Radiolytic spin labeling

It is not practical to cover every topic in this review in which radiation chemical techniques have contributed to the understanding of catalyst function or catalytic reactions. With this introduction as a cursory guide to relevant topics, we move on to a discussion of the radiolytic spin labeling technique for analyzing products of catalytic reactions in zeolites, which has been the main thrust of experiments directed at fundamental aspects of catalysis in our laboratory. 

Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z" ) created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. 

It sounds simple, but as mass spectroscopists know, ionization as an analysis technique creates very reactive species. It is not enough to be able to detect ions one must also have knowledge of how the ions react. Fortunately, many radical cations are quite stable in selected zeolites at low temperature. For example, the excellent stability of radical cations of simple aromatic hydrocarbons and olefins in MFI zeolites has contributed to the successful application of the radiolytic spin labeling technique to catalytic processes of considerable importance to the fuels and petrochemical industries -hydrocarbon transformations in zeolite ZSM5. ... 

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