STUDY OF METAL-OXIDE CATALYSTS IN THE OLEFIN OXIDATION FROM THEIR REDUCTION BEHAVIOR .2. REDUCTION OF ZNO WITH PROPYLENE

The reduction behavior of ZnO in a stream of propylene was investigated in detail. Propylene consumed the oxidic oxygen of a few lattice layers to produce 1,5-hexadiene, benzene, acetone, and CO2. The time courses of respective products were, however, not only different from each other but also dependent on the reduction conditions. With varying propylene partial pressure (PPR), the formation of CO2 was almost unaffected, suggesting that it is responsible to very reactive oxygen such as adsorbed oxygen. The other products showed dependences on PPR. Detailed analyses were carried out for the formation behavior of 1,5-hexadiene which showed a large formation maximum. It was shown that its time course taken under various PPR could be reduced to the same analytical form; the formation rate = k(Y′)2(Y′∞ - Y′)2 where Y′ and Y′∞ denote the amount of consumed oxygen at time t = t and t = ∞, respectively. This equation confirms that the increase of the propylene adsorption sites (exposed zinc atoms) combined with the decrease of the surface active oxygen during reduction brings about the rate maximum. The reaction orders of the equation, moreover, give the picture that the rate-determining step of the 1,5-hexadiene formation is associated with the reaction between two adsorbed propylene molecules and two oxidic oxygen atoms. © 1979 Academic Press, Inc. All rights reserved.