Synthesis and Structure of Ti-, Zr- and Hf-Containing Polyoxotungstates and Study of their Oxidation Catalysis Properties
Zusammenfassung der Projektergebnisse
Trying to understand the reactivity of Ti centers in the H2O2-based oxidation of alkenes is a highly relevant academic, but also industrial problem. In our work we have synthesized several Ti(IV)- containing POMs, such as [Ti2(OH)2As2W 19O67(H2O)]8- (1), [(α-Ti3GeW9O37OH)3(TiO3(OH2)3)]17- (4), [Ti8(C2O4)8P2W 18O76(H2O)4]18- (5), and [Ti6(C2O4)4P4W 32O124]20- (6). We also isolated the Zr(IV)- and Hf(IV)-containing polyanions [M6O4(OH)4 (H2O)2(CH3COO)5(AsW 9O33)2]11- (M = Zr, 2; Hf, 3). The compounds 1-6 were structurally fully characterized in the solid state (TGA, XRD, IR, Raman) and in solution (183W, 13C and 1H NMR, UV-vis). The catalytic properties of the new POMs have been studied in the selective liquid-phase oxidation of organic compounds with hydrogen peroxide, and relationships between the structure of the active centers, their activity and selectivity, and the mechanism of catalysis have been established. The experimental catalytic studies on 1 were complemented by computational methods (DFT). It was shown by DFT that formation of epoxide via a Ti–hydroperoxo intermediate is energetically more favorable than via a peroxo intermediate, and that protonation of the POM is crucial for the activation of the Ti–hydroperoxo species towards heterolytic oxygen-atom transfer, because it reduces the corresponding energy barrier. The latter conclusion was also demonstrated by experimental methods using 1. Hence, protonation is a powerful tool allowing for tuning the reactivity of Ti-POMs in H2O2-based oxidation of alkenes. Highly interesting and important future challenges include (i) isolation and comprehensive structural characterization of peroxo derivatives of 1 and studies of their reactivity, (ii) synthesis of new POM structures containing five- or even four-coordinate Ti atoms, and (iii) perform Ti-POM-based catalytic studies with alkyl hydroperoxides instead of hydrogen peroxide as oxidant, which might shed light on the differences in activation mechanisms of these two environmentally benign oxidants.
Projektbezogene Publikationen (Auswahl)
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Unique Catalytic Performance of the Polyoxometalate [Ti2(OH)2As2W 19O67(H2O)]8-: The Role of 5- Coordinated Titanium in H2O2 Activation. Eur. J. Inorg. Chem. 2009, 5134-5141
Kholdeeva, O. A.; Donoeva, B. G.; Trubitsina, T. A.; Al-Kadamany, G.; Kortz, U.
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Epoxidation of Alkenes with H2O2 Catalyzed by di-Titanium-Containing 19-Tungstodiarsenate(III): Experimental and Theoretical studies. Eur. J. Inorg. Chem. 2010, 5312-5317
Donoeva, B. G.; Trubitsina, T. A.; Antonova, N. S.; Carbó, J. J.; Poblet, J. M.; Al-Kadamany, G.; Kortz, U.; Kholdeeva, O. A.
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Hexazirconium- and Hexahafnium-Containing Tungstoarsenates(III) and Their Oxidation Catalysis Properties Chem. Eur. J. 2010, 16, 11797-11800
Al-Kadamany, G.; Mal, S. S.; Milev, B.; Donoeva, B.; Maksimovskaya, R.; Kholdeeva, O. A.; Kortz, U.
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Mechanistic insights into the alkene epoxidation with H2O2 by Ti- and other TM-containing polyoxometalates. The role of the metal nature and coordination environment. J. Am. Chem. Soc. 2010, 132, 7488-7497
Antonova, N. S.; Carbó, J. J.; Kortz, U.; Kholdeeva, O. A.; Poblet, J. M.
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Trimeric, Cyclic Ti10-Containing 27-Tungsto-3-Germanate [(α-Ti3GeW 9O37OH)3(TiO3(OH2)3)]17-. Compt. Rend. Chimie 2012, 15, 130-134
Al-Kadamany, G. A.; Bassil, B. S.; Kortz, U.
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The Oxalato-Titanium-Containing Tungstophosphate(V) Dimers, [Ti8(C2O4)8P2W 18O76(H2O)4]18- and [Ti6(C2O4)4P4W 32O124]20-. J. Clust. Sci. 2014, 25, 867-878
Al-Kadamany, G.; Bassil, B. S.; Raad, F.; Kortz, U.
