ESR study of cation radicals derived from benzene and methyl-substituted benzenes adsorbed on a synthetic mordenite: Formation of biphenyl cation radical from benzene

Abstract

This chapter describes the radiolytic spin labeling technique for analyzing products of catalytic reactions in zeolites. 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. The chapter discusses the proton- and Na+-exchanged zeolites. Proton-exchanged zeolites derive catalytic activity from bridging hydroxyl groups that are strong Bronsted acid sites and Na+-exchanged zeolites are inert adsorbents. The advantages of the radiolysis/electron paramagnetic resonance (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 fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (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 complex sequence of reactions has already occurred. Therefore, elucidation of the elementary reactions of the sequence necessitates in situ analysis at low temperatures. In addition, product selectivity can be more pronounced for reactions carried out under mild conditions.