Silica-based nanotubes and hollow nanospheres

: a transversal approach to understand their synthesis mechanism followed by applications in catalysis and drug release

Résumé

The development and deep understanding of materials with a high application potential represents a field of great interest. If considering chemical processes, the development of efficient methods based on the use of renewable or waste as starting materials while reducing energy consumption and waste generation emerged as a necessary achievement. The energy efficiency of most of the chemical transformations can be considerably enhanced by using a catalyst (i.e. working under lower temperature, reduction of reaction time). Moreover, the development of heterogeneous catalysts that can be easily separated from the reaction mixture (e.g. by simple filtration), reused in multiple cycles or even implemented in continuous flow processes, will contribute to enhance even more the sustainability. As such, there is a strong need to develop and more importantly, enhance our fundamental understanding concerning the synthesis and performances of new materials evolved in biomass upgrading processes.
Among the possible candidates, the thesis focused on porous low dimensional silica-based nanostructures presenting targeted morphology; nanotubes and hollow nanospheres. These solids display promising textural properties with high specific surface area (500 – 700 m2/g), large pore volume (about 2 cm3/g) and wide mesopores (14 – 27 nm). Their low dimensionality and mesoporosity are ideal features for their application as heterogeneous catalyst or vehicle for drug release, for example. To reach an accurate control of their morphology, the formation mechanism of silica nanotubes and hollow nanospheres is first investigated. The tuning of the stirring speed is evidenced as the easiest parameter to turn the system towards the formation of spheres or tubes. Then, silica nanotubes and nanospheres are applied for the enhanced release of curcumin. A slight effect of the silicate morphology is evidenced. Moreover, the possibility to insert heteroelement such as Hf, Ga and Zr at the targeted Si/M ratio, in the structure of silica nanotubes and/or nanospheres is deeply studied across the manuscript. M-doped silica nanostructures show Brønsted and Lewis acid sites, useful for the conversion of biomass derivatives. In particular, they show an excellent activity for the conversion of glycerol into solketal and the conversion of ethyl levulinate to ɣ-valerolactone. We highlight that the conversion of glycerol into solketal is favored by Brønsted acid sites while the conversion of ethyl levulinate to ɣ-valerolactone is enhanced with the presence of Lewis acid sites. By comparing the catalytic activity of M-doped nanotubes and hollow nanospheres, it is also shown that the morphology of a porous low D silicate is a key factor determining its catalytic activity. In conclusion, this work contributes to unveil the formation mechanism of silica nanotubes and hollow nanospheres, hence to an accurate control of their morphology. This PhD thesis also contributes improving the fundamental understanding on the structure-activity relationship of low D mesoporous silicates.

la date de réponse	14 sept. 2023
langue originale	Anglais
L'institution diplômante	Universite de Namur
Sponsors	Fund for Research Training in Industry and Agriculture (FRIA)
Superviseur	Carmela Aprile (Promoteur), Damien P. Debecker (Copromoteur), Stephane Vincent (Président), Sophie Hermans (Jury), Cèdric Boissiére (Jury) & Michel Dusselier (Jury)