Redox vanadium phosphate catalysts

A number of groups have prepared vanadium phosphate catalysts using hydro-thermal synthesis [92, 93, 128-130]. Using standard reaction mixtures, Dong and coworkers [128] showed that at elevated temperatures and pressures different materials are synthesized from those obtained under reflux conditions. Pressure did not seem to affect the product formed, but as the temperature increased to >200°C further reductions occurred and products formed. However, these materials were not found to have enhanced catalytic activity compared to traditionally prepared materials. At lower temperatures, hydrothermal syntheses have produced catalysts with comparable activity to those prepared under standard conditions [92, 93, 129, 130]. Taufiq-Yap and coworkers [129] found an enhancement in activity for hydrothermaUy prepared catalysts and suggested this was due to a modification in the redox behavior of the catalysts evidenced by TPO/TPR experiments. 

Buchanan JS, Sundaresan S. Kinetics of redox properties of vanadium phosphate catalysts for butane oxidation. Applied Catalysis 26 211-226, 1986. 

Vanadyl Phosphate Catalysts. — For the oxidation of C4-hydrocarbons to maleic anhydride, vanadyl phosphate catalysts with a variety of V P ratios and different additives have been proposed. Nakamura etal.123 observed for V P = 1 2, an average oxidation number of four for vanadium, highly aggregated vanadium ions and a high selectivity. Varma and Saraf124 also studied this reaction and on the basis of kinetic results propose a two-stage redox mechanism. They also concluded that maleic anhydride is hardly oxidized to carbon oxides, which are mainly formed in a side reaction from the original... 

The ion-exchange technique allows to prepare VO modified titanium phosphates. Different vanadium loadings can be obtained by properly controlling the operating exchange conditions and precursor phase. Vanadyl modified titanium phosphates catalysts were found active and selective towards SCR reaction, either as hydrogen or pyrophosphate phase. The results obtained in this paper indicate that the activity of the materi s can be relate to vanadyl species whose redox properties affect the catalytic behaviour. 

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