Influence of hierarchization on electron transfers in structured MFI-type zeolites

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.

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

“…In a previous paper, [18] we established that the formation of this pair is linked to the ratio between BSLP and strong Lewis sites that are involved in the charge transfer process.…”

“…As the CTC formation mechanism requires radical cation formation, [18] as the first step, the disappearance of the radical cation spectral signature is linked to its poor stabilization in the *BEA framework at a low Si/Al ratio. In addition, to obtain structural information about the nature of the adsorbed system in the channels, the samples were also characterized by Raman scattering spectroscopy.…”

“…In a previous paper, [18] we have shown that radical cation amount and its stability depend on the presence in the zeolite framework of strong Lewis acid sites in the vicinity of Brønsted sites named BSLPs. We have shown that the maximum radical amount is linked to the BSLP proportion with respect to the strong Lewis sites.…”

“…This phenomenon has already been postulated in previous papers16 in order to explain separated charge stabilization. When the Lewis site forms with a Brønsted one a BSLP, the CTC is formed by ion exchange and electron transfer [18] (Fig. 11c3).…”

“…[17] Recently, we have demonstrated that the strength of Lewis and Brønsted acidic sites and the proximity between these two types of sites are of great importance in the spontaneous ionisation process and radical cation stabilization. [18] In this paper, we examine the influence of the Si/Al ratio on intra-zeolite reactivity by characterizing the charge separated states formed after adsorption and spontaneous ionization of t-stilbene within the porous network of MFI-and *BEA-type zeolites. Our objective is to interpret the observed reaction mechanisms by linking them to the intrinsic physico-chemical properties of the host materials determined through the use of several complementary characterization techniques.…”