Generation and conversions of aromatic amine radical cations in acid zeolites

Incorporation of aromatic amines, including diphenylamine (DPA), carbazole (Cbz), diphenylbenzidine (DPB) and benzidine (Bzd), into acid zeolites generated the corresponding radical cations, DPA^·, Cbz^·, DPB^· and Bzd^·, as indicated by comparison of the characteristic long-wavelength diffuse reflectance spectra with spectra observed in solution. Samples of DPA^· sequestered in HZSM-5 are deprotonated; the resulting diphenylaminyl radical, DPN^·, is remarkably stable; diffuse reflectance or EPR spectra show that a significant population persists for longer than four years. In contrast, DPA^· adsorbed within HY zeolite, undergoes a clean conversion to Cbz^·. Non-acidic zeolites devoid of Brønsted and Lewis sites failed to generate radical cations to an extent detectable spectroscopically; these results clearly indicate that Brønsted and/or Lewis sites are responsible for the observed oxidation.