SPECTROSCOPIC STUDIES OF PROTONATED AROMATIC SPECIES AND RADICAL CATIONS IN H+-ZEOLITES

The formation of cation radicals and protonated species of all-trans-1,6,-diphenylhexatriene, anthracene, pyrene, and perylene in zeolites H+-ZSM-5, H+-mordenite, and H+-Y was studied using solid-state NMR and diffuse reflectance and fluorescence spectroscopies. The two types of species form concurrently, and cation radicals are more stable than the corresponding protonated species. The Bronsted acid sites are responsible for the formation of the protonated species, and the Lewis acid sites, associated nonframework Al atoms, for the formation of cation radicals. In contrast to solutions of strong acids, the transformation of protonated species into cation radicals does not occur in zeolites, where they revert to the original neutral molecules instead. Exposure of the samples to laboratory air accelerates this process, and only cation radicals are observed afterward. The stabilities of cation radicals and protonated species depend on the size of zeolitic cavities and channels and on the nature of the aromatic molecules. Further reactions of cation radicals are observed in zeolites with large interstitial space.