Ammonium molybdates, decompositions

Preparation of Molybdenum(VI) Oxide, a. Preparation of Molybdenum(VI) Oxide by the Thermal Decomposition of Ammonium Molybdate. Put a boat with 0.5 g of ammonium molybdate into a porcelain tube in an electric furnace. Connect one end of the... 

Calvin and coworkers, in their classical studies of photosynthetic processes, detected labeled triose phosphates and hexose phosphates on bi-dimensional chromatograms, using phenol-water and 1-butanol-pro-pionic acid-water as solvents.120121 In addition to the detection of radioactivity, an ammonium molybdate-nitric acid color reagent was used to detect phosphorus.16 Acidic-solvent developers were used to obviate decomposition of the esters. 

Table I lists the data for the sample of molybdene trioxide obtained by the thermal decomposition of ammonium molybdate at 450° for 5 hr. The rate of exchange of molecular oxygen is close to the rate of exchange...

In the absence of molybdenum, the blank dehydrated zeolites showed no CO hydrogenation activity even up to 400°C. In contrast, measurable quantities of aliphatic hydrocarbons were detected over the molybdenum-zeolite catalysts at 300°C and above. Figs. 1-2 show the time dependence of CO conversion over MOii g HY and Mo g CsY at 300°C. The conversion and product distribution were dependent on the reaction conditions, a typical set of results is illustrated in Table 1. The molybdenum-zeolites prepared by adsorption and decomposition of Mo(C0)g resembled closely the alumina-supported molybdenum catalysts prepared by decomposing Mo(C0)g on alumina (ref. 13). The results obtained presently could not match the figures reported by Brenner et aK (ref. 8), but this could be due to the significant differences in the reaction conditions used by the above authors. However, a comparison with the silica-molybdena catalyst (prepared by impregnation of ammonium molybdate) clearly indicates that the molybdenum-zeolites were more active on per molybdenum basis. The improved activity is due to the presence of zerovalent molybdenum (for LaY and HY, residual zerovalent molybdenum were responsible for the activity). 

Equations (II) to (IX) illustrate basic methods of preparation, but many variations are used, particularly in industry, to obtain an economic yield. Phthalic acid, phthalamide, phthalimide, and phthalic anhydride, together with urea, are often used instead of phthalonitrile, and catalysts such as ammonium molybdate or zirconium tetrachloride may be employed (249, 251, 269). The reaction between phthalonitrile and metals (finely divided or acid-etched) is usually very vigorous at 250°-300°C, sufficient heat being generated to maintain the reaction temperature. This is an illustration of the ease with which the phthalocyanine skeleton is formed. Even more surprising are the observations that palladium black (118) and gold (189) will dissolve in molten phthalonitrile. Reaction (III) between phthalonitrile and a finely divided metal, metal hydride, oxide, or chloride is perhaps the most generally employed. For the unstable phthalocyanine complexes such as that of silver (11), the double decomposition reaction... 

The isothermal method was used with equilibrium being attained in 3 days. Si02 was determined gravlmetrlcally by precipitation as H2SIO3 and decomposition at 1000°C. Phosphate was precipitated with ammonium molybdate. The phosphomolybdate was dissolved in alkali hydroxide and the excess hydroxide was titrated acidimetrlcally. 

Phosphorus may also be detected in the filtrate from the sodium decomposition, pronded that a 1 cc. portion of the filtrate be oxidized by boiling with a little concentrated nitric acid and subsequently tested with ammonium molybdate reagent. A more reliable test which is applicable also to quantitative work consists in fusing the organic compound (if non-volatile) with sodium carbonate and a small amount of potassium nitrate in a nickel crucible. The melt is dissolved in acid and tested with molybdate reagent in the usual manner. 

W.M Shaheen, M. M. Selim. Thermal decompositions of pure and mixed manganese carbonate and ammonium molybdate therahydrate Journal of Thermal Analysis and Calorimetry, 59,(3),(2000), 961-970. 

Yin Zhoulan, Li xinhai, Chen Qiyuan. Study on the kinetica of the thermal decompositions of ammonium molybdates Thermochimeca Acta, 352-353,(2000) 107-110. 

One of the simplest methods of preparation is by decomposition of a thermally unstable compound. The nitrate or chloride is often preferred, sulphates tend to decompose at higher temperatures. Where the presence of residual traces of anion is to be avoided, the metal salts of organic acids are particularly useful. Formates, oxalates, acetates etc, decompose at low temperatures and often reduce the metal at the same time. For the preparation of catalysts from anions, the ammonium salt is frequently used. Metallic salts of complex acids can be used as a source of metal oxide mixtures. Decomposition of the appropriate chromate, tungstate, molybdate or vanadate will produce the mixed oxide. 

Because the decompositions of many ammonium salts evolve NHj and HjO simultaneously (or conseeutively) in a 2 1 molar ratio, it is often convenient to represent the formula of such salts in the form m(NH4)20.n(metal oxide).xH20, where m, n and x are integers (see ammonium chromates, vanadates, tungstates and molybdates below). 

In the case of CoMo/A, which presents the same molybdate entities as in the non-promoted sample, their behaviour upon calcination is identical, and AlMoa evolves into an aluminium molybdate phase, and re-appears upon transfer to air. In the case of CoMo/Z, the calcination generally performed at 500°C to eliminate the ammonium counterions induces a decomposition of the cobaltomolybdate entity. Upon rehydration, the so freed molybdenum atoms associate with aluminium as in the impregnation of AHM to give AlMoe. Thus, the calcination-rehydration steps have a levelling effect on the structure of the promoted catalyst the same entities are finally identified whatever the support or the synthesis route. 

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