Isabelle Gener

Sorption of anthracene, phenanthrene and 9,10-dimethylanthracene on activated acid HZSM-5 zeolite. Effect of sorbate size on spontaneous ionization yield

In situ diffuse reflectance UV visible and Raman scattering spectroscopies were used to monitor the spontaneous ionization of anthracene, phenanthrene and 9,10-dimethylanthracene by direct exposure to thermally activated acid HZSM-5 zeolite. Calcination of the zeolite under oxygen is a prerequisite for the spontaneous ionization through the formation of Lewis acid sites. The framework structure of ZSM-5 zeolites contains two types of intersecting channels sufficiently wide to allow rod-shaped molecules like anthracene to pass through and to diffuse into the void space. As expected, molecular modeling calculations suggest that anthracene is able to penetrate into the void space of zeolite and that phenanthrene and dimethylanthracene cannot pass through due to steric constraints. The anthracene radical cation is found to be generated in high yield and to be stable over several months. The tight fit between the shape of anthracene and the pore size of the straight channels of the zeolite is considered to be the main factor responsible for the stabilization of the anthracene radical ion. Dimethylanthracene is found to generate dimethylanthracene radical cation in very low yield. Dimethylanthracene radical cation is probably located at the pore openings and is not stabilized in the void space. Ionization of phenanthrene occurs in low yield. However the resulting stable radical cation is identified as anthracene radical cation. The assumption is made that phenanthrene isomerizes readily at the openings of pores via its radical cation.

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

The effects of the extraframework cations M+ = Li+, Na+, K+, Rb+, Cs+ of aluminium rich M6.6ZSM-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 M6.6(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.

14-P-18-Generation of long-lived electron-hole pairs through sorption of Biphenyl into acidic ZSM-5 zeolites

Publisher Summary This chapter presents the generation of long-lived electron-hole pairs through the sorption of biphenyl into acidic ZSM-5 zeolites. The electron paramagnetic resonance (EPR), ultraviolet (UV)-visible investigations as well as Raman scattering results provide informative clues about the formation and nature of long-lived electron-hole pairs through spontaneous biphenyl (BP) ionization upon sorption in the void space of activated ZSM-5 zeolites. The transferred electron is trapped within the framework by electron accepting site while biphenyl radical cation captures one electron from the electron donating sites of zeolite framework to restore BP ground state and causes an electron deficient hole.

Adsorption of butane isomers on ferrierite: influence of the Si/Al ratio

The adsorption and the kinetics of diffusion of n-butane and i-butane on three HFER were studied as a function of the global Si/Al ratio from 6 to 19. The capacity of adsorption, the kinetics of diffusion and the heat of adsorption are higher for n-butane than for i-butane. N-butane is preferentially adsorbed into the micropores while i-butane is rather adsorbed into the micropores and mesopores. The sites of adsorption are energetically homogeneous for sample HFER-10 whereas they are heterogeneous for HFER-6 and HFER-19.

14-P-24-Raman spectroscopic study of 2,2′-bipyridine sorbed into ZSM5

Publisher Summary This chapter presents the Raman spectroscopic study of 2,2′-bipyridine (bpy) sorbed into ZSM5. The differences in the spectra obtained from the loaded zeolites have been rationalized in terms of probable conformation within the zeolite channels. Bpy conformation depends on the aluminum content of the framework, nature of cations, and zeolite acidity. Bpy occluded in silicalite-1 is found to be in the trans conformation. The nonbonding interactions between the extraframework sodium (Na + ) cation and occluded bpy stabilize the cisoid nonplanar conformer, whereas the coordination bonding between zinc (Zn 2+ ) and nitrogen (N) atoms constrains the cis planar conformation. In HZSM-5, bpy sorption results in both mono and diprotonation of bpy.