Charge transfers in hierarchically structured zeolites: A new challenge to take up for catalysis – HIZEOTREL

The work is divided into four tasks: zeolites and germanosilicates synthesis, preparation of different sets of modified zeolites, sample accurate physicochemical characterization (porosity, Brønsted and Lewis acidities, Si/Al and Ge/Al ratios, crystallite size, defects, ...) , aromatic molecules adsorption and spectroscopic characterization of radical species and resulting charge separated states.

Zeolites (ZSM-5, MOR, BEA) will be synthesized and modified. For each zeolite type, various series of sample were prepared in which only one physico-chemical properpy was gradually modified.
The electrostatic field progressive modification will be obtained by varying the nature and the number of charge balancing cations (exchange). The mesoporosity creation will be performing alkaline desilication treatments. The acidity modulation will be obtained by varying gradually the number framework aluminum atoms. This quantity can be adjusted either during the synthesis or by post-synthesis dealumination treatments. These treatments can modify selectively the acidity of Brønsted or Lewis with or without modification of the porosity according to the operating conditions. The size of crystallites will be checked during the synthesis. The series of samples will be characterized by numerous techniques: elementary analysis, electronic microscopies, 1H, 29Si and 27Al MAS-NMR, XRD, N2 adsorption at 77 K, adsorption and desorption of pyridine followed by FT-IR (acidity). The characterized samples will then be sent to the LASIR for the adsorption of aromatics. The characterization of adsorbed neutral molecules will be performed by XRD, diffuse reflectance UV-vis spectroscopy and FT Raman. The characterization of free radical species generated spontaneously (or photoinduced) will be carried out by UV-Vis absorption, resonance Raman spectroscopy, ESR.

T0+30 results
The syntheses of all the zeolites were carried out and the resulting samples fully characterized at IS2M. The post-synthesis modifications of the MFI, MOR and BEA type zeolites were carried out. We have several series of samples where a single physicochemical parameter varies: crystallite size, si / Al ratio, compensating cation, porosity, Bronsted acid sites.
The first results for the modifications by alkaline treatment show that the porosity modification will also influence the acidity and more particularly the spatial distribution of the acid sites. The proximity of Bronsted-Lewis pairs appears to strongly influence the charge transfer properties in H-ZSM-5 zeolite.

Proton NMR studies studies have been planed to better explain the observed phenomena.

International congress :
1. Communication par affiche. 06/2016. International Zeolite Congress, Rio de Janeiro. Electron transfers in hierarchically structured MFI zeolites.
2. Communication orale. 07/2017. 7eme International FEZA conference, Sofia (Bulgarie). Influence of hierarchization, Bronsted and Lewis acidities on electron transfer in structured MFI zeolites.

National congress :
1. Communication par affiche. Groupe Français des zéolithes.03/2016. Montpellier. Transferts de charges dans les zéolithes de structure MFI et MOR hiérarchisées et à pores extra-larges.
2. Communication orale. Groupe Français des zéolithes.03/2017. Paris. Influence de la hiérarchisation sur le transfert d’électrons dans les zéolithes structurées de type MFI

The phenomenon of charge separation resulting from the aromatic molecule adsorption in the zeolite pore volume has already been the subject of many publications. The ionization process responsible for this separated charge state can be initiated by photolysis, radiolysis, electrochemical or thermally as in the case of spontaneous ionization. Indeed, it is now well established that the adsorption of an aromatic molecule with a relatively low ionization potential in zeolite micropores can lead to its spontaneous ionization and to the creation of a separated charge state. Acidic zeolites are widely used in heterogeneous catalysis in the petrochemistry and refining. In conventional catalytic processes, the reaction is considered to proceed via the formation and the transformation of carbocations. Nevertheless, the identification of radical species in the coke of deactivated zeolite catalysts whereas these solids still exhibit an important catalytic activity opens a new way of thinking for catalysis. However, the complexity of the coke molecules do not allow a clear identification of the radical species or the separated charge states responsible for coke formation and/or catalytic activity enhancement. For a better understanding of these phenomena, it is crucial to elucidate the parameters that control their generation and their stability in order to develop more efficient heterogeneous catalysts or new hybrid materials for energy conversion. In this context, this project aims to investigate the influence of the physicochemical properties (Brønsted and Lewis acidity, Si/Al or (Si+Ge)/Al, crystallites size, pore size, mesoporosity, defects ...) of modified zeolites on the radical species formed (nature and lifetime) inside their pores from the adsorption of aromatic molecules which model coke molecules (naphthalene, anthracene, phenanthrene ...). The solids chosen for this project are zeolites (ZSM-5, MOR, BEA) widely used in the petrochemical industry and germanosilicates with extra large micropores. The idea is, for each family of material, to gradually change during the synthesis or by post-synthesis treatments (desilication, dealumination, cation exchange, degermination), a single property among those mentioned above to assess the impact of this property on the radical species generated during the aromatic molecule adsorption. Particular attention will be paid to the study of the influence of acidity, secondary porosity and crystallite size on the radical species formed. Indeed, the diffusion limitations often responsible for the catalytic activity decrease and the zeolite deactivation can be reduced by decreasing the zeolite crystallite size or by creating a wider secondary porosity (hierarchically structured zeolites by desilication or dealumination). However hierarchical zeolites have a large amount of structural defects due to the partial destruction of their framework. So, different structures of germanosilicates with extra-large pores and free of defects are also chosen for this study (IM-12, IM-17, ITQ-13, ITQ-22). These materials have an intrinsic Brønsted acidity interesting for catalysis applications which can be enhanced by increasing Al content. The behavior of radicals generated on such solids, not yet reported in the literature, could certainly help us to better understand the role of extraframework species in the generation of separated charge states. Thus, this project focuses on four main tasks: zeolite synthesis and modification (dealumination, desilication), physico-chemical characterizations, aromatic molecule adsorption and spectroscopic characterization of separated charge states. At the end of the project, all these tasks will hopefully allow us to assess the influence of each chemical or physical property on the spontaneous generation of radical cations and the subsequent separated charge states created in the pore volume.