Aluminium molybdates

Aluminium Molybdates.—Simple molybdates of aluminium have not been prepared, but by dissolving aluminium hydroxide in solutions of alkali tri- or para-molybdates, and allowing to crystallise, salts of the type 3R 20.Al20g.l2Mo03.aq. are obtained. These may be represented as derivatives of the alkali aluminates thus R3Al Mo20,)3.aq. The following have been described ... 

Aluminium molybdate, Al2(Mo04)3, is an important compound in catalyst technology. Although the crystal structure contains four equally populated inequivalent Al sites, an... 

Thus the presence of AlMoe at high Mo loadings on AHM/A should be ascribed to the high dispersion and to the interaction with the alumina support. Upon calcination, the presence of a surface aluminium molybdate Al2(Mo04)3 is shown by Raman spectroscopy, but AlMoe is recovered upon rehydration of the sample [7]. Pjg j. AlMoe, AI2O3... 

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. 

H. 8-Hydroxyquinaldine (XI). The reactions of 8-hydroxyquinaldine are, in general, similar to 8-hydroxyquinoline described under (C) above, but unlike the latter it does not produce an insoluble complex with aluminium. In acetic acid-acetate solution precipitates are formed with bismuth, cadmium, copper, iron(II) and iron(III), chromium, manganese, nickel, silver, zinc, titanium (Ti02 + ), molybdate, tungstate, and vanadate. The same ions are precipitated in ammoniacal solution with the exception of molybdate, tungstate, and vanadate, but with the addition of lead, calcium, strontium, and magnesium aluminium is not precipitated, but tartrate must be added to prevent the separation of aluminium hydroxide. 

Fluoride, in the absence of interfering anions (including phosphate, molybdate, citrate, and tartrate) and interfering cations (including cadmium, tin, strontium, iron, and particularly zirconium, cobalt, lead, nickel, zinc, copper, and aluminium), may be determined with thorium chloranilate in aqueous 2-methoxyethanol at pH 4.5 the absorbance is measured at 540 nm or, for small concentrations 0-2.0 mg L 1 at 330 nm. 

The reagent can be used, for example, on aluminium oxide, silica gel, kieselguhr. Si 50000, RP and cellulose layers. Sodium molybdate-impregnated phases and zirconium oxide layers are also suitable [1]. 

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

Pour the mother liquor into a number of test tubes and add a solution of a calcium, aluminium, or iron salt to each of them. What do you observe Write the equations of the reactions. What is the composition of commercial ammonium molybdate ... 

Metallic potassium and sodium are explosively converted into the hydroxides when brought into contact with concentrated solutions of hydrogen peroxide many of the heavier metals such as zinc and iron, and especially aluminium,3 are readily changed into their respective hydroxides, whilst chromium, arsenic, and molybdenum are oxidised respectively to chromic, arsenic, and molybdic acids. Colloidal tellurium yields telluric acid with very dilute solutions of peroxide4 the crystalline modification reacts slowly with 60 per cent, peroxide at 100° C. 

Cobalt and rhodium yield complex molybdates which are chemically" and crystallographically analogous to the corresponding complex molybdates of aluminium, iron, and chromium.1... 

The mitigation of corrosion can be achieved economically by the use of corrosion inhibitors. Chromate has been extensively used in an aqueous environment for the protection of aluminium, zinc and steel. Although chromates are cheap and effective, they are not acceptable because of their toxicity. Alternate inhibitors such as molybdates, organic inhibitors such as phosphonates, mixtures of phosphates, borates and silicates and surfactants like sulfonates have been used in place of chromates. Chromates are anodic inhibitors and help to form passive oxide on the metal surface. 

Preparation.—Although the purest molybdenum is obtained from wulfenite, the chief commercial source is molybdenite, which is converted into the trioxide by roasting in air either with or without the addition of sand, and, on dissolving the residue in ammonia, a solution of ammonium molybdate is obtained. This salt, freed from copper by treatment in ammoniacal solution with ammonium sulphide, and from aluminium by the addition of potassium carbonate, on ignition yields molybdenum dioxide alternatively, heating with excess of sulphur yields pure molybdenum disulphide, MoS, which on roasting, or by treatment with nitric acid, is converted into the trioxide MoOj. ... 

Mixtures of transition metal (Mo or W) sulphides dispersed on y-alumina supports are used in hydrotreatment processes to remove sulphur, nitrogen, oxygen and metals from oil fractions. The addition of phosphorus to these catalysts enhances the solubility and stability of molybdate and improves the thermal stability of the alumina support. Solid state P double-resonance NMR experiments ( P- AI REDOR and TRAPDOR) have been used to investigate the interaction between the impregnating phosphorus and the support surface (van Eck etal. 1995). The results showed that most of the phosphorus is in close contact with the aluminium, and that the layer of AIPO4 formed on the surface is not completely amorphous, but is slightly more ordered. 

Kieselgel 60, Merck 230-400 mesh. Thin-layer chromatography was performed on silica gel (Kieselgel 60 F254, Merck) aluminium-backed plates (0.2 mm thickness) and visualized by UV and/or dipping into a solution of 2.5 g ammonium molybdate and 1 g ceric sulfate in 10 mL sulphuric acid/90 mL water, followed by heating. Purchased from Aldrich Chemical Company, Inc., Milwaukee, WI, USA. 

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