ESR studies of organic radical cations in zeolites

An overview is presented of the formation of organic radical cations in zeolites. Attention is paid firstly to their deliberate production by radiolysis and then to the spontaneous oxidation by activated H-exchanged zeolites of adsorped organic substrates. The nature of the oxidation site arising from thermal pretreatment in oxygen is considered, and possible mechanistic details are outlined for the frequently observed oligomerisations of simple substrates. The broad conclusions are that radical cations are formed from that component of the organic product mixture with the lowest ionisation potential, and that the dominant molecular transformations are driven by proton (Brønsted) catalysis: it appears that there is little evidence that radical cations are involved particularly in heterogeneous catalysis by zeolites, but they may well precede the formation of neutral radicals which are implicated as reaction intermediates. The reorientational dynamics of alkene radical cations in zeolites, as determined by ESR spectroscopy, are also considered.