Spin labeling by ionization

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.  

How is it possible to distinguish radical cation reactions from genuine transformations catalyzed by the zeolite, and what types of processes tend to intrade on the identification of products of catalysis To answer the first question, we rely on the ion-exchangeability of zeolites. Zeolites are crystalline aluminosilicates, natural or man-made, that can adopt a remarkable range of channel-type and cage-type lattice architectures, depending on the 

When the primary species (products of catalysis) must be deduced from knowledge of the radical cation chemistry, interpretation becomes more 

The advantages of the radiolysis/EPR method for studying mechanisms of zeolite catalysis are due to the sensitivity and structural specificity of EPR, surpassing that of other in situ spectroscopies, such as FTIR and NMR, and the ability to identify products at low temperature. It is often the case that at high temperatures needed to evolve products from the zeolite for ex situ analysis, a 

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