LAboratoire de Spectrochimie Infrarouge et Raman – UMR 8516
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Molecular confinement and photo-controlled reactivity in porous media

Participants : V. De Waele , M. Hureau , A. Moissette , O. Poizat , K. Smirnov

Microporous materials (zeolites, aluminosilicates …) have porous structures able to incorporate molecular species of various sizes/shapes and to act as a confined reaction space with high electrostatic field. More and more inventive synthesis routes for obtaining innovative zeolite materials provide access to a wide variety of porous frameworks and internal surfaces. Our action aims at understanding the confinement phenomena in these host systems and their impact on the photoreactivity and reaction dynamics of guest molecules in order to optimize and/or broaden the scope of application of such materials. Recent works are particularly devoted to new emerging materials: hierarchical micro- meso-porous zeolites, nanostructured zeolites, nanozeolites, thin zeolitic films. In parallel, on a fundamental level, we develop a theoretical model of polarizable force field in molecular dynamics for studying the influence of the confinement on the electronic properties of the adsorbed molecules.

1. Charge transfer and charge separation mechanisms in [host zeolite network / guest organic molecule] hybrid systems

Stable interfacial charge separation states strongly absorbing in the sunlight domain of the spectrum have been shown to be formed upon intrazeolite sorption of polyaromatic molecules. We work on understanding and controlling the underlying chemical mechanisms. Two key issues are mainly addressed:

The control of the charge recovery after charge separation based on the grafting of an electron acceptor species at the surface of the zeolite particles, for applications in photovoltaic conversion. New nanostructured zeolite architectures such as nanosheets, where the mean diffusion distance for charge capture by the acceptor species is drastically reduced and, thus, the probability of deactivation by charge recombination restricted, are tested.

The influence of textural properties and confinement effect on the reactivity of host molecules in hierarchical acidic zeolites widely used in heterogeneous catalysis by petrochemical industries. The efforts focus especially on the effect of micro- meso-structuration on the formation efficiency and stability of the cationic products of spontaneous ionization of model aromatic compounds mimicking petroleum coke. Understanding the charge transfer processes arising after ionization is an essential task as these processes could be responsible for the catalysts deactivation in many industrial processes.

Fig. 9. Molecular modelling of the conformation of salicylidene aniline sorbed in silicalite 1 zeolite (Ref. P 2014-14).

Fig. 10. Schematized representation of the intrazeolitic electron transfer induced by photoexcitation of trans-stilbene in Mordenite zeolite grafted by TiO2 nanoclusters (Ref. P 2014-27).

Fig. 11. Spontaneous (left) and photoinduced (right) intrazeolitic charge-transfer processes of trans-stilbene dans la zéolithe Na6,6ZSM-5 (Ref. P 2012-56, P 2012-58).

2. Photo-activated thermochemical processes

Plasmon chemistry is the conversion of light energy into chemical energy based on the coupling of photo-excited metal nanoparticles hot electrons with the vibrational states of nearby reactants. Our studies aim at exploring the conditions of utilization of new materials made of zeolite nanocristals containing metal nanoparticles and assembled in the form of thin films for the plasmonic assisted photo-thermal activation of chemical reactions within the zeolite micropores. The zeolite porous volume can behave as chemical nanoreactors provided that energy can be selectively deposited at the nano-scale in a controlled way. In this regard, we examine the possibility of using metal nanoclusters incorporated in zeolite as photoactivatable local heat source. Using ultrafast laser pulses for exciting the nanoparticles can induce an instentaneous local temperature jump of the reactants in the porous volume, which can serve as thermal activation source for non-diffusion-limited chemical processes. This opens up possibilities for controlling optically the chemical reactivity in the zeolite nanocavities can be anticipated, thus offering a window of opportunity for real-time tracking reaction dynamics with high time resolution by using pump-probe transient spectroscopic techniques, in the same way as for photoreactions.

The main goal is to understand the elementary chemical processes in heterogeneous catalysis and to open up new selective reaction schemes favored by the combined effects of the pulsed energy deposition (as opposed to stochastic thermal processes) and reactants containment. This project is linked to the emergence of novel materials composed of an assembly of zeolite nanocristals promising for the realization of photosensitive and photoactivatable thin layers, the functionalization of which by metal nanoparticles (Ag, Cu, Pt …) is under control in the team.

Fig. 12. Ultrafast optical response of subnanometric Ag clusters confined in LTL zeolite, following excitation within the silver plasmon band. The energy of the hot electron gas is relaxed in part via electron-phonon coupling toward the metal clusters, but also in part directly to the surroundings, with the possibility to induce specific and selective chemical reaction processes (collaboration LCS, ENSICAEN, Caen et MACS, Institute Charles Gerhardt, Montpellier).