Dimolybdates

AMMONIUM compounds). Diammonium dimolybdate [27546-07-2] (NH 2 2 7 " ble commercially as the tetrahydrate and prepared from MoO and excess NH in aqueous solution at 100°C, has an infinite chain stmcture based on MoO octahedra. In aqueous solution the behavior of Mo(VI) is extremely pH-dependent (4). Above pH 7 molybdenum(VI) occurs as the tetrahedral oxyanion MoO , but below pH 7 a complex series of concentration-, temperature-, and pH-dependent equiUbria exist. The best known of these equiUbria lead to the formation of the heptamolybdate,... 

Ammonium ion forms isopolymolybdates, such as di-, tri-, or heptamolyb-dates with the molybdate anion. Only the dimolybdate, (NH4)2Mo20 , and ammonium heptamolybdate (NH4)6Mo7024 4H2O [12027—67—7], have commercial apphcations. 

Similarly, ammonium dimolybdate, (NH4)2Mo20t, crystallizes when aqueous solutions of M0O3 containing excess ammonia are evaporated at 100°C. Crystallization at low temperatures, however, forms ammonium heptamolyb-date, (NH4)eMo7024 4H2O, instead of dimolybdate. 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

Preparation of tricesium di-p-chloro-hexachloro-p-hydrido-dimolybdate-(3 ), Cs3[Mo2ClgH]. Molybdenum(II) acetate (10.0 g, 23.4 mmole) is placed in a three-necked, 500-mL flask containing a Teflon-coated stirrer bar. The flask is capped with three rubber septums, and a thermometer is inserted through one of the septums. After the flask and its contents have... 

At low concentrations hexavalent molybdenum in dilute acidic solutions is likely mononuclear.8 At higher acidity the ion Mo02(H20)4+ is probably present.8 In special situations the dimolybdate ion, Mo202, is formed whose structure is analogous to that of the better known dichromate ion, Cr202-. (Dinuclear MoVI ions are discussed separately below.)... 

Figure 11. Product distribution in MoOj/SiC>2 catalysts as determined from acidimetric titrations (a) silicomolybdic acid (o) dimolybdates ( ) polymolybdates. The estimation of MoOj (i i) is derived from the intensity ratio of the reflectance bands at 360 and 440 cm-1 [66].

In the disproportionation (metathesis) of olefins, i.e. the reaction 2Ri-CH=CH-R2 a R1-CH=CH-R1 + R2-CH=CH-R2, a M0O3/ AI2O3 catalyst is often applied. Its preparation involves impregnation of Y-AI2O3 extrudates with an aqueous solution of ammonium heptamolybdate (AHM) or an ammoniacal solution of ammonium dimolybdate (ADM). 

Molybdates.—Normal molybdates of the type R aMoO., exist in solution but are relatively unstable, and readily form acid salts or complex polymolybdates. Thus dimolybdates, R aMogOy, can be obtained by fusion of molybdenum trioxide with sodium or potassium nitrate trimolybdates, R gMojOio, and tetramolybdates, R Mo Oig, by heating molybdenum trioxide with an aqueous solution of sodium or potassium carbonate. Even more highly acid salts—for example, octa- and deca-molybdates—can be obtained. Solutions of normal molybdates, when treated with hj droehlorie add or nitric acid, yield a precipitate of acid molybdate this reaction does not, however, take place with sulphuric, acetic, oxalic, or tartaric acids. 

Ammonium Dimolybdate, (NH4)2Mo.j07.H20, exists as mono-clinic (prismatic) crystals. A compound (NH4).2Mo20,.3NH2 has been prepared by addition of ammonium hydroxide of medium strength to solid ammonium phosphomolybdate. This substance loses ammonia, yielding the dimolybdate. 

Cobalt Dimolybdate, 2CoMo20,.llH20.—The liquor remaining after removal of cobalt molybdate from mixed solutions of cobalt chloride and sodium dimolybdate (see above), when allowed to evaporate in the cold, deposits brown needle-shaped crystals of the above composition. When heated at 100° C. water is lost and the dihydrate CoMo20,.2H20 remains. ... 

Lithium Dimolybdate, Li2Mo20. 5H20, is obtained in groups of needle-shaped crystals by treating a solution of the normal salt with hydrochloric acid. The dimolybdate readily dissolves in hot water. On heating, it loses one-third of its water at 120° C., and the remainder only at red heat. 

Potassium Dimolybdate, KjMOgOj, is formed by fusion of suitable quantities of K2CO3 and M0O3, but is unstable the aqueous... 

Rubidium Dimolybdate, Rb2O.2MoO3.2H2O, crystallises in readily soluble prisms or plates from solutions containing some nitric acid. 

Sodium Dimolybdate, NaaMOjOj.is formed when suitable quantities of sodium carbonate or nitrate and molybdenum trioxide are fused together, and the mass washed free of carbonate or nitrate by cold water. The small crystals so obtained are only slightly soluble in either cold or hot water. From such a solution in hot water, prisms of the monohydrate, XajMojO. HjO, are deposited. ... 

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