E. Ya. Rode and G. V. Lysanova

SYNTHESIS AND PHYSICOCHEMICAL STUDY OF REDUCED MOLYBDENUM HYDROXIDE COMPOUNDS

(Presented by Academician I. V. Tananaev on 25 January 1962)

In the reduction of hexavalent molybdenum compounds in the presence of water, hydrated products of lower oxidation state are formed, known as molybdenum blues. The methods of preparation and the compositions of these compounds are very diverse (¹–⁵). They have found wide application as catalysts; being partially dehydrated, they possess semiconducting properties (²). Glemser and Lutz (⁴,⁵) accept the existence of the following compounds: $\mathrm{Mo_8O_{15}(OH){16}}$ ($\mathrm{MoO(OH)}\cdot H_2O}$); $\mathrm{Mo_4O_{102}$ ($\mathrm{MoO$); $\mathrm{Mo_2O_4(OH)}\cdot 0.25H_2O2}$ ($\mathrm{MoO$); $\mathrm{Mo_5O_7(OH)}\cdot 0.5H_2O{10}}$ ($\mathrm{MoO$); $\mathrm{Mo_5O_7(OH)}\cdot H_2O8}$ ($\mathrm{MoO$).}\cdot 0.8H_2O

We have carried out the synthesis under various conditions and a systematic study of hydroxide compounds of various oxidation states in the range from $\mathrm{MoO_3}$ to $\mathrm{MoO_2}$*. The starting products for the synthesis were: $\mathrm{MoO_3}$, obtained by thermal decomposition of recrystallized chemically pure ammonium molybdate, and $\mathrm{MoO_3\cdot 2H_2O}$. The synthesis conditions and the results of the physicochemical study are given in Table 1.

The syntheses, washing, and all investigations were carried out in a stream of nitrogen.

Table 1

* K. N. Egorychev took part in obtaining the experimental results.

A series of preparations was obtained with the more reduced products serving as the reductant. The water content in the preparations was determined by ignition in a stream of nitrogen at 450°, and the molybdenum content by precipitation as lead molybdate. Determination of the degree of reduction (x) in (\mathrm{MoO}_x) was carried out by potentiometric and volumetric methods.

To establish the physicochemical nature of the phases, as in the study of anhydrous compounds ((^6)), differential-thermal, thermogravimetric, and X-ray phase analyses were used in addition to chemical methods.

Fig. 1. Intensity—(\sin^2\theta) plots for individual phases:
(I) — (\mathrm{MoO}{2.20}\cdot0.9\mathrm{H_2O}), preparation 1;
(II) — (\mathrm{MoO}), preparation 3;}\cdot0.5\mathrm{H_2O
(III) — (\mathrm{MoO}{2.50}\cdot0.5\mathrm{H_2O}), preparation 6;
(IV) — (\mathrm{MoO}), preparation 9;}\cdot0.2\mathrm{H_2O
(V) — (\mathrm{MoO}{2.88}\cdot0.7\mathrm{H_2O}), preparation 11;
(VI) — (\mathrm{MoO}), preparation 12.}\cdot2\mathrm{H_2O

As a result, the existence of six types of reduced hydroxide compounds with individual thermograms, polytherms, and X-ray patterns was established. The most reduced product proved to be the burgundy-red preparation 1 (Table 1)—(\mathrm{MoO}_{2.20}\cdot0.9\mathrm{H_2O})—with an individual X-ray pattern (1) (Fig. 1). Thermogram (1) (Fig. 2) and polytherm (1) (Fig. 3) indicate the presence in the compound of water with different bond strengths. The portions of the endothermic effect at 136–190°, 236–306°, and 306–336° correspond: the first to the removal of adsorbed water, the second to the decomposition of the hydrate with (0.8\mathrm{H_2O}) (the stability region of which on polytherm (1) corresponds to the almost vertical segment (v)—(g)), and the third to the decomposition of the hydrate with (0.2\mathrm{H_2O}) (segment (d)—(e) of the polytherm). The exothermic effect at 364–390° corresponds to crystallization of the dehydrated product.

By partial oxidation of preparation 1, a series of products was obtained (preparations 2–4)—(\mathrm{MoO}{2.39-2.64}\cdot n\mathrm{H_2O}) ((n = 0.5–0.3\mathrm{H_2O})), the X-ray patterns of which (see X-ray pattern (II), Fig. 1, of preparation 3—(\mathrm{MoO}). On the basis of comparison of the values of (\sin^2\theta) and intensi-}\cdot0.5\mathrm{H_2O})) differ little from one another but are clearly distinct from X-ray pattern (I) of the preparation (\mathrm{MoO}_{2.20}\cdot0.9\mathrm{H_2O

From the intensities of the lines in the X-ray patterns it may be concluded that, in the course of oxidation of the preparation (\mathrm{MoO}{2.39}\cdot 0.5\mathrm{H_2O}) to the composition (\mathrm{MoO}}\cdot 0.3\mathrm{H_2O}), no noticeable change in the structure of the compounds occurs. Thermogram 2 (Fig. 2) and the powder pattern 2 (Fig. 3) of one of these oxidized products, preparation 3—(\mathrm{MoO{2.50}\cdot 0.5\mathrm{H_2O})—show that the firmly bound water contained in it is removed at (284—323^\circ), as a result of which decomposition of the compound and crystallization of the products formed occur (exothermic effect at (370^\circ)). Preparation 5 ((\mathrm{MoO})), obtained by another method (see Table 1), has an X-ray pattern close to the powder patterns of type II preparations (Fig. 1).}\cdot 0.8\mathrm{H_2O

Reduction of (\mathrm{MoO_3}) with zinc and concentrated HCl to a certain intermediate stage gave blue preparation 6—(\mathrm{MoO}{2.52}\cdot 0.5\mathrm{H_2O}) with X-ray pattern III (Fig. 1), different from the X-ray patterns of preparations 1—5. Oxidation of it to the composition (\mathrm{MoO}}\cdot 0.4\mathrm{H_2O}) (preparation 7) did not lead to a noticeable change in the appearance of the X-ray pattern. With still more prolonged oxidation in air, from preparations with an X-ray pattern of type III, preparations of composition close to (\mathrm{MoO{2.80}\cdot n\mathrm{H_2O}) with a new X-ray pattern IV (Fig. 1) can be obtained. Preparations of almost identical composition (\mathrm{MoO}) lattice takes place. As follows from consideration of thermogram 3 (Fig. 2), corresponding to preparation 9, removal of water from these compounds occurs at (284—322^\circ) in two stages (inflection at (312^\circ)).}\cdot 0.2\mathrm{H_2O}) with the same X-ray pattern IV were synthesized by reduction of (\mathrm{MoO_3}) with hydrogen at the moment of its evolution (zinc + dilute HCl—preparations 8 and 9), and also by reducing (\mathrm{MoO_3}) with powdered molybdenum in the presence of water by heating in a sealed ampoule at (110^\circ) (preparation 10). Comparison of the values of (\sin^2\theta) and the intensities of the lines in the powder patterns of preparations 8—10 and (\mathrm{MoO_3}) showed that, for small angles (\theta), the lines in the powder patterns of preparations 8—10 and (\mathrm{MoO_3}) coincide. With increasing angle (\theta), the displacement of the lines and the change in intensities increase. As a result, we came to the conclusion that, in the case of blue products of composition close to (\mathrm{MoO}_{2.80}\cdot n\mathrm{H_2O}), distortion of the (\mathrm{MoO_3

Fig. 2. Thermograms of hydroxy compounds: 1 — (\mathrm{MoO}{2.20}\cdot 0.9\mathrm{H_2O}) — preparation 1; 2 — (\mathrm{MoO}}\cdot 0.5\mathrm{H_2O}) — preparation 3; 3 — (\mathrm{MoO{2.83}\cdot 0.2\mathrm{H_2O}) — preparation 9; 4 — (\mathrm{MoO}) — preparation 12.}\cdot 0.7\mathrm{H_2O})—preparation 11; 5 — (\mathrm{MoO}_{2.89}\cdot 2\mathrm{H_2O

By prolonged heating of a mixture of (\mathrm{MoO_3}\cdot 2\mathrm{H_2O}) and Mo in a sealed ampoule at (110^\circ), preparation 11—(\mathrm{MoO}_{2.88}\cdot 0.7\mathrm{H_2O}) with an individu-

…by the individual Debye pattern V (Fig. 1). Thermogram 4 (Fig. 2) and polytherm 3 (Fig. 3) indicate the removal of water in two main stages: from 0.7 to 0.4 H₂O at 60–150° and complete dehydration at 150–290° (endothermic effects at 60–150° and 162–214°). Preparation 12, of the same degree of oxidation but more highly hydrated—MoO₂.₈₉·2H₂O—obtained by mixing hydrochloric-acid solutions of Mo^VI and Mo^V, gives an individual X-ray diffraction pattern VI (Fig. 1). As in the case of preparation 11, removal of water (thermogram 5, Fig. 2, and polytherm 4, Fig. 3) occurs in two stages. In both cases there is a break in the course of the thermogravimetric curves at about 190°, corresponding to the hydrate with 0.4H₂O.

Fig. 3. Curves of the dependence of H₂O content (%) on temperature: 1 — MoO₂.₂₀·0.9H₂O — preparation 1; 2 — MoO₂.₅₀·0.5H₂O — preparation 3; 3 — MoO₂.₈₈·0.7H₂O — preparation 11; 4 — MoO₂.₈₉·2H₂O — preparation 12.

Thus, the physicochemical study we have carried out of molybdenum hydroxy compounds has shown the presence of individual compounds possessing characteristic X-ray diffraction patterns (I–VI, Fig. 1), thermograms, and polytherms; some of them (preparations 1, 3, 6, 11, 12), with respect to degree of reduction and, in a number of cases, degree of hydration (preparations 1, 3, 6), coincide with compounds cited in the literature (4, 5); however, the Debye patterns of the compounds obtained by us do not coincide with the X-ray data reported by Glemser and Lutz (4). Comparison of the data obtained shows that part of the water in molybdenum blues is chemically bound, and moreover by bonds differing in strength. The presence of chemically bound water is a necessary condition for their existence, since removal of water from the preparations leads to decomposition. X-ray phase analysis of a number of preparations dehydrated at 450° showed the presence in them of a mixture of MoO₃ and MoO₂.

In thermograms 1 and 2 (Fig. 2), after the endothermic effect of removal of chemically bound water, an exothermic effect is recorded, corresponding to crystallization of the products of thermal decomposition—MoO₃ and MoO₂. In this case, with an increase in the degree of reduction and, consequently, with an increase in the amount of dioxide MoO₂ formed as a result of decomposition, the magnitude of the exothermic effect increases. With further increase in temperature, the oxides MoO₃ and MoO₂ formed react with one another, as described by us earlier (6), with formation of γ-Mo₄O₁₁ and ζ-Mo₉O₂₆ phases with thermal effects characteristic of their formation (Fig. 2, 1–5).

Institute of General and Inorganic Chemistry
named after N. S. Kurnakov
Academy of Sciences of the USSR

Received
19 I 1962

CITED LITERATURE

1. V. Auger, N. Iwanoff, C. R., 204, 1815 (1937); C. R., 205, 1910 (1937).
2. L. Sacconi, R. Cini, J. Chem. Phys., 18, 1124 (1950). Ann. Chim., 42, 706 (1952).
3. W. D. Treadwell, V. Schaeppi, Helv. chim. acta, 29, 771 (1946).
4. O. Glemser, G. Lutz, Z. anorg. Chem., 264, 17 (1951).
5. O. Glemser, Nachr. Acad. Wiss. Göttingen, math.-phys. Kl., 11a, math.-phys.-chem. Ableit., No. 7 (1955).
6. E. Ya. Rode, G. V. Lysanova, DAN, 145, No. 2 (1962).