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Long Lived Charge Separated States Induced by trans-Stilbene Incorporation in the Pores of Brønsted Acidic HZSM-5 Zeolites: Effect of Gallium on the Spontaneous Ionization Process

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journal contribution
posted on 10.06.2010, 00:00 by Alain Moissette, Raul F. Lobo, Hervé Vezin, Khalid A. Al-Majnouni, Claude Brémard
In situ CW-EPR, diffuse reflectance UV−visible spectroscopy and Raman scattering were used to monitor the spontaneous incorporation of trans-stilbene (t-St, C14H12) in the medium pore H2.2-GaZSM-5 zeolites [H2.2(GaO2)2.2(SiO2)93.8] by direct exposure under dry and inert atmosphere of solid t-St to dehydrated porous material without any solvent. The sorption of t-St with relatively low ionization potential (7.65 eV) occurs in Brønsted acidic H2.2-GaZSM-5 zeolites according to a complex and slow reaction sequence. First, charge separation occurs and t-St•+@H2.2-GaZSM-5•− radical pair is created, while long-lived t-St@H2.2GaZSM-5•−•+ electron−hole pair is formed through hole transfer. The analysis of the DRUVv spectra set recorded during the t-St sorption course shows the respective concentrations of all transient species as a function of time. In particular, note that system reorganization is observed through a second type of electron−hole pair. The broad and strong bands observed in the near-IR regions over extended periods of time are tentatively assigned to the electron and/or hole spectral signatures in slightly different environments. Applying pulsed X-band EPR techniques, we were able to reveal the structural surrounding of the unpaired electrons of charge-separated states through the proper assignment of electron couplings with a large number of nuclei such as 1H, 29Si, 69Ga, and 71Ga using the two-dimensional hyperfine-sublevel correlation experiment (2D-HYSCORE). The distance measurements deduced from dipolar coupling experiments provide a unique picture of the long distance distribution of unpaired electrons generated by spontaneous ionization of t-St upon incorporation within H2.2-GaZSM-5 zeolite. This result demonstrates that a large fraction of the unpaired electrons are ejected away from the initial site of ionization and that this compartmentalization plus the created electrostatic field hinder dramatically the propensity of charge recombination. The results for H-GaZSM-5 are compared to similar experiments conducted on H-AlZSM-5 zeolites.