Year: 2025
Type: dissertation
Abstract: 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 properties that are of great interest for applications in diverse fields such as energy, waste storage, or catalysis only emerge through structural defects. Nuclear Magnetic Resonance (NMR), unlike diffraction-based methods, is a local probe of the nucleus that allows for the characterization of each atomic environment, regardless of whether the material is ordered, disordered, or amorphous. In this manuscript, we focus on the study of active sites in various types of heterogeneous catalysts, particularly molybdenum-based catalysts used for olefin metathesis and a new type of amorphous alumino-silicate (AAS). These catalysts are central to numerous applications in sustainable chemistry, yet their structural characterization remains complex.In the first part of this work, we explored solid-state NMR of the 95Mo nucleus, an essential isotope for probing the local environment of molybdenum complexes involved in olefin metathesis. Despite the inherently low sensitivity of this nucleus, the initial spectra recorded on model complexes showed good resolution, with a wide range of chemical shifts and quadrupolar coupling constants. These results support the use of 95Mo NMR for the structural analysis of complex molybdenum-based molecular systems. The ultimate goal is to create a comprehensive database combining both NMR parameters and optimal acquisition conditions. This approach will gradually enable the study of real industrial catalytic systems, particularly molybdenum complexes grafted onto silica supports, and allow for a precise description of the active center using 95Mo NMR. These NMR measurements are complemented by Density Functional Theory (DFT) simulations to understand structure-property relationships and to build a structured database of various molybdenum environments encountered in catalysis.The second part of the thesis focuses on amorphous alumino-silicate materials (AAS), which are among the most effective solid acid catalysts used in refining and biomass conversion. Compared to microporous zeolites, AAS offers unrestricted mass transport, which leads to excellent performance in reactions involving large molecules. This makes them especially appealing for applications in the conversion of biomass into chemicals and renewable fuels. Due to their amorphous structure, these materials are challenging to characterize. Using high-field solid-state NMR spectroscopy on 1H, 29Si, and 27Al nuclei, we demonstrated that AAS contains two types of Brønsted acid sites: (i) pseudo-bridging silanols (SiOH···Al), and (ii) bridging silanols (Si-OH-Al). The former are stabilized by interaction with a neighboring Lewis acid site but exhibit lower acidity. Thus, the Brønsted acidity of pseudo-bridging silanols is lower than that of bridging silanols. Recently, it has been shown that AAS loaded with gold nanoparticles (Au/AAS) displays increased Brønsted acidity when exposed to light. This effect has been attributed to localized surface plasmon resonance (LSPR), which concentrates light energy near the nanoparticle surfaces, generating electric fields capable of inducing structural changes in the AAS. However, these changes have not yet been directly observed. The objective of this part of the study is therefore to monitor, using NMR, the structural modifications of the acid sites in Au/AAS systems under light irradiation
Author: Moussa, Céline
Institution: Unité de Catalyse et Chimie du Solide
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7128756263
Cites:
Cited by:
Related to: