Spontaneous ionization and electron transfer of polyaromatics by sorption in ZSM-5 zeolites

Abstract

This article reviews the recent works of the authors about the spontaneous ionization and subsequent electron transfer of polyaromatic molecules upon sorption into ZSM-5 zeolites. The results demonstrate once again that the tight fit between the size of rod shape polyaromatic molecules and the diameter of the straight channel can stabilize radical cation-electron pairs or electron–hole pairs over long periods. Applying diffuse reflectance UV–visible absorption (DRUVv) spectrometry, Raman scattering spectrometry and continuous wave electron paramagnetic resonance (CW-EPR) we were able to monitor with in situ conditions the sorption and ionization of biphenyl (BP), naphthalene (NPH) and anthracene (ANTH) in ZSM-5 zeolite samples with M n (AlO 2) n (SiO 2) 96– n composition per unit cell (UC) ( n = 0–6.6; M = H +, Li +, Na +, K +, Rb +, Cs +, Mg 2+, Ca 2+). Particular emphasis was placed on pulsed electron paramagnetic resonance (EPR) spectrometry, which has shown to be a powerful technique for a new insight of the ejected electron. The spontaneous ionization is an intrinsic property of the inner surface of the porous materials, which depends both on the ionization potential of sorbate and the polarization energy of the host at the sorption site. For molecules with relatively low ionization potential such as ANTH (IP 7.44 eV) the ionization can occur in both dehydrated acidic and non acidic M n ZSM-5 ( n > 2; M = H +, Li +, Na +, Mg 2+, Ca 2+) zeolites. However, the ionization yield was found to highly depend on the nature of the extraframework cation and was found to decrease from H + (100%) > Li + (30%) ~ Mg 2+ ~ Ca 2+ >> Na + >> K +, Rb +, Cs + (0%). It was established that the ejected electron is trapped as isolated electron in the oxygen framework in close proximity of Al atoms and extraframework cations with some pairing electronic effect with the ANTH •+ radical cation. Calcination of acidic H n ZSM-5 under molecular oxygen is a prerequisite for the spontaneous ionization of BP (IP 8.16 eV) and NPH (IP 8.14 eV) with higher ionization potential. The spontaneous ionization of molecules with relatively high ionization potential such as NPH and BP was effective upon sorption after generation of electron acceptor Lewis acid sites. The oxidizing power of radical cations BP •+ and NPH •+ initiates at room temperature subsequent electron abstraction from the framework and generates unusual long-lived electron–hole pairs. The oxidizing power of ANTH •+ is inefficient at room temperature but is effective at 450 K. The electron and positive hole are trapped in the oxygen framework in close proximity of Al and proton with electronic interactions with the occluded sorbate before the final charge recombination. To cite this article: S. Marquis et al., C. R. Chimie 8 (2005).