Influence of hierarchization on electron transfers in structured MFI-type zeolites. Issue 42 (22nd October 2018)

• Record Type: Journal Article
• Authors: Cremoux, T.; Batonneau-Gener, I.; Moissette, A.; Paillaud, J.-L.; Hureau, M.; Ligner, E.; Morais, C.; Laforge, S.; Marichal, C.; Nouali, H.
• Is Part Of: Physical chemistry chemical physics. Volume 20:Issue 42(2018)
• Page Start: 26903
• Page End: 26917
• Publication Date: 2018-10-22
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c8cp03485k
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : From an experimental study on H-ZSM-5 zeolite, it is shown that an optimal proportion of Brønsted Strong Lewis Pairs (BSLP) are responsible for the observed amount of adsorbed trans-Stilbene radical cations. A mechanism for their formation and the charge transfer complex (CTC) is also proposed. Abstract : H-ZSM-5 zeolite (Si/Al = 19.3) was hydrothermally synthesized. Alkaline and/or acid post-synthesis treatments were carried out to give rise to an interconnected mesoporous volume. The desilication treatment parameters have been tuned (temperature, organic base addition) to obtain a series of samples with increasing mesoporous volume and a constant number of acid sites. The physico-chemical properties of the resulting materials were fully characterized by many techniques (NMR, BET, PXRD, and pyridine thermal desorption followed by infrared spectroscopy). To assess the effect of post-treatments 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. The spectra obtained after t -St adsorption show clear differences depending on the applied post-treatments. It appears that the desilication treatments performed without acidic washing highly stabilize the radical cation resulting from the t -St spontaneous ionization. In contrast, by applying acidic washing after desilication, the ionization process becomes significantly weaker. The results show that the proportion of strong Lewis acid sites in the vicinity of Brønsted sites named Brønsted Strong Lewis Pairs (BSLP), are responsible for the amount of radical cations observed in the different samples. More precisely, it exists an optimal proportion of BSLP to achieve a high ionization rate. On the basis of the experimental results a mechanism for the formation of the t -St radical cation and the charge transfer complex (CTC) is proposed.