No abstract

The adsorption of trans-stilbene (t-St) on the acidic aluminum-, gallium-, and boron-containing zeolite beta (H-AlBEA, H-GaBEA, and H-BBEA, respectively) is investigated using UV/vis and timeresolved UV/vis spectroscopy, electron paramagnetic resonance spectroscopy, and Raman spectroscopy. On H-AlBEA, the results show a fast and spontaneous one-electron oxidation of t-St, quickly followed by the recapture of an electron from the zeolite framework by the t-St •+ radical cation and the formation of a long-lived charge-transfer complex (t-St H-AlBEA •−•+ ). This charge-transfer complex (I) evolves over a period of months into a spectroscopically distinct charge-transfer complex (II). Evidence for the (undetected) intermediate t-St •+ radical cation is obtained using time-resolved UV/vis spectroscopy. Similar electron-transfer processes have been observed in the acidic ferrierite, ZSM-5, and mordenite aluminosilicate zeolites. The key difference is that the rate of electron recapture by the t-St •+ radical cation is much faster than that in all of the other zeolites. This is in agreement with the trend observed in the ferrierite, ZSM-5, and mordenite zeolites: looser fit leads to a lower stability of the radical cation intermediate and leads to a rapid hole transfer to the zeolite framework to form a charge-transfer complex. In the case of boron-containing beta, the rate of formation of the radical cation is slow, and the yield is small. Gallium-containing zeolite beta shows intermediate behavior when compared with the Al and B forms of zeolite beta.

Diffuse reflectance UV-visible absorption and Raman scattering experimental data show evidence of the phenothiazine (PTZ) sorption and spontaneous ionization in the straight channels of three medium pore acid zeolites with various topologies (ferrierite (H-FER), H-ZSM-5 and mordenite (H-MOR)) but analogous Si/Al contents. The spectral data highlight the combined effects of confinement and local electrostatic field on the sorption and charge separation kinetics. The PTZ incorporation and ionization appeared to be quicker in the larger pore H-MOR than in H-ZSM-5 and in H-FER. However, sorption and ionization are almost complete in the three zeolites after about one year. The low ionization potential value of PTZ (I.P. = 6.73 eV) induced quasi instantaneous formation of the radical cation PTZ N+ in high yield within the internal space of each channel structure. Nevertheless, the higher confinement effect and higher polarizing effect offered by the 10-membered rings (10-MR) channels of H-FER favoured the PTZ second ionization to form the dication PTZ 2+ . The very long lifetimes of these charge separated states are probably due to the restricted mobility of PTZ in the narrow channels and to the compartmentalization of the trapped electron away from the initial site of PTZ ionization. However, a very slow charge recombination process is observed within the three zeolite morphologies after about one year. This reaction is only partial in the narrower pores of H-FER and H-ZSM-5 whereas the faster diffusion process within the larger pore H-MOR induces quasi total cation disappearance after 2 years. Therefore, the reaction mechanism indicates clearly that PTZ N+ and PTZ 2+ are only intermediates and that the thermodynamically stable end product is the occluded PTZ molecule.