Time on stream behaviour in the partial oxidation of propene over iron antimony oxide

The partial oxidation of propene has been studied over iron antimony oxide catalysts in the presence and absence of gaseous oxygen under isothermal and temperature programmed conditions. Temperature programmed reduction (TPR) using propene as reduction gas revealed that for temperatures up to 600 degrees C vacancies are created without phase transition. However, while during the reduction between 250 and 350 degrees C acrolein and CO2 are both formed, exclusively combustion products are formed between 500 and 600 degrees C, At higher temperatures the FeSbO4 rutile structure is destroyed during the reduction process. The activity is determined by the absolute amount of oxygen available because the activity drops when the reaction takes place in the absence of gaseous oxygen. However, even in the presence of gaseous oxygen the catalyst is partially reduced and the high initial conversion cannot be maintained. The high initial selectivity to acrolein drops remarkably with time on stream in the presence of gaseous oxygen which can be ascribed to the formation of electrophilic oxygen species during the reoxidation of the catalyst. A high initial selectivity to acrolein can only be maintained with time on stream in the presence of gaseous oxygen when excess Sb is present in the iron antimony oxide catalyst, This is due to the site isolation effect whereby Sb inhibits the reduction of the catalyst. (C) 1997 Elsevier Science B.V.