Influence of framework Si/Al ratio and topology on electron transfer on zeolites

H-ZSM-5 and H-*BEA zeolites were hydrothermally synthesized with different Si/Al ratios (~12 to ~40). The physico-chemical properties of the resulting materials were fully characterized by several techniques (NMR, BET, PXRD, pyridine thermal desorption followed by infrared spectroscopy). To assess the effect of zeolite type and Si/Al ratio on sample reactivity, the charge separation processes between the zeolite framework and adsorbed trans-stilbene (t-St) molecule were investigated by UV-visible diffuse reflectance and FT-Raman spectroscopy. The UV-visible absorption spectra obtained after t-St adsorption show clear difference depending on the zeolite type. It appears that the radical cation resulting from the t-St spontaneous ionization is more stabilized in MFI-type framework than in *BEA topology. However, the amount and stability of electron-hole pair resulting from the radical cation evolution to a charged transfer complex are more important with *BEA zeolite. On the basis of the experimental results and the sample physico-chemical properties, we found that the radical cation and the electron hole stabilities are strongly dependent on the hexacoordinated aluminum (Al(VI)) amount and more precisely on their environment i.e. the vicinity between Lewis sites and strong Lewis sites or Bronsted Strong Lewis Pair (BSLP).