Selective oxidation of benzene to phenol with nitrous oxide over MFI zeolites

Iron- and/or aluminum-containing MFI zeolites were prepared via various routes, including isomorphous substitution followed by thermal activation and grafting reactions with molecular Fe or Al precursors over [Al]MFI and [Fe]MFI, respectively. The effect of the iron and aluminum content on the performance in benzene oxidation with nitrous oxide at 623 K was studied for a suite of isomorphously substituted zeolites. The activity increased with increasing iron and aluminum content. Steamed zeolites were more active than their calcined counterparts. The extent of coking deactivation increased with initial activity and tallies with a consecutive reaction mechanism in which phenol further reacts to products that cannot leave the zeolite micropores. A very small amount of iron is involved in the selective oxidation reaction. A strong correlation is observed between the initial rate of phenol formation and the intensity of the infrared band at 1635 cm−1 due to NO2 interacting with extraframework FeOAl. On the other hand, nitrous oxide decomposition at higher temperatures is suggested to be mainly catalyzed by more clustered Fe species. Alternative preparation routes that use grafting of trimethylaluminum or iron chloride on otherwise inactive ferrosilicalite and aluminosilicalite, respectively, provide further support for the necessity of extraframework mixed iron-aluminum-oxo species. Steaming at high temperature is not only important for removal of the catalytically active metals from the zeolite framework, but also improves the formation of active sites. A case in point is the dispersion of Fe and Al in the micropores of silicalite-1, which renders an active, selective, and stable catalyst.