Impregnation of decamolybdocobaltate heteropolyanions over γ-alumina: detailed description of the physico-chemical phenomena.

In this work, the physicochemical phenomena occurring during equilibrium impregnation of Anderson-like decamolybdocobaltate H(4)Co(2)Mo(10)O(38)(6-) heteropolyanion aqueous solutions over γ-Al(2)O(3) were described in detail comprising chemical analysis, pH measurements, Raman, and UV-vis spectra. For a surface density lower than 2.5 Mo atoms nm(-2), the buffering effect of the support leads to decomposition of H(4)Co(2)Mo(10)O(38)(6-) into monomolybdates MoO(4)(2-) and Co(2+) cobalt cations that are then adsorbed by electrostatic and covalent interactions with γ-alumina. Between 2.5 and 3.8 Mo atoms nm(-2), MoO(4)(2-) monomers condense into heptamolybdates Mo(7)O(24)(6-) that are then adsorbed by electrostatic interactions and H(4)Co(2)Mo(10)O(38)(6-) becomes stable because of the lowering of the pH. Above 3.8 Mo atoms nm(-2), the quantities of adsorbed MoO(4)(2-) and Mo(7)O(24)(6-) become much smaller than that of electrostatically adsorbed H(4)Co(2)Mo(10)O(38)(6-). Adsorption of preserved H(4)Co(2)Mo(10)O(38)(6-) could be consecutive to the decomposition of the first molecules leading to prior adsorption of MoO(4)(2-) and Co(2+), and decrease in the buffering effect of γ-Al(2)O(3) and in the pH value. For dry impregnation, the same physicochemical phenomena occur considering a given Mo surface density. The methodology used in this work to rationalize the preparation of hydrotreatment catalysts from H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions can be transposed to any supported catalyst.

Decamolybdocobaltate heteropolyanions in aqueous solutions: chemistry driving their formation and domains of stability.

In the present study, aqueous solutions of decamolybdocobaltate H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions were prepared from molybdenum oxide, cobalt carbonate precursors and hydrogen peroxide used as oxidizing agent. The preparation was optimized adding a consecutive hydrothermal treatment at 150 °C to obtain pure H(4)Co(2)Mo(10)O(38)(6-) aqueous solutions for Co/Mo atomic ratio of 0.5. Combining quantitative Raman and UV-visible measurements and chemometric methods, it was demonstrated that a mixture of H(4)Co(2)Mo(10)O(38)(6-) and octomolybdate Mo(8)O(26)(4-) species is obtained for Co/Mo ratios lower than 0.5, and the relative quantities of H(4)Co(2)Mo(10)O(38)(6-) are determined by the presence of Mo(8)O(26)(4-) species and by the quantity of Co(2+) countercations available in the solutions to ensure the electroneutrality. As these quantities can be predicted for each Co/Mo ratio, this finding allows rationalization of the preparation of heterogeneous catalysts using impregnation by H(4)Co(2)Mo(10)O(38)(6-) aqueous solutions. Parameters relevant of the impregnation step such as the pH, the Co/Mo ratio, and the molybdenum concentration were varied to determine the domains of stability of H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions after formation. Stable from pH 1 to 4.5, this dimeric Anderson species is destabilized above pH 4.5; Co(2+), monomolybdate MoO(4)(2-) ions, and precipitates are then formed. For Co/Mo ratios lower than 0.5, the relative quantity of dimer does not vary with the pH and with a change of the Co/Mo ratio consecutive to the hydrothermal treatment. On the contrary, the coproduced Mo(8)O(26)(4-) species can be transformed into other isopolymolybdates varying the pH according to their domains of stability. For all of the ratios, H(4)Co(2)Mo(10)O(38)(6-) dimers were also shown to be stable in a wide range of molybdenum concentrations.