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Relationship between the state of oxidation, the activity and selectivity of catalysts based on VMgO for the ODH of n-butane

Abstract:

The Oxidative Dehydrogenation (ODH) of n-butane was studied under aerobic and anaerobic conditions on four catalysts based on a mixture of Vanadium and Magnesium Oxides (VMgO) with an atomic ratio Mg/V=4. The experimental results supported by the characterization techniques used (DRX, XPS, BET, TGA and Atomic Adsorption) made it possible to verify the reproducibility of the catalysts preparation method and to determine that activity and selectivity is a function of the catalysts oxidation state, that during the reaction process surface crystallinity changes are taking place, and that activity can be fully recovered by the catalyst re-oxidation. The main active phase formed on all the prepared catalysts was magnesium ortho-vanadate and it was found that the unique oxidation state of vanadium present in fresh catalysts was V+5. Working under anaerobic conditions it was detected the formation of the V +4 oxidation state and it was found that the ratio V +4/V+5 is a function of the catalyst reduction degree, which in our case never reached the level of producing V+3 In the presence of molecular oxygen only V+5 was observed. The presence of a second support (silica or á-alumina) had no effect on the vanadium species formed, but it was found that it promotes the stability of products formed by chemisorption of CO2, that it increases the dispersion of species formed and that it decreases the selectivity toward desired products.