ABSTRACT
The acrolein/diene ratio depends on the ratio of the rate constants. In the case of the oxidation of propene, its adsorption (which is equivalent to its activation to allylic intermediate) is rate determining [54]. In such conditions the surface of the catalyst is sparsely populated with allylic intermediates, which immediately react to acrolein, the selectivity to this product being thus very high. However, when the surface is contacted with allyl radicals generated in the gas phase by decomposition of allyl iodide, the coverage of the surface with adsorbed allylic intermediate may attain much higher values and the probability increases that two such species will be adsorbed at adjacent centers and recombine to form 1,5-hexadiene. In fact Table 7. S shows that the reaction of allyl iodide at the surface of Bi2MoOS results in the formation of certain amount of 1,5hexadiene and benzene, and consequently much lower selectivity to acrolein than that observed in the oxidation of propene. When active centers for oxygen insertion are removed by replacing molybdate anions with, say, phosphate, the reaction path to acrolein is eliminated and a high conversion to benzene is observed [55].
