ESR observation of redox processes on benzene-loaded NaPdY, NaPtY and NaCeY zeolites

Intense ESR signals assigned to the radical cation of the benzene dimer wer e observed after adsorption of benzene on calcined NaPdY, NaPtY and NaCeY z eolites. This reveals that benzene is oxidized by a single electron transfe r process, and since the transition metal cations are located inside the so dalite cages where they are not directly accessible to the benzene molecule s it implies that the electron transfer occurs over some distance and is po ssibly mediated by the zeolite framework. It also suggests that paramagneti c organic species, which are almost never discussed in mechanistic studies on zeolites, may be important transient intermediates in certain catalytic transformations of organic reactants. In ESR studies of radical cations in zeolites, oxidation by transition metal ions has so far not been used as a generation mechanism. Similar observations were made with 2,5-dimethylhexa- 2,4-diene in place of benzene. Copyright (C) 1999 John Wiley & Sons, Ltd.