Spontaneous charge separation induced by phenothiazinesorption within acidic HnZSM-5

Abstract

In situ diffuse reflectance UV-visible, Raman scattering and EPR experiments, carried out as a function of time after phenothiazine (PTZ) direct exposure to thermally activated acid H_nZSM-5 zeolite without any solvent, provide evidence of phenothiazine sorption and simultaneous spontaneous ionization. For comparison, phenothiazine behavior was investigated within non acidic zeolite and showed that most of the phenothiazine molecules were occluded as intact molecules and that ionization was very weak. The multivariate curve resolution analysis of the diffuse reflectance UV-visible spectra set recorded during the sorption process resolve the absorption spectra and respective concentrations of individual species involved in the sorption course. When PTZ entered through the acidic zeolite channels, PTZ˙⁺@H_nZSM-5˙⁻ radical pair is generated by the polarization energy. Subsequent PTZ²⁺ formation was fast but only partial. After months, equilibrium including PTZ˙⁺, PTZ²⁺ and occluded PTZ was reached within acid H_nZSM-5 but depends on the Si/Al ratio: the higher the Al content, the easier the spontaneous ionization. The close match between PTZ and the pore size of zeolites combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of PTZ˙⁺ and PTZ²⁺ and hinder efficiently the charge recombination. No evidence of Brønsted acid sites of H_nZSM-5 was found during the sorption of phenothiazine through generation of protonated species.