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2 Bulletin of the Polish Academy of Sciences. Chemistry

2 2001

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Thermal properties of the molybdenum - oxide based {Mo154} giant wheel type cluster with methanol ligands bonded to its surface

Bielański A., Małecka-Lubańska A., Krickemeyer E., Bögge H., Schmidtmann M., Müller A.

Bulletin of the Polish Academy of Sciences. Chemistry

2001

Vol. 49, No. 3

245-254

The thermal behaviour of the giant molybdenum ring cluster compound formula - containing H2O ligands partly substituted by methanol molecules - was studied using TG, DTG and DTA analyses simultaneously, and mass spectromet-ric analysis of the decomposition products. Methanol is released from the sample quite rapidly even at room temperature (and even before the experiments start) but small traces of methanol are still present which we tried to use for characterising the different stages of giant ring transformation. The initial rapid loss of mass of the sample was accompanied by the DTA and DTG peak at 75°C. A small DTA endothermic peak at 165°C was assigned to an MS maximum indicating the increased emission of methanol and formaldehyde. Double exothermic peaks at 352 and 366° C corresponded to the loss of water formed from the abundant OH groups. Simultaneously, peaks indicating trace emission of formaldehyde, methanol, carbon dioxide as well as traces of hydrocarbons containing 2-4 carbon atoms per molecule were registered mass spectrometrically. The exothermic effect was attributed to a kind of "recrystallisation" process of the completely dehydrated product.

The reaction of giant wheel polyoxomolybdate clusters with ethanol

Bielański A., Małecka-Lubańska A., Poźniczek J., Müller A., Krickemeyer E., Diemann E.

Bulletin of the Polish Academy of Sciences. Chemistry

2001

Vol. 49, No. 1

85-99

The conversion of ethanol at 100-280° C was used in order to characterise the redox and acid centres present in Na15{0.5[Mo(152)O(457)H14(H(2)O)68] • 0.5[Mo(154)O(462)H(2)14 (H(2)O)(70]}-ca. 400 H(2)O (1). It is concluded that Mo(VI)/Mo pairs at the external surface of the rings, of which the structure of 1 is composed, are the redox centres active in ethanol dehydrogenation to acetaldehyde. Simultaneous ethanol dehydration to diethyl ether or ethylene is a typical reaction occurring on acid centres. It is suggested that OH groups situated in (u3-0)(2)0(2) tetrahedrons inside the ring structure play such role. The formation of ethane predominating at highest temperatures is attributed to secondary reactions of primary ethylene.