Various Cu/HZSM5-zeolites were prepared and their catalytic properties were investigated by product analysis via GC/MS chromatography in order to trace down the mechanism of the gas phase one-step oxidation of benzene to phenol with molecular oxygen. Comparison of Cu free and Cu containing zeolites showed that the activation of O 2 takes place at copper centers of the zeolite and high copper loadings lead to high yields of deep oxidation products (CO, CO 2 ). No phenol was formed in the absence of BrØnsted acid sites, i.e. on Cu/KZSM5, revealing the bifunctionality of the Cu/HZSM5 zeolite. The yields of the various oxidation products and thus the selectivity towards phenol can be influenced by variation of the relative O 2 concentration in the reaction mixture, indicating the possibility of a stoichiometric use of O 2 . The role of the superoxide radical ion O 2•-as a reactive intermediate is discussed and a radical ionic reaction mechanism is suggested.

Hydroxyl radical intermediates are trapped in calcined Cu/HY zeolites in the presence of oxygen and water. This suggests that hydrogen peroxide is formed in situ from oxygen. Brønsted acids enhance the formation of the radicals.