Kinetics of electron–hole pair trapping via photoionization of biphenyl occluded in aluminium-rich ZSM-5 zeolites. Effects of extra-framework cations

Abstract

The effects of the extraframework cations M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ of aluminium rich M_6.6ZSM-5 zeolites on the long-lived photoproducts of occluded biphenyl (BP) have been investigated by conventional diffuse reflectance UV-visible (DRUVv) absorption and continuous wave electron paramagnetic resonance (CW-EPR). Many DRUVv and EPR spectra were recorded over two days after the UV laser photoionization (248 nm, 15 s, 30 mW cm⁻²) of dehydrated M_6.6(AlO₂)_6.6(SiO₂)_89.4 samples loaded with 1 BP per unit cell. The application of EPR measurements provided evidence of durable BP˙⁺-electron moiety through well resolved seven line signal (BP˙⁺) superimposed on a broad overlapping feature (trapped electron). The EPR spectra recorded during the development of the irradiated samples at room temperature indicated the BP˙⁺ disappearance and the persistence of broad signal. These persistent structureless signals were assigned to very long-lived electron–hole pairs. The application of the SIMPLISMA approach to DRUVv data sets recorded after the photolysis resolved two pure absorption spectra assigned to BP˙⁺ and electron–hole pairs (420–500 nm) for all the irradiated samples. All the decays of corresponding concentrations were found to fit accurately a model of first order dispersed heterogeneous kinetics. The BP˙⁺ disappearance rate constants were found to increase with larger alkali metal cations (Li⁺ ∼ Na⁺ < K⁺ ∼ Rb⁺ ≪ Cs⁺) and correlated to the electron-donating ability of the framework. The electron abstraction by BP˙⁺ did not induce direct charge recombination but generated persistent electron–hole pair over at most 1 h (Li⁺) and at least 1 min (Cs⁺). The electron–hole pair recombination went to completion over more than two days (M = Li⁺) and less than 1 h (M = K⁺). The recombination rate was found to be in relation with the electron donor properties of framework but probably involved other parameters.