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

Abstract

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 600 • C under O 2 of acidic H 6 ZSM-5 zeolite with chemical composition H 6 (AlO 2 / 6 (SiO 2 / 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 ž+ or TMB ž+ radical cations as major durable species and TMB 2+ 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 Brønsted acid sites of H 6 ZSM-5 was found during the sorption of TMPD or TMB through the generation of protonated species.

Spontaneous charge separation induced by phenothiazine sorption within acidic HnZSM-5

“…All the spectroscopic features recorded after TMB sorption were analogous to those exhibited in charge transfer bromide salts. 71…”

Sorption of anthracene, phenanthrene and 9,10-dimethylanthracene on activated acid HZSM-5 zeolite. Effect of sorbate size on spontaneous ionization yield

“…Spontaneous generation of long-lived radical cations after exposure of organic molecules to thermally activated acidic zeolites is well documented. [1][2][3][4][5][6][7] Microporous aluminosilicate materials like zeolites provide an appropriate environment to stabilize radical cations efficiently. This durable charge separation is due to electron acceptor sites, attributed to Lewis acid sites created by thermal treatment of zeolite, 5,8 and to structural restrictions of the mobility of the occluded organic moieties.…”

Resonance Raman Spectroscopy Investigation of the Interaction of Molecules Adsorbed on Solid Acid Surfaces

“…N, N, N′, N′ -tetramethyl-p-phenylenediamine (TMPD) and N, N, N′, N′-tetramethylbenzidine (TMB) were used as probes to characterize the nature of active sites in acid zeolite HZSM-5 [12]. These two amines are strong electron donors and are known to have low ionization potentials in the gas phase and to exhibit proton affinity.…”