Nature of Active Sites in the Oxidation of Benzene to Phenol with NO over H–ZSM-5 with Low Fe Concentrations

Selective oxidation of benzene with N2O to phenol was investigated at 350 and 450°C on H–ZSM-5 zeolites of different Si/Al composition, with concentrations of iron ranging from 30 to 2000 ppm, and with gallium (1000 ppm). Zeolites dehydrated at 480°C, dehydroxylated at 720°C, and steamed at temperatures ranging from 500 to 780°C as well as zeolites exchanged by NaCl and treated in NaOH solution were used. IR spectra of OH groups and of adsorbed d3-acetonitrile on zeolites were employed to determine the concentration of Brønsted and Al-Lewis sites. ESR spectra of Fe(III) ions and IR spectra of perturbed framework T–O–T bonds due to the presence of Fe(II) ions at cationic sites of the reduced zeolites were applied to determine the location of Fe ions in zeolites. The rate of phenol formation correlated with the concentration of Fe in the zeolite but was not affected by the presence of gallium. With all the steamed zeolites the rate of phenol formation increased substantially compared to that of dehydrated zeolites. In contrast, no correlation was found between the concentration of the Brønsted or Al-Lewis sites and the rate of phenol formation. The Fe ions with a complex oxo-structure were found to be the active sites for benzene oxidation with N2O to phenol. They are reflected in the characteristic ESR signals of Fe(III) at g 6.0 and 5.6, and in the IR band at 935 cm−1 of perturbed T–O–T framework bonds due to the Fe(II) ions at cationic sites present in the reduced zeolites.