Probing the spontaneous ionization of aromatic amines by adsorption in activated acidic zeolite HZSM-5

N, N, N', N'-tetramethyl-p-phenylenediamine (TMPD) and N, N, N', N'-tetramethylbenzidine (TMB) were used as probe molecules to examine their interactions with catalytically active sites generated after thermal treatment at 600degreesC under O-2 of acidic H(6)ZSM-5 zeolite with chemical composition H-6(AlO2)(6)(SiO2)(90). The aromatic amines were merely exposed as powders at room temperature under argon to activated acidic H(6)ZSM-5 molecular sieve. The course of adsorption and subsequent reactions were monitored by the application of diffuse reflectance, UV-visible absorption, electron paramagnetic resonance (EPR) and Raman scattering spectroscopy. The experimental investigations provided specific evidence of TMPD.+ or TMB.+ radical cations as major durable species and TMB2+ as minor species after complete adsorption. No direct evidence of trapped electrons within the zeolite framework resulting from the ionization was provided by EPR and electronic absorption spectroscopy. The aromatic amines through resonance Raman spectroscopy appear to be a sensitive and specific tool to probe the concentration, strength and accessibility of electron acceptor Lewis acid sites created during thermal treatment under O-2. In contrast, no evidence of Bronsted acid sites of H(6)ZSM-5 was found during the sorption of TMPD or TMB through the generation of protonated species. Copyright (C) 2002 John Wiley Sons, Ltd.