Oe. Figure 1 presents the EPR spectra of the products of heating the mixture CrO₃ + K₂Cr₂O₇. As is seen from the figure, the mixture heated at 600° shows an absorption spectrum sharply different from the spectrum of Cr₂O₃ and the spectra of various chromium chromates (⁴), which have one li-

line with \(g \approx 1.97\). The spectra obtained have a distinct fine structure, the resolution of which improves as the calcination temperature of the mixture is increased. It should be noted that these mixtures are strongly paramagnetic, and the paramagnetism* increases with increasing calcination temperature. Figure 2 shows how the EPR spectrum of a mixture calcined at 850° changes depending on the temperature at which it was recorded. Increasing the temperature leads to the contraction of the fine-structure components into a single line, while lowering the temperature again leads to the original spectrum. Figure 3 gives the spectra of the products of the interaction of \(\mathrm{CrO}_3\) with \(\mathrm{K}_2\mathrm{CrO}_4\), \(\mathrm{K}_3\mathrm{CO}_2\), KCl, and KOH at 600°. They also display a distinct fine structure and differ sharply from the spectra of ordinary \(\mathrm{Cr}_3\mathrm{O}_2\).

Fig. 2. Dependence of the form of the EPR spectrum for a mixture of \(\mathrm{CrO}_3 + \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\), calcined at 859°, on the measurement temperature: \(a\) — 25°; \(b\) — 43°; \(v\) — 54°; \(g\) — 80°.

Apparently, there are two explanations for these spectra:

1. The observed spectra are associated with hyperfine structure (h.f.s.) due to interaction with the nuclear spin of the isotope \(\mathrm{Cr}^{53}\).

2. The observed spectra are due to the fine structure (f.s.) of the \(\mathrm{Cr}^{3+}\) ion, analogous to that which exists in chrome alum.

The first assumption is unlikely, since in ordinary chromium compounds there is little \(\mathrm{Cr}^{53}\) isotope, and h.f.s. is observed only when compounds are enriched with the isotope \(\mathrm{Cr}^{53}\).

Fig. 3. EPR spectra of mixtures of \(\mathrm{CrO}_3\) with various substances, calcined at 600°: \(a\) — \(\mathrm{CrO}_3 + \mathrm{K}_2\mathrm{CO}_3\); \(b\) — \(\mathrm{CrO}_3 + \mathrm{KOH}\); \(v\) — \(\mathrm{CrO}_3 + \mathrm{K}_2\mathrm{CrO}_4\); \(g\) — \(\mathrm{CrO}_3 + \mathrm{KCl}\).

To test the second assumption, the EPR spectrum of powdered chrome-potassium alum, \(\mathrm{KCr(SO_4)_2 \cdot 12H_2O}\), shown in Fig. 4a, was recorded. It can be seen that this spectrum is very similar to the spectra obtained in the work. Apparently, topochemical reactions in the solid phase between \(\mathrm{CrO}_3\) and the substances considered lead to the formation of substances of the chrome-potassium alum type, for example, \(\mathrm{KCr(CrO_4)_2}\). As already mentioned, this compound was synthesized in work (2). The proposed assumption encounters one substantial difficulty. In the investigated compounds, \(\mathrm{KCr(CrO_4)_2}\) is found in a strongly paramagnetic (\(\alpha = \mathrm{Cr_2O_3}\)) and even ferromagnetic matrix, therefore—

* Mixtures calcined at 700 and 850° are ferromagnetic with \(\sigma = 0.14\) and \(\sigma = 0.16\), respectively. The figure gives data corresponding to \(H = 3500\) oersted.

the fine structure of the $\mathrm{Cr}^{3+}$ ion should not have been observed because of strong dipole–dipole interaction, as indeed occurs upon decomposition of chrome-potassium alum (Fig. 4b). If the spectra observed in the work are in fact associated with the fine structure of the $\mathrm{Cr}^{3+}$ ion, then there must exist an effective mechanism for shielding the $\mathrm{Cr}^{3+}$ ion in $\mathrm{KCr(CrO_4)_2}$ from magnetic interaction with the $\mathrm{Cr}^{3+}$ ions of chromium oxide.

Fig. 4. E.p.r. spectrum of chrome-potassium alum:
a — starting alum,
b — heated at 200°

The reactions studied also have very interesting topochemical features. The interaction of $\mathrm{CrO_3}$ with $\mathrm{K_2Cr_2O_7}$ at low temperatures (up to 350°) leads to products that were found in work (4) and described in detail in the monograph (5). In the e.p.r. spectra of these substances we also found narrow (100–130 oersted) e.p.r. lines and ferromagnetic properties. When a mixture of $\mathrm{CrO_3} + \mathrm{K_2Cr_2O_7}$ is heated to 450°, the e.p.r. signal disappears (Fig. 1a) and appears only at higher calcination temperatures (Fig. 1b, c, d). Since $\mathrm{K_2Cr_2O_7}$ melts at about 400°, it could be assumed that the products giving the e.p.r. spectra are obtained by reaction of $\mathrm{Cr_2O_3}$, formed during decomposition of $\mathrm{CrO_3}$, with molten $\mathrm{K_2Cr_2O_7}$. However, special experiments on the interaction of $\mathrm{Cr_2O_3}$ with $\mathrm{K_2Cr_2O_7}$ showed that this does not yield substances giving the spectra shown in Fig. 1. Moreover, the interaction of $\mathrm{CrO_3}$ with $\mathrm{K_2CrO_4}$, $\mathrm{K_2CO_3}$, $\mathrm{KCl}$, which do not melt at all under the experimental conditions, leads to spectra (Fig. 3) analogous to those shown in Fig. 1. The interaction in these systems, leading to products of the $\mathrm{KCr_3O_8}$ type, is probably due to the reaction of active products formed in the initial stages of decomposition of $\mathrm{CrO_3}$ (chromium chromates) with $\mathrm{K_2Cr_2O_7}$ and other salts. In this process compounds are formed which at high temperatures give products of the $\mathrm{KCr(CrO_4)_2}$ type.

Institute of Organic Chemistry
named after N. D. Zelinsky

Received
25 I 1963

References

1. V. I. Spitsyn, N. S. Afonskii, V. I. Tsirel'nikov, ZhNKh, 5, 1970 (1960).
2. L. Suckow, J. Fankuchen, R. Ward, J. Am. Chem. Soc., 74, 1678 (1952).
3. W. Klemm, Zs. anorg. u. allgem. Chem., 301, 323 (1959).
4. T. V. Rode, V. B. Kazanskii, Yu. I. Pecherskaya, ZhFKh, 35, 2370 (1961).
5. T. V. Rode, Oxygen Compounds of Chromium and Chromium Catalysts, Publishing House of the Academy of Sciences of the USSR, 1961.