[PhD Defense] - RM2I - Celine Moussa

Mme Anne Lesage 

Rapportrice

Université Claude Bernard Lyon

M. Eddy Dib

Rapporteur

CEMHTI Orléans

Mme Claire Marichal 

Examinatrice et Présidente

Université de Haute Alsace

M. Laurent Delevoye  

Directeur de thèse

Université de Lille 

M. Olivier Lafon  

Co-directeur de thèse

Université de Lille 

Solid-state NMR, quadrupole nuclei, solid catalysts, heterogeneous catalysis, molybdenum-based catalysts, olefin metathesis, amorphous silica-alumina, plasmonic photocatalysts, Brønsted acid, DFT

Rational inorganic synthesis methods enable the rapid development of new materials with innovative properties by employing a wide range of structural characterization techniques at the atomic scale. These techniques typically rely on the periodic arrangement of atoms in a crystal structure. However, many physical and chemical properties of materials only emerge through structural defects. In particular, these defects strongly influence the activity of heterogeneous catalysts. Solid-state nuclear magnetic resonance (NMR) spectroscopy, unlike diffraction-based techniques, allows probing the local environment of nuclei and hence, is well suited to characterize the atomic-level structure of defects, including active sites of heterogeneous catalysts. This PhD work focuses on the solid-state NMR characterization of active sites in two types of heterogeneous catalysts: molybdenum-based catalysts used for olefin metathesis and a new type of acid photocatalyst based on amorphous acidic aluminosilicate (AAS) functionalized by gold nanoparticles (Au/AAS). The rational improvement of the performances of these catalysts with numerous applications in sustainable chemistry is limited by the lack of knowledge about the atomic-level structure of their active sites. In the first part of this work, we explored how the local environment of Mo atoms in molybdenum-based catalysts can be probed by solid-state NMR of the 95Mo isotope. Despite the inherently low receptivity of this nucleus, we were able to record 95Mo NMR spectra of model Mo complexes with a wide range of chemical shifts and quadrupolar coupling constants. These experimental NMR parameters were compared to those calculated using Density Functional Theory (DFT) to investigate further the links between the local environment of 95Mo nuclei and their NMR spectra. These results support the use of 95Mo NMR for the structural analysis of complex molybdenum-based molecular systems. Furthermore, the obtained database of NMR parameters and optimal acquisition conditions will be useful to determine the structure of catalytic active sites in industrial Mo-containing heterogeneous catalysts, particularly molybdenum complexes grafted onto silica supports. The second part focuses on solid-state NMR characterization of Au/AAS acid photocatalysts, which exhibit increased Brønsted acidity when exposed to light. This effect has been attributed to localized surface plasmon resonance (LSPR) of gold nanoparticles, which could generate in their vicinity electric fields capable of inducing changes in the structure of Brønsted acid sites of AAS. However, there was a lack of experimental data about these structural changes. During my PhD, we developed and applied multinuclear (1H and 27Al) solid-state NMR experiments under light irradiation to evidence these structural changes due to LSPR in Au/AAS. In conclusion, this PhD work opens new avenues for the solid-state NMR characterization of two classes of heterogeneous catalysts: those containing molybdenum and plasmonic photocatalysts.