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THE THERMOLYSIS OF MANGANESE(II) AND COBALT(II) DIHYDROGENPHOSPHATE  SOLID  SOLUTION


Turko O.V., Antraptseva N.М., Bila G.M.

National University of Life and Environmental Sciences of Ukraine,

Kyiv, Ukraine


Heat treatment of crystal hydrates is one way to obtain anhydrous salts. This process is largely determined by the nature of the cation [1].

There is no literature concerning the thermal properties of the manganese and zinc dihydrogenphosphate solid solution. The necessity for studying this process, which is involved in the preparation of cyclotetraphosphates with polyfunctional properties, determined the goal of our study.

The goal of our study was to determine the sequence of thermal and structural transformations in the course of dehydration of the hydrated solid solution
Mn1-xCox(H2PO4)2·2H2O(0 <x< 1.00) and to quantify how the nature of the cation affects these transformations.

We studied dihydrogenphosphates of compositions Mn0.75Co0.25(H2PO4)2· ·2H2O, Mn0.5Co0.5(H2PO4)2·2H2O and Mn0.25Co0.75(H2PO4)2·2H2O. These samples were prepared, as in [2]. Thermal properties were studied, as in [3], on a Q-1500D derivatograph; the temperature determination error was ± 5°С.

Thermal analysis shows three major stages in the removal of water from
Mn1-xCox(H2PO4)2·2H2O (0 <x< 1.00) with various manganese and zinc concentrations, however some differences are observed in the thermal curves of these samples. These three stages appear in the DTA and DTG curves as three endotherms and in the TG curves as three weight loss steps. The resolution of the DTA peaks and the steps in the TG curves change adequately to the composition of the dihydrogenphosphate solid solution.

The thermal curves for Mn1-xCox(H2PO4)2·2H2O (0.5 < x < 1.0), which contain 1.23-9.30 wt % manganese, including Mn0.25Co0.75(H2PO4)2·2H2O, are similar to curves obtained for individual Zn(H2PO4)2·2H2O: the first (75-180°С) and second (185-225°С) dehydration stages are almost unresolved. The removal of two H2O moles in the range 75-180°С appears in the DTA curve as a set of endotherms; the peak temperatures of the strongest endotherms are 105, 125, 160, and 170°С. The same process for Mn0.5Co0.5(H2PO4)2·2H2O (9.80 wt % Mn) occurs at 85-190°С. This process is described by three endotherms (peaking at 110, 155, and 175°С), which clearly indicate the end of the first dehydration stage and the onset of the second stage (190-225°С). The thermal curves for the Mn0.56Co0.44(H2PO4)2·2H2O sample are in general analogous to the curves obtained for the thermolysis of individual Mn(H2PO4)2·2H2O [4]. Regarding the curves for Mn0.75Co0.25(H2PO4)2·2H2O and dihydrogenphosphates with x < 0.25, the only differences from the curves recorded in [4] are the less prominent stages in the TG curve and the 100C shift of the temperature.

Interpreting the results of the complex characterization of the products of partial and complete dehydration of Mn1-xCox(H2PO4)2·2H2O (0 <x< 1.00) solid solution, we found the following: despite the different trends of the thermal curves for dihydrogenphosphates with 0.5 ≤ x < 1.0 and 0 < x < 0.5, their composition is not the parameter governing the thermal transformation sequence. The thermolysis of Mn1-xCox(H2PO4)2·2H2O (0 <x< 1.00), regardless of their manganese and zinc concentrations, has the following scenario. At the first stage (at 85-190°С for the solid solution with x = 0.5), a heterogeneous mixture including a liquid phase (free H3PO4) along with the solid phase is formed as a result of removal of two coordinated water molecules. Free H3PO4 (6.7 wt % based on P2O5) was detected in the products of Mn0.5Co0.5(H2PO4)2·2H2O dehydration at 115°С; upon heating to 180°C, its amount increases to 14.4 wt %. Different H3PO4 amounts are produced in the dehydration of Mn0.75Co0.25(H2PO4)2·2H2O and Mn0.25Co0.75(H2PO4)2·2H2O: the higher the zinc concentration in the solid solution, the larger the acid amount in the thermolysis products and the lower the onset temperature of acid evolution.

The major component of the solid phase is the Mn1-xCox(H2PO4)2 solid solution, which was identified using X-ray diffraction data for Mn(H2PO4)2 [4]. Mn0.5Co0.5(H2PO4)2 is detected in the X-ray diffraction patterns of the samples prepared at 160-180°С. Its formation is the result of partial structural reorgani-zation of the starting Mn0.5Co0.5(H2PO4)2·2H2O: most of the diffraction reflections are conserved. The most complex structural transformations accompany the formation of the second component of the solid phase (the less protonated phosphate of the general formula Mn1–xCoxHPO4 mH2O, which is the product of disproportionation of the starting dihydrogenphosphate.

The second stage of Mn1-xCox(H2PO4)2·2H2O dehydration is elimination of 0.6-1.0 mol H2O. A distinctive feature of this stage is anionic condensation, which accompanies thermal transformations not only in the solid phase but also in the acid component of the dehydration products. The resulting condensed phosphates and condensed phosphoric acids have linear anion configurations. The degree of polycondensation n in the phosphates is five when Mn0.5Co0.5(H2PO4)2·2H2O is heated to 205°С and seven at 270°C; for the polyphosphoric acids n = 3 and 6, respectively.

In the X-ray diffraction patterns the above-described structural reorganizations appear as follows: a single crystalline phase with n = 2 (dihydrogendiphosphate solid solution Mn0.5Co0.5H2P2O7) is formed at 225-270°C together with a minor amount of diphosphate MnCoP2O7; the X-ray diffraction parameters of the latter are analogous to those for CoH2P2O7 and Co2P2O7 [3]. The condensed phosphates with n = 3-7 are X-ray amorphous. Their amount in the intermediate products of thermolysis of the x = 0.5 solid solution at 225°С is 11.1 wt % (based on P2O5); at 270°С this amount increases to 21.6 wt %. The diphosphate amount also increases to reach 30.1 wt % (55.6% of the total P2O5 amount) at 270°С.

The trends of the curves for the evolution of free phosphoric acids during subsequent thermolysis prove that a new source of acids appears at the second dehydration stage; this source is high-molecular hydrogenphosphates. Polyphosphoric acids of the general formula Hn+2PnO3n+1 (n = 2 - 6) are formed not only as a result of the condensation of the H3PO4 liberated at the first stage (at 80-190°С) but also as a result of the disproportionation of condensed phosphates. The conversion of the monophosphate anion to polyphosphate in the acid component of the products of dehydration of Mn0.5Co0.5(H2PO4)2·2H2O is 61% at 205°С and reaches its maximum (88%) at 270°С.

In the solid thermolysis products (the salt component), the most complex condensed phosphates (polyphosphates with n = 2-9 and the cyclophosphate with n = 4; the conversion of the monophosphate anion is 96-97%) are formed at higher temperatures. For the dihydrogenphosphate with x = 0.5 this temperature is 320°C.

Removal of the last 0.5 mol H2O from this mixture of high-condensed phosphates occurs in the range 320-370°С. This process is accompanied by significant structural reorganizations and physicochemical interactions that simplify the anionic composition of partially dehydrated oligophosphates. These transformations yield the only condensed phosphate (the cyclotetraphosphate with a ring anion) as the final thermolysis product. The crystal lattice of the cyclotetraphosphate is formed starting at 320°С. At 370°С (Mn0.5Co0.5)2P4O12 is the only crystalline phase (monoclinic sp. gr. C 2/c, Z = 4).

Thus, the formation of the solid solution (Mn1-xCox)2P4O12 (0<x<1.00) follows two parallel routes: one route involves dehydration of condensed hydrogenphosphates, and the other involves the interaction of intermediate free polyphosphoric acids and neutral oligophosphates. The quantitative ratio between the thermolysis routes is largely determined by the nature of the cation: the increasing zinc concentration of the solid solution increases the share of free phosphoric acids in the formation of the final thermolysis product, which is the cyclotetraphosphate (Mn1-xCox)2P4O12.


References:

1. Shchegrov L.N. Divalent Metal Phosphates. – Kiev: Naukova Dumka, 1987. [in Russian].

2. Antraptseva N. M., Ryabtseva N. V., and Shchegrov L. N., Zh. Neorg. Khim. 38 (10), 1595 (1993).

3. Shchegrov L. N. and Antraptseva N. M., Ukr. Khim. Zh.51 (2), 127 (1985).

4. Shchegrov L.N., Antraptseva N.M., and Ponomareva I.G., Izv. Akad. Nauk SSSR, Neorg. Mater. 25 (2), 308 (1989).