Proximity Effect on the Reactivity of Dioxygen Activated over Distant Binuclear Fe Sites in Zeolite Matrices

Distant binuclear cationic M(II) centers in transitionmetal-exchanged zeolites were shown to activate dioxygen by its splitting at room temperature to form a pair of very active oxygen species (i.e., alpha-oxygens) able to subsequently oxidize methane to methanol at room temperature. Selective oxidations of methane and other hydrocarbons are of extreme importance because of their potential for the transformation of hydrocarbons to valuable products. The reactivity of the alpha-oxygens with dihydrogen was investigated to obtain insight into the reactivity of these unique species. The reduction of Fe(IV)=O centers of pairs of distant alpha-oxygen atoms is a model reaction that allows for the study of the effect of the proximity of the other Fe(IV)=O site on the reactivity of the alpha-oxygen. The reduction by dihydrogen is also the key reaction for the quantification of these unique sites by temperature-programmed reduction (TPR) techniques. Our study reveals that (i) there is no direct concurrent reaction of both the Fe(IV)=O centers of pairs of the distant alpha-oxygen atoms with a molecule of dihydrogen; (ii) first, one Fe(IV)=O site of a pair of the distant alpha-oxygen atoms reacts with H-2(g) to form a water molecule, which is adsorbed on the Fe(II) cation while the other Fe(IV)=O site is intact. Afterward, one of the two H atoms of the adsorbed water molecule migrates to yield two Fe(III)OH groups, which subsequently react with another molecule of dihydrogen to give two water molecules, each adsorbed on one Fe(II) cation; (iii) an isolated Fe(IV)=O site is reduced by the same mechanism as the first Fe(IV)=O site of a pair of the distant alpha-oxygen atoms to yield H2O adsorbed on the Fe(II) cation; and (iv) lower reducibility of the Fe(IV)=O centers of pairs of the distant alpha-oxygen atoms with respect to the isolated Fe(IV)=O sites.

Automated summary

Distant binuclear cationic M(II) centers in transitionmetal-exchanged zeolites demonstrate the activation of dioxygen and subsequent oxidation of methane to methanol at room temperature. The reactivity of the alpha-oxygen species with dihydrogen provides valuable insights into their unique characteristics.