Add time:07/31/2019         Source:sciencedirect.com

The effect of condensing metal atoms (tungsten and vanadium) on the catalytic properties of 12-heteropolyphosphoric acids, as heterogeneous oxidation catalysts, has been investigated. While both the reducibility by isobutyric acid and carbon monoxide and reoxidizability by gaseous oxygen of molybdovanadophosphoric acids (H3+n[PMo12−nVnO40], n = 0, 1, 2, and 3) paralleled their highest oxidation-reduction potential and showed the maximum at H4[PMo11V1O40], the reoxidizability of molybdotungstophosphoric acids (H3[PMO12−nWnO40], n = 0, 1, 2, … 6, and 12) increased with lowering their highest oxidation-reduction potential and their reducibility decreased markedly with increasing the content of tungsten atom irrespective of the change in their highest oxidation-reduction potential. The preferential formation of V4+ in the reduced molybdovanadophosphoric acids and that of Mo5+ in the reduced molybdotungstophosphoric acids, which were revealed by ESR, also suggested different mechanistic roles of condensing metal atoms between tungsten and vanadium atoms. These results are discussed as the effect of condensing metal atoms on the reducibility and reoxidizability of 12-heteropolyphosphoric acids, and it is shown that unlike the tungsten atoms as a MoO bond strength modifier the vanadium atoms play the role of an electron reservoir: their redox properties decisively affect the reducibility and reoxidizability of molyb-dovanadophosphoric acids. It is also shown that potassium, rubidium, and cesium salts of H4 [PMo11V1O40] are fairly selective catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid.

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