Nature of Zeolite Activity, a-Sites

Zeolite activity, being a quite untypical phenomenon for oxidation catalysis, gave rise to several fundamental questions. The origin of the activity caused most discussion. 

The O radicals on FeZSM-5 are called the a-oxygen, denoted further as Oa. Reaction (7.12) proceeds with an activation energy of 42 kj mol-1 and is described by the following kinetic equation  

Desorption of a-oxygen into the gas phase starts at above 520 K. The concentration of a-oxygen (and hence the concentration of a-sites, Ca) can be reliably measured based on the amount of N2 evolved by (7.12), the isotopic exchange of 1802 with Oa, or its reaction with CO at room temperature [113]. 

226 7 Nitrous Oxide as an Oxygen Donor in Oxidation Chemistry and Catalysis 

A specificity of N20 oxidant compared to 02 is one of the most interesting points arising from benzene oxidation over FeZSM-5 zeolites. The specificity is clearly seen from the results presented in Table 7.6 [ 118]. With nitrous oxide, benzene conversion is 27% at 623 K, whereas with dioxygen it is only 0.3% at 773 K. Moreover, the reaction route changes totally N20 leads to selective formation of phenol, while 02 leads only to the products of complete oxidation. 

To provide effective performance in the oxidation of benzene to phenol, FeZSM-5 should have a rather small iron content, all of which is ideally in the form of a-sites. 

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