Preparation and properties of the heteropolyoxometalates of large organic cation with molybdotungstosilicic acids

Abstract

Some new heteropolyoxometalates of large organic cations with molybdotungstosilicic acids (general formula: (CTMA)₄SiMo _xW_12−xO₄₀⋅mH₂O⋅nDMF, x=0,2,4,6,8,10,12) were prepared by the reaction of cetyltrimethylammonium bromide (CTMAB,C₁₆H₃₃N(CH₃)₃Br) with H₄SiMo _xW_12−xO₄₀(x=0,2,4,6,8,10,12) in aqueous solution and recrystallization in DMF, and characterized by elemental analysis, IR spectra,TG-DTA and XRD techniques. The IR spectrum confirms the presence of Keggin structure and organic cations in these compounds, and it is indicated that the stretching vibration of the M-Od, M-Ob-M and Si-Oa becomes more red-shifted when molybdenum is gradually substituted for the tungsten atom. In particular, the thermal decomposition of the heteropolyoxometalates was studied in nitrogen atmosphere. The TG-DTA curves show that their thermal behaviors not only contain the release of water molecule, DMF molecule, CTMA and its fragments but also contains simultaneous collapse of Keggin anion. Their end products of the thermal decomposition are the mixture of WO₃, MoO₃ and SiO₂. And from the final decomposition temperature of view, it is found that the thermal stability of these compounds gradually is decreased when the number of molybdenum atoms is increased.

Keywords

heteropolyoxometalates / molybdotungstosilicic acid / cetyltrimethylammonium bromide / IR spectral and thermal study

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