Benzene hydroxylation over FeZSM-5 catalysts: which Fe sites are active?

FeZSM-5 with a wide range of Fe content (0.015-2.1 wt%) were studied in the benzene hydroxylation to phenol with nitrous oxide (C6H6:N2O = 1:5) at low temperatures (< 550 K). Catalysts were activated before the reaction by steaming and/or calcinations in He (1323 K). High selectivity of benzene-to-phenol transformation (>98%) was obtained within 3 h without any deactivation of the catalyst. Three types of Fe(II) sites were formed in the zeolites extraframework due to activation and are attributed to: (1) Fe(II) sites in mononuclear species, (2) oligonuclear species with at least two oxygen-bridged Fe(II) sites, and (3) Fe(II) sites within Fe2O3 nanoparticles. The degree of nuclearity of Fe(II) species was observed to increase with iron content and activation temperature/time. The total amount of Fe(II) sites was monitored by the transient response method of the N2O decomposition (523 K) accompanied by the formation of surface atomic oxygen (O)(Fe). Only mono- and oligonuclear Fe(II) sites active in CO oxidation seem also to be responsible for the FeZSM-5 activity in benzene hydroxylation. Their amount was measured by the transient response of CO2 during CO oxidation on zeolites preloaded by (O)(Fe). The turnover frequencies in the benzene oxidation were constant independently of the catalyst activation in the isomorphously substituted zeolites. The Fe(II) ions in nanoparticles (inactive in hydroxylation) are probably irreversibly reoxidized by N2O to Fe(III), which are known to be responsible for the total oxidation of benzene. (C) 2004 Elsevier Inc. All rights reserved.