Ammonium molybdate tetrahydrate: introduce and the process of the pyrolytic decomposition

Introduction

Ammonium molybdate tetrahydrate is a colorless or light yellow prismatic crystal with a molecular weight of 1235.86.(Figure 1) The relative density is 2.498. Ammonium molybdate tetrahydrate is soluble in water (4 g/100 mL water), strong bases, and strong acids, but insoluble in alcohols or acetone. Its aqueous solution is weakly acidic (pH=5). Ammonium molybdate tetrahydrate in the air is prone to weathering and loss of crystal water and some ammonia. When heated to 90 ℃, it will lose one crystal water. When reaching 190 ℃, ammonium molybdate tetrahydrate will decompose into ammonia, water, and molybdenum trioxide.[1] Many domestic scholars have used thermogravimetric differential thermal analysis of ammonium molybdate tetrahydrate to study the particle size distribution of decomposition products of ammonium molybdate tetrahydrate in air at different temperatures, and explore the search for segmented control of thermal decomposition conditions of ammonium paramolybdate. This article mainly describes the relevant research results.

Process of the pyrolytic decomposition

The pyrolytic decomposition of ammonium molybdate tetrahydrate was analyzed by TG - DSC and laser granulometer. The results show that there are three stages in the process of pyrolytic decomposition in the air. The crystal water in ammonium molybdate tetrahydrate got lost from room temperature to 222.6 ℃. The partial ammonia ion  was decomposed from 222.6 ℃ to 248.7 ℃ , and metastable molybdenum oxide was formed. Metastable molybdenum oxide turned into molybdic oxide from 248.7℃ to 322. 5℃. At the same time, the influence of temperature on the particle size of the decomposition was the greatest from322.5 ℃ to 500.0℃, greater from 248.7℃ to 322.5℃ and not obvious from room temperature to 322.5 ℃.At the same time，the pattern of pyrolysates of ammonium molybdate tetrahydrate was changed hardly below 300℃．But，its pattern was changed into irregular flake above 300℃，and relatively regular flake MoO3 was obtained at last. The main reason was that the pyrolysates of ammonium molybdate tetrahydrate were interacted，migrated and recombined with the increase of temperature，and MoO3 was volatilized and deposited under high temperature. [2-3]

Thermal decomposition of ammonium molybdate tetrahydrate in 30~600℃ air was analyzed by TG-DSC. The results show that there are three stages during weight loss of ammonium molybdate tetrahydrate at 30~600℃, and weight loss tends to finish when the temperature is higher than 340℃. The initial temperature for weight loss is about 100 ℃. The first stage is 100-173 ℃, the second stage is 173-259 ℃, and the third stage is 259-340 ℃. XRD pattern shows that the end product of thermal decomposition is pure orthorhombic molybdenum trioxide. The mechanism of thermal decomposition of ammonium molybdate tetrahydrate in the air was proposed and theoretical verification was carried out. lt is considered that ammonium molybdate tetrahydrate only loses four crystal water molecules in the first stage of its thermal decomposition, while NH3 and H20 are lost by the molar ratio of 2:1 in the other three stages. Otherwise, through enthalpy change values of decomposition reaction in the various stages, approximate molar formation enthalpy was estimated of ammonium molybdate tetrahydrate and the related decomposition products under the experimental conditions.

The thermal decomposition mechanism of ammonium molybdate tetrahydrate can be described as follows:

Phase 1: 5(NH₄)₆Mo₇O₂₄•4H₂O(s)=4(NH₄)₄Mo₅O₁₇(s)+2NH₃(g)+21H₂O(g) (1)

Phase 2: 4(NH₄)₄Mo₅O₁₇(s)=5(NH₄)₂Mo₄O₁₃(s)+6NH₃(g)+3H₂O(g) (2)

Phase 3: (NH₄)₂Mo₄O₁₃(s)=4MoO₃(s)+2NH₃(g)+H₂O(g) (3)

The approximate standard molar enthalpies of formation ΔfHm for (NH₄) ₆Mo₇O₂₄ · 4H₂O (s), (NH₄) ₄Mo₅O₁₇ (s), and (NH₄) ₂Mo₄O₁₃ (s) under experimental conditions were calculated to be -3344.20, -1630.52, and -1090.56 kJ/mol, respectively, based on DSC experimental data of ammonium paramolybdate.[4]

Toxicity protection

Ammonium molybdate tetrahydrate is toxic, with an LD50 of 333 mg/kg. The maximum allowable concentration of its aerosol is 2 mg/m³, which is 4 mg/m³ in the dust state. Operators are required to wear gas masks and dust-proof work clothes during work. Dust generating equipment should be sealed and covered, while ensuring ventilation in the workplace.

In summary, ammonium molybdate tetrahydrate is an important inorganic fine chemical product and inorganic chemical intermediate. It can derive many molybdenum compounds and has great potential for application.

References

[1]Zhang H. Research progress in production of ammonium paramolybdate [J].China Molybdenum Industry.2014, 37(002):49-54.

[2]Fu Xm,Yang Zz, Liu Zw. Research on the process of the pyrolytic decomposition of ammonium paramolybdate [J].China Molybdenum Industry. 2011,35(3): 30-35.

[3]Fu Xm, Liu Zw.Research on Pyrolytic Decomposition and Pattern Evolution of the Pyrolysate of Ammonium Paramolybdate [J]. Chinese journal of rare metals. 2011,35(1):113-117.

[4]Zhou XP, Ma L, etal. Thermal Decomposition of Ammonium Paramolybdate and its Thermodynamic Analysis[J].Rare Metals and Cemented Carbides. 2015,(3):25-28.

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