Study of Te and V as counter-cations in Keggin type phosphomolybdic polyoxometalate catalysts for isobutane oxidation

Keggin-type phosphomolybdic acid and cesium salts with protons partially substituted by tellurium and by both tellurium and vanadium have been prepared, characterized using several techniques and tested as catalysts in the partial oxidation of isobutane into methacrylic acid (MAA). The results showed that tellurium when introduced as counter-cation was present as Te⁴⁺ capping the Keggin anion and was randomly distributed in the acid or in the cesium salt. This cation induced a positive effect on the selectivity to MAA and methacrolein (MA) without significant effect on the activity except in the acid at low loading where it also increased the activity. The co-substitution of protons by vanadyl cations had a slight effect on the selectivity but increased the activity especially at low level of substitution, which led to a very efficient catalyst. Selectivity to MAA and MA and isobutane conversion rate of 65 and 17% respectively were reached at 350 °C and were both very constant with time on stream. The catalytic results obtained in both stationary and transient conditions allowed to propose a reaction mechanism very close to one already proposed with four intermediates amongst which one is common to both MAA and MA. These results were used to understand the catalytic effect of tellurium and vanadium.