Long Lived Charge Separated States Induced by trans-Stilbene Incorporation in the Pores of Brønsted Acidic HZSM-5 Zeolites: Effect of Gallium on the Spontaneous Ionization Process

Abstract

In situ CW-EPR, diffuse reflectance UV-visible spectroscopy and Raman scattering were used to monitor the spontaneous incorporation of trans-stilbene (t-St, C 14 H 12 ) in the medium pore H 2.2 -GaZSM-5 zeolites [H 2.2 (GaO 2 ) 2.2 (SiO 2 ) 93.8 ] by direct exposure under dry and inert atmosphere of solid t-St to dehydrated porous material without any solvent. The sorption of t-St with relatively low ionization potential (7.65 eV) occurs in Brønsted acidic H 2.2 -GaZSM-5 zeolites according to a complex and slow reaction sequence. First, charge separation occurs and t-St •+ @H 2.2 -GaZSM-5 •radical pair is created, while long-lived t-St@H 2.2 GaZSM-5 •-•+ electron-hole pair is formed through hole transfer. The analysis of the DRUVv spectra set recorded during the t-St sorption course shows the respective concentrations of all transient species as a function of time. In particular, note that system reorganization is observed through a second type of electron-hole pair. The broad and strong bands observed in the near-IR regions over extended periods of time are tentatively assigned to the electron and/or hole spectral signatures in slightly different environments. Applying pulsed X-band EPR techniques, we were able to reveal the structural surrounding of the unpaired electrons of chargeseparated states through the proper assignment of electron couplings with a large number of nuclei such as 1 H, 29 Si, 69 Ga, and 71 Ga using the two-dimensional hyperfine-sublevel correlation experiment (2D-HYSCORE). The distance measurements deduced from dipolar coupling experiments provide a unique picture of the long distance distribution of unpaired electrons generated by spontaneous ionization of t-St upon incorporation within H 2.2 -GaZSM-5 zeolite. This result demonstrates that a large fraction of the unpaired electrons are ejected away from the initial site of ionization and that this compartmentalization plus the created electrostatic field hinder dramatically the propensity of charge recombination. The results for H-GaZSM-5 are compared to similar experiments conducted on H-AlZSM-5 zeolites.

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

“…Even if the disappearance of the radical cation can take place directly by geminated recombination, the evolution of the relative concentrations of the radical cation and CTC demonstrates an electron transfer process that reflects the oxidizing power of the radical cation (E ox = 1.75 V/SCE) with respect to the electron donor character of the zeolite framework. [42] As described in many studies carried out with this type of polyaromatic molecule, [1] the radical cation captures another electron belonging to the internal surface of the zeolite by transfer of electronic hole to generate a long-lived electron/hole pair that spectrally leads to a charge transfer complex band. Fig.…”

Comparison between Spontaneous and Photoinduced Ionization Mechanisms for p-Quaterphenyl in M-ZSM-5 (M = H+, Na+) Zeolites

“…Only the observation of a broad signal in the near infrared region around 1400-1600 nm could be associated with such trapped electron as already reported for analogous zeolite systems after spontaneous ionization. 24,27 The evolutions of the spectral concentrations C k (t) as function of time of both the extracted species involved in the p-QPh sorption in H-ZSM-5 zeolite were also calculated by the MCR method using the spectra obtained for one year and are presented on figure 2B. Curve a shows that the spectral concentration of the radical cation remained very low during the first week following the mixing.…”

“…In the absence of spontaneous ionization, the molecules are incorporated in preferential sorption sites and require photoexcitation to be ionized. [27][28][29][30][31][32][33][34][35][36][37][38] However, previous works have demonstrated the analogy between the reaction mechanisms occurring after the photo or spontaneous ionization including radical stabilization, charge shifting reactions and final charge recombination. 22,28,32,39,40,41 In both cases, the combined effects of the confinement and of the internal electrostatic field play a major role in the electron transfer kinetics but the development of applicative systems require thorough control of such parameters and clear understanding of the observed reactivity.…”

Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

“…The electron trapped in the AlO 4 HC À site exhibits no evident absorption in the visible range, but shows an intense broad band around 1500 nm in the NIR region, as reported for incorporation of alkali metals in zeolites. [27,28] The resolved spectra b and c in Figure 2 with bands at 512 and 552 nm and at 619, 672, and 750 nm, respectively, have no counterpart in solution chemistry of PP n , PP n C + , PP n C À , or PP n 2 + (n = 2-6). [7] No oligomerization reaction was detected for neutral molecule PP n or radical cation PP n C + .…”