Generation and migration of electrons and holes during naphthalene sorption in acidic Al-ZSM-5 zeolites
Résumé
In situ CW EPR, diffuse reflectance UV-visible and Raman scattering spectroscopies were used to monitor the spontaneous ionization of naphthalene by direct exposure to thermally activated acid HZSM-5 zeolite (Hn(AlO2)n(SiO2)96-n). Calcination of the zeolite is a prerequisite for the spontaneous ionization. It is shown through EPR and UV-visible results that the maximum yield of naphthalene radical cation NPH+ increases gradually as the temperature of thermal treatment under O2 increases and can reach 50%. The tight fit between the shape of NPH+ and the pore size of straight channels of zeolites combined with aluminum content of zeolite appeared to be the most important factors responsible for the stabilization of the charge separation of naphthalene in NPH+ and trapped electron. Complete disappearance as a function of time of NPH+ with the persistence of paramagnetic species in high yield indicates that a supplementary electron-transfer reaction between NPH+ and zeolite framework is taking place during the migration of NPH+ and trapped electron. All the experimental data support the formation of positive holes within the zeolite framework stabilized by occluded NPH in ground state. The ZSM-5 zeolite participates as an electron donor in electron-transfer reaction with occluded NPH+ preferentially to the direct charge recombination with trapped electron. The spontaneous initial charge separation evolves to long-lived charge separated state as an electron-hole pair. Deactivation of the long-lived electron-hole pair is observed over a long period at room temperature. The pulsed EPR results provide evidence of the fate of ejected electron and electron-hole pair as isolated electrons. This technique demonstrates interaction between isolated electron and 1H and 13C nuclei of occluded naphthalene. Unfortunately, no evidence was found for interaction of unpaired electron with 27Al and 29Si nuclei of zeolite framework as previously described for analogous system using biphenyl.